Neuro Disorders: Inborn Errors of Metabolism

Introduction

Inborn Errors of Metabolism are also called as inherited metabolic diseases, and as the name implies they are hereditary or genetically mediated disorders related to the metabolism. Metabolism is the process of breakdown of complex substances like proteins, fat, carbohydrates or other substances in the body so these diseases affect the utilization of above nutrients in the body and result in specific disease processes depending upon the particular metabolic pathway affected.

Majority of these are autosomal recessive type of inherited disorders and they manifest mostly during the childhood, especially the very early life. At birth many of them may look ok but as day passes more & more patients will start clinically manifesting.

Clinical manifestations

The clinical manifestations (symptoms and signs) are protean and the list is very huge due to a very big list of these conditions. Below some common manifestations are listed;

• Irritability, refusal of feeding
• Failure to thrive
• Nausea/vomiting
• Seizures
• Not picking up developmental milestones
• Losing the already acquired milestones
• Liver enlargement, jaundice
• Skin and skeletal manifestations etc

Not every disease will present with each and very manifestation mentioned above and the type of manifestations will depend upon individual disease conditon.

Classification/types

The categories under which these diseases can be lumped are also large and few common such categories are listed below with some commonly known examples for each category.

• Lysosomal storage diseases (e.g. Fabry Disease, Mucopolysaccharoidoses)
• Carbohydrate (e.g. glycogen storage diseases), amino acid (e.g. phenyl ketonurira), organic acid, fatty acids metabolism diseases
• Mitochondrial diseases (e.g. MERRF syndrome)
• Peroxisomal dysfunction (adrenoleukodystropy)
• Nucleic acid metabolic related diseases – i.e. purines & primidines (e.g. Lesch Nyhan syndrome)
• Steroid metabolism (e.g. CAH or congenital-adrenal-hyperplasia)

Diagnosis & management

The following investigations are frequently necessary (decided on case to case basis);

• Urine analysis (for amino acids, organic acids etc)
• Blood tests (for amino acids, lactic acid etc)
• CT and/or MRI of the brain
• EEG
• Biopsy
• Enzyme assays
• Genetic studies etc

When it comes to the treatment still there are no curative options available for most of these condtions. However by early detecting some of these conditions and making appropriate dietary changes damage to organs & tissues can be prevented significantly.

At research level treatments in the form of gene therapy, enzyme replacement therapy, bone marrow/stem cell transplantation etc are taking place, and for certain diseases if instituted very early in the disease a lot of damage can be prevented. Unfortunately these are extremely expensive therapies and available at only few centers so a routine use is not feasible yet.

A pediatric and/or adult neurologist is commonly involved with the care of patients with inborn errors of metabolism.



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