Charcot-Marie-Tooth disease (CMT) is an inherited neurological condition characterized by damage to peripheral nerves, which control sensation and movement.
How nerve conduction studies are performed
A nerve conduction study primarily measures the velocity or speed at which an electric signal travels along nerve fibers. It also determines the intensity of the signal.
During a nerve conduction study, two electrode patches are attached to the skin over a nerve. One electrode stimulates the nerve with an electrical impulse and the other electrode records the signal. The speed is determined by measuring the distance between the two electrodes and the time it takes for the signal to travel from one electrode to the other.
The test is usually performed for several nerves and at different locations along the nerves, for instance at distal parts of the limbs (near the hands and feet) and more proximal sections, toward the body core. Both sensory nerves that transmit sensation and motor nerves that control movements are tested during a nerve conduction study.
How nerve conduction study results help to diagnose CMT
Nerve conduction studies are used together with other tests to diagnose CMT. Abnormal findings in a nerve conduction study are not specific for CMT, as other disorders can give similar results. Genetic tests are usually used to confirm the diagnosis.
Nerve conduction studies can also help identify the type of CMT a patient may have. In CMT types in which the myelin sheath has degenerated, the conduction velocity tends to be lower, whereas degeneration of the axons or nerve fibers results in a reduction of signal intensity.
In CMT1, for instance, the myelin sheath is damaged. This type of disease is characterized by a low nerve conduction velocity of less than 38 meters per second. The slowing in conduction velocity is uniform and diffuse because there is a general problem with myelination. Different nerves of the upper and lower limbs and proximal and distal regions show a similar decrease in conduction velocity. This characteristic helps to distinguish CMT1 from other disorders that are caused by inflammation, since focal inflammation causes a reduction in some parts of the nerve but not in others.
In CMTX, in which the formation of the myelin sheath is impaired, the slowing of conduction velocity is less uniform. For this reason, CMTX is sometimes misdiagnosed as an inflammatory nerve disorder called chronic inflammatory demyelinating polyradiculoneuropathy (CIPD). CMTX is an X-linked disorder, which means that the mutated gene in this subtype is located on the X-chromosome. Women have two X-chromosomes, whereas men have only one. For this reason, men are more severely affected and develop a more pronounced form of the disease. Men with CMTX have a nerve conduction velocity between 25 and 45 meters per second, whereas women have an almost normal velocity.
In CMT2, the structure and function of axons are affected. In these patients, nerve conduction velocity is usually not impaired, but the signal is weaker.
In dominant-intermediate CMT (DI-CMT), the axons and the myelin sheath are affected, and the nerve conduction velocity is said to be “intermediate.” The velocity in DI-CMT can, however, vary substantially between cases and can overlap with velocities recorded in both CMT1 and CMT2. For this reason, DI-CMT is difficult to diagnose based on nerve conduction studies.
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