Diagnosing CMT

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.

A patient gets his "knee-jerk" reflex tested at a CMT Center of Excellence.

A patient gets his “knee-jerk” reflex tested at a CMT Center of Excellence.

First Symptoms

CMT is usually first noticed when someone develops lower leg weakness and foot deformities such as foot drop, 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, the patient may be asked to walk on his heels or to move part of his leg against an opposing force. Sensory loss is often identified when the doctor tests for deep tendon reflexes, such as the knee-jerk, which are reduced or absent in CMT patients.

Family History

Since CMT is primarily hereditary, it is important to tell the doctor 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

Electrodiagnostic testing used to diagnose CMT usually includes a nerve conduction velocity test (NCV), 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 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 mild discomfort and a topical anesthetic or mild sedative can be administered, especially if the patient is young and frightened of the procedure.

Generally, electrodiagnostic tests are performed by a neurologist or by a physician 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.

Genetic Testing

Genetic tests, done by drawing blood, are available to test for many, but not all, common chromosomal defects causing CMT. A positive genetic test can provide definitive diagnosis and provide useful information for family planning. However, a negative result does not rule out CMT, since some types cannot yet be tested using DNA sampling. Currently, 27 types can be identified by DNA testing, including: 1A, 1B, 1C, 1D, 1E, 1F, 1X, 2A, 2B, 2E, 2F, 2I, 2J, 2K, 4A, 4C, 4E, 4F, 4J, HNPP, CHN, and DSN. Click here for more genetic testing information.