Evaluating the use and limitations of the Danish National Patient Register in register-based research using an example of multiple sclerosis

Identifying people living with cystic fibrosis in the Danish National Patient Registry: A validation study

BACKGROUND

The Danish National Patient Registry (DNPR) serves as a valuable resource for scientific research. However, to ensure accurate results in cystic fibrosis (CF) studies that rely on DNPR data, a robust case-identification algorithm is essential. This study aimed to develop and validate algorithms for the reliable identification of CF patients in the DNPR.

METHODS

Using the Danish Cystic Fibrosis Registry (DCFR) as a reference, accuracy measures including sensitivity and positive predictive value (PPV) for case-finding algorithms deployed in the DNPR were calculated. Algorithms were based on minimum number of hospital contacts with CF as the main diagnosis and minimum number of days between first and last contact.

RESULTS

An algorithm requiring a minimum of one hospital contact with CF as the main diagnosis yielded a sensitivity of 96.1 % (95 % CI: 94.2 %; 97.4 %) and a PPV of 84.9 % (82.0 %; 87.4 %). The highest-performing algorithm required minimum 2 hospital visits and a minimum of 182 days between the first and the last contact and yielded a sensitivity of 95.9 % (95 % CI: 94.1 %; 97.2 %), PPV of 91.0 % (95 % CI: 88.6 %; 93.0 %) and a cohort entry delay of 3.2 months at the 75th percentile (95th percentile: 38.7 months).

CONCLUSIONS

The DNPR captures individuals with CF with high sensitivity and is a valuable resource for CF-research. PPV was improved at a minimal cost of sensitivity by increasing requirements of minimum number of hospital contacts and days between first and last contact. Cohort entry delay increased with number of required hospital contacts.

Neonatal outcomes in women with Multiple Sclerosis - Influence of disease activity: A Danish nationwide cohort study

BACKGROUND

Maternal Multiple Sclerosis (MS) has been associated with an increased risk of adverse birth outcomes. We hypothesized that active disease during conception and pregnancy plays an important role in this context, which this study aims to address.

METHODS

We used the Danish registers to conduct a nationwide cohort study. Information on maternal disease activity during pregnancy was retrieved using proxies from the linked registers (hospitalization, magnetic resonance imaging of the brain, and use of systemic corticosteroids during pregnancy). Neonates, exposed in utero to maternal disease activity constituted the exposed cohort and the unexposed cohort constituted neonates without in utero exposure to maternal disease activity. The examined outcomes were preterm birth, small for gestational age, low 5-minute Apgar score, and major congenital anomalies. In logistic regression models we estimated the odds ratios (OR) with adjustment for confounders such as maternal age, comorbidities, parity, smoking, calendar year of birth, and disease-modifying treatment.

RESULTS

Among the study population of 2492 children of mothers with MS we identified 273 (11 %) neonates exposed to maternal disease activity during pregnancy, and 2219 (89 %) neonates without exposure to disease activity. The adjusted odds ratios (aOR) for preterm birth, small for gestational age, low 5-minute Apgar score, and major congenital anomalies among children born to women with disease activity during pregnancy were 0.92 (95 % confidence interval (95 % CI) 0.53-1.60), aOR 1.19 (95 % CI 0.62-2.26), aOR 2.57 (95 % CI 0.93-7.15) and aOR 0.93 (95 % CI 0.48-1.83), respectively.

CONCLUSIONS

Women with MS having disease activity during pregnancy did not have a statistically significantly increased risk of adverse neonatal outcomes compared to women with MS without disease activity, which is overall reassuring results. We believe, that this will be useful knowledge for patients and clinicians in planning a pregnancy and preparing a birth plan.

Smoking during pregnancy and risk of multiple sclerosis in offspring and mother: A Danish nationwide register-based cohort study

BACKGROUND

The association between intra-uterine exposure to maternal smoking and risk of multiple sclerosis (MS) has been little studied and with conflicting results.

OBJECTIVE

To examine the risk of MS in offspring exposed intra-uterine to maternal smoking. In addition, to re-examine prior observations of an elevated risk of MS among smokers, assuming that self-reported smoking during pregnancy reflects the woman's general smoking habits.

METHODS

The study cohort included all Danish women, pregnant in the period 1991-2018, (n = 789,299) and singletons from these pregnancies (n = 879,135). Nationwide information on maternal smoking during pregnancy and MS cases in the study cohort were obtained from the Medical Birth Register and the National Patient Register. Cox regression analysis was used to estimate hazard ratios (HRs) for the association between smoking and MS risk.

RESULTS

Women who smoked during pregnancy had a 42% increased risk of developing MS compared with non-smoking women (HR = 1.42 (1.32-1.52), n = 1,296). The risk of MS among singletons of women who smoked during pregnancy was 38% higher than that among singletons born to non-smoking women (HR = 1.38 (1.08-1.76), n = 110).

CONCLUSION

Our observations add further to the evidence implicating smoking in the development of MS and suggest that intra-uterine exposure to tobacco smoke may increase MS risk.

Patients with multiple sclerosis: COVID-19 related disease activity and hospitalisations based on a nationwide cohort study

BACKGROUND

It has not previously been clarified if COVID-19 triggers disease activity and increases the risk of hospitalisation with COVID-19 in patients with multiple sclerosis. We examined the association between COVID-19 and the use of systemic corticosteroids prescriptions and hospital contacts at neurological departments as proxies of disease activity among patients with multiple sclerosis. Furthermore, we examined whether patients with multiple sclerosis were more likely to be hospitalised with COVID-19 compared to references.

METHODS

This study was based on nationwide health registries with data on the Danish population of 2,222,946 individuals with a positive COVID-19 PCR test. To study disease activity our study population consisted of all patients with multiple sclerosis and a positive COVID-19 PCR test. Our proxies for disease activity were compared after versus before a positive COVID-19 PCR test using a binomial regression model. Adjustments were made for age, sex, comorbidity, length of multiple sclerosis diagnosis, calendar period, vaccination, and immunomodulatory treatment. To study the risk of hospitalisation with COVID-19 in patients with multiple sclerosis our study population here consisted of all Danish citizens with a COVID-19 positive PCR test. In logistic regression models we estimated odds ratio (OR) for hospitalisation with COVID-19 in patients with multiple sclerosis relative to patients affected with other autoimmune diseases (inflammatory bowel disease/rheumatoid arthritis), and relative to individuals from the general population. Adjustments were made for age, sex, comorbidity, vaccination, and calendar period. To examine the impact of disease-modifying treatment, the risk of hospitalisation with COVID-19 was estimated in those with disease-modifying treatment versus those without any disease-modifying treatment.

RESULTS

We included 7358 patients with multiple sclerosis and a positive COVID-19 PCR test. The adjusted incidence rate ratio (aIRR) for having corticosteroid prescriptions after COVID-19 in patients with multiple sclerosis was 0.93 (95 % CI 0.78 - 1.10), and the aIRR for hospital contacts at neurological departments/admissions with multiple sclerosis as primary diagnosis after COVID-19 infection was 1.10 (95 % 1.00-1.22). Adjusted OR (aOR) for hospitalisation with COVID-19 in the 30 days after a positive COVID-19 PCR test was 3.21 (95 % CI 2.75-3.74) compared to patients with other autoimmune diseases and the aOR was 5.34 (95 % CI 4.65-6.14) for patients with multiple sclerosis compared to all other individuals in the general population with a positive test. We found no increased risk of hospitalisations with COVID-19 in patients with multiple sclerosis using disease-modifying treatment 6 months prior to a positive COVID-19 PCR test compared to patients with multiple sclerosis without disease-modifying treatment (aOR 0.94 (95 % CI 0.69-1.27)).

CONCLUSIONS

In this nationwide cohort of patients with multiple sclerosis, COVID-19 did not seem to trigger multiple sclerosis disease activity (based on proxy variables). We found a significantly increased risk of being hospitalised with COVID-19 in the first 30 days after a positive COVID-19 PCR test in patients with multiple sclerosis irrespective of the type of reference population. In patients with multiple sclerosis, the use of disease-modifying treatment did not increase the risk of hospitalisation with COVID-19.

Siblings reduce multiple sclerosis risk by preventing delayed primary Epstein-Barr virus infection

Epstein-Barr virus infection, and perhaps almost exclusively delayed Epstein-Barr virus infection, seems to be a prerequisite for the development of multiple sclerosis. Siblings provide protection against infectious mononucleosis by occasionally preventing delayed primary Epstein-Barr virus infection, with its associated high risk of infectious mononucleosis. Each additional sibling provides further protection according to the age difference between the index child and the sibling. The closer the siblings are in age, the higher the protection, with younger siblings being more protective against infectious mononucleosis than older siblings. If the hypothesis that delayed Epstein-Barr virus infection is necessary for the development of multiple sclerosis is true, then the relative risk of multiple sclerosis as a function of sibship constellation should mirror the relative risk of infectious mononucleosis as a function of sibship constellation. Such an indirect hypothesis test is necessitated by the fact that age at primary Epstein-Barr virus infection is unknown for practically all people who have not experienced infectious mononucleosis. In this retrospective cohort study using nationwide registers, we followed all Danes born during the period 1971-2018 (n = 2 576 011) from 1977 to 2018 for hospital contacts with an infectious mononucleosis diagnosis (n = 23 905) or a multiple sclerosis diagnosis (n = 4442), defining two different end points. Relative risks (hazard ratios) of each end point as a function of sibship constellation were obtained from stratified Cox regression analyses. The hazard ratios of interest for infectious mononucleosis and multiple sclerosis could be assumed to be identical (test for homogeneity P = 0.19), implying that having siblings, especially of younger age, may protect a person against multiple sclerosis through early exposure to the Epstein-Barr virus. Maximum protection per sibling was obtained by having a 0-2 years younger sibling, corresponding to a hazard ratio of 0.80, with a 95% confidence interval of 0.76-0.85. The corresponding hazard ratio from having an (0-2 years) older sibling was 0.91 (0.86-0.96). Our results suggest that it may be possible essentially to eradicate multiple sclerosis using an Epstein-Barr virus vaccine administered before the teenage years. Getting there would require both successful replication of our study findings and, if so, elucidation of why early Epstein-Barr virus infection does not usually trigger the immune mechanisms responsible for the association between delayed Epstein-Barr virus infection and multiple sclerosis risk.

Past, Present, and Future Labor Market Participation Among Patients Admitted to Hospital With Concurrent Substance Use and Mental Health Disorder, and What We Can Learn From It

OBJECTIVE

The aim of the study is to investigate the labor market participation of patients with concurrent substance use and mental health disorder before treatment, as well as the strength of the barriers to re-enter the labor market.

METHODS

The study population is composed of individuals with concurrent substance use and mental health disorder at a psychiatric unit in 1996, 2001, 2006, 2011, and 2016. The number of self-supported years 20 years back in time, as well as present and subsequent employment, was calculated.

RESULTS

From 1996 to 2016, there was a decrease by 43.8% in self-supported years before treatment and by 36.4% in the fraction of patients working at the time of admission.

CONCLUSIONS

The results point to increasing difficulties with regard to labor market attachment among individuals with mental illnesses.

Pregnancy loss and risk of multiple sclerosis and autoimmune neurological disorder: A nationwide cohort study

Background

The loss of one or more pregnancies before viability (i.e. pregnancy loss or miscarriage), has been linked to an increased risk of diseases later in life such as myocardial infarction and stroke. Recurrent pregnancy loss (i.e. three consecutive pregnancy losses) and multiple sclerosis have both been linked to immunological traits, which could predispose to both occurrences. The objective of the current study was to investigate if pregnancy loss is associated with later autoimmune neurological disease.

Methods

This register-based cohort study, included the Danish female population age 12 or older between 1977–2017. Women were grouped hierarchically: 0, 1, 2, ≥3 pregnancy losses, primary recurrent pregnancy loss (i.e. not preceded by a delivery), and secondary recurrent pregnancy loss (i.e. preceded by a delivery). The main outcome was multiple sclerosis and additional outcomes were amyotrophic lateral sclerosis, Guillain-Barré syndrome, and myasthenia gravis. Bayesian Poisson regression estimated incidence rate ratios [IRR] and 95% credible intervals [CI] adjusted for year, age, live births, family history of an outcome, and education.

Results

After 40,380,194 years of follow-up, multiple sclerosis was diagnosed among 7,667 out of 1,513,544 included women (0.5%), median age at diagnosis 34.2 years (IQR 27.4–41.4 years), and median age at symptom onset 31.2 years (IQR 24.8–38.2). The adjusted IRR of multiple sclerosis after 1 pregnancy loss was: 1.03 (95% CI 0.95–1.11), 2 losses: 1.02 (95% CI 0.86–1.20), ≥3 non-consecutive losses: 0.81 (95% CI 0.51–1.24), primary recurrent pregnancy loss: 1.18 (95% CI 0.84–1.60), secondary recurrent pregnancy loss: 1.16 (95% CI 0.81–1.63), as compared to women with no pregnancy losses. Seven sensitivity analyses and analyses for additional outcomes did not show significantly elevated adjusted risk estimates.

Conclusions

In this nationwide study, pregnancy loss was not significantly associated with autoimmune neurological disorder.

Treatment Patterns in Danish Patients with Atopic Dermatitis Before and After Hospital Referral

Introduction

A number of treatments for atopic dermatitis (AD) are available; however, long-term treatment patterns and healthcare consumption in patients with AD are poorly described.

Methods

We conducted a registry-based longitudinal drug utilization study among Danish patients with AD that were referred to their first-ever visit at hospital-based dermatology clinics. Their first visit was in the period between 1 January 2005 and 31 December 2012, and patients were followed up to 5 years after their first visit.

Results

In total, 8213 people with a first-time hospital dermatologist contact for AD were included in the study (3514 aged 0–9 years, 1501 aged 10–19 years, 3198 aged 20 years or older). At first visit, a baseline history of moderately potent topical corticosteroid (TCS) use was seen among 46.6% of children (0–9 years), whereas potent or very potent TCS use was more frequently among older individuals (e.g., 51.1% and 25.6% of people aged 50 years or older had used potent and very potent TCS, respectively). The median (interquartile range) annual number of visits to general practitioners was 4 (2–7) for children and 5 (2–8) for adults, in the 12 months prior to referral. Three years after referral, these numbers had decreased to 2 (1–4) and 3 (1–6), respectively. In the first year after referral, 6% of patients were prescribed systemic corticosteroids, whereas other systemic therapies were used in 5% or less.

Conclusions

After referral, low proportions of patients received systemic treatment, or potent TCS. These findings highlight considerable differences in treatment patterns between general practitioners and private practice dermatologists, compared with hospital-based dermatologists, and emphasize the need for better adherence to evidence-based treatment guidelines.

The Danish Multiple Sclerosis Registry

OBJECTIVES

The Danish Multiple Sclerosis Registry is the oldest operative and nationwide MS registry. We present The Danish Multiple Sclerosis Registry with its history, data collection, scientific contribution, and national and international research collaboration.

MATERIALS AND METHODS

Detailed description of data collection, completeness, quality optimizing procedures, funding, and legal, ethical and data protection issues are provided.

RESULTS

The total number of registered cases with clinical isolated syndrome and multiple sclerosis since 1956 was by start of May 2020 30,023 of whom 16,515 cases were alive and residing in Denmark, giving a prevalence rate of about 284 per 100,000 population. The mean annual number of new cases receiving an MS diagnosis was 649 per year in the period 2010 to 2019. In total, 7,945 patients (48.1%) are receiving disease modifying therapy at the start of May 2020.

CONCLUSIONS

Multiple Sclerosis registers are becoming increasingly important, not only for epidemiological research but also by quantifying the burden of the disease for the patients and society and helping health care providers and regulators in their decisions. The Danish Multiple Sclerosis Registry has served as data source for a number of scientific publications including epidemiological studies on changes in incidence and mortality, cohort studies investigating risk factors for developing MS, comorbidities and socioeconomic outcomes in the MS population, and observational studies on effectiveness of disease modifying treatments outside the narrow realms of randomized clinical trials.

Risk of neuroinflammatory events in arthritis patients treated with tumour necrosis factor alpha inhibitors: a collaborative population-based cohort study from Denmark and Sweden

Objectives

To investigate whether tumour necrosis factor alpha inhibitors (TNFis) are associated with an increased risk of neuroinflammatory diseases among patients with arthritic diseases.

Methods

Cohorts of patients with rheumatoid arthritis (RA, n=25 796), psoriatic arthritis (PsA, n=8586) and ankylosing spondylitis (AS, n=9527) who initiated a TNFi treatment year 2000–2017 were identified from nationwide clinical rheumatology registers in Sweden and Denmark. Information on demyelinating disease and inflammatory neuropathy diagnoses was retrieved from prospective linkage to National Patients Register. A Cox proportional hazard model was used to estimate HRs and 95% CI comparing TNFi exposed and non-exposed, by disease and country.

Results

Among 111 455 patients with RA, we identified 270 (Sweden) and 51 (Denmark) events (all types of neuroinflammatory diseases combined), corresponding to crude incidence rates (per 1000 person-years) of 0.37 (Sweden) and 0.39 (Denmark) in TNFi-treated patients vs 0.39 (Sweden) and 0.28 (Denmark) in unexposed patients, and an age-sex-calendar-period-adjusted HR (95% CI) of 0.97 (0.72 to 1.33) (Sweden) and 1.45 (0.74 to 2.81) (Denmark) in TNFi exposed compared with non-exposed patients. For a total of 64 065 AS/PsA patients, the corresponding numbers were: 196 and 32 events, crude incidence rates of 0.59 and 0.87 in TNFi-treated patients vs 0.40 and 0.19 in unexposed patients, and HRs of 1.50 (1.07 to 2.11) and 3.41 (1.30 to 8.96), for Sweden and Denmark, respectively. For multiple sclerosis, the patterns of HRs were similar.

Conclusions

Use of TNFi in AS/PsA, but not in RA, was associated with increased risk of incident neuroinflammatory disease, though the absolute risk was below one in 1000 patients/year.

Incidence of Free of Charge Physiotherapy in a Danish National Cohort of Stroke, Parkinson's Disease, Multiple Sclerosis and Rheumatoid Arthritis Patients

Background

Denmark is a welfare state with a publically funded healthcare system that includes the right to free of charge physiotherapy (FCP) for patients with chronic or progressive disease who fulfill strict criteria. The aim of this study was to investigate the incidence of referral to FCP in patients with a hospital diagnosis of stroke, multiple sclerosis (MS), Parkinson’s disease (PD) and rheumatoid arthritis (RA) between 2007 and 2016.

Methods

The study was register-based and included data from The Danish National Patient Registry and The National Health Service Registry. The study population included the four largest disease groups receiving FCP in Denmark. The incidence of receiving FCP was reported as the cumulated incidence proportion (CIP).

Results

The study showed that FCP was mainly initiated within the first 2 years after diagnosis. The 2-year CIP was 8% for stroke patients, 53% for PD patients, 49% for MS patients, and 16% for RA patients. The proportion of patients referred to FCP generally increased over the period of the study due to more patients being referred from medical specialists in primary care.

Conclusion

This study found substantial differences in the incidence of referral to FCP in a Danish population of stroke, PD, MS and RA patients.

Incidence of Immune-Mediated Inflammatory Diseases Among Patients With Inflammatory Bowel Diseases in Denmark

BACKGROUND & AIMS

It is not clear whether the co-occurrence of immune-mediated inflammatory diseases (IMIDs) affects the course of inflammatory bowel diseases (IBD). We investigated the occurrence of IMIDs in relation to onset of IBD and the effects of concurrent IMIDs on IBD outcomes in a nationwide study of the Danish population.

METHODS

We used a nationwide cohort of all individuals diagnosed with IBD, including Crohn's disease (CD) or ulcerative colitis (UC), in Denmark from 2007 through 2016 (n = 14,377). Patients were match with individuals without IBD from the general population (controls, n = 71,885). All cohort members were followed from birth until 2016, their migration, or their death. The occurrence of IMIDs was assessed using the Danish national patient register and Registry of Medicinal Products Statistics.

RESULTS

A total of 3,235 patients with a diagnosis of IBD (22.5%) has also received a diagnosis of an IMID; most IMIDs occurred before the onset of IBD (n = 2,600, 80.3%). The most common IMIDs observed were psoriasis, asthma, type 1 diabetes, and iridocyclitis. Patients with IBD treated with infliximab were at reduced risk of developing IMIDs (CD adjusted odds ratio [aOR], 0.52; 95% CI, 0.34-0.81 and UC aOR, 0.47; 95% CI, 0.29-0.76). Co-occurrence of IMIDs increased the risk of surgery in patients with CD that developed IMIDs after CD onset (aOR, 2.30; 95% CI, 1.46-4.20) but not in UC.

CONCLUSIONS

In a nationwide study of the Danish population, 22.5% of patients with IBD also had at least 1 concurrent IMID. Co-occurrence of IMIDs increased the risk of surgery in patients with CD.

Diagnostic Value of Oligoclonal Bands in Children: A Nationwide Population-Based Cohort Study

OBJECTIVE

We evaluated the diagnostic value of cerebrospinal fluid oligoclonal bands in individuals less than 18 years of age.

METHODS

In a nationwide population-based setting, we retrieved data on 2055 children's oligoclonal band examination, including concordant cerebrospinal fluid biomarkers, during 1994 to 2017. Case ascertainment was by review of medical records and diagnostic codes. We used Fisher's exact test to explore distribution differences of oligoclonal band positivity in acquired demyelinating syndromes (ADS) before and after age 12 years and calculated the sensitivity, specificity, positive predictive value, and negative predictive value of oligoclonal bands to distinguish ADS from the other diagnostic groups.

RESULTS

Median age at oligoclonal band examination was 15.2 years (range = 1.8 to 18.0), and 10% had presence of cerebrospinal fluid oligoclonal bands. Oligoclonal band positivity was the highest in ADS (52%), but it was age dependent: 21% in children with ADS before age 12 years and 68% in children aged 12 through 17 years (P < 0.0001) owing to the higher incidence of multiple sclerosis in the latter. Cerebrospinal fluid oligoclonal bands were not predictive of ADS before age 12 years compared with the other diagnostic groups. However, cerebrospinal fluid oligoclonal bands in children aged 12 through 17 years were highly predictive of ADS compared with central nervous system infections and non-ADS immune-mediated central nervous system diseases (positive predictive value: 0.89; 95% confidence interval = 0.82 to 0.94; P < 0.0001), but negative oligoclonal bands were not discriminatory (negative predictive value: P = 0.17).

CONCLUSIONS

In a clinical setting, cerebrospinal fluid oligoclonal band examination may be of higher yield in children aged 12 through 17 years if there is clinical suspicion of multiple sclerosis, and in such circumstances a positive test supports a diagnosis of multiple sclerosis.

Incidence and risk of necrotizing enterocolitis in Denmark from 1994-2014

Introduction. We suspected that the incidence of NEC in Denmark had increased during the last 20 years but hypothesized that this could be explained by the increased neonatal survival. Methods. We conducted a retrospective, observational cohort study of all registered liveborn infants in Denmark in the period from January 1, 1994 to December 31, 2014. Data were obtained from the Medical Birth Registry, National Patient Register, and Cause of Death register in Denmark. The primary outcome was the registration of NEC (ICD-10: DP77.9) during a hospital admission within 6 months after birth. The statistical analysis used ‘death before NEC’ as a competing risk. Results. The cohort consisted of 1,351,675 infants, of which 8,059 died. There was a strongly significant decreasing risk of death over the period for the all infants (p<0.0001 in all gestational age groups). In total, 994 infants were diagnosed with NEC which lead to an incidence of 7.4 per 10,000 live-born infants. During the observation period, the incidence increased from 6.3 to 7.9 per 10,000 births (p = 0.006). When accounting for ‘death before NEC’ as a competing risk, the increase could be explained by the increased neonatal survival. There was, however, a GA-group/epoch interaction (p = 0.008) in the cause-specific hazard ratios with a trend towards an increasing risk of NEC in the most preterm infants and a decreasing risk of NEC in the term infants. Conclusion. While the overall incidence of NEC increased over the study period, the overall risk of NEC did not increase when considering the increased survival. Nevertheless, there seemed to be an increased risk of NEC in the most premature infants which was masked by a decreased risk in the term infants. This study suggests that research to prevent NEC in the most preterm infants is more important now than ever.

Pediatric autoimmune encephalitis in Denmark during 2011-17: A nationwide multicenter population-based cohort study

BACKGROUND

The incidence of pediatric autoimmune encephalitis (AIE) is unknown. Our aim was to assess the incidence of pediatric AIE in Denmark 2011-17.

METHODS

In a nationwide population-based setting, we retrieved data on all children tested for AIE before age 18 years. We reviewed medical records in a) children with AIE antibodies (n = 18) to assess whether children fulfilled the AIE consensus criteria, b) children tested negative for AIE antibodies who were registered with an AIE diagnostic code to estimate the incidence of "antibody negative but probable AIE", and c) a reference cohort (n = 596) to determine the positive predictive value of International Classification of Diseases (ICD) codes used for anti-NMDAR encephalitis.

RESULTS

375 children were tested for AIE 2011-17 (median age 11.1 years; 54% girls); 18 children (5%) had AIE antibodies (percentage tested positive): CSF GAD₆₅-IgG (3.1%), plasma NMDAR-IgG (2.8%), CSF NMDAR-IgG (1.8%), plasma GAD₆₅-IgG (1.0%), and plasma CASPR2-IgG (0.4%). Five children fulfilled the criteria for probably/definite anti-NMDAR encephalitis (incidence: 0.07/100,000 person-years; 95% CI = 0.03-0.17), and 4 children with anti-GAD₆₅ associated AIE (incidence = 0.055/100,000 person-years, 95% CI = 0.021-0.15). The incidence of "antibody negative but probable AIE" was 0.055/100,000 person-years (95% CI = 0.021-0.15). The positive predictive value of ICD diagnostic codes used for anti-NMDAR encephalitis was 8%.

CONCLUSIONS

We diagnosed only children with anti-NMDAR, anti-GAD₆₅, and "antibody negative but probable AIE". Before examining AIE antibodies, clinical presentation, paraclinical studies (CSF, EEG, and MRI), and incidence of pediatric AIEs should be considered. Updating the ICD to include AIE codes is warranted.

Impact of red meat, processed meat and fibre intake on risk of late-onset chronic inflammatory diseases: prospective cohort study on lifestyle factors using the Danish 'Diet, Cancer and Health' cohort (PROCID-DCH): protocol

INTRODUCTION

Chronic inflammatory diseases (CIDs) (Crohn's disease, ulcerative colitis, psoriasis, psoriatic arthritis, rheumatoid arthritis and multiple sclerosis) are diseases of the immune system that have some shared genetic and environmental predisposing factors, but still few studies have investigated the effects of lifestyle on disease risk of several CIDs. The primary aim of this prospective cohort study is to investigate the impact of fibre, red meat and processed meat on risk of late-onset CID, with the perspective that results of this study can contribute in supporting future diet recommendations for effective personalised prevention.

METHODS AND ANALYSIS

The study will use data from 57 053 persons from the prospective Danish cohort study 'Diet, Cancer and Health' together with National Health Registry data. The follow-up period is from December 1993 to December 2018. Questionnaire data on diet and lifestyle were collected at entry to the Diet, Cancer and Health study. The outcome CID is defined as having a diagnosis of one of the CIDs registered in the National Patient Registry or, for multiple sclerosis, in the Danish Multiple Sclerosis Registry during follow-up and being treated with a drug used for the specific disease. The major outcome of the analyses will be to detect variability in risk of late onset of any CID and, if power allows, disease risk of late onset of each CID diagnosis between persons with different fibre and red meat, and processed meat intake. The outcome will be adjusted for age, sex, body mass index, physical activity, energy, alcohol, fermented dairy products, education, smoking status, hormone replacement therapy and comorbidity.

ETHICS AND DISSEMINATION

The study is approved by the Danish Data Protection Agency (2012-58-0018). The core study is an open register-based cohort study. The study does not need approval from the Ethics committee or Institutional Review Board by Danish law. Study findings will be disseminated through peer-reviewed journals, patient associations and presentations at international conferences.

TRIAL REGISTRATION NUMBER

NCT03456206; Post-results.

Low HDL Cholesterol and High Risk of Autoimmune Disease: Two Population-Based Cohort Studies Including 117341 Individuals

BACKGROUND

HDL is quantitatively the most important lipoprotein in most species and mechanistic evidence points toward a role for HDL in normal immune function. We tested the hypothesis that concentrations of HDL cholesterol are associated with risk of autoimmune disease.

METHODS

From 2 studies of the general population-the Copenhagen General Population Study and the Copenhagen City Heart study-we included 107954 and 9387 individuals with baseline measurements of HDL cholesterol. These were followed with the national Danish Patient Registry from baseline in 2003-2015 or 1991-1994 through 2017, during which time 4078 and 1101 individuals developed autoimmune disease in the 2 studies.

RESULTS

In the Copenhagen General Population Study, compared to individuals with HDL cholesterol ≥2.0 mmol/L (77 mg/dL), the multifactorially adjusted hazard ratios for any autoimmune disease were 1.06 (95% CI, 0.94-1.19) for individuals with HDL cholesterol of 1.5-1.99 mmol/L (58-77 mg/dL), 1.18 (95% CI, 1.04-1.35) for individuals with HDL cholesterol of 1.0-1.49 mmol/L (39-58 mg/dL), and 1.84 (95% CI, 1.52-2.22) for individuals with HDL cholesterol <1.0 mmol/L (39 mg/dL) (P for trend <0.001). These results were similar when excluding events within 5 years of baseline, in women and men separately, for events at baseline, irrespective of low-grade inflammation or triglyceride concentrations, for the apolipoprotein A1 part of HDL, and for more restrictive end point definitions. Finally, the Copenhagen City Heart Study provided independent confirmation.

CONCLUSIONS

Low HDL cholesterol level is associated with high risk of autoimmune disease in individuals from the general population. Our observational findings cannot determine causality.

Prior High Tibial Osteotomy Does Not Affect the Survival of Total Knee Arthroplasties: Results From the Danish Knee Arthroplasty Registry

BACKGROUND

High tibial osteotomy (HTO) is a joint preserving treatment of unicompartmental osteoarthritis in the knee. In cases with insufficient or deteriorating clinical results patients may undergo a total knee arthroplasty (TKA). The influence of prior HTO on TKA survival is debated.

METHODS

We conducted a population-based registry study comparing 1044 primary TKAs in patients with prior HTO to 63,763 de novo TKAs inserted from 1997 to 2015. Implant survival was estimated by Kaplan-Meier analysis with revision of any kind as endpoint. Patient and surgery characteristics, including choice of implant design, were compared and their influence on TKA survival was estimated by Cox regression. Finally, indications of revision were compared between the groups.

RESULTS

TKA following HTO had an inferior survival with a 10-year estimated survival of 91% compared to 94% for de novo TKA, corresponding to a crude hazard ratio (HR) of 1.73 (P < .001). However, after adjustment for differences in gender and age this risk diminished (HR 1.19, P = .09). The choice of implant constraint was similar between the groups and in both groups posterior stabilized TKA was associated with inferior survival with an adjusted HR of 1.46 (P = .03) in post-HTO TKA when compared to cruciate-retaining TKA.

CONCLUSION

TKA following HTO had a crude inferior survival when compared to TKA without prior surgery of any kind. The inferior survival was explainable by patient characteristics, defined by male gender and lower age, rather than the prior HTO. However, when the prior HTO resulted in the use of posterior stabilized TKA the survival decreased.

Pediatric acquired demyelinating syndromes: a nationwide validation study of the Danish National Patient Register

Objective

To validate the Danish National Patient Register’s (NPR) diagnoses of pediatric acquired demyelinating syndromes (ADS) including multiple sclerosis (MS).

Study design and setting

We identified ADS diagnostic groups using International Classification of Diseases (ICD) codes and reviewed medical records to validate the NPR diagnoses during 2008–2015.

Results

Among 409 children in the study, 184 children had a validated and final ADS diagnosis after reviewing medical records as follows: optic neuritis (ON; n=46), transverse myelitis (TM; n=16), acute disseminated encephalomyelitis (ADEM; n=50), clinically isolated syndrome (CIS) including dissemination in space (CIS [DIS]) but not dissemination in time (n=6), neuromyelitis optica spectrum disorder (NMOsd; n=5), and MS (n=61). During the mean follow-up of 4.6 years, 33% of children initially diagnosed with monophasic ADS progressed to MS. Positive predictive value (PPV) was 0.71 (95% confidence interval [CI] =0.62–0.80) for ON, 0.64 (95% CI =0.43–0.82) for TM, 0.93 (95% CI =0.84–0.98) for MS, 0.27 (95% CI =0.19–0.35) for CIS, 0.43 (95% CI =0.10–0.82) for NMOsd, and 0.15 (95% CI =0.10–0.20) for ADEM. Assuming complete coverage for non-MS ADS, the sensitivity was 0.99 (95% CI =0.93–1.00) for ON, 0.83 (95% CI =0.36–1.00) for CIS (DIS), and 0.80 (95% CI =0.56–0.94) for TM, but only 0.58 (95% CI =0.43–0.72) for ADEM and 0.60 (95% CI =0.15–0.95) for NMOsd.

Conclusion

PPV was high for MS and considered acceptable for ON and TM; therefore, these ICD revision 10 (ICD-10) codes from the NPR are useful for epidemiological studies. Conversely, PPV was low for CIS and ADEM; NMOsd was inconclusive.

Occupational exposure to extremely low-frequency magnetic fields and risk for central nervous system disease: an update of a Danish cohort study among utility workers

PURPOSE

Evidence of whether exposure to extremely low-frequency magnetic fields (ELF-MF) is related to central nervous system diseases is inconsistent. This study updates a previous study of the incidence of such diseases in a large cohort of Danish utility workers by almost doubling the period of follow-up.

METHODS

We investigated the risks for dementia, motor neurone disease, Parkinson disease, multiple sclerosis and epilepsy among 32,006 men employed at the 99 utility companies that supplied Denmark with electricity during the period 1900-1993. Cases were identified in the Danish National Patient Registry and the cohort was followed during 1982-2010. Exposure was estimated from a job-exposure matrix based on company records of job title and area of work and cohort members were allocated to one of three categories (<0.1, 0.1-0.99 and ≥1.0 µT).

RESULTS

For dementia, multiple sclerosis and epilepsy the incidence rate ratios (IRR) were close to unity, but higher for motor neurone disease [IRR 1.24, 95% confidence interval (CI) 0.86-1.79] and lower for Parkinson disease (IRR 0.81, 95% CI 0.67-0.97) among workers exposed to ≥0.1 µT compared with the Danish population. For the highest level of exposure (≥1.0 µT), IRRs of 1.44, 1.78, 1.40 and 1.34 were observed for dementia, motor neurone disease, multiple sclerosis and epilepsy, respectively.

CONCLUSIONS

We observed elevated risks of dementia, motor neurone disease, multiple sclerosis and epilepsy and lower risks of Parkinson disease in relation to exposure to ELF-MF in a large cohort of utility employees.

Exploring the concurrent validity of the nationwide assessment of permanent nursing home residence in Denmark - A cross-sectional data analysis using two administrative registries

BACKGROUND

Many register studies make use of information about permanent nursing home residents. Statistics Denmark (StatD) identifies nursing home residents by two different indirect methods, one based on reports from the municipalities regarding home care in taken place in a nursing home, and the other based on an algorithm created by StatD. The aim of the present study was to validate StatD's nursing home register using dedicated administrative municipality records on individual nursing home residents as gold standard.

METHODS

In total, ten Danish municipalities were selected. Within each Danish Region, we randomly selected one municipality reporting to Stat D (Method 1) and one not reporting where instead an algorithm created by StatD was used to discover nursing home residents (Method 2). Method 1 means that municipalities reported to Stat D whether home care has taken place in a nursing home or in a private home. Method 2 is based on an algorithm created by Stat D for the municipalities where Method 1 is not applicable. Our gold standard was the information from the local administrative system in all ten selected municipalities. Each municipality provided a list with all individuals > 65 years living in a nursing home on January 1st, 2013 as well as the central personal number. This was compared to the list of individuals >65 living in nursing home facilities in the same ten municipalities on January 1st, 2013 retrieved from StatD.

RESULTS

According to the data received directly from the municipalities, which was used as our gold Standard 3821 individuals were identified as nursing home residents. The StatD register identified 6,141 individuals as residents. Additionally, 556 of the individuals identified by the municipalities were not identified in the StatD register. Overall sensitivity for the ten municipalities in the StatD nursing home register was 0.85 (95% CI 0.84-0.87) and the PPV was 0.53 (95% CI 0.52-0.54). The municipalities for which nursing home status was based on the StatD algorithm (method 2) had a sensitivity of 0.84 (95% CI 0.82-0.86) and PPV of 0.48 (95% CI 0.46-0.50). Both slightly lower than the reporting municipalities (method 1) where the sensitivity was 0.87(95% CI 0.85-0.88) and the PPV was 0.57 (95% CI 0.56-0.59). Additionally, the sensitivity and PPV of the Stat D register varied heavily among the ten municipalities from 0.51 (95% CI 0.43-0.59) to 0.96 (95% CI 0.95-0.98) and PPV correspondingly, from 0.14 (95% CI: 0.11-0.17) to 0.73 (95% CI 0.69-0.77).

CONCLUSIONS

The overall PPV of StatD nursing home register was low and differences between municipalities existed. Even in countries with extensive nation-wide registers, validating studies should be conducted for outcomes based on these registers.

The Association Between Maternal Chronic Inflammatory Bowel Disease and Long-term Health Outcomes in Children-A Nationwide Cohort Study

BACKGROUND

Very little is known about the long-term impact of maternal inflammatory bowel disease (IBD) on the offspring's future health. We aimed to examine whether children born to mothers with IBD had an increased risk of long-term morbidities.

METHODS

In this nationwide register-based cohort study, including all children born alive in Denmark between 1989 and 2013, we investigated the association between maternal IBD and 15 selected disease categories of physical and mental conditions in the offspring. We used multivariate regression models estimating the hazard ratio (HR) of long-term child morbidities.

RESULTS

The cohort comprised 9238 children born to women with IBD (exposed), and 1,371,407 born to women without IBD (unexposed). Median follow-up time among the exposed was 9.7 years (interquartile range [IQR] 4.9-15.7) and 13.8 (interquartile range 7.4-19.9) among the unexposed. In children, who in utero had been exposed to maternal ulcerative colitis, the HR of IBD in the offspring was 4.63 (95% confidence interval, 3.49-6.16). For maternal Crohn's disease, the HR of IBD in the offspring was 7.70 (95% confidence interval, 5.66-10.47). For all other diseases in the offspring, we did not find a significantly increased risk associated with maternal IBD.

CONCLUSIONS

Children born to mothers with IBD are more likely to be diagnosed with IBD themselves, compared with children born to women without IBD. Our data otherwise do not provide evidence for an increased risk of any of the other examined diseases in the offspring.

Pediatric-onset multiple sclerosis and other acquired demyelinating syndromes of the central nervous system in Denmark during 1977-2015: A nationwide population-based incidence study

BACKGROUND

The incidence of acquired demyelinating syndromes (ADS) including multiple sclerosis (MS) has never been investigated in a Danish pediatric population.

OBJECTIVES

We estimated the nationwide age- and sex-specific incidence of pediatric ADS including MS.

METHODS

Data were sourced from the Danish Multiple Sclerosis Registry, providing cases of pediatric MS for 1977-2015, and the National Patient Register, providing cases of ADS during 2008-2015. All medical records were reviewed to validate the register-based diagnoses.

RESULTS

We identified 364 cases of pediatric MS occurring during 1977-2015 (incidence rate = 0.79 per 100,000 person-years). MS was exceptionally rare before puberty, but the incidence rose considerably from 9 years in girls and 11 years in boys. The female-to-male ratio was 2.5; the median age at onset was 16 years (range = 7-17 years). The MS incidence rate was relatively stable through the study period. During 2008-2015, we identified 219 ADS cases. The incidence was 2.29 per 100,000 person-years with considerable differences in the age peaks for the separate ADS.

CONCLUSION

The incidence rates of MS and other ADS in Denmark were higher than those reported for some other European countries. Referral bias and classification differences may account for this disparity, in particular the age-intervals and the definition of onset.

Age at Menarche and Risk of Multiple Sclerosis: A Prospective Cohort Study Based on the Danish National Birth Cohort

Few studies have addressed the possible association between age at menarche and multiple sclerosis (MS), and results are conflicting. We studied this issue in a large prospective cohort study. The study cohort comprised 77,330 women included in the Danish National Birth Cohort (1996-2002). Information on menarcheal age was ascertained at the first interview, which took place in the 16th week of pregnancy. Women were followed for MS from the first interview to December 31, 2011. Associations between age at menarche and risk of MS were evaluated with hazard ratios and 95% confidence intervals using Cox proportional hazards regression models. Overall, 226 women developed MS during an average follow-up period of 11.7 years. Age at menarche among women with MS was generally lower than that among women without MS (Wilcoxon rank-sum test; P = 0.002). We observed an inverse association between age at menarche and MS risk. For each 1-year increase in age at menarche, risk of MS was reduced by 13% (hazard ratio = 0.87, 95% confidence interval: 0.79, 0.96). Early age at menarche appears to be associated with an increased risk of MS. The mechanisms behind this association remain to be established.

Alcohol consumption and the risk of postoperative mortality and morbidity after primary hip or knee arthroplasty - A register-based cohort study

Objective

To investigate the implications of low and moderate preoperative alcohol consumption on postoperative mortality and morbidity after primary hip and knee arthroplasty.

Methods

A total of 30,799 patients who underwent primary hip or knee arthroplasty between January 1^st, 2005 and October 8^th, 2011 with information on preoperative alcohol consumption (0 grams of pure alcohol/week, >0–168 g/week, >168–252 g/week, and >252 g/week) were identified through the Danish Anesthesia Database. The 90-day and 1-year risks of mortality (primary outcomes), 1-year risk of prosthetic infection, and 30-day risks of cardiovascular disease and deep venous thrombosis (secondary outcomes) were estimated by Cox regression analysis.

Results

We identified 285 (0.9%) deaths within the first 90 days and 694 (2.3%) within the first year. Within the first 30 days, 209 (0.7%) and 270 (0.9%) patients had acquired cardiovascular disease and deep venous thrombosis, respectively, and 514 (1.7%) patients developed prosthetic infection within the first year. The adjusted mortality models yielded hazard ratios of 0.55 (95% confidence interval [CI] 0.41 to 0.74) at 90 days and 0.61 (95% CI 0.51 to 0.73) at 1 year for the group consuming >0–168 g/week when compared to abstainers. Adjusted hazard ratios showed that the group consuming >0–168 g/week had a 0.91 (95% CI 0.75 to 1.11) risk of prosthetic infection, 0.68 (95% CI 0.50 to 0.92) risk of cardiovascular disease and 0.88 (95% CI 0.67 to 1.15) risk of deep venous thrombosis when compared to abstainers.

Conclusions

This study demonstrates that low-to-moderate alcohol consumption prior to primary hip or knee arthroplasty is associated with lower risks of mortality at both 90 days and 1 year after surgery and of cardiovascular disease after 30 days. More research from longitudinal studies is needed to identify specific causal relations and explanations.

Prevalence of maternal chronic diseases during pregnancy - a nationwide population based study from 1989 to 2013

INTRODUCTION

There is substantial evidence of a negative impact of maternal chronic disease during pregnancy on reproductive outcomes. Knowledge of the prevalence of chronic diseases during pregnancy is limited, but essential for a focused preventive effort regarding optimal disease control during pregnancy. We aimed to analyze the prevalence of chronic diseases during pregnancy.

MATERIAL AND METHODS

This register-based cohort study included all women giving birth in Denmark between 1989 and 2013 based on data from Danish health registers. Maternal chronic diseases included 23 disease categories of both physical and mental health conditions recorded within a period of 10 years before childbirth.

RESULTS

We included 1 362 200 childbirths during the study period. The overall prevalence of maternal chronic disease increased from 3.71% in 1989 to 15.76% in 2013. The most frequently registered chronic diseases were chronic lung diseases/asthma (1.73%), thyroid disorders (1.50%) and anxiety and personality disorders (1.33%). Taking increasing maternal age at birth into account, the relative risk for women to have a chronic disease from 2009 to 2013 was 4.14 (95% CI 4.05-4.22), compared with mothers giving birth from 1989 to 1993.

CONCLUSIONS

We found an increasing prevalence of maternal chronic disease during pregnancy and more than a four-fold increased risk of maternal chronic disease during pregnancy for childbirths in the period 2009 through 2013, compared with 1989 through 1993. The main limitation of our study is related to a potentially greater awareness and hence more careful registration of maternal chronic disease over time and thereby an increased tendency to register diseases.

Danish Prostate Cancer Registry - methodology and early results from a novel national database

Background

Systematized Nomenclature of Medicine (SNOMED) codes are computer-processable medical terms used to describe histopathological evaluations. SNOMED codes are not readily usable for analysis. We invented an algorithm that converts prostate SNOMED codes into an analyzable format. We present the methodology and early results from a new national Danish prostate database containing clinical data from all males who had evaluation of prostate tissue from 1995 to 2011.

Materials and methods

SNOMED codes were retrieved from the Danish Pathology Register. A total of 26,295 combinations of SNOMED codes were identified. A computer algorithm was developed to transcode SNOMED codes into an analyzable format including procedure (eg, biopsy, transurethral resection, etc), diagnosis, and date of diagnosis. For validation, ~55,000 pathological reports were manually reviewed. Prostate-specific antigen, vital status, causes of death, and tumor-node-metastasis classification were integrated from national registries.

Results

Of the 161,525 specimens from 113,801 males identified, 83,379 (51.6%) were sets of prostate biopsies, 56,118 (34.7%) were transurethral/transvesical resections of the prostate (TUR-Ps), and the remaining 22,028 (13.6%) specimens were derived from radical prostatectomies, bladder interventions, etc. A total of 48,078 (42.2%) males had histopathologically verified prostate cancer, and of these, 78.8% and 16.8% were diagnosed on prostate biopsies and TUR-Ps, respectively.

Future perspectives

A validated algorithm was successfully developed to convert complex prostate SNOMED codes into clinical useful data. A unique database, including males with both normal and cancerous histopathological data, was created to form the most comprehensive national prostate database to date. Potentially, our algorithm can be used for conversion of other SNOMED data and is available upon request.

Revision of infected knee arthroplasties in Denmark

Background and purpose - The surgical treatment of periprosthetic knee infection is generally either a partial revision procedure (open debridement and exchange of the tibial insert) or a 2-stage exchange arthroplasty procedure. We describe the failure rates of these procedures on a nationwide basis. Patients and methods - 105 partial revisions (100 patients) and 215 potential 2-stage revision procedures (205 patients) performed due to infection from July 1, 2011 to June 30, 2013 were identified from the Danish Knee Arthroplasty Register (DKR). Failure was defined as surgically related death ≤ 90 days postoperatively, re-revision due to infection, or not reaching the second stage for a planned 2-stage procedure within a median follow-up period of 3.2 (2.2-4.2) years. Results - The failure rate of the partial revisions was 43%. 71 of the partial revisions (67%) were revisions of a primary prosthesis with a re-revision rate due to infection of 34%, as compared to 55% in revisions of a revision prosthesis (p = 0.05). The failure rate of the 2-stage revisions was 30%. Median time interval between stages was 84 (9-597) days. 117 (54%) of the 2-stage revisions were revisions of a primary prosthesis with a re-revision rate due to infection of 21%, as compared to 29% in revisions of a previously revised prosthesis (p = 0.1). Overall postoperative mortality was 0.6% in high-volume centers (> 30 procedures within 2 years) as opposed to 7% in the remaining centers (p = 0.003). Interpretation - The failure rates of 43% after the partial revision procedures and 30% after the 2-stage revisions in combination with the higher mortality outside high-volume centers call for centralization and reconsideration of surgical strategies.

Milk intake is not associated with ischaemic heart disease in observational or Mendelian randomization analyses in 98,529 Danish adults

BACKGROUND

Observationally, reports on the association between milk intake and risk of ischaemic heart disease (IHD) and myocardial infarction (MI) have produced conflicting results; and no previous large-scale study using the lactase persistent/non-persistent LCT-13910 C/T genotype as a largely unconfounded proxy for milk intake free of reverse causation has been conducted. We tested the hypothesis that milk intake observationally and genetically through the LCT-13910 C/T genotype is associated with risk of IHD and MI in a Mendelian randomization design.

METHODS

We included 98,529 White individuals of Danish descent, aged 20-100 years, from three studies of the general population. Information on IHD (N = 10,372) and MI (N = 4188) were obtained from national Danish registries. First, we investigated observational associations between milk intake and incident IHD and MI. Second, we confirmed the association between the rs4988235 genetic variant LCT-13910 C/T, associated with lactase persistence/non-persistence, and milk intake. Finally, we tested whether LCT-13910 C/T genotype was associated with risk of IHD and MI as well as with cardiovascular risk factors.

RESULTS

During a mean follow-up time of 5.4 years, the observational hazard ratio for a 1 glass/week higher milk intake was 1.00 [95% confidence interval (CI): 1.00,1.01] for both IHD and MI. Median milk intake was 3 glasses/week (interquartile range: 0-7) in lactase CC non-persistent individuals compared with 5 glasses/week (0-10) in lactase TC/TT persistent individuals (P = 3*10(-60)). In the dominant genetic model comparing lactase TC/TT persistent individuals with lactase CC non-persistent individuals, the odds ratio was 1.00 (0.92,1.09) for IHD and 0.96 (0.84,1.09) for MI. Finally, in the dominant genetic model genotype was not associated convincingly with plasma levels of total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, triglycerides or glucose, nor with blood pressure.

CONCLUSION

Milk intake was not associated with risk of IHD or MI, observationally or genetically.

Self-rated health in women prior to clinical onset of multiple sclerosis: A study within the Danish National Birth Cohort

BACKGROUND

It has been suggested that onset of multiple sclerosis (MS) is preceded by a clinically silent period of up to 10 years.

OBJECTIVES

Examine whether such a period should be associated with poor self-rated health (SRH).

METHODS

Information on SRH before pregnancy was ascertained among 80,848 women participating in the Danish National Birth Cohort (DNBC) 1996-2002. Women were followed for MS from enrolment in DNBC in the 16th week of pregnancy until 31 December 2011. Associations between SRH and MS were evaluated by means of hazard ratios (HR) with 95% confidence intervals (CIs) using Cox proportional hazard models.

RESULTS

During on average 11.7 years of follow-up, 239 women were diagnosed with MS. Overall, neither women with fair (HR = 1.09 (95% CI = 0.83-1.41), n = 113) nor poor pre-pregnancy SRH (HR = 0.94 (95% CI = 0.47-1.87), n = 9) were at an increased risk of MS compared with women reporting very good pre-pregnancy SRH. Supplementary analyses showed no significant differences in MS risk in consecutive periods of follow-up.

CONCLUSION

In this first prospective cohort study assessing MS risk as a function of SRH, we found no indication of a long period of poor SRH prior to MS. Our findings based on pregnant women may not necessarily apply to all women.

The Danish National Patient Registry: a review of content, data quality, and research potential

Background

The Danish National Patient Registry (DNPR) is one of the world’s oldest nationwide hospital registries and is used extensively for research. Many studies have validated algorithms for identifying health events in the DNPR, but the reports are fragmented and no overview exists.

Objectives

To review the content, data quality, and research potential of the DNPR.

Methods

We examined the setting, history, aims, content, and classification systems of the DNPR. We searched PubMed and the Danish Medical Journal to create a bibliography of validation studies. We included also studies that were referenced in retrieved papers or known to us beforehand. Methodological considerations related to DNPR data were reviewed.

Results

During 1977–2012, the DNPR registered 8,085,603 persons, accounting for 7,268,857 inpatient, 5,953,405 outpatient, and 5,097,300 emergency department contacts. The DNPR provides nationwide longitudinal registration of detailed administrative and clinical data. It has recorded information on all patients discharged from Danish nonpsychiatric hospitals since 1977 and on psychiatric inpatients and emergency department and outpatient specialty clinic contacts since 1995. For each patient contact, one primary and optional secondary diagnoses are recorded according to the International Classification of Diseases. The DNPR provides a data source to identify diseases, examinations, certain in-hospital medical treatments, and surgical procedures. Long-term temporal trends in hospitalization and treatment rates can be studied. The positive predictive values of diseases and treatments vary widely (<15%–100%). The DNPR data are linkable at the patient level with data from other Danish administrative registries, clinical registries, randomized controlled trials, population surveys, and epidemiologic field studies – enabling researchers to reconstruct individual life and health trajectories for an entire population.

Conclusion

The DNPR is a valuable tool for epidemiological research. However, both its strengths and limitations must be considered when interpreting research results, and continuous validation of its clinical data is essential.

Long-term prognosis and causes of death after spondylodiscitis: A Danish nationwide cohort study

BACKGROUND

Data on long-term prognosis after spondylodiscitis are scarce. The purpose of this study was to determine long-term mortality and the causes of death after spondylodiscitis.

METHODS

A nationwide, population-based cohort study using national registries of patients diagnosed with non-post-operative pyogenic spondylodiscitis from 1994-2009, alive 1 year after diagnosis (n = 1505). A comparison cohort from the background population individually matched for sex and age was identified (n = 7525). Kaplan-Meier survival curves were constructed and Poisson regression analyses used to estimate mortality rate ratios (MRR).

RESULTS

Three hundred and sixty-five patients (24%) and 1115 individuals from the comparison cohort (15%) died. Unadjusted MRR for spondylodiscitis patients was 1.76 (95% CI = 1.57-1.98) and 1.47 (95% CI = 1.30-1.66) after adjustment for comorbidity. No deaths were observed in 128 patients under the age of 16 years. Siblings of patients did not have increased long-term mortality compared with siblings of the individuals from the comparison cohort. This study observed increased mortality due to infections (MRR = 2.57), neoplasms (MRR = 1.40), endocrine (MRR = 3.72), cardiovascular (MRR = 1.62), respiratory (MRR = 1.71), gastrointestinal (MRR = 3.35), musculoskeletal (MRR = 5.39) and genitourinary diseases (MRR = 3.37), but also due to trauma, poisoning and external causes (MRR = 2.78), alcohol abuse-related diseases (MRR = 5.59) and drug abuse-related diseases (6 vs 0 deaths, MRR not calculable).

CONCLUSIONS

Patients diagnosed with spondylodiscitis have increased long-term mortality, mainly due to comorbidities, particularly substance abuse.

Registers of multiple sclerosis in Denmark

There are two nationwide population-based registers for multiple sclerosis (MS) in Denmark. The oldest register is The Danish Multiple Sclerosis Registry (DMSR), which is an epidemiological register for estimation of prevalence and incidence of MS and survival, and for identifying exposures earlier in life that may affect the risk of MS. This register has no systematic follow-up data except for survival. The DMSR has over the years published nationwide incidence- and prevalence data from Denmark and has been involved in a number of 'historical prospective' studies to elucidate the association between a number of different environmental exposures in the past and the subsequent risk of MS. Some of these studies have been able to exonerate suspected risk factors. The other register, the nationwide Danish Multiple Sclerosis Treatment Register, is a follow-up register for all patients who have received disease-modifying treatments since 1996. It has, in particular, contributed to the knowledge of the role of antibodies against the biological drugs used for the treatment of MS.

Incidence rates of atopic dermatitis, asthma, and allergic rhinoconjunctivitis in Danish and Swedish children

BACKGROUND

Several studies have shown that the prevalence of the frequent chronic conditions of atopic dermatitis, asthma, and allergy has increased substantially for reasons not fully understood. Atopic diseases affect quality of life in both children and their family members.

OBJECTIVE

Using national registers, we sought to establish up-to-date incidence rates of atopic dermatitis, asthma, and allergic rhinoconjunctivitis in the Danish and Swedish child populations.

METHODS

Children born in Denmark from 1997 to 2011 or born in Sweden from 2006 to 2010 participated in this cross-national, population-based cohort study. Incidence rates of atopic dermatitis, asthma, and allergic rhinoconjunctivitis in the Danish and Swedish child cohorts were ascertained through disease-specific dispensed prescribed medication, specific hospital contacts, or both.

RESULTS

In both countries the incidence rate of atopic dermatitis was stable during the study periods. The incidence rate of asthma increased until 2006 and stabilized for the rest of the study period in Denmark and increased in Sweden. The incidence rate of allergic rhinoconjunctivitis decreased in both countries.

CONCLUSION

The study revealed similar trends, with stable incidence rates of atopic dermatitis in both Danish and Swedish children, an increase and then stabilization in asthma incidence rates in Denmark and an increase in Sweden, and a decrease in allergic rhinoconjunctivitis incidence rates. At age 5 years, one third of all children were affected with at least one of the conditions of atopic dermatitis, asthma, or allergic rhinoconjunctivitis.

Major stressful life events in adulthood and risk of multiple sclerosis

OBJECTIVE

It is unclear whether psychological stress is associated with increased risk of multiple sclerosis (MS). We studied the association between major stressful life events and MS in a nationwide cohort study using death of a child or a spouse or marital dissolution as indicators of severe stress.

METHODS

We created two study cohorts based on all Danish men and women born 1950-1992. One cohort consisted of all persons who became parents between 1968 and 2010, and another cohort consisted of all persons who married between 1968 and 2010. Members of both cohorts were followed for MS between 1982 and 2010 using data from the National Multiple Sclerosis Registry. Associations between major stressful life events and risk of MS were evaluated by means of MS incidence rate ratios (RR) with 95% confidence interval (CI) obtained in Poisson regression analyses.

RESULTS

During approximately 30 million person-years of follow-up, bereaved parents experienced no unusual risk of MS compared with parents who did not lose a child (RR=1.12 (95% CI 0.89 to 1.38)). Likewise, neither divorced (RR=0.98 (95% CI 0.89 to 1.06)) nor widowed (RR=0.98 (95% CI 0.71 to 1.32) persons were at any unusual risk of MS compared with married persons of the same sex.

CONCLUSIONS

Our national cohort study provides little evidence for a causal association between major stressful life events (as exemplified by divorce or the loss of a child or a spouse) and subsequent MS risk.

High nationwide incidence of multiple sclerosis in Sweden

Over recent years increased MS incidence, primarily in women, has been reported. We recently reported an unexpectedly high MS prevalence of 189/100,000 in Sweden. In the present study we estimated the nationwide age- and gender-specific MS incidence and the sex ratio in Sweden between 2001 and 2008. MS patients were identified by linking two nationwide health data registers, and the Swedish population register. The earliest registered date of MS diagnosis was determined. By logistic regression, the probability of the date of MS diagnosis being within the incidence period, depending on age and time was estimated for a subset of patients and applied to other patients. By Poisson regression, the hazard functions for the incidence of MS diagnosis were estimated. The expected number of MS patients was 7,361.4. The incidence in the average population of 9,054,658 was 10.2 per 100,000 person-years, and 6.2 and 14.0 per 100,000 person-years for men and women, respectively. The crude female to male ratio was 2.26. No increase of incidence or change of sex ratio was observed from 2001 to 2008. In conclusion, the average MS incidence in Sweden from 2001 to 2008 was 10.2 per 100.000, which was considerably higher than previous regional Swedish estimates of 4.3–6.4. No increase of female to male ratio of MS during the study period was observed. We provide supplementary data that can be used as tools for examining excess MS risk in different study materials.

Remigration of migrants with severe disease: myth or reality?--a register-based cohort study

BACKGROUND

'Remigration bias' is often referred to when explaining low mortality outcomes among migrants compared with local-born. The hypothesis suggests that severely ill migrants tend to return to their country of origin, but it has hitherto not been tested in a large-scale epidemiological study. Consequently, we studied whether migrants with severe disease were more likely to emigrate compared with migrants without severe disease.

METHODS

A historic prospective cohort study was conducted based on all adult refugees and family reunification immigrants (n = 114,331) who obtained residence permission in Denmark between 1 January 1993 and 31 December 2010. Migrants with severe disease were identified through the Danish National Patient Register. Emigrations during follow-up (1 January 1993 to 31 December 2011) were identified using the Danish Civil Registration System. Hazard ratios were calculated for emigration among migrants with different levels of disease severity, adjusting for sex, age and income.

RESULTS

Results showed progressively fewer emigrations with increasing disease severity. Migrants with low (HR = 0.92; 95% CI: 0.80-1.06), moderate (HR = 0.84; 95% CI: 0.67-1.06) and high (HR = 0.70; 95% CI: 0.55-0.90) disease severity had fewer emigrations to 'country of origin' compared with migrants without disease. For emigration to 'any country', results likewise showed fewer emigrations among migrants with low (HR = 0.86; 95% CI: 0.78-0.95), moderate (HR = 0.73; 95% CI: 0.62-0.87) and high (HR = 0.64; 95% CI: 0.53-0.76) disease severity. This tendency was also observed by nationality and migrant status. On the disease-specific level, no uniform tendencies in emigration patterns were identified.

CONCLUSION

The results do not generally support the hypothesis of 'remigration bias' but rather suggest the opposite. Accordingly, remigration bias does not appear to explain lower mortality of migrants.

Stressful life-events in childhood and risk of multiple sclerosis: a Danish nationwide cohort study

Background:

Current knowledge concerning the association between exposure to stressful life-events (SFLEs) in childhood and later risk of multiple sclerosis (MS) is sparse.

Objectives:

We studied the associations between SFLEs in childhood and subsequent risk of MS in a nationwide cohort of 2.9 million Danes born from 1968 to 2011.

Methods:

A SFLE in childhood was defined as exposure before age 18 years to parental divorce, parental death, or death of a sibling, using information from the Danish Civil Registration System. MS cases in the cohort were identified in the Danish Multiple Sclerosis Registry. Associations of SFLE with MS risk were evaluated by incidence rate ratios (RR) of MS obtained in log-linear Poisson regression models.

Results:

Persons exposed to any SFLE in childhood were at 11% elevated risk of MS (RR = 1.11; 95% confidence interval: 1.03–1.20), compared to non-exposed persons. Stratification by subtype of SFLE showed that parental death and death of a sibling were not associated with MS risk. However, persons exposed to parental divorce were at 13% increased risk of developing MS compared to non-exposed (RR = 1.13; 1.04–1.23).

Conclusions:

Associations of SFLEs in childhood with risk of MS are weak. However, parental divorce is somehow associated with modestly increased risk of MS.

Body mass index and risk of autoimmune diseases: a study within the Danish National Birth Cohort

BACKGROUND

A possible aetiological link between obesity and certain autoimmune diseases (ADs) has been suggested. We investigated the associations between body mass index (BMI, kg/m2) and 43 ADs.

METHODS

75,008 women participating in the Danish National Birth Cohort were followed during a median time of 11 years. Diagnoses on ADs were retrieved from the Danish National Patient Register. Cox proportional hazard ratios (HRs) with 95% confidence intervals (CIs) were calculated adjusting for potential confounders (smoking, alcohol, parity and socio-occupational status).

RESULTS

During follow-up, 2430 women (3.2%) developed a total of 2607 new-onset ADs. Risk of any autoimmune disease was increased in obese women (HR, 1.27; 95% CI, 1.11 to 1.46) compared with normal weight women (18.5-≤25 kg/m2). Obese women (BMI≥30 kg/m2) were at increased risk of sarcoidosis (HR 3.59; 95% CI, 2.31 to 5.57) and type 1 diabetes mellitus (HR 2.67; 95% CI, 1.71 to 4.17). Risk of dermatitis herpetiformis increased by 14% (95% CI, 1% to 30%) per BMI unit. Conversely, risk of celiac disease and Raynaud's phenomenon decreased by 7% (95% CI, 1% to 13%) and 12% (95% CI, 4% to 19%) per BMI unit, respectively. Further associations between BMI and risk of psoriasis, rheumatoid arthritis and Crohn's disease were suggested.

CONCLUSIONS

BMI was found to be associated with several Ads. This was most pronounced between obesity and risk of sarcoidosis and and risk of type 1 diabetes mellitus. These novel findings need confirmation and the possible role of adipose tissue-derived immunological changes in the development of autoimmune reactions needs consideration.

Same-sex marriage, autoimmune thyroid gland dysfunction and other autoimmune diseases in Denmark 1989-2008

Autoimmune diseases have been little studied in gay men and lesbians. We followed 4.4 million Danes, including 9,615 same-sex married (SSM) persons, for 47 autoimmune diseases in the National Patient Registry between 1989 and 2008. Poisson regression analyses provided first hospitalization rate ratios (RRs) comparing rates between SSM individuals and persons in other marital status categories. SSM individuals experienced no unusual overall risk of autoimmune diseases. However, the risk of autoimmune thyroid dysfunction was increased, notably Hashimoto's thyroiditis (women(SSM), RR = 2.92; 95% confidence interval (CI) 1.74-4.55) and Graves' disease (men(SSM), RR = 1.88; 95% CI 1.08-3.01). There was also an excess of primary biliary cirrhosis (women(SSM), RR = 4.09; 95% CI 1.01-10.7), and of psoriasis (men(SSM), RR = 2.48; 95% CI 1.77-3.36), rheumatic fever (men(SSM), RR = 7.55; 95% CI 1.87-19.8), myasthenia gravis (men(SSM), RR = 5.51; 95% CI 1.36-14.4), localized scleroderma (men(SSM), RR = 7.16; 95% CI 1.18-22.6) and pemphigoid (men(SSM), RR = 6.56; 95% CI 1.08-20.6), while Dupuytren's contracture was reduced (men(SSM), RR = 0.64; 95% CI 0.39-0.99). The excess of psoriasis was restricted to same-sex married men with HIV/AIDS (men(SSM), RR = 10.5; 95% CI 6.44-15.9), whereas Graves' disease occurred in excess only among same-sex married men without HIV/AIDS (men(SSM), RR = 1.99; 95% CI 1.12-3.22). Lesbians and immunologically competent gay men in same-sex marriage face no unusual overall risk of autoimmune diseases. However, the observed increased risk of thyroid dysfunction in these lesbians and gay men deserves further study.

Statin treatment and risk of recurrent venous thromboembolism: a nationwide cohort study

OBJECTIVES

Statins may decrease the risk of primary venous thromboembolism (VTE), that is, deep vein thrombosis (DVT) and pulmonary embolism (PE) but the effect of statins in preventing recurrent VTE is less clear. The aim of this study was therefore to investigate the association between statin therapy and risk of recurrent VTE.

DESIGN

A prospective cohort study.

SETTING

All hospitals in Denmark.

PARTICIPANTS

All patients with a hospital diagnosis of VTE in Denmark during 1997-2009 associated with a warfarin or heparin prescription were identified.

MAIN OUTCOME MEASURES

Adjusted HR of recurrent hospitalised VTE (ie, fatal or non-fatal DVT or PE) associated with use of statins.

RESULTS

44 330 patients with VTE were included in the study. Of these 3914 were receiving statin therapy at baseline. Patients receiving statins were older (68±11 compared to 62±18 years), had more comorbidity and used more medications. The incidence rate for recurrent VTE was 24.4 (95% CI 22.8 to 26.2) per 1000 person-years among statin users and 48.5 (95% CI 47.4 to 49.7) per 1000 person-years among non-statin users. Statin use was associated with a significantly lower risk of a recurrent VTE, adjusted HR 0.74 (95% CI 0.68 to 0.80), compared with no statin use. The association between statin use and risk of recurrent VTE was significantly affected by age. Among younger individuals (≤80 years), statin use was associated with lower risk of recurrent VTE, HR 0.70 (95% CI 0.65 to 0.76) whereas in older individuals (>80 years) statin use was significantly associated with higher risk of recurrent VTE, HR 1.28 (95% CI 1.02 to 1.60), p for interaction=<0.0001.

CONCLUSIONS

Statin use was associated with a decreased risk of recurrent VTE.

Multiple sclerosis and risk of Parkinson's disease: a Danish nationwide cohort study

BACKGROUND AND PURPOSE

Case reports have observed a co-occurrence of multiple sclerosis (MS) and Parkinson's disease (PD) and it has been hypothesized that MS lesions could affect dopaminergic pathways causing parkinsonism. Our aim was to examine the association between MS and PD in a historically prospective cohort study using Danish nationwide register data.

METHODS

Multiple sclerosis patients identified in the Multiple Sclerosis Registry were followed for PD from 1977 to 2011 in the National Patient Register. As measures of relative risk, ratios of observed to expected incidence rates of first hospitalization for PD amongst persons with MS were used, i.e. standardized incidence ratios (SIRs) with 95% confidence intervals (CIs).

RESULTS

Amongst 15,557 MS patients 26 cases of PD were observed versus 26.51 expected, reflecting no overall increased risk of PD (SIR 0.98, 95% CI 0.67-1.44). Similar estimates were seen for female (SIR 0.99, 95% CI 0.58-1.67) and male MS patients (SIR 0.97, 95% CI 0.55-1.72). Likewise, no increased risk of PD amongst MS patients was observed in a robustness analysis backdating the date of diagnosis of PD by 5 years to account for the time lag between disease onset and first hospital contact with PD (SIR 0.57, 95% CI 0.32-1.00).

CONCLUSION

Our data do not suggest an increased risk of PD amongst patients with MS.

The use of epidemiological multiple sclerosis registers in research: the Danish MS Registry

INTRODUCTION

The claim of detection of several environmental risk factors for multiple sclerosis (MS), some of them new, makes the research of population-based MS registers for critical review or confirmation of alleged associations more relevant than ever before.

AIMS

To present examples of the use and important contributions from the Danish MS Registry (DMSR) over decades.

METHODS

The DMSR has through more than six decades registered virtually all patients with MS in Denmark, using multiple sources of notification and has been used for descriptive epidemiology, follow-up studies, studies of comorbidity and 'historical prospective' studies of proposed risk factors for MS.

RESULTS

Based on research from DMSR, we have found that female incidence of multiple sclerosis in Denmark has increased considerably; that patients with MS loose their working ability and their spouses/partners at a much higher rate than the background population; that patients with MS have a considerable excess mortality which seem to have decreased over several centuries decades - not centuries, also before the era of disease modifying treatment; that fewer patients with MS than expected from the population get diagnosed with or die from cancer; that infectious mononucleosis increases the risk of MS; that head trauma and a number of occupational exposures, for example, nurses, utility workers, exposure to solvents do not carry an enhanced risk of MS.

CONCLUSION

The DMSR has, as an example of long-lasting population-based registers, proven to be an effective tool for studying MS epidemiology. In future, the need for this kind of registers will continue, as biology or immunology cannot stand alone.