neurologic disorders
Electromyography (EMG) and Nerve Conduction Studies
Nerve conduction studies
In nerve conduction studies, a peripheral nerve is stimulated with electrical shocks at several points along its course to a muscle, and the time to initiation of contraction is recorded. The time an impulse takes to traverse a measured length of nerve determines conduction velocity. The time required to traverse the segment nearest the muscle is called distal latency. Similar measurements can be made for sensory nerves. In general, nerve conduction is much faster in larger myelinated nerves than in unmyelinated and thinly myelinated nerves. Thus, conduction velocity is slowed more when larger myelinated fibers are damaged. When unmyelinated axons are damaged, the amplitude of the action potential is decreased, but conduction velocity is relatively unimpaired. In neuropathy, conduction is often slowed, and the response pattern may show a dispersion of action potentials due to unequal involvement of myelinated and unmyelinated nerve fibers. When a specific neuropathy primarily affects large myelinated fibers, the predominant effect is a decrease in conduction velocity. However, when neuropathies affect only small umyelinated or thinly myelinated fibers (or when weakness is due to a muscle disorder), the amplitude of action potentials is decreased and nerve conduction velocities are typically normal. A nerve can be repeatedly stimulated to evaluate the neuromuscular junction for fatigability; eg, a progressive decremental response occurs in Myasthenia Gravis . Techniques focusing on single-fiber conduction may be more sensitive for detecting changes associated with neuromuscular junction disorders such as myasthenia gravis. 