(Continuous myokymia, neuromyotonia, pseudomyotonia, Gamstorp-Wohlfart syndrome, Isaacs-Mertens syndrome)

Very rare in children. Usually begins after the age of 40 years. Syndrome of continuous muscular activity, due to the presence of

autoantibodies against the presynaptic VGKC potassium channels: this produces hyperexcitability of the nerve membrane at the level of the peripheral nerves. Auto-antibodies are generally of autoimmune origin (myasthenia gravis, Hashimoto thyroiditis, celiac disease, pernicious anemia ...) but can be paraneoplastic in origin (thymoma, lung, Hodgkin, ovary, bladder). It is therefore an acquired form of neuromyotonia.

An inherited form [MIM 160 120] is due to the autosomal dominant transmission of a mutation of the KCNA1 gene (12p13): tachycardia and sweating are also present; occasionally, this mutation is associated with a congenital diaphragmatic hernia. Other mutations of the same gene produce episodic ataxia with myokymia.

Clinical presentation:

-         muscle rigidity is exaggerated by voluntary movements,  

-         spontaneous twitching, cramps

-         hypertrophy of muscle groups (calves, forearms, hands), probably secondary to their hyperactivity

-         multiple muscle contractions at the level of limbs and the face (subcutaneous rippling)

-         pseudomyotonia in 30% of cases: slow muscle relaxation after a voluntary contraction (hand, eyes, jaw) 

-         muscle activity persists during sleep.

-        frequent sweating. 

Biology: moderate elevation of CPK level; look for other autoantibodies and undiagnosed cancer. Muscle biopsy: non specific. EMG: neuromyotonic and myokimic discharges.

Treatment: carbamazepine or phenytoin, sometimes gabapentin; sometimes: plasmapheresis, IV gammaglobulins, corticosteroids or immunosuppression.

Anesthetic implications: 

monitor curarization: some cases of increased sensitivity to nondepolarizing myorelaxants have been described (for the acquired forms, according to the level of the circulating antibodies or an associated myasthenia). Avoid succinylcholine. Increased risk of postoperative respiratory complications (inhalation triggered by a myotonic crisis of the laryngeal muscles). Propofol has been used successfully (no pseudomyotonic crisis), in contrast with what has been described in case of myotonic dystrophy. It seems that induction of general anesthesia stops the muscular contractions.

References : 

-         Ahmed A, Simmons Z. 
Isaacs syndrome: a review. Muscle & Nerve 2015; 52: 5-12. 

-        Singh H, Tewari A, Bansal A, Garg et al.
Anaesthesia for a patient with Isaac’s syndrome and myasthenia gravis.
Br J Anaesth 2009 ; 103 : 460-1

-        Ginsburg G, Forde R, Martyn JA, Eikermann M.
Increased sensitivity to a nondepolarizing muscle relaxant in a patient with acquired neuromyotonia.
Muscle Nerve 2009 ; 40 : 139-42.

-        Kim YM, Lee SH, Han CS, Choi EM et al.
Anesthetic experience using total intravenous anesthesia in a patient with Isaacs’ syndrome.
Korean J Anesthesiol 2013; 64: 164-7

-        Asai A, Kako E, Hasegawa T, Sobue K.
Peripheral nerve block provides effective analgesia for a patient with peripheral nerve hyperexcitatibility syndrome: Isaacs syndrome case report.
A&A Practice 2018 ; 11 : 268-9 

-        Okutani H, Okano Y, Hirose M.
Painful myokynia after surgery in a patient with Isaac’s syndrome: a case report.
JA Clinical Reports 2020; 6:14, doi 10.1186/s40981-020-00321-y

-        Moody AE, Beutler BD, Moody CE, Chang C et al.
Anaesthesia and orphan disease: a patient with neuromyotonia undergoing single lung ventilation.
Eur J Anaesthesiol 2020; 37:731-3

Updated: May 2021