Background
Electrophysiological studies include nerve conduction velocity (NCV) testing and needle electromyography (EMG). These procedures are used as part of the diagnostic evaluation of the peripheral nervous system and the localization of abnormalities along peripheral nerves or motor neurons.1,2 NCV and EMG studies are particularly useful for diagnosing radiculopathy and entrapment neuropathy.
Historical Overview
In 1771, Luigi Galvani demonstrated that electrical stimulation of muscle produced contraction. In 1849, Emil du Bois-Reymond discovered that it also was possible to record electrical activity during a voluntary muscle contraction. This electrical response was not quantified until the development of improved recording equipment including the cathode ray oscilloscope by Gasser and Erlanger in 1922 and the amplifier and oscillograph by Matthews in 1928. The first modern EMG machine was built by Jasper in 1942.
In 1929, Adrian and Bronk developed a method to record a single motor unit action potential (MUAP) in human subjects by connecting concentric and bipolar needle electrodes to an amplifier and a loud speaker. In 1938, Denny-Brown and Pennybacker recorded and described fasciculations, or action potentials of single contracting or spontaneously firing motor units. They also described fibrillations by separating fasciculations from the action potentials of single, dennervated muscle fibers. Subsequently, Larrabee, Hodes and German measured the combined potential of individual muscle fibers from the surface of the muscle after stimulating the supplying nerve. This became known as a compound muscle action potential (CMAP). In 1957, Lambert and Eaton differentiated pre-synaptic and post-synaptic neuromuscular transmission disorders after describing the electrophysiological features of a newly recognized disease they called the myasthenic syndrome.
Description
For NCV studies, the function of peripheral nerves is evaluated by electrically stimulating the nerve and recording the response at the muscle or the nerve.1,2 When compared with normative data, the responses can yield information about demyelination and axonal loss. NCVs include motor, sensory and mixed nerve testing. The results from NCV studies can be affected by a number of variables, including technique used, skin temperature (positively correlated) and age (inversely correlated).1,2
A motor NCV test is performed by applying a single supramaximal stimulus on at least two proximal points along the nerve. The resulting compound muscle action potential (CMAP) is recorded by a surface electrode on the innervated muscle. NCV is calculated as the distance (ie, length between the stimulating and recording electrodes) divided by latency (ie, time from the onset of the stimulus to the onset of the response; NCV Figure). In patients with carpal tunnel syndrome, for example, distal latency is prolonged because the carpal canal is located distally to the most distal site of median nerve stimulation.3
For EMG studies, muscle function is evaluated by inserting a needle electrode into a muscle and recording the intrinsic electrical activity of muscle fibers1,2 (EMG Figure). There are three components to the needle EMG: observation at rest, motor unit potential (MUP) after minimal voluntary contraction and the recruitment pattern of MUPs after maximal contraction. The amplitude, duration, shape and rate of MUPs are indicators of motor unit function.2 Normally, MUP amplitude and duration are relatively constant. Abnormal activities include fibrillations from single muscle fibers and fasciculations from groups of muscle fibers. In patients with carpal tunnel syndrome, focal demyelination after prolonged entrapment can produce fibrillation, positive sharp waves (PSWs) and reduced recruitment on EMG.2
Diagnoses
Carpal tunnel syndrome
Tests should include motor, sensory and mixed NCV studies for the median and ulnar nerves of the affected arm. The most sensitive test is sensory NCV across the palm-to-wrist segment. Other sensitive tests include the finger-to-wrist NCV and distal motor latency. A CTS diagnosis can be confirmed with 85–91% sensitivity and 95% specificity.4 To document any secondary axonal degeneration, a needle EMG in the abductor pollicis brevis is recommended.2
Cubital tunnel syndrome
Tests should include motor, sensory and mixed NCV studies for the median and ulnar nerves of the affected arm. The sensory and mixed NCVs for the finger-to-wrist and wrist-to-elbow segments are frequently abnormal. An inching approach on the ulnar motor nerve, from 6 cm below the medial epicondyle to ≥4 cm proximal to it, helps localize the area of compression. The sensitivity for identifying the site of compression ranges from 83% to 96% of patients.5
Cervical radiculopathy
The most important test is the EMG.2,6 With nerve root compression, sensory NCVs are normal because the dorsal root ganglion (sensory somata) is distal to the site of compression. Similarly, motor NCVs will not reflect demyelination because the area of injury is proximal to the tested nerve sites. EMG will detect degenerative changes in muscles innervated by the specific nerve root. Because the radicular pattern of muscle involvement leads to a specific root level, the paraspinal muscles also must be evaluated. If the paraspinals are involved, the diagnosis of radiculopathy can be confirmed. However, a negative EMG does not rule out radiculopathy, especially if only the sensory part of the nerve is affected or there is no motor axonal loss. The sensitivity of NCV/EMG in radiculopathy is about 80–85%.2
Colles’ fracture
For Colles’ fracture, NCV/EMG is only required when the swelling from the fracture is causing clinical signs and symptoms of carpal tunnel syndrome. In this situation, electrophysiological studies can be used to verify the secondary diagnosis of carpal tunnel syndrome and its severity.
Rheumatoid arthritis
For rheumatoid arthritis (RA), NCV/EMG is only required when the flexor tenosynovitis from the RA is causing clinical signs and symptoms of carpal tunnel syndrome. In this situation the electrophysiological studies can be used to verify the secondary diagnosis of carpal tunnel syndrome and its severity.
