[MIM 608 390]

(PAM for Potassium-Aggravated Myotonias or SCM for Sodium Channel Myotonia)

This group includes three rare diseases, of autosomal dominant transmission, caused by a 'gain of function'mutation in the SCN4A gene [on 17q 23], which codes for the Nav1.4 sodium channel  of the skeletal muscle. These myopathies are generally aggravated by the intake of K.

These are:

o        myotonia fluctuans: onset in the first or second decade of life; extraocular  , bulbar and limb muscles stiffening following the ingestion of K or exercise (especially if a period of rest of 20 to 40 minutes is observed between two physical exercises);  high CPK levels (2 to 3 x normal)

o        myotonia permanens: permanent stiffening of the facial, respiratory and bulbar muscles that worsens following exercise or ingestion of K. Begins before the age of 10 years and is accompanied by hypertrophy of the muscles of the neck and shoulders; contracture of the intercostal muscles can induce respiratory difficulties; high CPK levels. May present as SNEL (episodic and severe spasms in the neonatal period) that combines neonatal hypotonia, stridor and breathing difficulties, rapidly changing to hypertonia: responds to flecainide.

o        acetazolamide-responsive myotonia : generalized myotonia occuring after fasting, exposure to cold or the ingestion of K; begins before the age of 10 years and affects the extraocular muscles, the muscles of mastication and muscles of the proximal limb; these episodes are often painful and respond to acetazolamide. Moderately high CPK levels.

Other mutations of the same gene can produce congenital paramyotony and hyper- or hypokalemic periodic paralysis .

Anesthetic implications:

the risk of malignant hyperthermia seems no greater than in the general population.

To be avoided:

-        hypothermia and contact with cold surfaces;

-        succinylcholine

-        anticholinesterases (neostigmine)

Monitor the kaliemia. Perioperative risk of muscle rigidity and rhabdomyolysis.

References : 

-         Parness J, Bandschapp O, Girard T. 
The myotonias and susceptibility to malignant hyperthermia. 
Anesth Analg 2009, 109: 1054-64.

-         Heatwole CR, Statland JM, Logigian EL. 
The diagnosis and treatment of myotonic disorders. 
Muscle Nerve 2013; 47: 632-48.

-        Portaro S, Rodolico C, Sinicropi S, Musumeci O, Valenzise M, Toscano A.
Flecainide-responsive myotonia permanens with SNEL onset: a new case and literature review.
Pediatrics 2016; 137: e 20153289

-        Matsumoto N, Nishimoto R, Matsuoka Y, Takeda Y, Moritasmu H.
Anesthetic management of a patient with sodium-channel myotonia : a case report.
JA Clinical Reports 2019 ; 5 :77

-        Stunnenberg BC, LoRusso S, Arnold WD, Barohn RJ, Cannon SC et al.
Guidelines on clinical presentation and management of nondystrophic myotonias.
Muscle Nerve 2020; 62:430-44.

Updated: January 2021