J Clin Neurol.  2014 Jan;10(1):69-71.

Myasthenia in Acquired Neuromyotonia

• Byun JI
• Moon HJ
• Hong YH

Abstract

BACKGROUND
Acquired neuromyotonia (NMT) forms part of the spectrum of acquired peripheral nerve hyperexcitability syndrome, and is thought to be caused by antibodies to voltage-gated potassium channels (VGKC). Exertional weakness is unusual unless autoimmune myasthenia gravis (MG) is superimposed.
CASE REPORT
A case of acquired NMT accompanied by exertional weakness without coexistence of seropositive MG is reported herein.
CONCLUSIONS
Clinical and electrophysiological observations suggest that the cholinergic overactivity in NMT can compromise the safety factor sufficiently to cause a defect in neuromuscular junction transmission.

Keyword

neuromyotonia; myasthenia; neuromuscular junction

MeSH Terms

Antibodies
Isaacs Syndrome*
Myasthenia Gravis
Neuromuscular Junction
Peripheral Nerves
Potassium Channels, Voltage-Gated
Antibodies
Potassium Channels, Voltage-Gated

Figure

• 

  Fig. 1 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).

Reference

1. Maddison P. Neuromyotonia. Clin Neurophysiol. 2006; 117:2118–2127.

2. Auerbach A, Akk G. Desensitization of mouse nicotinic acetylcholine receptor channels. A two-gate mechanism. J Gen Physiol. 1998; 112:181–197.

3. Singh G, Avasthi G, Khurana SB. Continuous muscle fibre activity: description of neuroelectromyographic findings including single fibre EMG. Electromyogr Clin Neurophysiol. 1998; 38:121–127.

4. van Dijk JG, Lammers GJ, Wintzen AR, Molenaar PC. Repetitive CMAPs: mechanisms of neural and synaptic genesis. Muscle Nerve. 1996; 19:1127–1133.

5. Torbergsen T, Stålberg E, Brautaset NJ. Generator sites for spontaneous activity in neuromyotonia. An EMG study. Electroencephalogr Clin Neurophysiol. 1996; 101:69–78.

6. Valli G, Barbieri S, Cappa S, Pellegrini G, Scarlato G. Syndromes of abnormal muscular activity: overlap between continuous muscle fibre activity and the stiff man syndrome. J Neurol Neurosurg Psychiatry. 1983; 46:241–247.

7. Maddison P, Mills KR, Newsom-Davis J. Clinical electrophysiological characterization of the acquired neuromyotonia phenotype of autoimmune peripheral nerve hyperexcitability. Muscle Nerve. 2006; 33:801–808.