Neurologic symptoms are the presenting and most prominent clinical problems associated with many inherited metabolic disorders. However, neurologic problems in general are common, especially psychomotor retardation, and deciding whom to investigate, and the type of testing to be done, is often difficult. The age of onset and clinical course often provide important clues to the metabolic nature of the disorder. This is also one situation in which delineation of the extent of the pathology is often invaluable. Besides determining the range of pathology within the nervous system, it is important to establish the extent to which other organs and tissues are involved in order to make a rapid diagnosis of inherited metabolic disease.

Careful and comprehensive clinical assessment, along with imaging studies, electrophysiologic investigation, and histopathologic and ultrastructural information from selected biopsies help to establish the distribution and type of abnormalities within the nervous system. Some patterns of abnormalities are so typical of certain disorders that metabolic studies are required only to confirm the diagnosis. Similarly, the pattern and degree of involvement of other organs and tissues is sometimes sufficiently characteristic to suggest a specific course of metabolic investigation. On one hand, for example, the presence of retinitis pigmentosa, hepatocellular dysfunction, and renal tubular dysfunction, in a child with psychomotor retardation, muscle weakness and seizures, strongly suggest the possibility of a mitochondrial defect.