"This 18-month old female presented with developmental delay. By this age myelination should be essentially

complete, yet there is residual T2 high signal in the subcortical white matter due to disruption of normal myelin formation. The differential diagnosis is depends on the head size.

With megalencephaly the possibilities are:

• Alexander's disease
• Canavan's disease
• van der Knapp’s leukodystrophy (but there is no cystic change)

Without megalencephaly: - in utero infection including Toxoplasma, Rubella, cytomegalovirus and herpes simplex virus. - mitochondrial cytopathy - Pelizeus-Merzbacher disease (but patient is female and PMD is X-linked recessive) - 18q deletion syndrome (the karyotype was normal) - Tuberous sclerosis (there were no other stigmata)

The more common leukodystrophies, metachromatic leukodystrophy and adrenoleukodystrophy, typically spare the subcortical U-fibres.