Overview of Charcot-Marie-Tooth Disease
Charcot-Marie-Tooth (CMT) disease, also known as hereditary motor and sensory neuropathy (HMSN), is an inherited, degenerative nerve disorder that causes muscle weakness and atrophy in the feet, legs, hands, and forearms. It is characterized by progressive loss of use and sensation in the limbs.
Charcot, Marie, and Tooth are the names of the physicians who identified the disease and described its symptoms. It is not the same as Charcot foot disease, a neuropathic joint disease that is a common complication of diabetes mellitus.
In CMT, the myelin coating on motor and sensory nerves gradually deteriorates, resulting in poor transmission of nerve impulses. The feet and legs are the first to show the affects of myelin deterioration, or demyelination. Muscles fail to receive stimulation from the nerves and then begin to waste away (atrophy). Atrophy in the small muscles in the feet and hands causes the fingers and toes to curl.
Anatomy of the Nerves
Peripheral nerves extend from the spinal cord throughout the body. Nerve cells, or neurons, carry impulses to and from the brain via the spinal cord. Motor neurons signal muscles to move; sensory neurons transmit sensations, such as heat, pain, and surface texture to the brain.
Most neurons are made up of:
The axon is an elongated structure that conducts signals from the soma to synaptic terminals. Each neuron has one axon, which is wrapped in multiple layers of a substance called myelin. Axons transmit signals at a constant speed, called nerve conduction velocity (NCV), which is determined by the diameter of the axon and by the thickness of its myelin sheath. Myelin is critical in nerve signal conduction.
Types of Charcot-Marie-Tooth Disease
There are two forms of CMT. One form involves degeneration of the myelin sheath that surrounds a nerve's axon. The other involves impairment of the axon. There are several types of CMT within each form. The most commonly diagnosed type is CMT1. In patients with CMT1, there is axonal demyelination resulting in reduced motor and sensory nerve conduction. Loss of stimulation by the affected nerves causes muscle weakness and atrophy.
Incidence & Prevalence of Charcot-Marie-Tooth Disease
Approximately 125,000 people in the United States have Charcot-Marie-Tooth disease. CMT occurs slightly more often in men than in women and is not prevalent in any one race. Signs of the disease usually appear before the age of 30.
Cause of Charcot-Marie-Tooth Disease
CMT is caused by an inherited genetic mutation. There are rare cases in which the mutation occurs spontaneously within one egg or sperm.