Nerve Conduction Test
Definition: evaluation of peripheral nerves by nerve conduction testing involves measurement of the velocity of an electrical stimulus in a target nerve.
Method: the test is done by pre cutaneous stimulation of a nerve at two separate points along its course and recording the electrical response (evoked action potentials) on the skin (for sensory nerves) or an associated muscle(for motor nerves). This test helps to evaluate both generalized and localized neuropathies that might affect peripheral nerves or muscle function. Nerve conduction testing is usually performed by a specially trained physiatrist or neurologist, often at the same time as electromyography. Referrals should include a clinical history and the reason for the precedure.
Normal in: nerve conduction is normal in primary muscle disorders, myasthenia gravis radiculopathies, and amyotrophic lateral sclerosis.
Abnormal in: detection of abnormalities in all extremities suggests generalized disorders such as polyneuropathy, mononeuritis multiplex, or guillain barre syndrome. If only localized abnormalities are found, then mechanical problems with nerve entrapment (e.g., carpal tunnel syndrome) or localized inflammatory problems such as mononeuritis multiplex should be considered. Measurement to confirm a diagnosis of carpal or tarsal tunnel syndrome can be limited to the involved extremity, whereas a more generalized problem requires evaluation of all limbs.
Confounding factors: a cardiac pacemaker (or implanted defibrillator) may influence evoked potentials. Results are unreliable or unobtainable in agitated or uncooperative patients. Results are not affected by medications.
Indications: nerve conduction velocity testing is a noninvasive procedure appropriate for evaluating neuropathies. Specifically, nerve conduction velocity is useful in determining the extent of nerve involvement (that is polyneuropathy versus mononeuritis multiplex) and whether a demyelinating or axonal process is present. It is often useful in distingushing neuropathic from myopathic causes of muscle weakness. Diagnostic findings may occur in patients with entrapment syndromes (e.g., carpal tunnel syndrome), guillain Barre syndrome, Eaton Lambert syndrome, and myasthenia gravis.
Motor nerve conduction velocity’s are recorded from surface electrodes taped over muscles distally in the limb. Normative control values for motor nerve conduction velocities at different ages are used for comparison. This difference is attributable to the degree of myelination, which increases with age over the early developmental years. Although nerve conduction velocities are fairly uniform from age 3 years though adulthood, nerve conduction volocities can vary based on several conditions. To study motor conduction, the nerve is supramaximally stimulated at two or more points along its course where it is most superficial.
Sensory nerve conductions are generally unaffected by lesions proximal to the dorsal root ganglion even though there is sensory loss. Sensory testing is good for localizing a lesion either proximal or distal to the dorsal root ganglia. In addition, sensory nerve potential is lower in amplitude than compound motor action potentials and can be obscured by electrical activity or artifacts. Sensory axons are evaluated in four ways: stimulating and recording from a cutaneous nerve, recording from a cutaneous nerve while stimulating a mixed nerve, recording from a mixed nerve, while stimulating a cutaneous nerve and recording from the spinal column while a cutaneous nerve or mixed nerve is stimulated. Variables measured include onset latency, peak latency, and peak to peak amplitude.