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.
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