Thomsen and Becker disease

Thomsen disease and Becker disease are two forms of myotonia congenita, a group of inherited skeletal muscle disorders caused by mutations in the CLCN1 gene, which encodes the voltage-gated chloride channel ClC-1 in skeletal muscle. Thomsen disease (autosomal dominant myotonia congenita) and Becker disease (autosomal recessive myotonia congenita, also called generalized myotonia congenita) share the hallmark feature of myotonia — involuntary sustained muscle contraction and delayed relaxation after voluntary movement. This primarily affects the skeletal muscular system, causing muscle stiffness that is often most pronounced after a period of rest and tends to improve with repeated movement, a phenomenon known as the 'warm-up' effect. Thomsen disease typically presents in infancy or early childhood with generalized muscle stiffness. Affected individuals often have a muscular (hypertrophic) body habitus. The condition is generally milder than Becker disease. Becker disease usually manifests later in childhood and tends to be more severe, with more pronounced myotonia and the potential for transient episodes of muscle weakness. In Becker disease, muscle hypertrophy is also common, but patients may experience transient weakness particularly in the lower extremities upon initiating movement. Both forms can cause difficulty with grip release, eyelid closure, and initiating movements such as standing or walking after rest. There is currently no cure for either form of myotonia congenita. Treatment is symptomatic and primarily involves the use of sodium channel blockers such as mexiletine, which is considered the first-line pharmacological therapy for reducing myotonia. Other medications that may be used include lamotrigine, carbamazepine, and phenytoin. Physical activity and regular exercise are generally encouraged, as they can help reduce stiffness. Avoidance of cold temperatures and prolonged rest may also help manage symptoms. Genetic counseling is recommended for affected families. The prognosis is generally favorable, as myotonia congenita does not significantly reduce life expectancy, though it can impact quality of life.

Data sourced from FDA regulatory filings and ClinicalTrials.gov. Updated periodically.

Common questions about Thomsen and Becker disease