Nerve conduction test carpal tunnel syndrome

by Nathan Wei, MD, FACP, FACR

Nathan Wei is a nationally known board-certified rheumatologist and author of the Second Opinion Arthritis Treatment Kit. It's available exclusively at this website... not available in stores.

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The wrist consists of eight carpal bones that form the floor and a fibrous band, the transverse carpal ligament that forms the roof.

The space between the ligament and the wrist bones is called the carpal tunnel. The median nerve passes through the carpal tunnel to provide the innervation to the thumb, index, and middle fingers of the hand. Any condition that causes swelling within the carpal tunnel can compress the median nerve. Irritation of the median nerve in this manner causes tingling and numbness of the thumb, index, and the middle fingers, a condition known as "carpal tunnel syndrome."

Common conditions that can lead to carpal tunnel syndrome include obesity, pregnancy, hypothyroidism, arthritis, diabetes, and trauma. Tendon inflammation resulting from repetitive activity, such as typing or moving the mouse at the computer, can also cause carpal tunnel symptoms. Carpal tunnel syndrome from repetitive maneuvers is termed a repetitive stress injury. Some rare diseases can cause deposition of abnormal materials in and around the carpal tunnel, leading to nerve irritation. These diseases include amyloidosis, sarcoidosis, multiple myeloma and leukemia.

Patients with carpal tunnel syndrome feel numbness and tingling of the hand in the distribution of the median nerve (the thumb, index, middle, and part of the fourth fingers). This is often worse at night and can awaken patients from sleep. The reason symptoms are worse at night may be related to the flexed-wrist sleeping position and/or fluid accumulating around the wrist and hand while lying down.

As the disease progresses, patients can develop a burning sensation as well as cramping and weakness of the hand. Loss of grip strength can lead to frequent dropping of objects from the hand. Chronic carpal tunnel syndrome can also lead to wasting (atrophy) of the hand muscles, particularly those at the base of the thumb.

The diagnosis of carpal tunnel syndrome is suspected based on the symptoms. Examination of the neck, shoulder, elbow, including pulses, and reflexes can be performed to exclude other conditions that can mimic carpal tunnel syndrome. The wrist can be examined for swelling, warmth, tenderness, deformity, and discoloration. Sometimes tapping the palm side of the wrist can reproduce tingling of the hand, and is referred to as Tinel's sign. Symptoms can also at times be reproduced by flexing the wrist (referred to as Phalen's maneuver). Squeezing the wrist as it enters the palm may reproduce the symptoms also (median nerve compression test).

The diagnosis is strongly suggested when a nerve conduction velocity test is abnormal. A nerve conduction velocity test (NCV), is an electrical test that is used to detect abnormal nerve conduction. In this test, the nerve is electrically stimulated at one electrode while a second electrode detects the electrical impulse 'down stream' from the first. This is usually done with surface patch electrodesthat are placed on the skin over the nerve at various locations. The distance between electrodes and the time it takes for electrical impulses to travel between electrodes are used to calculate the speed of impulse transmission (nerve conduction velocity).

This test involves measuring the rate of speed of electrical impulses as they travel down a nerve. In carpal tunnel syndrome, the impulse slows as it crosses through the carpal tunnel.

The choice of treatment for carpal tunnel syndrome depends on the severity of the symptoms and any underlying disease which might be causing the symptoms.

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