Asked if they’ve had nerve conduction studies previously, some patients in my neurological practice answer, “I’m not sure.”
My response: “Then you probably haven’t.” Nerve conduction studies are generally memorable. And it’s not because they’re particularly painful or otherwise unpleasant. (They’re not.) Rather, it’s because they’re completely unlike any other medical test in existence.
The uniqueness of nerve conduction studies is also their advantage. They have the ability to look at certain medical conditions from a point of view completely different from that of other tests, and can therefore discover and pinpoint problems that are invisible to other tests.
Nerve conduction studies are usually requested to help diagnose nerve and muscle disorders. They are often paired with electromyographic (EMG) studies, performed during the same testing session by the same physician and making use of the same equipment.
Nerve conduction studies evaluate the physiology and functioning of the peripheral nerves. Unlike scans or x-rays that evaluate anatomy and structure, nerve conduction studies look at the nerves’ performance.
How is this done? In a nutshell, a brief electrical pulse or shock is applied through a pair of electrode probes to the skin overlying a nerve, generating a precisely timed set of nerve-impulses. With another electrode taped to the skin over another portion of the same nerve or over a muscle connected to the nerve being studied, the physician can measure how quickly nerve-impulses travel from the point of stimulation to the point of recording.
In healthy nerves the nerve-impulses travel at a rate of 40-60 meters per second (120-180 feet per second) so that only a few thousandths of a second are required to cover a distance of several inches. In order to capture a signal traveling that fast, an oscilloscope is required.
The two most important features of the recording are the length of time required to traverse the nerve-segment in question and the size or amplitude of the resulting electrical response. Electrical responses recorded from nerves are typically 2-50 millionths of a volt in amplitude, while responses from muscles are typically 1-20 thousandths of a volt.
The body’s peripheral nerves are like telephone cables bundling together numerous individual fibers. Some of the nerve-fibers carry instructions from the brain and spinal cord to the muscles, causing them to contract. These are called motor nerve-fibers. Sensory nerve fibers, often intermixed with motor fibers in the same nerve-bundle, carry messages in the opposite direction, informing the spinal cord and brain about stimuli—such as touch, pain and temperature—generated in the skin, joints and other peripheral tissues.
By rearranging the locations of the stimulating and recording electrodes, the physician can separately measure conduction in the motor and sensory fibers even when they are intermixed in the same nerve-bundle.
Nerve conduction studies can help diagnose pinched, damaged or under-functioning nerves. One pattern of nerve impairment is called “mononeuropathy,” a term meaning “one sick nerve.” It involves dysfunction that is restricted to just a single location in a single nerve-bundle.
Carpal tunnel syndrome, in which the median nerve is pinched within the wrist, is the most common mononeuropathy in the arm, and can produce numbness, weakness or pain in the hand. Nerve conduction studies are exquisitely sensitive in detecting this condition. Another mononeuropathy in the arm involves the ulnar nerve at the portion of the elbow often called the “crazy bone.”
Nerve conduction studies can also help diagnose mononeuropathies in the legs. One such condition, in which the peroneal nerve is impaired near the knee, can cause a foot-drop. Another condition, involving the tibial nerve at the ankle, is called a tarsal tunnel syndrome, and is analogous to the carpal tunnel syndrome of the arm.
The other main pattern of nerve impairment is called “polyneuropathy,” a term meaning “many sick nerves.” In polyneuropathy there is a more widespread pattern of dysfunction instead of just at single locations. This can be seen in people with diabetes, excessive alcohol consumption and a number of other medical conditions. Nerve conduction studies are adept at identifying this category of nerve impairment as well.
As is the case with every other medical test, nerve conduction studies are not perfect. The diagnosing physician considers the strengths and weaknesses of this form of testing along with other clinical information in order to arrive at an accurate diagnosis.
by Gary Cordingley
