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In distinction to other myelin disorders, hypomyelination is a permanent deficiency in myelin deposition rather than myelin destruction (i.e. demyelination) or abnormal myelin deposition (i.e. dysmyelination).
Clinical findings are non-specific but patients often present with developmental delay, cerebellar signs or spasticity. Some disorders have additional clinical findings.
The exact pathogenesis varies depending on the exact underlying disorder, but they represent inborn errors of metabolism.
They can be etiologically subcategorised into those with and without typical peripheral nervous system involvement (usually based on nerve conduction studies):
- with typical peripheral nervous system involvement
no typical peripheral nervous system involvement
- Pelizaeus-Merzbacher disease and Pelizaeus-Merzbacher like disease
- Tay syndrome (trichothiodystrophy with hypersensitivity to sunlight)
- Salla disease
- H-ABC (hypomyelination with atrophy of basal ganglia and cerebellum)
- serine synthesis defects
- oculogenital dysplasia
- 18q syndrome
- early onset neuronal degenerative disorders
- GM1 gangliosidosis
- spastic paraplegia type 2
The imaging diagnosis of hypomyelination is made on the basis of 1:
- unchanged myelination pattern not appropriately progressed for the patient's corrected gestational age on two successive brain MRI scans carried out at least 6 months apart
- at least one MRI scan should have been acquired after the age of 12 months
The underlying disorder is usually diagnosed by genetic testing but ancillary MRI features seen in some of these disorders may narrow the differentials for testing.
Please refer to the article on normal myelination for details on MRI assessment of myelination.