There are four general classifications within this spectrum of diseases. In order of severity, they are:
- Connatal PMD, which is the most severe type and involves delayed mental and physical development and severe neurological symptoms;
- Classic PMD, in which the early symptoms include muscle weakness, involuntary movements of the eyes (nystagmus), and delays in motor development within the first year of life;
- Complicated SPG2, which features motor development issues and brain involvement, and,
- Pure SPG2, which includes cases of PMD that do not have neurologic complications.
Noticeable changes in the extent of myelination can be detected by MRI analyses of the brain. Additional symptoms of PMD may include slow growth, tremor, failure to develop normal control of head movement, and deteriorating speech and mental function.