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Gulf War and Health: Insecticides and Solvents, Volume 2
peripheral neuropathy develop over weeks or months. A slowly progressive course suggests a hereditary or metabolic cause. Nerve conduction and electromyography are used to confirm the diagnosis and to determine the type of pathophysiologic effect, especially whether the neuropathy is demyelinating or axonal.
Measuring nerve conduction is a key method for testing the functioning of a peripheral nerve. Its purpose is to localize where the pathology is along the length of the nerve. It also aids in characterizing the pathology—namely, whether it affects axons, cell bodies, or the myelin sheath (Aminoff, 1987). Testing often involves stimulating a nerve at one point along its path and recording the electrical impulse from the nerve at another point.
Nerve-conduction testing typically measures conduction velocity, expressed as meters/second. Conduction velocity is calculated by dividing the distance between the stimulating and recording points by the time from stimulation to onset of recorded impulse (the latency). Nerve conduction testing also measures the amplitude of the compound action potential of a sensory nerve or the muscular wave (M wave) for a motor nerve. Compound action potential is the sum of individual impulses from axons within the nerve. It is recorded at the surface of a sensory nerve after the nerve has been electrically stimulated. The M wave is the compound action potential recorded from the surface of a muscle after stimulation of a motor nerve. The M wave refers to the sum of individual impulses from axons that control muscular contraction.
If the axon is affected, the amplitude (maximal voltage, in microvolts) of the compound action potential or M wave is generally smaller than normal. Nerve impulses can be conducted only by the remaining undamaged axons and this reduces the magnitude of the compound action potential. If the myelin sheath is affected, the conduction velocity is slower than normal, and other electrophysiological parameters can be affected too.
Nerve conduction studies examine functioning of the peripheral nervous system, and a similar type of testing—known as evoked potentials—is used to examine the functioning of both the peripheral and the central nervous systems. Studies using evoked potentials examine the characteristics of electrical waveforms generated by a stimulus delivered to a sensory receptor or nerve or applied directly to a particular area of the brain, spinal cord, or muscle. There are many types of evoked potentials—including auditory evoked potentials, brainstem auditory evoked potentials, and visual evoked potentials—and each is designed to uncover and localize pathology in distinct parts of the nervous system.
Electromyography (EMG) is used to test motor unit function; a motor unit consists of the motor neuron, its axon, and the muscle cell it innervates. The EMG helps to define the type of neurotoxic insult or neuromuscular disorder.
A typical test uses a recording electrode inserted through the skin into the muscle to measure electrical activity. Thousands of motor units are in the legs. Fewer units are in the head and neck. A reduced number of motor units is evidence of denervation (the loss of connection between the motor neuron, its axon, and the muscle fiber it supplies). Denervation of motor units is necessary before muscle cells begin to develop abnormal, spontaneously discharging potentials. Therefore, the timing of an abnormal electromyogram in relation to a toxic exposure is very important in the interpretation of the results. As toxic neuropathy develops and clinical signs appear, greater EMG changes are recordable. Denervated muscle fibers manifest spontaneous electrical discharges, called fibrillations,