Nerve conduction study
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 is 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 paresthesias (numbness, tingling, burning) and/or 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.
- Peripheral neuropathy
- Carpal tunnel syndrome
- Ulnar neuropathy
- Guillain-Barré syndrome
- Facioscapulohumeral muscular dystrophy
The nerve conduction study consists of the following components:
Motor NCS are performed by electrical stimulation of a peripheral nerve and recording from a muscle supplied by this nerve. The time it takes for the electrical impulse to travel from the stimulation to the recording site is measured. This value is called the latency and is measured in milliseconds (ms). The size of the response - called the amplitude - is also measured. Motor amplitudes are measured in millivolts (mV). By stimulating in two or more different locations along the same nerve, the NCV across different segments can be determined. Calculations are performed using the distance between the different stimulating electrodes and the difference in latencies.
Sensory NCS are performed by electrical stimulation of a peripheral nerve and recording from a purely-sensory portion of the nerve, such as on a finger. Like the motor studies, sensory latencies are also measured in ms. Sensory amplitudes are measures in microvolts (ųV). The sensory NCV is calculated based upon the latency and the distance between the stimlating and recording electrode.
F-wave study uses stimulation of a motor 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 site of the stimulating electrode in the limb to the spinal cord and back to the limb in the same nerve that was stimulated. The F-wave study evaluates 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 reflex electrical discharge from a muscle in the limb. This also evaluates conduction between the limb and the spinal cord, 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.
In 1998 a small-pain-fibers (spf-NCS) method was cleared by the FDA. This method uses an electrical stimulus with a neuroselective frequency to determine the minimum voltage causing conduction. Rather than comparing the data with population averages on a bell-shaped curve, which at best has about 65% sensitivity, the patient is his own control. In a three year LSU Pain Center study it was found that the nerve requiring the greatest voltage to cause conduction of the A-delta (Fast Pain) fibers identified nerve root pathology with 95% sensitivity. Besides being painless, the test is fast. A new version, uses a potentiometer to objectively measure the amplitude of the action potential at a distant site along the nerve being tested. The previous version relied on the patient reporting a sensation when the nerve fired. The spf-NCS does not require myelin loss to detect function change, so velocity is not measured.
Interpretation of nerve conductions
The interpretation of nerve conduction studies is complex, but in general, different pathological processes result in changes in latencies, motor and/or sensory amplitudes, or slowing of the conduction velocities to differing degrees. For example, slowing of the NCV usually indicates there is damage to the myelin. Another example, slowing across the wrist for the motor and sensory latencies of the median nerve indicates focal compression of the median nerve at the wrist, called carpal tunnel syndrome. On the other hand, slowing of all nerve conductions in more than one limb indicates generalized sick nerves, or generalized peripheral neuropathy. People with diabetes mellitus often develop generalized peripheral neuropathy.
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 to anyone. Patients with a permanent pacemaker or other such implanted stimulators such as deep brain stimulators or Spinal Cord 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. The muscle will be sore at the site of the needle.
Other special nerve conduction studies that are occasionally performed include double stimuli and repetitive stimulation.
- American Association of Neuromuscular & Electrodiagnostic Medicine
- American Board of Electrodiagnostic Medicine
- Information about measuring the NCV
- EMG and Nerve Conduction education, training, and expert analysis of NCV reports
- Details of NCV from National Institutes of Health
- WebMD summary of EMG and NCS