Evaluation of myotonia and therapeutic implications: about a child with myotonia congenita (Becker disease)

Introduction: We report about a patient with autosomal recessive congenital myotonia (Becker disease). The disorder is caused by mutations involving the chloride channel of the muscles (CLCN1-gene), which is responsible for shutting off electrical excitation in the muscles, causing hyperexcitability in muscle fiber membranes. Symptoms include delayed relaxation of the muscles after voluntary contraction (myotonia), and may also include stiffness, hypertrophy (enlargement) of the muscles and transient weakness. Tha myotonia is relieved by repetitive movement (warm-up effect).

Case presentation: Medical history: 15 year old boy presented with progressive motor problemes since age 12: slow walking, low endurance, frequent falls (twice a week), difficulting climbing stairs und opening hands, stiffness (specially in the shoulders). The symptoms are related neither to temperature nor to meals.

Family history: Both parents are from Turkey and consanguineous.

Video: Proximal muscle weakness, myotonia after clenching the fist, warm-up effect. Muscle strech tendon reflexes and cranial nerve examination were normal. Elbow joints contracture (aprox. 20°), contracture of Achilles tendon, scapulae alata, kyphosis. Diminished motor performance.

Laboratory tests: CK normal/slightly increased. EMG (M. quadriceps) described as „normal“. Muscle biopsy: mild myopatic changes. ECG and Echocardiography: normal. Molecular genetic analysis of DM1 and DM2: normal.

Current laboratory tests: EMG of several muscles: severe myotonic discharges. Molecular genetic analysis of myotonia congenital (Becker disease): homozygous mutation in exon 6 of CLCN1-gene.

We initiated a therapy due to the severe motor limitation. Deterioration with Mexiletin (drug of first choice). Pronounced subjective and objective improvement of myotonia and indurance with carbamazepine.

Conclusions: EMG and warm-up effect are essential criteria in the evaluation of myotonia. Molecular genetic analysis shows mutation in the CLCN1-gene. A treatment should be initiated if motor limitations are severe. Carbamazepine can show a positive effect.