A combination of lower leg weakness and foot deformities is a red flag for Charcot-Marie-Tooth disease (CMT) but isn’t sufficient for diagnosis. When a patient has those symptoms, a neurologist will usually start with a physical exam to look for further signs of distal weakness and sensory loss.
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.
To look for sensory loss, the neurologist will usually test the patient’s deep tendon reflexes (like the knee-jerk reflex), which are reduced or absent in most people with CMT.
During this initial evaluation, the neurologist also will ask about the patient’s family history. A family history of CMT-like symptoms, combined with signs of nerve damage from the individual’s physical exam, strongly point to CMT or another hereditary neuropathy.
Lack of a family history doesn’t rule out CMT, but might prompt the neurologist to ask about diabetes, overexposure to certain drugs and other potential causes of neuropathy.
Next, if the diagnosis is still consistent with CMT, the neurologist may arrange for genetic testing. These tests, done by drawing a blood 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 result doesn’t rule out CMT.
The neurologist also may perform a nerve conduction velocity (NCV) test, which measures the strength and speed of electrical signals transmitted through nerves. It’s 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.