CMT is caused by defects in specific genes (genetic mutations) often inherited in an autosomal dominant pattern. This means that for some forms of CMT, if one parent has CMT, each child has a 50% chance of inheriting the gene and the disease.
CMT can be classified in a number of ways:
- Type I (demyelinating)—This type affects the coating of the nerve called the myelin sheath, causing nerve impulses to travel more slowly. It usually occurs in childhood or adolescence and is the most common type of CMT.
- Type II (axonal)—This type affects the part of the nerve called axons. Although the speed is normal, the size or amount of impulses is less than normal. This type of CMT is less common and occurs after adolescence.
Type III—Also called Dejerine-Sottas disease, this is a more rare, severe, early onset form of CMT. It is sometimes considered to be a subtype of CMT Type I. Symptoms may include:
- Delayed ability to walk due to weakness of the leg muscles closest to the trunk
- Severe sensory problems
- Hearing loss
- Type IV (demyelinating autosomal recessive)—This is similar to Type I, but often less severe. The inheritance pattern also makes it less likely to be inherited by an affected individual's children.