Neuromyotonia is a rare condition (prevalence of less than 1 in 1,000,000) of peripheral nerve hyperexcitability. There is a hereditary form through an autosomal dominant mutation of the KCNA1 gene on chromosome 12p13.
This form commonly features tachycardia, excessive sweating and it is sometimes associated with a congenital diaphragmatic hernia. There is also an autosomal recessive form of the disease involving the HINT1 gene. There is also an acquired form in which most patients have autoantibodies to Caspr2 and LGI1, which are proteins associated with the presynaptic voltage gated potassium channel (VGKC). This causes a hyperexcitability of the nerve membranes. These autoantibodies are either of autoimmune origin (association with myasthenia, Hashimoto’s thyroiditis or pernicious anaemia) or a paraneoplastic syndrome (thymoma, lung, ovarian or bladder cancer, Hodgkin’s lymphoma). Neuromyotonia typically presents with myokymia (muscle twitching), stiffness (30 % present with pseudomyotonia, manifesting as delayed relaxation following muscle contraction) and muscle cramps. Less common manifestations may include easy fatigability, hyperhidrosis and ataxia. Onset of disease can happen at any age and is sometimes associated with myasthenia gravis and thymoma as noted above. It can follow either a progressive course or a relapsing/remitting pattern. Diagnosis is made by a combination of clinical symptoms such as myokymia in the presence of electromyography (EMG) showing doublets/triplets discharges. Other imaging such as computerised tomography or MRI may be utilised to investigate the presence of thymoma or other malignancies associated with the paraneoplastic form of neuromyotonia. Treatment with anticonvulsants, such as phenytoin, carbamazepine or gabapentin is common to relieve some of the pain associated with the abnormal muscle firing. Plasma exchange, IVIG (intravenous immunoglobulin) and steroids have been reported to alleviate symptoms in some patients. The presence of a malignancy may require surgical work up and intervention in affected patients.