Pelizaeus-Merzbacher disease (PMD) is a rare, progressive, degenerative disease of the central nervous system in which coordination, motor and intellectual function are impaired. The disease is part of a group of gene-linked conditions known as the leukodystrophies, which affect the growth of the myelin sheath - the fatty layer that wraps around and protects nerve fibers in the brain. The disease is caused by a mutation in the gene that controls the production of a myelin protein called proteolipid protein-1 (PLP1). PMD is inherited as an X-linked recessive trait; the affected individuals are male and the mothers are carriers of the PLP1 mutation. The severity and onset of the disease varies widely depending on the type of PLP1 mutation. PMD is part of a spectrum of diseases related to PLP1, which includes spastic paraplegia type 2 (SPG2).
Within this spectrum of diseases there are four general classifications. In order of severity, these are:

• Connatal PMD, the most severe type involving delayed mental and physical development and severe neurological symptoms;
• Classic PMD, where the early symptoms include muscle weakness, involuntary eye movements (nystagmus), and delays in motor development in the first year of life;
• Complicated SPG2, with motor development problems and brain involvement, and,
• Pure SPG2, including cases of PMD that have no neurological complications.

Noticeable changes in the degree of myelination can be detected by MRI analyzes of the brain. Additional symptoms of PMD may include slow growth, tremor, failure to develop normal head movement control, and deterioration in speech and cognitive function.