Neuromyotonia

Neuromyotonia

Neuromyotonia is neurogenic muscle stiffness (cf. myotonia, myogenic muscle stiffness) which reflects peripheral nerve hyperexcitability. Clinically this is manifest as muscle cramps and stiffness, particularly during and after muscle contraction, and as muscular activity at rest (myokymia, fasciculations). Tendon areflexia and abnormal postures of hands and feet may also be observed.
A syndrome of ocular neuromyotonia has been described in which spasms of the extraocular muscles cause a transient heterophoria and diplopia.
Physiologically neuromyotonia is characterized by continuous motor unit and muscle fibre activity which is due to peripheral nerve hyperexcitability; it is abolished by curare (cf. myotonia). Neuromyotonia may be associated with autoantibodies directed against presynaptic voltage-gated K+ channels. Around 20% of patients have an underlying small-cell lung cancer or thymoma, suggesting a paraneoplastic etiology in these patients. Neuromyotonia has also been associated with mutations within the voltage-gated K+ ion channel gene.
Carbamazepine and phenytoin may help the stiffness and areflexia.

References

Browne DL, Gancher ST, Nutt JG, et al. Episodic ataxia-myokymia syndrome is associated with a point mutation in the human potassium channel gene, KCNA1. Nature Genetics 1994; 8: 136-140
Ezra E, Spalton D, Sanders MD, Graham EM, Plant GT. Ocular neuromyotonia. British Journal of Ophthalmology 1996; 80: 350-355 Hart IK, Waters C, Vincent A, et al. Autoantibodies detected to expressed K+ channels are implicated in neuromyotonia. Annals ofNeurology 1997; 48: 238-246
Isaacs H. A syndrome of continuous muscle-fibre activity. Journal ofNeurology, Neurosurgery and Psychiatry 1961; 24: 319-325
Maddison P. Neuromyotonia. Practical Neurology 2002; 2: 225-229

Cross References

Fasciculation; Myokymia; Myotonia; Paramyotonia; Pseudomyotonia; Stiffness