Paramyotonia congenita (PC) is a rare congenital autosomal dominant neuromuscular disorder characterized by "paradoxical" myotonia.

Paramyotonia congenita (as well as hyperkalemic periodic paralysis and the potassium-aggravated myotonias) is caused by mutations in a sodium channel, SCN4A.

The result of these alterations in channel kinetics is that there is prolonged inward (depolarizing) current following muscle excitation.

These lead to a general increase in cellular excitability,[citation needed] as shown in figure 1.

There is no large difference between these two groups except that patients with no known mutation have attacks precipitated less by cold but more by hunger, are much more likely to have normal muscle biopsies, and show less decreased compound muscle action potentials when compared to patients with known mutations.

Diagnosis of paramyotonia congenita is made upon evaluation of patient symptoms and case history.

[citation needed] Some patients do not require treatment to manage the symptoms of paramyotonia congenita.

[citation needed] Paramyotonia congenita is considered an extremely rare disorder, though little epidemiological work has been done.

Many individuals with PC herald from the Ravensberg area in North-West Germany, where a founder effect seems to be responsible for most cases.

[citation needed] Originally thought to be separate from hyperkalemic periodic paralysis and the sodium channel myotonias, there is now considerable disagreement as to whether these disorders represent separate entities or overlapping phenotypes of a complex disorder spectrum.