Charcot Marie Tooth

The nervous system is effectively a two-way system.  Messages from the brain travel via the upper motor neurones, down the spinal cord, through the lower motor neurones, such as the sciatic nerve in the leg, to the muscles.  In the sensory system, the messages travel upward from the sensory input through the first sensory neurones, up the spinal cord and through to the brain.    

Peripheral nerves are in the limbs and are composed of two types of nerve fibre – motor nerves and sensory nerves.   It is the peripheral nervous system that is affected by CMT.  Peripheral nerves can be thought of as electrical cables; the fibres (like wires) run down the middle and are wrapped in insulating material known as myelin.  If the myelin becomes damaged, the nerve impulses tend to be conducted more slowly than usual.   If the fibres (known as axons) are damaged, the speed of condition is normal but the size of the signal is reduced.

 There are two main types of CMT:

Demyelinating CMT

This involves a defect in the insulating myelin that wraps around the peripheral nerves.  As the myelin degenerates around the nerves, the impulses are carried along the nerve much more slowly than they should be.   This problem, on it's own, does not cause disability.

Over time, this problem with the myelin causes defects to become apparent in the axons themselves.  The muscles then do not receive the proper messages from the brain, and start to weaken, causing the various symptoms of CMT.  Only at this stage, does the condition start to become disabling.

This may be the reason why some people can have the genetic defect, and slow nerve condition, but never develop any recognisable symptoms.  There are many other factors in the genetic make-up of individuals controlling and managing the nervous system, and in these individuals, there is obviously another genetic factor involved in keeping the muscles functioning normally.

Axonal CMT

This involves a defect in the axon, whereby the nerves carry the messages at the proper speed, but at a greatly reduced strength.  As before, this lack of proper stimulation in the muscles causes weakness and atrophy. 

There are at the last count at least thirty different genetic types of CMT – each genetic defect causing a slightly different problem in the nerves – see the section on Types of CMT

A third, more rare type involves the nerves of the spinal cord – Spinal CMT (otherwise known as Distal Spinal Muscular Atrophy).  In this type, only the motor nerves are affected but the effects are similar to the other two types.

It is important to understand that although there are many different types of CMT, with  many different genetic causes, the symptoms they cause are very similar, as is the management of the condition.

Page Last Updated 17 November 2009