A new distinct pattern of hypomyelination has been recently described in four boys.[26] Hypomyelinating leukoencephalopathy affecting early myelinating structures is presumed to be X-linked. All patients presented a specific MRI involvement, with hypomyelination (mild T2-hyperintensity, T1-hyper-, iso- or mild hypo-intensity) of structures that are known to myelinate early in life. In particular, the optic radiations and the frontoparietal periventricular white matter were involved in all cases; the brainstem and the cerebellar white matter were also hypomyelinated, as well as the thalamus. The posterior limb of the internal capsule showed a striped altered signal in T2-weighted images in the majority of cases. The clinical presentation is characterized by onset around the age of 6–20 months with nystagmus. Cerebellar signs (ataxia and dysarthria) were reported in all patients.
It is interesting to note that this entity differs from all other hypomyelinating disorders. In fact, hypomyelinating leukodystrophies usually present with a pattern of hypomyelination in which the early myelinating structures are the most myelinated in comparison to regions of late myelination. In other words, the myelination sequence is respected in hypomyelinating leukodystrophies, except in hypomyelinating leukoencephalopathy affecting early myelinating structures where the structures involved are those of early myelination.