ΝEUROPHYSIOLOGICAL INVESTIGATION OF DISORDERS OF NEUROMUSCULAR TRANSMISSION

Abstract

Neurophysiological study is essential for the diagnosis of myasthenia gravis or other diseases of neuromuscular junction. This article reviewed the basic principles of two main methods, repetitive nerve stimulation (RNS) and single-fiber electromyography (SF-EMG). RNS is the most frequently performed method. In patients with myasthenia, depolarization of some muscle fibers fails resulting in decremental muscle response during consecutive electrical nerve stimuli, at a frequency of 1-5Hz. An amplitude decrement of 10-15% between the 1st compound muscle action potential and the smallest of the subsequent 4th to 9th potentials is considered abnormal. Tetanic stimulation and voluntary exercise are provocative techniques which enhance the diagnostic yield of RNS. Unlike the findings in myasthenia, a post-activation or posttetanic facilitation of compound muscle action potential by more than 100% is indicative of myasthenic syndrome. Specificity is close to 95%, whereas sensitivity is relative low (80% in generalized myasthenia, less than 50% in ocular myasthenia and 85% in myasthenic syndrome). Limb temperature and treatment with pyridostigmine can affect the results. Clinically affected muscles, particularly facial and proximal limb muscles are selected for this test. SFEMG is technically more difficult, time consuming and dependent on the skill of the neurophysiologist. It requires the use of a needle electrode to record a pair of action potentials of 2 single muscle fibers belonging to the same motor unit. During consecutive discharges, the variability of interpotential interval in a pair is called jitter. Increased jitter (occasionally with impulse blocking) is suggestive of a neuromuscular fiber transmission defect. Twenty pairs of fibers should be examined in a muscle and the commonly selected muscles for this examination include the extensor digiti communis, the frontalis and the orbicularis oculis. Normal jitter values vary considerably, depending on the type of needle used (SF or classical concentric), muscle examined, subject’s age, methodology applied (voluntary muscle activation or intramuscular, axonal stimulation). Treatment with pyridostigmine does not normalize SFEMG results and serial measurements of jitter can be useful in following the course of disease. Compared to RNS, SF-EMG has a lower specificity since it can be positive in pre and post synaptic disorders as well as in other neuromuscular diseases, but its sensitivity is particularly high, 94 to 99% even in ocular myasthenia. In other words, normal jitter in a weak muscle excludes the possibility of neuromuscular junction disorder..