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recessive","evidence":"PCS","frequency_raw":"2/2","hpo_id":"HP:0002486","is_excluded":false,"label":"Myotonia","onset_id":null,"reference":"PMID:34938096","source_release":"2026-06-23"},"sourceGroups":["published"]},{"frequencyKind":"count","key":"faa3677381adae5fc6fa166256a049bc6b0bf2bc4604bd61a9c9d1073a147688","raw":{"aspect":"P","body_system":"head_and_neck","disease_id":"OMIM:255700","disease_name":"Myotonia congenita, autosomal recessive","evidence":"PCS","frequency_raw":"7/27","hpo_id":"HP:0025605","is_excluded":false,"label":"Lid lag on downgaze","onset_id":null,"reference":"PMID:18337100","source_release":"2026-06-23"},"sourceGroups":["published"]},{"frequencyKind":"range","key":"fdd8570495fea7bb01c89b2af473055ae3936c8d3712cd2583c1f7bcc4e79564","raw":{"aspect":"P","body_system":"musculoskeletal","disease_id":"ORPHA:614","disease_name":"Thomsen and Becker disease","evidence":"TAS","frequency_raw":"HP:0040283","hpo_id":"HP:0009063","is_excluded":false,"label":"Progressive distal muscle weakness","onset_id":null,"reference":"ORPHA:614","source_release":"2026-06-23"},"sourceGroups":["orphanet"]}],"referenceLabels":{},"references":{"causeHtml":"<ul><li><a href=\"https://medlineplus.gov/genetics/condition/myotonia-congenita\">Myotonia congenita</a></li></ul>","identityNote":"MONDO cross-references identify native condition entries. Ancestor membership does not transfer phenotype values between groups.","importantHtml":""},"sections":[{"bodyHtml":"<p>Mutations in the CLCN1 gene cause myotonia congenita. The CLCN1 gene provides instructions for making a protein that is critical for the normal function of skeletal muscle cells. For the body to move normally, skeletal muscles must tense (contract) and relax in a coordinated way. Muscle contraction and relaxation are controlled by the flow of charged atoms (ions) into and out of muscle cells. Specifically, the protein produced from the CLCN1 gene forms a channel that controls the flow of negatively charged chlorine atoms (chloride ions) into these cells. The main function of this channel is to stabilize the cells&#x27; electrical charge, which prevents muscles from contracting abnormally.</p><p>Mutations in the CLCN1 gene alter the usual structure or function of chloride channels. The altered channels cannot properly regulate ion flow, reducing the movement of chloride ions into skeletal muscle cells. This disruption in chloride ion flow triggers prolonged muscle contractions, which are the hallmark of myotonia.</p><p class=\"section-source\"><a href=\"https://medlineplus.gov/genetics/condition/myotonia-congenita/#causes\">MedlinePlus Genetics: Causes</a></p>","key":"forms","title":"What causes Myotonia congenita?"},{"bodyHtml":"<p>The two major types of myotonia congenita are known as Thomsen disease and Becker disease. These conditions are distinguished by the severity of their symptoms and their patterns of inheritance. Becker disease usually appears later in childhood than Thomsen disease and causes more severe muscle stiffness, particularly in males. People with Becker disease often experience temporary attacks of muscle weakness, particularly in the arms and hands, brought on by movement after periods of rest. They may also develop mild, permanent muscle weakness over time. This muscle weakness is not seen in people with Thomsen disease.</p><p class=\"section-source\">Source: <a href=\"https://medlineplus.gov/genetics/condition/myotonia-congenita\">MedlinePlus Genetics</a></p>","key":"overview","title":"Condition overview"}],"sharedSources":[],"slug":"myotonia-congenita","sourceDocuments":[{"name":"Myotonia congenita","reviewed":"2020-02","sections":{"causes":{"fetchedAt":"2026-10-20T20:23:39.856296+00:00","label":"Causes","pageSha256":"e4724c9794212d057e7f7eb32db97b3cb6bcad3bdd922a3c0ec86f4cb127204d","sourceUrl":"https://medlineplus.gov/genetics/condition/myotonia-congenita/#causes","status":"available"}},"source_release":"2026-10-01","url":"https://medlineplus.gov/genetics/condition/myotonia-congenita"}],"sourceNote":"MedlinePlus descriptions and Causes; original HPO annotations.","takeawayHtml":"Cause explanations are from MedlinePlus Genetics. Feature values below retain their original sources and patient groups.","title":"Myotonia congenita"},"schemaVersion":1}