Myokymia of lower limbs for over one year

Abstract

A 51-year-old man came to our outpatient for one-year history of progressive myokymia in both legs. He had initially noted a “continuous muscle-fiber activity” of lower limbs in July 2013. Two months later, similar symptoms progressively affected muscles in both thighs and fundament, which was persistent during sleep. The frequency, duration and severity were gradually increased. He suffered from a feeling of tiredness and fatigue in both legs after long-time walking. In the local hospital, spinal MRI showed protrusion of C_5-6, L_4-5, L₅-S₁ intervertebral disc. One day in Oct 2013, his legs subsequently became generally stiff and painful to the point of a stiff paralysis and presented excessive sweating after drunk. This attack lasted for 30 min. Then he came to our clinic. Routine blood tests were normal. Electromyographic examination showed neurogenic damages of left thoracic paraspinal muscles. Abnormal F wave of left lower limb and discrete waveform of four limbs could be seen. Nerve conduction velocity (NCV) and sympathetic skin response (SSR) were unremarkable. Treatments with oral phenytoin sodium and carbamazepine were all ineffective. The attack occurred only a few times a year, especially after drunk, as well as the frequency, duration and severity roughly similar. He was admitted in our hospital in Oct 2014. The patient had a 9-year history of high blood pressure and the family history was normal. Neurological examination revealed generalized leg muscle hypertrophy, especially the gastrocnemius muscle, despite lack of physical exercise. Cranial nerves were intact. Hyperhidrosis was noted along with abundant myokymia that were evident in lower limbs. Muscle bulk and strength of upper limbs were normal. The deep tendon reflexes of lower limbs were unobtainable. Sensory and cerebellar examination revealed no dysfunction. Laboratory records showed that serum A type Sjögren's syndrome antibody (SSA) and Ro52 antibody was positive. Cerebrospinal fluid (CSF) contained protein 0.56 g/L (0.15-0.45 g/L), and IgG 49.8mg/L. Cytology, myelin basic protein (MBP), glucose, specific oligoclonal band (SOB), Gram staining and culture, voltage-gated potassium channel (VGKC) antibody, N⁃methyl⁃D⁃aspartate receptor (NMDAR) antibody, GM1 antibody and so on were all negative. Reexamination EMG showed typical fasciculations, doublets and triplets of spontaneous motor unit potentials (MUPs). Repetitive nerve stimulation (RNS) and motor-evoked potential (MEP) were unremarkable.

This is a unique patient with clinical and electrophysiological features of Isaacs syndrome (neuromyotonia, NMT) including myokymia, muscle stiffness and continuous motor unit activity in association with autonomic dysfunction. NMT is a disorder of generalized peripheral nerve hyperexcitability (PNH), manifesting as spontaneous, continuous muscle activity of peripheral nerve origin. It is characterized clinically by muscle twitching at rest (visible myokymia) and cramps, which can be triggered by voluntary or induced muscle contraction, and impaired muscle relaxation, or pseudomyotonia. Patients may exhibit excessive sweating, paraesthesia or mild muscle weakness. An episodic syndrome of hyperhidrosis associated with VGKC-complex antibodies has also been described. However, this patient did not exhibit neuromuscular manifestations. Unlike acquired NMT, this patient did not respond to monotherapy of carbamazepine and phenytoin. But low dose carbamazepine combined with IVIG later showed some benefit. The patient was clinically similar to autoimmune NMT except for the paroxysmal course and marked pain, which was a notable and extreme feature. The nature of this disease exhibits frequent paroxysmal attacks since genetic channelopathies exhibit an episodic feature. However, antibodies to VGKC-complex were not identified, so the pathogenesis of the newly described clinical syndrome was still unknown.

 DOI: 10.3969/j.issn.1672-6731.2015.09.016