Types of CMT.

What are the different types of CMT?

CMT Type 1 Group:: Type 1 is comprised of types A B and C (disorders of the myelin nerve sheath causing slow nerve conduction velocity). There is autosomal dominant inheritance. This means that each child of a CMT parent has a 50% chance of inheriting the disorder.
In CMT Type 1A, the genetic defect occurs in a duplicated region of DNA coding on chromosome 17 in about 60-80% of cases. In rare cases of other CMT families have point mutations in the myelin PMP 22 gene. About 95% of CMT type 1 families have type 1A.
The gene responsible for type 1B has been found on the long arm of chromosome 1 in a myelin protein (myelin protein zero (MPZ).
There are very few families with CMT type 1C. The chromosomal location is not known.

Isolated or Sporadic CMT Type 1:: Isolated cases may occur as a result of new dominant mutations (CMT 1A and B, or other CMT varieties) The parents may show no signs of CMT. Some cases are more severe and are called Dejerine-Sottas neuropathy.

X-Linked CMT:: In about 25% of CMT Type 1 families, the genetic defect occurs on the X chromosome. All the female offspring of an affected male with X-linked CMT will be carriers for CMT. Each daughter of a CMT mother has a 50% chance of being a carrier and each son has a 50% chance of inheriting CMT. The gene defect in X-linked CMT has been identified on Connexin 32, a gap junction problem.

CMT Type II Group:: Affects the nerve cell bodies in the spinal cord. Most families with CMT II follow an autosomal dominant inheritance pattern with each child of a CMT parent having a 50% chance of inheriting the disease. About 40-50% of CMT II families have their genetic defect on chromosome 1. At least two known genes cause this type of CMT. Hereditary Sensory Neuropathy is a form of CMT II.

Autosomal Recessive CMT:: Both parents of one or more affected children are clinically normal "gene carriers" and may be distant blood relatives. The parents have a 25% recurrence risk to have an affected child. Gene loci for recessive CMT have been mapped on chromosomes 8 and 5 but so far no gene mutations have been found.

Spinal CMT and Type V:: These varieties have dominant inheritance and little or no sensory involvement. CMT V has upper motor neuron signs diagnosable by a neurologist. Nerve conduction velocities can be normal or slowed. Onset is usual in childhood with a marked tendency to toe walk, high arched feet, ankle stiffness and cramps. There are probably a number off different genetic sub types. These genes are being mapped.

Complicated Types of CMT:: CMT can be associated with deafness, visual defects, spasticity or ataxia.