[Effect of posture and postural fixation in repetitive nerve stimulation test]

Repetitive nerve stimulation (RNS) test is recognized as being useful in the diagnosis of myasthenia gravis. We investigated incremental response at high rate stimulation. Healthy 20 median nerves were stimulated with 120% of supramaximal intensity. Compound muscle action potential (CMAP) was recorded with a surface electrode on the abductor pollicis brevis. In a single stimulus the amplitude of CMAP in the passive and voluntary flexion position of the thumb increased by 8% and 34% compared with that of the mid-position. Nevertheless, the negative potential areas (NPA) of CMAP slightly decreased by 2% and 5%, because of the briefer duration by 10% and 30%. Additionally, the size of the first 4 CMAPs in the non-fixed condition was compared with in the fixed by the examiner's hand in RNS test. With 10 and 20/sec stimulation, NPA decreased by 5% and 12% by the comparison of the fourth value with the first, though the amplitude increased by 9% and 24%. With 20/sec stimulation the fixed condition reduced the increase of amplitude by 24% to 18% and the decrease of NPA by 12% to 4%. Incremental response at high rate stimulation is a classical clinical finding, however, in our study NPA decreased while the amplitude increased, which may relate to the briefer duration. The appreciation of NPA will be desired to reduce the influence of synchronization. Moreover, muscle fiber was investigated in two conditions, shortened in passive flexion and fibrillated in voluntary flexion. Consequently, besides synchronization induced by fibrillation, the position change increasing the elasticity of muscle could play a role in the incremental response. The effect of fixation demonstrated this suggestion and movement of muscles must be minimized by every possible means in RNS test.