Neuromyotonia

Signs and symptoms

Patients may experience muscle cramps, stiffness, myotonia-like slow relaxation, walking difficulties, hyperhidrosis, myokymia, fasciculations, fatigue, exercise intolerance, and myoclonic jerks. Neuromyotonic discharges occur continuously or in decrementing clusters, starting and stopping abruptly, and are unaffected by voluntary activity. Symptoms are most prominent in the calves, legs, trunk, face, and neck, and at least a third of people also have sensory symptoms.

Causes

Neuromyotonia has three known causes: acquired, paraneoplastic, and hereditary. The acquired type, which makes up about 80 percent of cases, is thought to be autoimmune. It involves antibodies that attach to potassium channels in motor nerves, causing constant overactivity.

These antibodies have been found to impair potassium channel function in lab-grown nerve cells. While the precise cause is still unclear, evidence increasingly points to an autoimmune origin. Symptoms usually appear between ages 15 and 60, and most people develop them before turning 40.

Diagnosis

Diagnosis is clinical and begins with ruling out more common conditions at the general practitioner level, including a basic neurological exam. A neurologist or neuromuscular specialist may perform needle electromyography and nerve conduction studies, which are the most important tests, along with chest CT, blood work for voltage-gated potassium channel antibodies, acetylcholine receptor antibody, serum immunofixation, TSH, ANA, ESR, and EEG.

EMG shows doublet, triplet, or multiplet single unit discharges with high irregular intraburst frequency, often with fibrillation potentials and fasciculations. Because NMT is rare, correct diagnosis may take years. It can be misdiagnosed as ALS or multiple sclerosis; a trained neuromuscular specialist is recommended for accurate diagnosis.

Prognosis

The long-term prognosis is uncertain and depends on the underlying cause, such as autoimmune or paraneoplastic. Increased understanding of NMT and autoimmunity has led to novel treatments, and many patients respond well, with significant symptom relief. Some cases of spontaneous remission have been noted.

Symptoms generally do not deteriorate into anything more serious, and with correct treatment they are manageable. A very small proportion of cases may develop central nervous system findings leading to Morvan's syndrome. In paraneoplastic cases, the underlying cancer determines prognosis, but most NMT cases are autoimmune and not associated with cancer.

Did You Know?

• Some studies have linked NMT with certain types of cancers, mostly lung and thymus.
• Plasma exchange and IVIg treatment may provide short-term relief for some forms of the acquired disorder.
• Anticonvulsants including phenytoin and carbamazepine usually provide significant relief from stiffness, muscle spasms, and pain.