Charcot-Marie-Tooth Disease (CMT)
A comprehensive history and physical examination remain the core of ascertainment of and evaluation for cases of CMT.
During this initial evaluation, a neurologist will ask about a patient’s family history. A family history of CMT-like symptoms, combined with signs of nerve damage from an individual’s physical exam, could strongly point to CMT or another hereditary neuropathy. Lack of a family history does not rule out CMT but might prompt a neurologist to ask about diabetes, overexposure to certain drugs, and other potential causes of neuropathy.
To look for sensory loss, a neurologist will usually test a patient’s deep tendon reflexes (such as the knee-jerk reflex), which are reduced or absent in most people with CMT.
As a test for leg weakness, a neurologist might ask patients to walk on their heels or move part of their leg against an opposing force.
The neurologist also may perform a nerve conduction velocity (NCV) test, which measures the strength and speed of electrical signals transmitted through nerves. The test is done by placing surface electrodes, similar to those used for electrocardiograms, on the skin at various points over a nerve. One electrode delivers a mild shock that stimulates an electrical response in the nerve, and the others record this response as it travels through the nerve. Delayed responses are a sign of demyelination and small responses are a sign of axonopathy. Thus, NCV is often used to distinguish between CMT1 and CMT2.
Other procedures sometimes used to diagnose CMT include electromyography (EMG), which measures the electrical signals in muscles, and less commonly, nerve biopsy, which involves the removal and examination of a small piece of nerve.
Next, if the diagnosis is still consistent with CMT, a neurologist may arrange for genetic testing. These tests, done by drawing a blood sample or capturing a saliva sample, are designed to detect the most common genetic defects known to cause CMT. Many, but certainly not all, of the genetic mutations underlying CMT can be detected with a DNA blood test. For more on getting a definitive genetic diagnosis, see The Genie's Out of the Bottle: Genetic testing in the 21st century.
A positive genetic test result can provide a definite diagnosis and useful information for family planning. But once again, a negative genetic test result does not rule out CMT.