PMC3896653_01
Coexistence of myasthenia and neuromyotonia. A: Repeated measures of maximal handgrip strength demonstrating the presence of exertional muscle weakness. Strength was measured six times, with a 5-second rest between trials, revealing decreases of 30% and 55% on the right and left sides, respectively. The uppermost line was obtained from a healthy subject of the same sex and body weight. B: Low-frequency repetitive nerve stimulations of the ulnar nerve showing markedly abnormal reductions of compound muscle action potential (CMAP) amplitudes (28% and 43% at 2- and 3-Hz stimulation frequencies, respectively). C and D: Stimulation-induced M-wave and F/H-wave afterdischarges in the peroneal and tibial nerves. E: A 5-second continuous needle electromyogram recording from the biceps brachii muscle, showing high-frequency waning continuous motor unit discharges. F: Synaptic repetitive CMAP (arrow) along with prominent M-wave and F-wave afterdischarges at the abductor digiti minimi muscle after supramaximal stimulation of the ulnar nerve. G: Moderate reductions of CMAP amplitudes in response to lowfrequency repetitive nerve stimulation of the ulnar nerve at 1 week after treatment (19% and 15% at 2- and 5-Hz stimulation frequencies, respectively). Note the increased CMAPs and smaller decrements compared to those obtained before treatment (B).
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F1
Neurological examination revealed mild limb weakness with remarkable fatigability (Fig. 1A), diffuse fasciculation, and myokymia. Repetitive nerve stimulation (RNS) of the ulnar nerve at low frequencies revealed marked reductions in compound muscle action potential (CMAP) amplitudes (Fig. 1B), but there was neither postexercise facilitation nor abnormal increase at a stimulation frequency of 50 Hz. The findings of a conventional nerve conduction study were normal except for abnormal stimulation-induced afterdischarges (Fig. 1C and D). Needle electromyography revealed varying degrees of neuromyotonic (Fig. 1E) and myokymic discharges with normal motor unit potentials. However, a follow-up nerve conduction study at that time was not fully consistent with the clinical recovery, revealing the occurrence of repetitive CMAPs (repCMAPs) along with more prominent afterdischarges (Fig. 1F). Abnormal CMAP decrements continued to be present in a follow-up RNS study, but to a lesser degree with larger CMAPs (Fig. 1G).