Charcot-Marie-Tooth disease (CMT) is an inherited neurological disorder characterized by damage to the peripheral nerves, which control sensation and movement.
Nerve conduction studies are used as a diagnostic tool to detect the degeneration of axons (nerve fibers) and the myelin sheath, the insulating sheath that surrounds nerve fibers and allows proper transmission of electric signals.
What nerve conduction studies involve
A nerve conduction study primarily measures the velocity or speed at which an electric signal travels along nerve fibers.
During the 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 involves several spots on the limbs, either those more distal (near the hands and feet) or more proximal, meaning toward the body core. Both sensory nerves that transmit sensation and motor nerves that control movements can be tested during a nerve conduction study.
Nerve conduction studies are usually performed along with electromyography (EMG) to help determine whether a problem exists in nerves or muscles. Notably, EMG identifies whether the muscle is responding correctly to the stimulus coming from the nerve.
How these studies help to diagnose CMT
Nerve conduction studies are used together with other tests to diagnose CMT. Abnormal findings in a nerve conduction study, however, are not specific for CMT. Genetic tests are typically needed to confirm a CMT diagnosis.
Nerve conduction studies also can help to identify a specific type of CMT in a patient. In CMT types in which the myelin sheath has degenerated, the conduction velocity tends to be lower, whereas degeneration of nerve fibers (axons) may result in normal values.
In CMT1, for instance, the myelin sheath is damaged. This disease type is characterized by a low nerve conduction velocity of less than 38 meters per second (m/s; normal values are greater than 50 m/s). The slowing in conduction velocity is uniform and diffuse. Different nerves of the upper and lower limbs, and of proximal and distal regions, show a similar decrease in conduction velocity, which helps to distinguish CMT1 from inflammatory diseases affecting peripheral nerves.
In CMTX the formation of the myelin sheath is impaired, and the slowing of conduction velocity is less uniform. This also happens in an inflammatory nerve disorder called chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), with misdiagnoses of CMT cases as CIDP previously reported.
CMTX is an X-linked disorder, which means that the mutated gene is located on the X chromosome. Since men have only one X chromosome, they typically develop a more pronounced form of the disease. Men with CMT1X have reduced nerve conduction velocity, whereas women tend to have values in the normal range.
In CMT2, the structure and function of axons are affected. In these patients, nerve conduction velocity is preserved or only mildly slowed, but nerve action potential (electrical impulse) is slower.
In dominant intermediate CMT (DI-CMT), the axons and the myelin sheath may be affected, and nerve conduction velocity is said to be “intermediate” 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.
Last updated: Oct. 28, 2021
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