Diagnosis of congenital myotonia can as well be confirmed my molecular genetic analysis of ClCN-1 gene responsible for function of ClC-1 channels.
- Risk to Family Members — Autosomal Dominant Inheritance
Parents of a proband. The majority of individuals diagnosed with autosomal dominant myotonia congenita have an affected parent. A proband with autosomal dominant myotonia congenita may potentially have the disorder as the result of a new gene mutation. The proportion of cases caused by de novo is very low.
Sibs of a proband. The risk to the sibs of the proband depends on the genetic status of the proband’s parents. If a parent of the proband is affected, the risk to the sibs is 50%. When the parents are clinically unaffected, the risk to the sibs of a proband appears to be low.
Offspring of a proband. Each child of an individual with autosomal dominant myotonia congenita has a 50% chance of inheriting the mutation.
Other family members of a proband. The risk to other family members depends on the status of the proband’s parents. If a parent is affected, his or her family members are at risk.
- Risk to Family Members — Autosomal Recessive Inheritance
Parents of a proband. The parents of an individual with autosomal recessive myotonia congenita are obligate heterozygotes and therefore each carry one mutant allele. Heterozygotes (carriers) are asymptomatic. Occasionally, the parents of a proband with autosomal recessive myotonia congenita show subtle evidence of myotonia on EMG testing.
Sibs of a proband. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
Offspring of a proband. The offspring of an individual with autosomal recessive myotonia congenita are obligate heterozygotes (carriers) for a disease-causing mutation.
Other family members of a proband. Each sib of the proband’s parents is at a 50% risk of being a carrier.
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