Myasthenia in Acquired Neuromyotonia
A 19-year-old man presented with generalized muscle stiffness and cramps of 5 weeks duration. He also noticed moderate limb weakness, and complained of difficulty climbing stairs, elevating his arms, and handwriting. Repeated exertions relieved the stiffness, but made the weakness worse. He had paresthesia and hyperhidrosis in both hands and feet. Neurological examination revealed mild limb weakness with remarkable fatigability (), 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 (), 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 (). Needle electromyography revealed varying degrees of neuromyotonic () and myokymic discharges with normal motor unit potentials. Antibody tests against VGKC, acetylcholine (ACh) receptor, and muscle-specific kinase were all negative. Computed tomography of the chest revealed no evidence of thymus abnormalities. All other laboratory tests yielded no other unusual findings.\nWith a presumed diagnosis of seronegative generalized MG superimposed on NMT, this patient was treated with high-dose intravenous immunoglobulin (IVIg) along with oral carbamazepine and a low dose of pyridostigmine bromide (30 mg three times per day). Within 2 days, even before the IVIg treatment was completed, the patient had responded so dramatically that not only the muscle stiffness but also the weakness had improved to a near-premorbid state. 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 (). The repCMAPs disappeared after discontinuation of pyridostigmine, but the abnormal afterdischarges persisted in the peroneal and tibial nerves. Abnormal CMAP decrements continued to be present in a follow-up RNS study, but to a lesser degree with larger CMAPs (). The patient continued to receive carbamazepine treatment only, and his clinical improvement was maintained during the next 2 years of follow-up.