A nerve conduction study (NCS) is a test commonly used to evaluate the function, especially the ability of electrical conduction, of the motor and sensory nerves of the human body. Nerve conduction velocity (NCV) is a common measurement made during this test. The term NCV often used to mean the actual test, but this is improper use of the term since velocity is only one measurement out of the entire test.
Nerve conduction studies are used mainly for evaluation of numbness and weakness of the arms and legs. The type of study is determined by the problem. Some of the common disorders which can be diagnosed by nerve conduction studies include:
The nerve conduction study consists of the following components:
Motor NCS is performed by electrical stimulation of a peripheral nerve and recording from a muscle supplied by this nerve. By stimulating in two or more different locations along the nerve, the NCV in different segments can be determined. Calculations are performed using the distance between the different stimulating electrodes and the difference in time of the response of the muscle to stimulation.
Sensory NCS is performed by electrical stimulation of a peripheral nerve and recording from a purely-sensory portion of the nerve, such as on a finger. The sensory NCV is calculated from the time from stimulation to recording the action potential in the nerve and knowing the distance between the stimulating and recording electrodes.
F-wave study uses stimulation of a muscle nerve and recording of action potentials from a muscle supplied by the nerve. This is not a reflex, per se, in that the nerve potential travels from the limb to the spine and back to the limb in the same nerve. The F-wave study evaluated conduction velocity of nerves between the limb and spine, whereas the motor and sensory nerve conduction studies evaluate conduction in the limb, itself.
H-reflex study uses stimulation of a nerve and recording the reflexive electrical discharge from a muscle in the limb. This also evaluates conduction between the limb and the spine, but in this case, the afferent impulses (those going towards the spinal cord) are in sensory nerves while the efferent impulses (those coming from the spinal cord) are in motor nerves.
Interpretation of nerve conductions
The interpretation of nerve conduction studies is complex but in general, slowing of the NCV indicates a problem in function of the nerves. For example, slowing of the motor and sensory NCV of the median nerve at the wrist indicates carpal tunnel syndrome, a common disorder. On the other hand, slowing of all nerve conductions in more than one limb indicates peripheral neuropathy, another common neurologic disorder often due to diabetes mellitus.
Patient risk
Nerve conduction studies are very helpful to diagnose certain diseases of the nerves of the body. The test is not invasive, but can be a little painful due to the electrical shocks. However, the shocks are associated with such a low amount of electrical current that they are not dangerous for most people. Patients with a permanent pacemaker or other such implanted stimulators must tell the examiner prior to the study. This does not prevent the study, but special precautions are taken.
The nerve conduction study is often combined with electromyography (EMG) where small electrodes on needles are inserted into selected muscles. This is a little painful, but not markedly so.
Other special nerve conduction studies that are occasionally performed include double stimuli and repetitive stimulation.
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