Developmental Delay, Hypomyelination, and Nystagmus: Case and Approach.

Pelizaeus-Merzbacher-like disease (PMLD, OMIM #608804) is an autosomal recessive hypomyelinating leukodystrophy caused by homozygous variants in the gene. It usually presents in the first months of life with nystagmus, developmental delay, and diffuse hypomyelination on brain magnetic resonance imaging (MRI). We report a case of a 3-year-old boy that presented with nystagmus and global developmental delay. MRI showed diffuse hypomyelination, including the cerebellum. Pelizaeus-Merzbacher disease (PMD) was suspected; however, no pathological variants of the gene were found. Exome sequencing found variants in the gene, leading to a diagnosis of PMLD. The combination of global developmental delay, hypomyelination, and nystagmus in a child should raise suspicion of PMD and PMLD. Unlike PMD, however, hypomyelination of the brainstem and cerebellum are frequently seen and brainstem auditory evoked potentials are usually normal in PMLD. The latter has an overall better prognosis than the former as well. Epidemiological studies on leukodystrophies have found conflicting results on which disease is more common. However, PMLD is a rare leukodystrophy and both PMLD and PMD should be considered in any child with developmental delay, hypomyelination, and nystagmus.