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

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.

Cesarean section and offspring's risk of multiple sclerosis: a Danish nationwide cohort study

BACKGROUND

Apart from a recent study reporting a 2- to 3-fold increased risk of multiple sclerosis (MS) among women and men who were delivered by Cesarean section (C-section), little attention has been given to the possible association between mode of delivery and the risk of MS.

OBJECTIVES

We studied the association between C-section and risk of MS, in a cohort of 1.7 million Danes born from 1973 to 2005.

METHODS

Information on C-section and MS was obtained from the Danish Medical Birth Register and the Danish MS Register, respectively. The association between C-section and MS was evaluated by means of MS incidence rate ratios (RR) with 95% confidence intervals (CI) obtained in log-linear Poisson regression analyses.

RESULTS

There were 930 cases of MS in the study cohort, of whom 80 (9%) were delivered by C-section. Overall, we found there was no significant association between C-section and risk of MS (RR = 1.17; 0.92-1.46). Analyses stratified by sex revealed no unusual risk of MS for women (RR = 1.08: 0.80-1.42) nor men (RR = 1.37: 0.91-1.98). A supplementary sibling-matched Cox regression analysis likewise suggested there was no excess risk of MS in persons delivered by C-section (HR = 1.03; 0.63-1.69).

CONCLUSIONS

Mode of delivery appears to be unimportant in relation to MS development in the offspring.

Mobile phones and multiple sclerosis--a nationwide cohort study in Denmark

We investigated the risk of, prognosis of and symptoms of multiple sclerosis (MS) among all Danish residents who owned a mobile phone subscription before 1996. Using the Danish Multiple Sclerosis Registry and Civil Registration System, study subjects were followed up for MS through 2004. Poisson models were used to calculate incidence rate ratios (IRR, age range: 18–64 years) and mortality rate ratios (MRR, age range: 18+) and to compare presenting symptoms among subscribers and all non-subscribers. A total of 405 971 subscription holders accrued four million years of follow up, with men accounting for 86% of the observation time. Among subscription holding men, the IRR of MS was close to unity, overall as well as 13+ years after first subscription (IRR 1.02, 95% CI: 0.48–2.16). Among women, the IRR was 3.43 (95% CI: 0.86–13.72) 13+ years after first subscription, however, based on only two cases. Presenting symptoms of MS differed between subscribers and non-subscribers (p = 0.03), with slightly increased risk of diplopia in both genders (IRR: 1.38, 95% CI: 1.02–1.86), an increased risk of fatigue among women (IRR: 3.02, 95% CI: 1.45–6.28), and of optic neuritis among men (IRR: 1.38, 95% CI: 1.03–1.86). Overall the MRR was close to one (MRR: 0.91, 95%CI 0.70–1.19) among MS-patients with a subscription and although we observed some increased MRR estimates among women, these were based on small numbers. In conclusion, we found little evidence for a pronounced association between mobile phone use and risk of MS or mortality rate among MS patients. Symptoms of MS differed between subscribers and nonsubscribers for symptoms previously suggested to be associated with mobile phone use. This deserves further attention, as does the increased long-term risk of MS among female subscribers, although small numbers and lack of consistency between genders prevent causal interpretation.

The risk of fracture in incident multiple sclerosis patients: The Danish National Health Registers

Background:

Patients with multiple sclerosis (MS) may be at increased risk of fractures owing to osteoporosis and falling.

Objective:

To evaluate the risk of fracture in incident MS patients drawn from a dedicated MS registry compared with population-based controls.

Methods:

We conducted a population-based cohort study (1996–2007) utilising the Danish National Health Registers that were linked to the Danish MS Registry and the Danish MS Treatment Registry. Incident MS patients (2963 cases) were 1:6 matched by year of birth, gender, calendar time and region to persons without MS (controls). Cox proportional hazards models and logistic regression were used to estimate the risk of fracture in MS. Time-dependent adjustments were made for age, history of diseases and drug use.

Results:

Compared with controls, patients with MS had no overall increased risk of fracture (adjusted hazard ratio (adj. HR): 1.0, 95% CI: 0.9–1.2). However, the risk of femur/hip fracture (adj. HR: 1.9, 95% CI: 1.1–3.4) was significantly increased compared to controls. As compared with unexposed patients, MS patients who had been exposed to a short course of methylprednisolone in the prior year had no significantly increased risk of osteoporotic fracture (adj. HR: 1.2, 95% CI: 0.5–2.9). Disabled MS patients with Expanded Disability Status Scale [EDSS] scores between 6 and 10, had a 2.6-fold increased risk of osteoporotic fracture (adjusted odds ratio (adj. OR): 2.6, 95% CI: 1.0–6.6) compared to patients with an EDSS score between 0 and 3.

Conclusion:

Patients with MS had a higher risk of femur/hip fracture than controls. Disability status is probably more important than glucocorticoid use in the aetiology of MS and osteoporotic fracture.