Proposal for a simplified classification of IMD based on a pathophysiological approach: A practical guide for clinicians

Challenges and opportunities in neurometabolic disease treatment with enzyme delivery

INTRODUCTION

Neurometabolic disorders remain challenging to treat, largely due to the limited availability of drugs that can cross the blood-brain barrier (BBB) and effectively target brain impairment. Key reasons for inadequate treatment include a lack of coordinated knowledge, few studies on BBB status in these diseases, and poorly designed therapies.

AREAS COVERED

This paper provides an overview of current research on neurometabolic disorders and therapeutic options, focusing on the treatment of neurological involvement. It highlights the limitations of existing therapies, describes innovative protocols recently developed, and explores new opportunities for therapy design and testing, some of which are already under investigation. The goal is to guide researchers toward innovative and potentially more effective treatments.

EXPERT OPINION

Advancing research on neurometabolic diseases is crucial for designing effective treatment strategies. The field suffers from a lack of collaboration, and a strong collective effort is needed to enhance synergy, increase knowledge, and develop a new therapeutic paradigm for neurometabolic disorders.

Pediatric palliative care for metabolic diseases: 20-year epidemiological survey of outpatients at a Brazilian quaternary hospital

The interface between pediatric palliative care (PPC) and inborn metabolic diseases (IMD) remains incipient, though these conditions fill the state of art of complex chronic diseases, eligible to this health approach. We analyzed the medical records of PPC clinic during the years 2001 to 2021 and the IMD outpatients. We established a parallel with the world scientific literature concerning the epidemiology of PPC and IMD. Among outpatients, 14% were diagnosed with IMD, which were referred to the PPC service earlier compared to Non-IMD cases. The Group 3 (complex molecules) was the most frequent (64.7%), following by Group 1 representing by small molecules (21.6%), the latter having a lower median age at diagnosis when compared to the former (0.7 vs. 5.2 years, p = 0.001). The sphingolipidoses were the pathologies most frequent in our cohort, in line with what was observed in the literature. There were no differences between IMD groups in terms of diagnosis and PPC referral age, however in Non-IMD conditions, the age of diagnosis were earlier than IMD. Nevertheless, IMD group showed lower age of referral to PPC. The IMD comprises large fraction of outpatients in the PPC setting, thus further studies are needed in this field.

The interplay between glucose and ketone bodies in neural stem cell metabolism

Glucose is the primary energy source for neural stem cells (NSCs), supporting their proliferation, differentiation, and quiescence. However, the high demand for glucose during brain development often exceeds its supply, leading to the utilization of alternative energy sources including ketone bodies. Ketone bodies, including β-hydroxybutyrate, are short-chain fatty acids produced through hepatic ketogenesis and play a crucial role in providing energy and the biosynthetic components for NSCs when required. The interplay between glucose and ketone metabolism influences NSC behavior and fate decisions, and disruptions in these metabolic pathways have been linked to neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. Additionally, ketone bodies exert neuroprotective effects on NSCs and modulate cellular responses to oxidative stress, energy maintenance, deacetylation, and inflammation. As such, understanding the interdependence of glucose and ketone metabolism in NSCs is crucial to understanding their roles in NSC function and their implications for neurological conditions. This article reviews the mechanisms of glucose and ketone utilization in NSCs, their impact on NSC function, and the therapeutic potential of targeting these metabolic pathways in neurological disorders.

Diagnosis of inborn errors of metabolism through prenatal exome sequencing with targeted analysis for fetal structural anomalies

OBJECTIVES

The value of prenatal exome sequencing (pES) for fetuses with structural anomalies is widely reported. In England, testing is conducted through trio exome sequencing and analysis of a gene panel. Over a 30-month period testing of 921 pregnancies resulted in a genetic diagnosis in 32.8% of cases (302/921). Here we review cases diagnosed with an inborn error of metabolism.

METHODS

Diagnoses of inborn errors of metabolism (IEM) were classified according to the ICIMD classification system. Genetic diagnoses were assessed against Human Phenotype Ontology terms, gestation of scan findings and literature evidence.

RESULTS

35/302 diagnoses (11.6%) represented IEM. Almost half affected metabolism of complex macromolecules and organelles (n = 16), including congenital disorders of glycosylation (n = 8), peroxisome biogenesis disorders (n = 4), and lysosomal storage disorders (n = 4). There were eight disorders of lipid metabolism and transport, the majority being genes in the cholesterol biosynthesis pathway, eight disorders of intermediary metabolism, of which seven were defects in "energy" processes, and two diagnoses of alkaline phosphatase deficiency.

CONCLUSIONS

Review of pES diagnoses and ultrasound scan findings is key to understanding genotype-phenotype correlations. IEM are genetically heterogeneous and may present with variable scan findings, which makes an individual diagnosis difficult to suspect. Diagnosis during pregnancy is particularly important for many IEM with respect to prognosis and early neonatal management.

Spectrum of Organic Aciduria Diseases in Tunisia: A 35-year Retrospective Study

Background

Organic aciduria diseases (OADs) occur worldwide, with differences in prevalence and patterns between populations.

Objectives

To describe the spectrum of OADs identified in Tunisia over a 35-years period.

Materials and Methods

This retrospective study included patients who were diagnosed with OADs between 1987 and 2022 in the Laboratory of Biochemistry, Rabta Hospital, Tunisia. Organic acids were analyzed using gas chromatography-mass spectrometry.

Results

A total of 30,670 urine samples were analyzed for OADs, of which 471 were positive for OADs. The estimated incidence of OADs in Tunisia was 6.78 per 100,000 live births. Methylmalonic (n = 146) and propionic (n = 90) acidurias were the most common OADs (estimated incidence: 2.10 and 1.30 per 100,000 live births, respectively). There were 54 cases of L-2-hydroxyglutatric acidurias and 30 cases of pyroglutamic acidurias, which makes it one of the highest in the world. The main clinical features were hypotonia (65%) and feeding difficulties (41%). Age at diagnosis was highly variable, ranging from 1 day to 49 years. Only 27% of the patients were diagnosed within the first month of life. The prevalence of OADs was highest in the Center-East and Southeast regions.

Conclusions

In Tunisia, OADs are relatively frequent, but there are shortcomings regarding the diagnosis of these disorders. The frequency and health/social impact of these disorders warrant the need for implementing newborn screening programs and suitable patient management.

3D Printing of Dietary Products for the Management of Inborn Errors of Intermediary Metabolism in Pediatric Populations

The incidence of Inborn Error of Intermediary Metabolism (IEiM) diseases may be low, yet collectively, they impact approximately 6-10% of the global population, primarily affecting children. Precise treatment doses and strict adherence to prescribed diet and pharmacological treatment regimens are imperative to avert metabolic disturbances in patients. However, the existing dietary and pharmacological products suffer from poor palatability, posing challenges to patient adherence. Furthermore, frequent dose adjustments contingent on age and drug blood levels further complicate treatment. Semi-solid extrusion (SSE) 3D printing technology is currently under assessment as a pioneering method for crafting customized chewable dosage forms, surmounting the primary limitations prevalent in present therapies. This method offers a spectrum of advantages, including the flexibility to tailor patient-specific doses, excipients, and organoleptic properties. These elements are pivotal in ensuring the treatment's efficacy, safety, and adherence. This comprehensive review presents the current landscape of available dietary products, diagnostic methods, therapeutic monitoring, and the latest advancements in SSE technology. It highlights the rationale underpinning their adoption while addressing regulatory aspects imperative for their seamless integration into clinical practice.

Clinical Presentation of Inherited Metabolic Diseases in Newborns Hospitalised in an Intensive Care Unit

BACKGROUND

The first clinical manifestations of inherited metabolic diseases occur in the neonatal period in up to half of cases, often with nonspecific symptoms, making their recognition challenging. This study aimed to characterise inherited metabolic disease cases with neonatal presentation requiring admission to the paediatric intensive care unit in a Portuguese reference centre for inherited metabolic diseases.

MATERIAL AND METHODS

An observational study with retrospective data collection was performed, including all newborns with an inherited metabolic disease admitted to the pediatric intensive care unit between June 2011 and June 2022. Three 'pathophysiological' groups were defined: cases due to small molecules, energy deficiency and complex molecules.

RESULTS

Twenty newborns, with a median age at admission of 7.5 days, were included. Thirteen (65%) were female, sixteen (80%) had a small molecule disorder, and four (20%) had diseases of energy defects. Neurological manifestations were the most common, with most newborns presenting symptomatically in the first week of life. There was no difference between the groups in neurological, cardiac, and hepatic involvement and shock at presentation. A symptom-free interval was more frequent in patients with small molecule disorders than the others (p=0.01). The main metabolic changes found were altered plasma amino acids (n=13) and organic aciduria (n=10), creatine kinase elevation (n=13), hyperlactatemia (n=12), metabolic acidosis with increased anion gap (n=8) and hyperammonaemia (n=7). Newborn screening of metabolites helped make a diagnosis in 60% of cases. Five newborns died due to multiorgan failure (n=3) or refractory cardiogenic shock (n=1), and in one, therapeutic efforts were limited due to an adverse neurological prognosis.

CONCLUSION

Although the symptoms and signs are often nonspecific, we should suspect inherited metabolic disease when a newborn presents with neurological symptoms after a symptom-free period, however short it might be. Newborns with suspected inherited metabolic disease should be evaluated with simple biochemical tests, and newborn screening should be urgently expanded to start specific treatment earlier, reducing mortality and morbidity.

Metabolomic Studies in Inborn Errors of Metabolism: Last Years and Future Perspectives

The inborn errors of metabolism (IEMs or Inherited Metabolic Disorders) are a heterogeneous group of diseases caused by a deficit of some specific metabolic pathways. IEMs may present with multiple overlapping symptoms, sometimes difficult delayed diagnosis and postponed therapies. Additionally, many IEMs are not covered in newborn screening and the diagnostic profiling in the metabolic laboratory is indispensable to reach a correct diagnosis. In recent years, Metabolomics helped to obtain a better understanding of pathogenesis and pathophysiology of IEMs, by validating diagnostic biomarkers, discovering new specific metabolic patterns and new IEMs itself. The expansion of Metabolomics in clinical biochemistry and laboratory medicine has brought these approaches in clinical practice as part of newborn screenings, as an exam for differential diagnosis between IEMs, and evaluation of metabolites in follow up as markers of severity or therapies efficacy. Lastly, several research groups are trying to profile metabolomics data in platforms to have a holistic vision of the metabolic, proteomic and genomic pathways of every single patient. In 2018 this team has made a review of literature to understand the value of Metabolomics in IEMs. Our review offers an update on use and perspectives of metabolomics in IEMs, with an overview of the studies available from 2018 to 2022.

Vaccination strategies for people living with inborn errors of metabolism in Brazil

OBJECTIVE

Through a literature review, make recommendations regarding immunizations in people living with Inborn Error of Metabolism (IEM) in Brazil, assess the possible impact on metabolic decompensations after immunization, and if this specific population may have an impaired immune response to vaccines.

SOURCE OF DATA

The MeSH Terms vaccination OR vaccine OR immunization associated with the term inborn error of metabolism AND recommendation were used in combination with search databases. Only articles published after 1990, in the languages English, Spanish, French or Portuguese, human-related were included.

SYNTHESIS OF DATA

A total of 44 articles were included to make the following recommendations. Individuals with IEMs need to be up to date with their immunizations. Regarding which vaccines should be offered, children and adults should follow the routine immunization schedules locally available, including the COVID-19 vaccines. The only exception is the rotavirus vaccine for hereditary fructose intolerance. The benefit of immunization outweighs the very low risk of metabolic decompensation. Since not all patients will have an adequate immune response, measuring antibody conversion and titers is recommended CONCLUSIONS: All patients should receive age-appropriate immunizations in their respective schedules without delays. The only situation when vaccination may be contraindicated is with oral rotavirus vaccine in hereditary fructose intolerance. Monitoring the levels of antibodies should be done to detect any immune dysfunction or the necessity for boosters. A personalized immunization schedule is ideal for patients with IEMs. The reference organizations could improve their recommendations to address all IEMs, not only some of them.

Lipidomics-Paving the Road towards Better Insight and Precision Medicine in Rare Metabolic Diseases

Even though the application of Next-Generation Sequencing (NGS) has significantly facilitated the identification of disease-associated mutations, the diagnostic rate of rare diseases is still below 50%. This causes a diagnostic odyssey and prevents specific treatment, as well as genetic counseling for further family planning. Increasing the diagnostic rate and reducing the time to diagnosis in children with unclear disease are crucial for a better patient outcome and improvement of quality of life. In many cases, NGS reveals variants of unknown significance (VUS) that need further investigations. The delineation of novel (lipid) biomarkers is not only crucial to prove the pathogenicity of VUS, but provides surrogate parameters for the monitoring of disease progression and therapeutic interventions. Lipids are essential organic compounds in living organisms, serving as building blocks for cellular membranes, energy storage and signaling molecules. Among other disorders, an imbalance in lipid homeostasis can lead to chronic inflammation, vascular dysfunction and neurodegenerative diseases. Therefore, analyzing lipids in biological samples provides great insight into the underlying functional role of lipids in healthy and disease statuses. The method of choice for lipid analysis and/or huge assemblies of lipids (=lipidome) is mass spectrometry due to its high sensitivity and specificity. Due to the inherent chemical complexity of the lipidome and the consequent challenges associated with analyzing it, progress in the field of lipidomics has lagged behind other omics disciplines. However, compared to the previous decade, the output of publications on lipidomics has increased more than 17-fold within the last decade and has, therefore, become one of the fastest-growing research fields. Combining multiple omics approaches will provide a unique and efficient tool for determining pathogenicity of VUS at the functional level, and thereby identifying rare, as well as novel, genetic disorders by molecular techniques and biochemical analyses.

The landscape of CRISPR/Cas9 for inborn errors of metabolism

Since its discovery as a genome editing tool, the clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9 (CRISPR/Cas9) system has opened new horizons in the diagnosis, research, and treatment of genetic diseases. CRISPR/Cas9 can rewrite the genome at any region with outstanding precision to modify it and further instructions for gene expression. Inborn Errors of Metabolism (IEM) are a group of more than 1500 diseases produced by mutations in genes encoding for proteins that participate in metabolic pathways. IEM involves small molecules, energetic deficits, or complex molecules diseases, which may be susceptible to be treated with this novel tool. In recent years, potential therapeutic approaches have been attempted, and new models have been developed using CRISPR/Cas9. In this review, we summarize the most relevant findings in the scientific literature about the implementation of CRISPR/Cas9 in IEM and discuss the future use of CRISPR/Cas9 to modify epigenetic markers, which seem to play a critical role in the context of IEM. The current delivery strategies of CRISPR/Cas9 are also discussed.

A narrative review of metabolomics in the era of "-omics": integration into clinical practice for inborn errors of metabolism

BACKGROUND AND OBJECTIVE

Traditional targeted metabolomic investigations identify a pre-defined list of analytes in samples and have been widely used for decades in the diagnosis and monitoring of inborn errors of metabolism (IEMs). Recent technological advances have resulted in the development and maturation of untargeted metabolomics: a holistic, unbiased, analytical approach to detecting metabolic disturbances in human disease. We aim to provide a summary of untargeted metabolomics [focusing on tandem mass spectrometry (MS-MS)] and its application in the field of IEMs.

METHODS

Data for this review was identified through a literature search using PubMed, Google Scholar, and personal repositories of articles collected by the authors. Findings are presented within several sections describing the metabolome, the current use of targeted metabolomics in the diagnostic pathway of patients with IEMs, the more recent integration of untargeted metabolomics into clinical care, and the limitations of this newly employed analytical technique.

KEY CONTENT AND FINDINGS

Untargeted metabolomic investigations are increasingly utilized in screening for rare disorders, improving understanding of cellular and subcellular physiology, discovering novel biomarkers, monitoring therapy, and functionally validating genomic variants. Although the untargeted metabolomic approach has some limitations, this "next generation metabolic screening" platform is becoming increasingly affordable and accessible.

CONCLUSIONS

When used in conjunction with genomics and the other promising "-omic" technologies, untargeted metabolomics has the potential to revolutionize the diagnostics of IEMs (and other rare disorders), improving both clinical and health economic outcomes.

Imaging of Inherited Metabolic and Endocrine Disorders

"Inherited metabolic disorders represent a large group of disorders of which approximately 25% present in neonatal period with acute metabolic decompensation, rapid clinical deterioration, and often nonspecific imaging findings. Neonatal onset signifies the profound severity of the metabolic abnormality compared with cases with later presentation and necessitates rapid diagnosis and urgent therapeutic measures in an attempt to decrease the extent of brain injury and prevent grave neurologic sequela or death. Here, the authors discuss classification and clinical and imaging findings in a spectrum of metabolic and endocrine disorders with neonatal presentation."

Biochemical testing for inborn errors of metabolism: experience from a large tertiary neonatal centre

Inborn errors of metabolism are an individually rare but collectively significant cause of mortality and morbidity in the neonatal period. They are identified by either newborn screening programmes or clinician-initiated targeted biochemical screening. This study examines the relative contribution of these two methods to the identification of inborn errors of metabolism and describes the incidence of these conditions in a large, tertiary, neonatal unit. We also examined which factors could impact the reliability of metabolic testing in this cohort. This is a retrospective, single-site study examining infants in whom a targeted metabolic investigation was performed from January 2018 to December 2020 inclusive. Data was also provided by the national newborn screening laboratory regarding newborn screening diagnoses. Two hundred and four newborns received a clinician-initiated metabolic screen during the time period examined with 5 newborns being diagnosed with an inborn error of metabolism (IEM) (2.4%). Of the 25,240 infants born in the hospital during the period examined, a further 11 newborns had an inborn error of metabolism diagnosed on newborn screening. This produced an incidence in our unit over the time described of 6.34 per 10,000 births. This number reflects a minimum estimate, given that the conditions diagnosed refer to early-onset disorders and distinctive categories of IEM only. Efficiency of the clinician-initiated metabolic screening process was also examined. The only statistically significant variable in requiring repeat metabolic screening was early day of life (z-score = - 2.58, p = 0.0098). A total of 28.4% was missing one of three key metabolic investigation parameters of blood glucose, ammonia or lactate concentration with ammonia the most common investigation missing. While hypoglycemia was the most common clinical rationale for a clinician-initiated metabolic test, it was a poor predictor of inborn error of metabolism with no newborns of 25 screened were diagnosed with a metabolic disorder.

CONCLUSION

Clinician-targeted metabolic screening had a high diagnostic yield given the relatively low prevalence of inborn errors of metabolism in the general population. Thoughts should be given to the rationale behind each targeted metabolic test and what specific metabolic disease or category of inborn error of metabolism they are concerned along with commencing targeted testing.

WHAT IS KNOWN

• Inborn errors of metabolism are a rare but potentially treatable cause of newborn mortality and morbidity. • A previous study conducted in a tertiary unit in an area with limited newborn screening demonstrated a diagnostic yield of 5.4%.

WHAT IS NEW

• Clinician-initiated targeted metabolic screening has a good diagnostic performance even with a more expanded newborn screening programme. • Further optimisation could be achieved by examining the best timing and also the rationale of metabolic testing in the newborn period.

1H-Nuclear Magnetic Resonance Analysis of Urine as Diagnostic Tool for Organic Acidemias and Aminoacidopathies

The utility of low-resolution 1H-NMR analysis for the identification of biomarkers provided evidence for rapid biochemical diagnoses of organic acidemia and aminoacidopathy. 1H-NMR, with a sensitivity expected for a field strength of 400 MHz at 64 scans was used to establish the metabolomic urine sample profiles of an infant population diagnosed with small molecule Inborn Errors of Metabolism (smIEM) compared to unaffected individuals. A qualitative differentiation of the 1H-NMR spectral profiles of urine samples obtained from individuals affected by different organic acidemias and aminoacidopathies was achieved in combination with GC-MS. The smIEM disorders investigated in this study included phenylalanine metabolism; isovaleric, propionic, 3-methylglutaconicm and glutaric type I acidemia; and deficiencies in medium chain acyl-coenzyme and holocarboxylase synthase. The observed metabolites were comparable and similar to those reported in the literature, as well as to those detected with higher-resolution NMR. In this study, diagnostic marker metabolites were identified for the smIEM disorders. In some cases, changes in metabolite profiles differentiated post-treatments and follow-ups while allowing for the establishment of different clinical states of a biochemical disorder. In addition, for the first time, a 1H-NMR-based biomarker profile was established for holocarboxylase synthase deficiency spectrum.

Movement Disorders and Liver Disease

The association of movement disorders with structural or functional hepatic disease occurs in three principal scenarios: (1) combined involvement of both organ systems from a single disease entity, (2) nervous system dysfunction resulting from exposure to toxic compounds in the setting of defective hepatic clearance, or (3) hepatic and/or neurological injury secondary to exposure to exogenous drugs or toxins. An important early step in the workup of any patient with combined movement disorders and liver disease is the exclusion of Wilson's disease. Diagnostic delay remains common for this treatable disorder, and this has major implications for patient outcomes. Thereafter, a structured approach integrating variables such as age of onset, tempo of progression, nature and severity of liver involvement, movement disorder phenomenology, exposure to drugs/toxins and laboratory/neuroimaging findings is key to ensuring timely diagnosis and disease‐specific therapy. Herein, we provide an overview of disorders which may manifest with a combination of movement disorders and liver disease, structured under the three headings as detailed above. In each section, the most common disorders are discussed, along with important clinical pearls, suggested diagnostic workup, differential diagnoses and where appropriate, treatment considerations.

Energy metabolism in childhood neurodevelopmental disorders

Whereas energy function in the aging brain and their related neurodegenerative diseases has been explored in some detail, there is limited knowledge about molecular mechanisms and brain networks of energy metabolism during infancy and childhood. In this review we describe current insights on physiological brain energetics at prenatal and neonatal stages, and in childhood. We then describe the main groups of inborn errors of energy metabolism affecting the brain. Of note, scarce basic neuroscience research in this field limits the opportunity for these disorders to provide paradigms of energy utilization during neurodevelopment. Finally, we report energy metabolism disturbances in well-known non-metabolic neurodevelopmental disorders. As energy metabolism is a fundamental biological function, brain energy utilization is likely altered in most neuropediatric diseases. Precise knowledge on mechanisms of brain energy disturbance will open the possibility of metabolic modulation therapies regardless of disease etiology.

Inherited disorders of complex lipid metabolism: A clinical review

Over 80 human diseases have been attributed to defects in complex lipid metabolism. A majority of them have been reported recently in the setting of rapid advances in genomic technology and their increased use in clinical settings. Lipids are ubiquitous in human biology and play roles in many cellular and intercellular processes. While inborn errors in lipid metabolism can affect every organ system with many examples of genetic heterogeneity and pleiotropy, the clinical manifestations of many of these disorders can be explained based on the disruption of the metabolic pathway involved. In this review, we will discuss the physiological function of major pathways in complex lipid metabolism, including nonlysosomal sphingolipid metabolism, acylceramide metabolism, de novo phospholipid synthesis, phospholipid remodeling, phosphatidylinositol metabolism, mitochondrial cardiolipin synthesis and remodeling, and ether lipid metabolism as well as common clinical phenotypes associated with each.

Evaluation of Body Composition, Physical Activity, and Food Intake in Patients with Inborn Errors of Intermediary Metabolism

Children with inborn errors of intermediary metabolism (IEiM) must follow special diets that restrict their intake of essential nutrients and may compromise normal growth and development. We evaluated body composition, bone mineral density, physical activity, and food intake in IEiM patients undergoing dietary treatment. IEiM patients (n = 99) aged 5–19 years and healthy age- and sex-matched controls (n = 98) were recruited and underwent dual-energy X-ray absorptiometry to evaluate anthropometric characteristics and body composition. Data on food intake and physical activity were also collected using validated questionnaires. The height z-score was significantly lower in IEiM patients than controls (−0.28 vs. 0.15; p = 0.008), particularly in those with carbohydrate and amino acid metabolism disorders. Significant differences in adiposity were observed between patients and controls for the waist circumference z-score (−0.08 vs. −0.58; p = 0.005), but not the body mass index z-score (0.56 vs. 0.42; p = 0.279). IEiM patients had a significantly lower total bone mineral density (BMD) than controls (0.89 vs. 1.6; p = 0.001) and a higher risk of osteopenia (z-score < −2, 33.3% vs. 20.4%) and osteoporosis (z-score < −2.5, 7.1% vs. 0%), but none presented fractures. There was a significant positive correlation between natural protein intake and BMD. Our results indicate that patients with IEiM undergoing dietary treatment, especially those with amino acid and carbohydrate metabolism disorders, present alterations in body composition, including a reduced height, a tendency towards overweight and obesity, and a reduced BMD.

Clinico-radiological phenotyping and diagnostic pathways in childhood neurometabolic disorders-a practical introductory guide

Inborn errors of metabolism (IEM) although individually rare, together constitute a significant proportion of childhood neurological disorders. Majority of these disorders occur due to deficiency of an enzyme in a specific metabolic pathway, leading to damage by accumulation of a toxic substrate or deficiency of an essential metabolite. Early diagnosis is crucial in many of these conditions to prevent or minimise brain damage. Whilst many of the neuroimaging features are nonspecific, certain disorders demonstrate specific patterns due to selective vulnerability of different structures to different insults. Along with clinical and biochemical profile, neuroimaging thus plays a pivotal role in differentiating metabolic disorders from other causes, in providing a differential diagnosis or suggesting a metabolic pathway derangement, and on occasion also helps make a specific diagnosis. This allows initiation of targeted metabolic and genetic work up and treatment. Familiarity with the clinical features, relevant biochemical features and neuroimaging findings of common metabolic disorders to facilitate a prompt diagnosis cannot thus be overemphasized. In this article, we describe the latest classification scheme, the clinical and biochemical clues and common radiological patterns. The diagnostic algorithm followed in daily practice after clinico-radiological phenotyping is alluded to and illustrated by clinical vignettes. Focused sections on neonatal metabolic disorders and mitochondrial disorders are also provided. The purpose of this article is to provide a brief overview and serve as a practical primer to clinical and radiological phenotypes and diagnostic aspects of IEM.

An international classification of inherited metabolic disorders (ICIMD)

Several initiatives at establishing a classification of inherited metabolic disorders have been published previously, some focusing on pathomechanisms, others on clinical manifestations, while yet another attempted a simplified approach of a comprehensive nosology. Some of these classifications suffered from shortcomings, such as lack of a mechanism for continuous update in light of a rapidly evolving field, or lack of widespread input from the metabolic community at large. Our classification-the International Classification of Inherited Metabolic Disorders, or International Classification of Inborn Metabolic Disorders (ICIMD)-includes 1450 disorders, and differs from prior approaches in that it benefited from input by a large number of experts in the field, and was endorsed by major metabolic societies around the globe. Several criteria such as pathway involvement and pathomechanisms were considered. The main purpose of the hierarchical, group-based approach of the ICIMD is an improved understanding of the interconnections between many individual conditions that may share functional, clinical, and diagnostic features. The ICIMD aims to include any primary genetic condition in which alteration of a biochemical pathway is intrinsic to specific biochemical, clinical, and/or pathophysiological features. As new disorders are discovered, we will seek the opinion of experts in the advisory board prior to inclusion in the appropriate group of the ICIMD, thus guaranteeing the continuing relevance of this classification via regular curation and expert advice.

Clouds over IMD? Perspectives for inherited metabolic diseases in adults from a retrospective cohort study in two Swiss adult metabolic clinics

Background

Inherited metabolic diseases (IMD) are complex medical conditions. Thanks to improvements in diagnosis and treatment, a growing number of pediatric IMD patients reach adulthood. Thus, clinical care of adults with IMD has emerged as a new and challenging reality. This purpose of this study of adults with IMD in an adult metabolic clinic at two academic hospitals (Lausanne and Geneva) was to help inform decisions on the future organization of health care for this group of patients.

Methods

All adult patients with a biochemical and/or genetic diagnosis of IMD followed at the clinics were included in the study. Electronic patient records were reviewed for clinical features, diagnostic studies, treatment and long-term outcome. Data of undiagnosed patients referred for suspected IMD were analyzed separately.

Results

126 patients were included in the study. The most prevalent group of diseases was small molecules disorders with 82 (65%) patients, followed by energy defects disorders with 29 (23%) patients and complex molecules disorders with 15 (12%) patients. Overall, 64% of patients were diagnosed before, and 36% after the age 16 years. Among the 126 cases, 51% suffered from medical complications. 79% of the patients were receiving a specific treatment for their disease. Among the 138 undiagnosed patients referred for suspicion of IMD, investigations lead to a genetic diagnosis in 24 (17%) patients. 19 had confirmation of an IMD, 5 were found to have another genetic condition.

Conclusions

This retrospective study reveals significant features of adult IMD cohort. The disorders are heterogeneous, and there is no one-size-fits-all approach – treatment must be tailored to fit each specific disorder in each individual patient. Even patients who are followed at the dedicated clinic are not protected from metabolic decompensations and/or chronic organ-specific complications. While it is commonly assumed that patients with IMD are more stable once they become adults, our data show that the diseases continue to exact a lifelong toll. A coordinated monitoring of target organs by a multidisciplinary team is needed. To ensure that the success in diagnosis and treatment of individuals with IMD is sustained, there is a clear requirement for adequately staffed adult IMD clinics.

Laboratory Diagnosis of Lysosomal Diseases: Newborn Screening to Treatment

The goal of screening programs for inborn errors of metabolism (IEM) is early detection and timely intervention to significantly reduce morbidity, mortality and associated disabilities. Phenylketonuria exemplifies their success as neonates are identified at birth and then promptly treated allowing normal neurological development. Lysosomal diseases comprise about 50 IEM arising from a deficiency in a protein required for proper lysosomal function. Typically, these defects are in lysosomal enzymes with the concomitant accumulation of the enzyme's substrate as the cardinal feature. None of the lysosomal diseases are screened at birth in Australia and in the absence of a family history, traditional laboratory diagnosis of the majority, involves demonstrating a deficiency of the requisite enzyme. Diagnostic confusion can arise from interpretation of the degree of residual enzyme activity causative of disease and is impractical when the disorder is not due to an enzyme deficiency per se. Advances in mass spectrometry technologies has enabled simultaneous measurement of the enzymes' substrates and their metabolites which facilitates the efficiency of diagnosis. Employing urine chemistry as a reflection of multisystemic disease, individual lysosomal diseases can be identified by a characteristic substrate pattern complicit with the enzyme deficiency. Determination of lipids in plasma allows the diagnosis of a further class of lysosomal disorders, the sphingolipids. The ideal goal would be to measure biomarkers for each specific lysosomal disorder in the one mass spectrometry-based platform to achieve a diagnosis. Confirmation of the diagnosis is usually by identifying pathogenic variants in the underlying gene, and although molecular genetic technologies can provide the initial diagnosis, the biochemistry will remain important for interpreting molecular variants of uncertain significance.