Pseudomyopathic Changes in Needle Electromyography in Lambert-Eaton Myasthenic Syndrome

Lambert-Eaton myasthenic syndrome (LEMS) is a rare presynaptic disorder of the neuromuscular junction in association with cancer and subsequently in cases in which no neoplasm has been detected (O’Neill et al., 1988). The diagnosis of LEMS is based on the combination of fluctuating muscle weakness, diminished or absent reflexes, and a more than 60% increment of compound muscle action potential (CMAP) amplitude after brief exercise or 50？Hz stimulation for 1？s in a repetitive nerve stimulation (RNS) test (Oh et al., 2005). On the other hand, needle electromyography (EMG) findings related to LEMS have not been well described. Here, we report a case of LEMS, which showed apparent myopathic changes in needle EMG findings. Furthermore, we retrospectively examined the needle EMG findings in 8 patients with LEMS. In six of the 8 patients, the EMG findings showed myopathy-like findings. Although the findings of needle EMG indicated myopathic changes at a glance, the motor unit potential (MUP) returned to normal after a sustained strong muscle contraction. We propose the name “pseudomyopathic changes” for this phenomenon. 1. Case Report A 69-year-old female (patient 1 in Table 1) developed a gait disturbance 6 months before admission and suffered from the gradual progression of weakness in her extremities. On admission, neurological examination showed proximal muscle weakness and reduced deep tendon reflex. Routine laboratory test results were unremarkable. Antinuclear antibodies, anti-acetylcholine (ACh) receptor antibody, and anti-voltage-gated calcium channel (VGCC) antibodies were negative. The results of chest and abdominal CT were unremarkable. In a nerve conduction study, CMAPs had slightly low amplitudes, and nerve conduction velocities were normal in the median and ulnar nerves. Needle EMG showed early recruitment and polyphasic MUPs with short durations and low amplitudes, suggesting the diagnosis of myopathy. Interestingly, the myopathic EMG findings were improved after sustained strong muscle contraction for 10？s; the MUP sizes returned to normal after a sustained strong muscle contraction, and the early recruitment of MUPs in the weak contraction was clearly normalized in the biceps brachii muscle (Figure 1). Thereafter, we performed an RNS test because she complained of easy fatigability. The RNS test showed an obvious incremental response of the CMAP amplitude of the abductor digiti minimi muscle after high-rate stimulation of the ulnar nerve, indicating the diagnosis of LEMS [1–3]. We examined the patient for systemic malignancies, and the