Charcot-Marie-Tooth, or CMT, is the most commonly inherited peripheral neuropathy and is found world-wide among all races and ethnic groups. Discovered in 1886 by three physicians, Jean-Martin Charcot, Pierre Marie, and Howard Henry Tooth, CMT affects an estimated 2.8 million people.
CMT is usually first noticed when someone develops lower leg weakness and foot drop, and foot deformities such as hammertoes and high arches, but that alone will not lead to diagnosis. A visit to a neurologist will start with a physical exam to look for further signs of distal weakness and sensory loss. To identify weakness, a person may be asked to do a heel-walk or to move part of the leg against an opposing force. Sensory loss is often identified when the doctor pokes a person with a pin or puts a tuning fork on the toe and these sensations are not felt, or are only felt a little.
Since CMT is a genetic condition, it often runs in families. It is important to tell the doctor or genetic counselor about any other family members with CMT-like symptoms. Lack of family history does not rule out CMT, but it will cause the doctor to look further to rule out other causes of neuropathy, such as diabetes or exposure to certain chemicals or drugs.
Further testing may be electrodiagnostic, genetic, or both.
Electrodiagnostic testing used to diagnose CMT usually includes a nerve conduction study (NCS), which measures the strength and speed of electrical signals moving down the peripheral nerves. Delayed responses are a sign of demyelination (type 1) and small responses of strength are a sign of axonopathy (type 2). An electromyogram (EMG) is also used to measure the electrical signals’ strength in the muscles of the arms or legs. NCVs and EMGs may cause discomfort for some. For those people, it may be possible to modify the study, perhaps testing fewer nerves than the doctor requested. In limited circumstances, sedation can be used for the NCS, but it reduces the usefulness of an EMG.
Generally, electrodiagnostic tests are performed by a lab technician, a neurologist or by a doctor specializing in physical medicine and rehabilitation. Click here to find a physician who is board-certified to conduct needle EMGs and nerve conduction studies, or click here for more information about electrodiagnostic testing.
More than 100 different genetic causes to CMT have been identified. Genetic tests, done by drawing blood, or in some cases through saliva, are available to test for many, but not all, genetic changes causing CMT. A positive genetic test can provide definitive diagnosis and useful information for family planning. However, a negative result does not rule out CMT since some types cannot yet be tested using DNA sampling. Click here for more genetic testing information.