Myotonic Disorders

Myotonia Definition: A prolonged failure of relaxation with after- discharge on the EMG Features: Requires strong contraction for its elicitation More evident after a period of relaxation Tends to disappear with repeated contractions Can be demonstrated by tapping a muscle--percussion myotonia EMG: When the needle is moved in the muscle it elicits a characteristic high frequency repetitive discharge which wax and wane in frequency and amplitude producing a “dive bomber” sound on the audio monitor

Disorders with myotonia 1. Myotonica dystrophica (Steinert) 2. Congenital Myotonia (Thomsen) 3. Paramyotonia (von Eulenberg)

Myotonic dystrophy (Steinert) This is the most common inherited muscle disorder affecting adults Autosomal dominant 15 cases per live births and M:F is 1:1 Varies from a mild late onset form of the disease to a severe congenital disorder. Severity depends on the number of CTG repeats in the DMPK gene on chromosome 19 The classical form usually present in adolescence or early adulthood The most frequent symptoms are muscle weakness and stiffness affecting the distal limbs and myotonia

Myotonic dystrophy Systemic features Ocular : Cataract, ptosis, retinal degeneration Cardiac:Conduction disease, arryhtmias,cardiomyopathy Gastrointestinal: Dysphagia, colonic dysmotility, megacolon Diaphragmatic weakness can cause dyspnea Endocrine: Diabetes due to insulin resistance, testicular atrophy, abnormal GH release Immune: reduced serum immunoglobulins Miscellaneous: Frontal balding, temporal wasting, cranial hyperostosis

Myotonic dystrophy Laboratory features: CPK mildly elevated EMG: myotonic discharges “dive bomber” Genetic screening: CTG repeat on chromosome 19 on leukocyte DNA Treatment: Supportive care Phenytoin, procainamide and mexiletine for myotonia

Myotonia Congenita (Thomsen) Genetics: Autosomal dominant and recessive Both are caused by mutations in the chloride channel gene of skeletal muscle. Men more frequently effected than women in the ratio of 3:1 Pathogenesis: The stiffness and myotonic discharges are caused by repeated muscle membrane depolarization following activation preventing muscle relaxation

Myotonia congenita Clinical features: Symptoms first usually appear in adolescence Main symptoms are painless stiffness of muscles particularly of the legs. Stiffness is provoked by rest after exercise. Striking feature is the sometimes generalized stiffness following a sudden surprise or startle resulting in a fall Muscle stiffness improves after repeated exercise (warm up phenomenon) Patients appear athletic and muscles are hypertrophied

Paramyotonia congenita Autosomal dominant Mutation in the sodium channel gene (SCN 4A) Symptoms usually begin in childhood Persistent eye closure after crying, protracted facial grimacing after washing with cold water. Myotonia induced by exercise and cold water Patients appear muscular Treatment is avoidance of cold temperatures, exercise Mexiletine for myotonia