Paramyotonia congenita (PMC) is a muscle disorder which causes muscle stiffness (myotonia) that appears during exercise and becomes much more severe upon exposure to cold. The myotonia which occurs during attacks of PMC may be so severe that the patient cannot inhale and exhale properly, and movement becomes difficult.
The medical term myotonia means that the muscle fibers are slow to relax after contraction. The ‘para’ part of the term paramyotonia stands for paradoxical because while myotonia normally goes away when the muscles are exercised or ‘warmed up’, paradoxical myotonia becomes more severe with movement, thus ‘paramyotonia’.
Myotonia can make it difficult for the patient to let go of objects which have been picked up (like a pen or drinking glass). The muscles may feel tight or stiff. One patient described his myotonia like feeling like he is wearing a very tight wet suit during attacks. For some patients myotonia is painless, for others it is painful. Myotonic patients may look like athletes but their “well-developed” muscles are often quite weak.
Some patients have PMC alone, while others have both PMC and Hyperkalemic periodic paralysis. HyperKPP and PMC are caused by a mutation in a sodium channel on the same gene and are considered to be different forms of the same disorder.
The difference between PMC and HyperKPP are:
1) PMC patients are usually extremely sensitive to cold or chilling.
2) PMC patients develop muscle stiffness as they exercise, not during rest after exercise.
3) PMC episodes tend to last longer than HyperKPP episodes – days rather than hours. In some PMC patients long-term episodes (weeks to months) of significant weakness can be triggered by unusual exertion. Such patients must be extremely careful to avoid muscular fatigue.
PMC can be associated with either hypokalemia or hyperkalemia, but it appears more commonly in association with hyperkalemia. There is a form of PMC in which episodes are triggered by a fall in potassium. Patients with HyperKPP may have this form of PMC, and thus may be sensitive to the movement of serum potassium in either direction.
What causes Paramyotonia Congenita?
PMC is caused by a genetic mutation in the sodium channel of the muscle membrane. This mutation makes the person with PMC extremely sensitive to cold, to the exchange of sodium and potassium ions which occurs during exercise and to shifts in potassium levels that wouldn’t bother the average person. The person with PMC may get muscle stiffness and weakness with even slight changes in potassium level, or in response to chilling or exercise.
How did I get it?
In a majority of cases PMC is inherited from a parent, but in some cases it just happens, like any other genetic mutation, for reasons which are unclear. It usually, but not always, accompanies HyperKPP.
Will I pass this on to my children?
There is a 50% chance that a child of an affected person will inherit the gene mutation, but the degree to which a child is affected may vary from one child to the next. One child may be mildly affected, the next seriously affected and the next unaffected. Even identical twins may be affected to different degrees. A child with PMC is often affected from an early age, often during the first year of life.
What triggers attacks of PMC?
The hyperkalemic forms may be triggered by potassium-rich food. Cantaloupes, apricots, dried figs, kiwi fruit, peaches, raisins, banana and prunes are all high in potassium as are orange and pineapple juices and apricot and peach nectars. High potassium vegetables include; Artichoke, lentils and beans, parsnips, potato, pumpkin, spinach, broccoli, Brussel sprouts, cauliflower, tomato juice/puree and V-8 juice. Most nuts are high in potassium as is peanut butter. Chocolate is also high in potassium.
Other triggers for PMC episodes include activity or exercise. Vigorous exercise may provoke weakness which lasts for months. Weather changes trigger episodes in some patients. There is more than one type of PMC. In some types patients must avoid potassium and in some forms they require potassium.
What tests are used to diagnose PMC?
The myotonia may be found during electromyographic (EMG) examination. During this test fine needles are inserted into the muscle and the muscle’s electrical signals are recorded. The muscle is chilled for this test, as chilling brings on the myotonia. Evidence of myotonia supports the diagnosis of PMC, but PMC is the most difficult type of myotonia to detect by EMG. At times diagnosis is not as easily accomplished, and further testing is necessary. There is more information about diagnosis in other articles on this website. See The Exercise EMG.
Where do I go for the best treatment?
The best place for treatment is almost always the patient’s own family physician, if that physician is willing to read about the condition and learn how to manage it. Every patient responds a bit differently to treatment and the members of our group who are, on the whole, the most satisfied with their care are those who are cared for by the person who cares for their overall health. It’s very important to take responsibility for one’s own health. Once a diagnosis is reached the patient must assume the day-to-day work of managing the condition.
Who is doing research on PMC?
There are a number of research teams working on the PPs. Research is concentrating on the genetics and mechanics of the channelopathies, rather than on therapy and management. There’s been nothing new in the way of therapy for many years, although we have learned how to apply therapies much more effectively, and we’ve made great strides in management.
Can I have genetic test for PMC?
Genetic testing is available, but it is not yet reliable enough to diagnose all cases of PMC. While a genetic test may be able to say you have PMC, a negative result does NOT rule out the disorder, as there are a number of variants which are not yet identified.
What medications are prescribed for PMC?
Treatments for PMC must be individualized depending on symptoms. Patients must learn to ‘read’ their symptoms and take the appropriate management steps to lower or raise potassium levels as needed. PMC probably presents the biggest management challenge of all of the periodic paralyses and requires both flexibility and a team approach between patient and physician.
The medications prescribed depend on the type of PMC the patient has, and if the patient has another type of periodic paralysis as well as PMC. The carbonic anhydrase inhibitors ‘Diamox’ (acetazolomide) and Keveyis (dichlorphenamide) are often prescribed for the other periodic paralyses, but may make a PMC patient’s symptoms worse. Diuretics which reduce the level of potassium in the blood are often used in PMC, especially when it is accompanied by HyperKPP. These are drugs like the thiazides (Hydrodiuril, hydrochlorothiazide) or furosemide (Lasix). Florinef is used occasionally when the patient needs to retain sodium and fluid as well as excrete potassium. Low doses of Mexiletine (Mexitil) and Paxil (paroxetine) are used to help reduce myotonia. Small amounts of potassium supplementation are often helpful in cases of PMC, even when patients take diuretics to help them excrete potassium.
Will I be able to lead a normal life?
Most people with PMC lead reasonably normal lives, though they must be cautious not to get chilled or overextend themselves physically. Many cases are mild, and even in those with frequent symptoms can be helped with medication and attention to diet and lifestyle. There is no denying that PMC does require adaptations to patients’ lives but most patients manage to lead well-rounded and fulfilling lives.
Will I end up in a wheelchair?
This is a hard question to answer. If activities like climbing stairs and walking long distances become very fatiguing it may be wise to use a chair to conserve physical strength and avoid falls. Any number of our older members use a wheelchair outside the home, but are mobile and on their feet inside the house. Very few rely on a wheelchair full time unless their condition is complicated by other problems.
Will my life be cut short?
There is no evidence that PMC is life-threatening.
reviewed May 2022