Charcot-Marie-Tooth disease (CMT) is one of the most common inherited conditions affecting nerves. It is characterized by damage to peripheral nerves, which connect the brain and spinal cord to the rest of the body and supply movement and sensation to the arms and legs.
CMT is characterized by muscle weakness in the arms, legs, hands, and feet, but these symptoms alone are not enough for a doctor to confirm the diagnosis of CMT.
For that purpose, doctors may recommend various tests. These include genetic tests, nerve conduction studies, electromyography, and — less frequently — nerve biopsy.
They may also help in family planning, as they can determine if two people are carriers of a CMT-causing mutation that could be passed down to their biological children.
Genetic tests also may be included in prenatal screening, performed during pregnancy to check if a fetus will develop CMT.
Results are interpreted considering the patient’s symptoms, physical exam, family history, and the results of other laboratory tests. A negative result does not exclude CMT, as an unidentified gene may be missed by DNA sampling.
A genetic counselor can help patients determine the best genetic test for them and to explain test results. A counselor also may help to determine whether other family members may have inherited CMT and whether there is risk of transmitting the disease to their offspring.
Nerve conduction studies
This test helps detect the degeneration of nerve fibers and myelin, the insulating sheath that surrounds nerve fibers and allows proper transmission of electric signals. The test primarily measures the speed at which an electric signal travels along nerve fibers.
In nerve conduction studies, two electrode patches are placed on the skin over a peripheral or motor nerve. These electrodes produce and record small electric impulses that stimulate the nerve.
Results of nerve conduction studies can help identify which CF type a patient has, as CMT types with myelin degeneration have lower conduction velocity than those characterized by degeneration of nerve fibers.
Nerve conduction studies often are performed with EMG, a method that measures the electrical activity between muscles and specialized nerve cells called motor neurons. Together, these procedures help determine if a problem exists in nerves or muscles.
In EMG, a nerve conduction study is followed by a second part focusing on muscle health and function. A needle electrode is inserted through the skin to measure muscle signals during contraction and at rest.
A nerve biopsy is less common in CMT diagnosis and used mostly when the results of all other methods are inconclusive. In a nerve biopsy, a small incision is made to remove a small piece of peripheral nerve for analysis. Generally, the biopsy is taken from the calf of the lower leg.
Here, doctors may find particular abnormalities in the peripheral nerves, such as abnormal myelination and nerve fiber degeneration, that are specific to disease type.
While this method entails a very low risk of causing nerve damage, its use should be discussed case by case because it is an invasive procedure.
Last updated: Nov. 24, 2021
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