Nerve conduction velocity
From Freepedia
Nerve conduction velocity (NCV) is a measurement made during a Nerve conduction study (NCS), a test commonly used to evaluate function of the motor and sensory nerves of the body. Some people use the term NCV to mean the actual test, but this is improper use of the term since velocity is only one measurement.
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
- Sensory NCS
- F-wave study
- H-reflex study
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 cconduction 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 (towards the spinal cord) impulses are in sensory nerves while the efferent (back from the spinal cord) impulses 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.
What patients need to know about NCS: Nerve conduction studies are very helpful to diagnose certain disorders of the nerves of the body. The test is not invasive, but can be a little painful from the electrical shocks. However, the shocks are associated with such a low amount of current that they are not dangerous for most people. If you have a permanant pacemaker or other implanted stimulator, you must tell the examiner. 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 also a little painful, but not markedly so. Special nerve conduction studies are occasionally performed which are not detailed here, including double stimuli and repetitive stimulation. Your doctor should explain these in depth before the exam.
External link:
- Information about measuring the NCV
- Details of NCV from National Institutes of Health
- WebMD summary of EMG and NCS
