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Paramyotonia congenita (PC) is a rare congenital autosomal dominant neuromuscular disorder characterized by "paradoxical" myotonia. [2] This type of myotonia has been termed paradoxical because it becomes worse with exercise whereas classical myotonia, as seen in myotonia congenita , is alleviated by exercise.
Myotonia congenita is caused in humans by loss-of-function mutations in the gene CLCN1. This is the gene encoding the protein CLCN1, that forms the ClC-1 chloride channel, critical for the normal function of skeletal muscle cells. This gene is also associated with the condition in horses, goats, and dogs.
Paramyotonia congenita (Online Mendelian Inheritance in Man (OMIM): 168300), a form which often accompanies hyperkalemic periodic paralysis, but may present alone. The primary symptom of paramyotonia congenita is muscle contracture which develops during exercise or activity.
Myotonia is the defining symptom of many channelopathies (diseases of ion channel transport) such as myotonia congenita, paramyotonia congenita and myotonic dystrophy. [3] [4] Brody disease (a disease of ion pump transport) has symptoms similar to myotonia congenita, however, the delayed muscle relaxation is pseudo-myotonia as the EMG is normal ...
The Na v 1.4 voltage-gated sodium channel is encoded by the SCN4A gene. Mutations in the gene are associated with hypokalemic periodic paralysis , hyperkalemic periodic paralysis , paramyotonia congenita , and potassium-aggravated myotonia .
Congenital myotonia can be inherited as an autosomal dominant trait (with incomplete penetrance) or a recessive trait, resulting in the varying severity of the condition. [15] [16] In affected goats, the CLCN1 gene contains a missense mutation; the amino acid alanine is replaced with a proline residue. [16]
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A severe form of DM1, congenital myotonic dystrophy, may appear in newborns of mothers who have DM. Congenital myotonic dystrophy can also be inherited via the paternal gene, although it is said to be relatively rare. Congenital means that the condition is present from birth. [8]