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At a glance

It is a rare neuromuscular disorder causing severe neuromyotonia. It is caused by rapid bursts of nerve impulses, resulting in painful muscle twitching, cramping, increase sweating, and delayed muscle relaxation. Other features include speech, swallowing, and breathing problems due to pharyngeal or laryngeal muscles dysfunction. Rare Syndrome with peripheral nerve lesions characterized by muscle stiffness, cramps, and continuous vibrating muscle movements (myokymia).


Hereditary Continuous Muscle Fiber Activity Syndrome; Isaacs Syndrome; Neuromyotonia Syndrome; Pseudomyotonia Syndrome.


This medical condition was originally described by Hyam Isaac, a South African Neurophysiologist, and Hans-Georg Mertens, a German Neurologist. Dr Mertens held the chair of neurology at the University of Würzburg, Germany, from 1968 to 1989.


The true incidence remains undetermined. The clinical onset has been shown to be between the ages of 15 and 60. However, most individuals are experiencing symptoms before age 40.

Genetic inheritance

This medical condition is either hereditary or acquired. The acquired form results from exposure to toxins such as gold and mercury. However, the most frequent inherited pathway is as autosomal dominant.


Unclear; the role of autoimmunity, specifically antibodies against the potassium channel-inducing hyperexcitability of the peripheral nerve, has been advocated. Probably caused by mutations in the potassium channel gene KCNA1 located on chromosome 12p13.


Characterized by muscular stiffness and cramping, particularly in the limbs with continuous fine, vibrating muscle movements (myokymia).

Clinical aspects

Neuromuscular signs dominate the disease. Progressive stiffness, cramping, weakness, and constant writhing movements of the muscles under the skin persist during sleep or under general anesthesia without muscle relaxant. Calf hypertrophy can be seen. Electromyography (EMG) shows specific anomalies (spontaneous discharges with rhythmical and continuous firing). Muscle biopsies are abnormal. Clinical examination finds hypertonia, spasticity, speech defect, ataxia, and sometimes seizures. Hyperhidrosis and tachycardia, because of autonomic nervous system involvement, are possible. Association with diaphragmatic hernia has been reported.

Precautions before anesthesia

Evaluate muscular dysfunction (clinical, EMG), neurologic function (clinical, electroencephalogram [EEG]), and autonomic nervous system involvement (clinical, heart rate variability).

Anesthetic considerations

Careful intraoperative positioning is needed. Perioperative blood pressure monitoring is necessary because of autonomic system dysregulation. Regional anesthesia is not contraindicated but probably should be avoided or the benefit must be clearly established because of peripheral nervous lesion (peripheral blocks) and autonomic nervous system involvement (perimedullar blockade). In a case report, anesthesia was conducted safely using neuromuscular blockade with continuous neuromuscular monitoring to ensure proper reversal of muscle paralysis at the end of the procedure.

Pharmacological implications


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