Inborn errors and demyelination: MRI and the diagnosis of white matter disease.

The progress and extent of myelination can be assessed using magnetic resonance imaging (MRI). Myelination is delayed or diminished in several inherited metabolic abnormalities presenting in early life. Only minimal myelination of the CNS occurs in Pelizaeus-Merzbacher disease. Dysmyelination tends to produce fairly symmetrical lesions affecting white matter. In many mitochondrial enzyme and some lysosomal defects, the grey matter is also involved. The appearances and in particular the distribution on MRI and/or CT are characteristic in some conditions and the diagnosis is limited in others. Demyelination due to inflammatory disorders typically causes multifocal white matter lesions, recurrent in multiple sclerosis, monophasic in acute disseminated encephalomyelitis, extending in progressive multifocal leukoencephalopathy and classically involving the pons or corpus callosum in myelinolysis. Hypoxic ischaemic lesions may be metabolically induced and simulate primary demyelinating disorders. Mitochondrial enzyme defects in particular may present with stroke-like appearances. In many of these conditions, diagnosis is biochemical, but imaging has a significant role in suggesting the diagnosis, and documenting progression, response to therapy or complications.