Paramyotonia congenita

Symptoms and signs

Patients report muscle stiffness that may progress to focal weakness, and this stiffness does not resolve with continued movement. Symptoms are exacerbated or induced by cold environments; for instance, eating ice cream can stiffen the throat.

Exercise such as squatting or repetitive fist clenching consistently triggers myotonia or weakness. Some individuals note that specific foods like carrots or watermelon can provoke symptoms. Permanent weakness is not part of the canonical definition, though this criterion is not always strictly applied in the literature.

Pathophysiology

Paramyotonia congenita results from mutations in the SCN4A gene, which encodes a sodium channel. These mutations impair fast inactivation of the channel, and some also alter activation and deactivation. The consequence is prolonged inward depolarizing current after muscle excitation and the emergence of a window current due to shifts in voltage sensitivity. These changes increase cellular excitability, as illustrated by theoretical simulations: normal muscle produces a single action potential in response to a depolarizing pulse, whereas myotonic muscle fires repetitively throughout the stimulus.

In a study of 26 kindreds, 71% had a mutation in SCN4A, while 29% had no known mutation. Patients without a known mutation differ in that attacks are less often cold-precipitated but more often hunger-precipitated, they are more likely to have normal muscle biopsies, and they show less reduction in compound muscle action potentials.

Diagnosis

Diagnosis relies on evaluation of symptoms and case history, requiring myotonia that increases with exercise or movement and typically worsens in cold. Patients with permanent weakness are usually not classified as having PC.

Electromyography can help distinguish PC from myotonia congenita. Clinicians may provoke episodes of myotonia or weakness to differentiate PC from hyperkalemic periodic paralysis or potassium-aggravated myotonias. Genomic sequencing of SCN4A provides definitive diagnosis.

Epidemiology

Paramyotonia congenita is extremely rare, with limited epidemiological data. Prevalence is higher in European-derived populations and lower among Asians. In Germany, estimated prevalence ranges from 1:350,000 to 1:180,000. However, cases are not uniformly distributed; a founder effect in the Ravensberg area of North-West Germany yields a local prevalence of about 1:6,000.

Did You Know?

• Some patients do not require treatment; others find mexiletine or acetazolamide helpful.
• Avoidance of triggering events is an effective prevention strategy.
• Originally thought to be separate from hyperkalemic periodic paralysis, there is now disagreement whether these are distinct disorders or overlapping phenotypes.