12 - Electrodiagnostic Examination

Abstract

Nerve conduction studies: The functional state of the peripheral nervous system (PNS), assessed by nerve conduction study (NCS) and electromyography (EMG), clarifies a clinical diagnosis, contributes to a prognosis, and can document the effectiveness of a treatment. For motor NCS, a stimulating electrode is placed on the skin overlying that nerve and a recording electrode over the mid-region (innervation zone) of a muscle it innervates. The surface waveform recorded, compound motor action potential (CMAP), is produced by summation of individual muscle fibre depolarisation potentials. The stimulus–response latency from two separate stimulation sites along the same nerve enables a motor nerve conduction velocity to be calculated by first subtracting one latency from the other and then dividing by the distance that separates those stimulation points. Preferred nerve trunks in the upper limb are the median and ulnar nerves, and in the lower limb the deep peroneal and tibial nerves. For sensory NCS, antidromic stimulation of a cutaneous nerve is accompanied by a proximal recording from two sites over the parent nerve trunk; subtraction reveals the sensory nerve conduction velocity (SNCV). Some spinal nerve roots are accessed by activating their muscle spindle reflex arcs (H-reflex) at appropriate levels and represent the electrophysiological correlate of a muscle stretch reflex. Electromyography is a technique in which the recording electrode is within a needle inserted into a muscle; voluntary activation of the muscle allows motor unit action potentials (MUAPs) to be recorded. Each MUAP shape represents the summated action potentials of muscle fibres belonging to one motor unit. The normal overlap and interdigitating of muscle fibres of different motor unit territories allows sampling of several MUAPs simultaneously when volitionally activated. Abnormal EMG waveforms are associated with dysfunction at different levels of the motor unit (e.g., neuropathies, motor neuron disease, myopathy, or neuromuscular junction disorder) and can be a dynamic process.