WHAT IS ELECTRONEUROMYOGRAPHY?
Clinical electroneuromyography includes observing, analyzing, and interpreting the bioelectrical activity of muscles and nerves as they respond to electrical stimulation, needle provocation, and voluntary activation. Most often, the testing consists of a combination of nerve conduction studies (NCS) and electromyography (EMG), although additional tests may occasionally be appropriate. These may include somatosensory-evoked potentials, brain stem auditory-evoked potentials, visual-evoked potentials, intraoperative monitoring, and repetitive stimulation. Of these additional electrophysiological tests, somatosensory-evoked potentials and repetitive stimulation are more commonly accomplished on a routine basis. However, this overview focuses on clinical NCS and EMG studies.
ANATOMY AND PHYSIOLOGY REVIEW
Peripheral Nerve Structure
The anatomic unit of the nervous system is the neuron, with its various processes or nerve fibers.1 By contrast, the functional unit of the neuromuscular system is the motor unit, consisting of the anterior horn cell, the nerve root, the plexus, individual nerve fibers, the neuromuscular junction, and all the muscle fibers innervated by that axon (Fig. 17-1).
The functional unit of the neuromuscular system is the motor unit, which consists of the anterior horn cell, the nerve root, the brachial or lumbosacral plexus, individual nerve fibers, the neuromuscular junction, and all the muscle fibers innervated by that axon.
ENMG testing assesses various aspects of the neuron and the motor unit. EMG testing examines the delineated components of the motor unit, allowing the practitioner to determine the location of a neuromuscular impairment.
Results of ENMG testing cannot be used to identify the actual cause of a nerve or muscle impairment, although it is often assumed to do so. For instance, if testing reveals that a patient has slowed neural conduction in the median nerve across the carpal tunnel of the wrist and the opponens pollicis muscle demonstrates evidence of denervation with EMG examination, we would refer to this condition as carpal tunnel syndrome. However, the ENMG testing identifies only the location (carpal tunnel) and severity (mild, moderate, severe) of the condition, not its cause. The cause could be an abnormally thickened flexor tendon putting pressure on the nerve, a thickened transverse carpal ligament (flexor retinaculum), a bony impingement from advanced arthritis or from a fracture, or a temporary generalized edema accumulation such as can occur during the second and third trimesters of pregnancy. Imaging studies (radiograph, magnetic resonance imagine [MRI], etc.) may be able to identify the anatomical or structural cause of a condition, but ENMG is restricted to identifying the location of a neuromuscular impairment.
Peripheral nerves are composed of a variety of elements and cell types. One naturally thinks of the axon, which is composed of neurofilaments and microtubules along with mitochondria interspersed in the cytoplasm (Fig. 17-2). Myelinated nerve fibers are surrounded by layers ...