The motor nerves carry messages from the brain and spinal cord to our muscles, telling them to contract. The sensory nerves convert specific external stimuli, such as touch and pain, into signals that are transmitted back to the brain. CMT causes the motor and sensory nerves to become damaged and eventually die. This leads to weakness and wasting of the muscles below the knees and often those of the hands. It can also cause numbness or loss of feeling in the hands and feet (the ‘sensory’ component).
CMT is also referred to as peroneal muscular atrophy, as the peroneal muscles on the outer side of the calves are particularly affected. Other names include Dejerine-Sottas disease and hereditary hypertrophic neuropathy. CMT is the favoured and most commonly-used name. There are two main types of CMT – type 1 and type 2. They are classified according to exactly which part of the nerve is damaged. If you think of a nerve as an electrical cable, the part of the nerve called the axon is the metal wire that conducts the electrical signal (see figure below). Around the axon is a protective myelin sheath – the plastic insulation you’d find around an electrical cable. In CMT type 1, it is the myelin sheath that becomes damaged and in type 2 it is the axon itself that is damaged. This nerve damage results in muscle weakness and wasting, as well as loss of sensation.
These two main types of CMT are further divided into sub-types, depending on how they are inherited and the gene that is involved.