Paramyotonia congenita


Paramyotonia congenita

Description, Causes and Risk Factors:

Abbreviation: PC.

A non-progressive myotonia induced by exposure of muscles to cold; there are episodes of intermittent flaccid paralysis, but no atrophy or hypertrophy of muscles; autosomal dominant inheritance caused by mutation in the sodium channel gene (SCN4A) on chromosome 17q. This is a disorder allelic to hyperkalemic periodic paralysis. There is a variant autosomal dominant form [MIM*168350] in which cold is not a provoking factor.

Paramyotonia congenita is a disorder that affects muscles used for movement (skeletal muscles). Beginning in infancy or early childhood, people with this condition experience bouts of sustained muscle tensing (myotonia) that prevent muscles from relaxing normally. Myotonia causes muscle stiffness that typically appears after exercise and can be induced by muscle cooling. This stiffness chiefly affects muscles in the face, neck, arms, and hands. Unlike many other forms of myotonia, the muscle stiffness associated with paramyotonia congenita tends to worsen with repeated movements.

Most people — even those without muscle disease — feel that their muscles do not work as well when they are cold. This effect is dramatic in people with paramyotonia congenita. Exposure to cold initially causes muscle stiffness in these individuals, and prolonged cold exposure leads to temporary episodes of mild to severe muscle weakness that may last for several hours at a time.

Mutations in the SCN4A gene cause paramyotonia congenita. The SCN4A 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 contractions are triggered by the flow of positively charged atoms (ions), including sodium, into skeletal muscle cells. The SCN4A protein forms channels that control the flow of sodium ions into these cells.

Mutations in the SCN4A gene alter the usual structure and function of sodium channels. The altered channels cannot properly regulate the flow of sodium ions into skeletal muscle cells. The resulting increase in ion flow interferes with normal muscle contraction and relaxation, leading to episodes of myotonia or muscle weakness.

This condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. In many cases, an affected person has one parent with the condition.

Prevalence is generally higher in European derived populations and lower among Asians. Epidemiological estimates have been provided for the German population. Here, it was estimated that the prevalence of PC is between 1:350,000 (0.00028%) and 1:180,000 (0.00056%). It should be noted, however, that the German population of patients with PC is not uniformly distributed across the country. Many individuals with PC herald from the Ravensberg area in North-West Germany, where a founder effect is seems to be responsible for most cases. The prevalence here is estimated at 1:6000 or 0.017%.

Paramyotonia congenita has an excellent prognosis. Although annoying, it does not cause significant disability, and the patient usually learns to make lifestyle adjustments that prevent exacerbations (for example, dressing warmly and avoiding exposure to cold).

Symptoms:

Paramyotonia congenita causes episodes of muscle stiffness and weakness — mostly in the face, neck and upper extremities — that can last from minutes to hours. The stiffness is sensitive to exercise and cold. During brief exercise, overexcitation of muscles can cause stiffness, and with prolonged exercise, the overexcitation can occasionally lead to a fatigue-like weakness or even complete paralysis.

Cold exposure can have similar effects, but some people experience muscle stiffness, weakness, or sometimes, temporary paralysis even when they are warm.

Diagnosis:

Diagnosis may include electromyographic (EMG) testing involves placing a needle electrode into a muscle and measuring its electrical activity. EMG testing in paramyotonia congenita may reveal differences between electrical activity in a warm muscle and electrical activity in a cooled muscle. There are a number of genetic defects that are associated with the chloride channel defect of paramyotonia congenita, some of which can be revealed through genetic testing.

Treatment:

Paramyotonia congenita (PC) can present with one of two problems (or a combo of both). The main thing with PC is to determine whether the patient is troubled by episodic weakness, myotonia, or both. The treatments for these two problems are different and if the patient has troubling symptoms with both, should be treated for both. One can treat the myotonia with Mexilitine as a first line agent (barring contraindications) and episodic weakness with Daranide or Diamox. We continue to study PC and to make progress but this has not yet led to any new insights into NEW drugs that might help even more. This kind of advance always lags behind studies that help us understand the disease mechanism.

NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.

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