Pelizaeus-Merzbacher disease, classic form

Pelizaeus-Merzbacher disease (PMD), classic form, is a rare X-linked leukodystrophy caused by mutations in the PLP1 gene, which encodes proteolipid protein 1, a major structural component of central nervous system myelin. The disease belongs to the group of hypomyelinating leukodystrophies, in which the brain fails to produce adequate myelin — the insulating sheath around nerve fibers that is essential for efficient signal transmission. The classic form is the most common presentation of PMD and is sometimes referred to as Type I PMD. The classic form typically presents in infancy with nystagmus (involuntary rhythmic eye movements), which is often the earliest recognizable sign, appearing within the first few months of life. As the child develops, additional neurological features emerge, including hypotonia that gradually transitions to spasticity, cerebellar ataxia, titubation (head trembling), impaired motor development, and progressive movement difficulties. Cognitive development is variably affected but is generally delayed. Affected individuals usually achieve limited motor milestones such as sitting or assisted standing but rarely achieve independent ambulation. The disease follows a slowly progressive course, and many patients survive into the third decade of life or beyond, though with significant disability. The central nervous system is the primary body system affected, with MRI characteristically showing diffuse hypomyelination of the brain white matter. Diagnosis is confirmed through genetic testing of the PLP1 gene, where duplications are the most common mutation type, followed by point mutations and deletions. There is currently no cure or disease-modifying treatment for classic PMD. Management is supportive and multidisciplinary, including physical therapy, occupational therapy, speech therapy, antispasticity medications, and management of seizures if they occur. Experimental approaches including cell-based therapies and gene therapy are under investigation but remain in early stages of clinical development.

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