Paramyotonia congenita (PMC) is a type of periodic paralysis that causes people to have attacks of muscle stiffness (myotonia) when they are active, such as when exercising.
Normally, myotonia happens after exercising, not during it. However, if someone has paramyotonia congenita, they will experience myotonia while they are exercising. Because that is the opposite of how myotonia normally works, it’s like a paradox, which is where the “para” in “paramyotonia” comes from. “Congenita” means that this is a condition someone is born with.
It is possible for someone to have paramyotonia congenita combined with another type of periodic paralysis, often hyperkalemic or hypokalemic periodic paralysis. This means that muscle stiffness attacks can be followed by muscle weakness, or vice-versa. Avoiding cold environments is a way to reduce the chances of having myotonia or muscle weakness attacks. Medications can help as well, but a medication that is good for one paramyotonia congenita patient may hurt another, so customizing the right medication for each individual patient is important.
Periodic paralysis is a rare genetic disorder that is characterized by temporary attacks of extreme muscle weakness or paralysis. These attacks can be triggered by a variety of things, with the most common triggers being resting after exercise, cold temperatures, and having too much or too little potassium. There are several types of periodic paralysis. Understanding the different types can be difficult, but we are here to give you guidance.
Periodic paralysis is caused by muscle cells that do not have the right balance of chemicals to work properly. Muscle cells have openings called channels that open and close to let particles such as sodium, potassium, and calcium go in and out. These particles are called ions, and muscle cells can only work if the right kinds of ions enter and exit the cells at the right times. When the channels don’t open and close the way they’re supposed to, the ions become imbalanced, and muscles aren’t able to function normally.
Paramyotonia congenita (PMC) is a type of periodic paralysis that causes people to have attacks of muscle stiffness (myotonia) when they are active, such as when exercising. Normally, myotonia happens after exercising, not during it. However, if someone has paramyotonia congenita, they will experience myotonia while they are exercising. Because that is the opposite of how myotonia normally works, it’s like a paradox, which is where the “para” in “paramyotonia” comes from. “Congenita” means that this is a condition someone is born with.
People who have paramyotonia congenita suffer from attacks of myotonia, which is a stiff, rigid feeling of the muscles. Attacks can last for days, or even weeks. The way all muscles move is by contracting, but with myotonia, muscles don’t quickly relax after contracting. This results in muscles that contract too much for too long, making them feel stiff. This is caused by a mutation in sodium channels in muscle cell membranes. Myotonia can cause some patients to develop large muscles that look like an athlete’s, but those large muscles are surprisingly weak.
Myotonia presents itself in various ways. For example, it can make it hard for a patient to let go of a doorknob after they have squeezed their fingers around the doorknob to turn it. Another example of myotonia is difficulty making facial expressions because the facial muscles are stiff. Mild myotonia attacks may go away by “working it off,” or repeatedly making small movements of the stiff muscles. But some myotonia attacks can be severe — chest muscles may feel stiff, causing breathing difficulties. Myotonia can sometimes be painful, but other times, it occurs without pain.
There is a type of paramyotonia congenita called pure paramyotonia. This condition causes patients to have myotonia attacks and muscle weakness attacks. People who have pure paramyotonia may have attacks of myotonia and weakness at the same time, or myotonia only, or weakness only. In some cases, one type of attack comes right after the other. The upper body is most likely to be affected, including the face and neck. It is possible to have both pure paramyotonia and hyperkalemic periodic paralysis.
There is another type of paramyotonia congenita known as paramyotonia congenita von Eulenburg. Patients with this condition have myotonia and weakness, and the weakness is often triggered by low blood potassium levels (hypokalemia). It’s possible to have both paramyotonia congenita von Eulenburg and hyperkalemic periodic paralysis, which means that muscle weakness can be triggered by blood potassium being too low or too high. Some hyperkalemic patients experience muscle weakness attacks triggered by high potassium, and then their potassium levels overcorrect, falling too low. The low potassium may trigger a myotonia attack.
A major trigger for muscle stiffness attacks in paramyotonia congenita is overexertion. This could be through intense exercise or other types of exhausting activities such as moving heavy furniture around. The “para” in paramyotonia refers to the paradox of myotonia occurring during exertion instead of after it, like in other types of myotonia. In other words, too much strenuous movement makes the myotonia attack worse.
Cold is another major trigger for muscle stiffness attacks. It is difficult for people with this disorder to live in areas with a cold climate. However, it doesn’t take extremely cold temperatures to cause an attack. Even in a warm climate, a myotonia attack can happen if the patient sits near an air conditioner or drinks an ice-cold beverage. Cold sensitivity is usually a major problem for people with paramyotonia congenita.
Low potassium can trigger muscle stiffness in patients with paramyotonia congenita von Eulenburg. Also, patients with any type of paramyotonia congenita may have hyperkalemic periodic paralysis, too. That disorder will cause them to have weakness attacks that are triggered by high potassium levels, so foods high in potassium should be avoided. There are other triggers for hyperkalemic periodic paralysis as well.
If you have paramyotonia congenita, keeping a trigger diary is highly recommended in order to figure out what triggers your attacks. After you have an attack, you should record in your trigger diary the time of the attack, which body parts were affected, whether you had weakness or stiffness, what you were eating or doing before the attack, how long the attack lasted, and what you did to recover from it. The diary will be very useful as time goes on because you’ll be able to read through it and discover your triggers.
When a person has paramyotonia congenita, they usually experience symptoms beginning in infancy or childhood. Symptoms do not get worse as the person ages. However, what makes things so complicated is that receiving a proper diagnosis is difficult. Doctors must determine which type of paramyotonia the patient has, and they must determine whether or not the patient also has a periodic paralysis disorder such as hyperkalemic periodic paralysis.
Many doctors don’t know anything about periodic paralysis or paramyotonia congenita, so they may give patients an incorrect diagnosis of another more commonly known disorder. Patients may go years without the correct diagnosis, and during that time, they will not receive the right kind of treatment. Having the correct diagnosis is crucial.
Giving doctors your family’s health history and your trigger diary can help them come closer to a paramyotonia congenita diagnosis, and there are other diagnostic tools available, too. Paramyotonia congenita is a genetic disorder, and commercial lab genetic tests can be performed that can detect the disorder in some patients. However, genetic testing is not perfect and does not always detect all cases. A muscle biopsy is sometimes needed for doctors to diagnose periodic paralysis. This means that the doctor must remove and examine a small piece of the patient’s muscle tissue during or after an attack to see if the muscle tissue is abnormal. Another diagnosis method is the Compound Muscle Action Potential test, which measures the electrical activity of muscles. This test can’t always detect periodic paralysis, but it can do so in a lot of cases.
The way paramyotonia congenita is treated depends on whether or not the patient only has myotonia or they have both myotonia and muscle weakness attacks. Regarding myotonia attacks, mexiletine and lamotrigine are two medications that have worked to reduce this symptom in some patients. This is because they block the mutated sodium channels in muscle cell membranes.
The diuretic acetazolamide may be effective in reducing the likelihood of muscle weakness attacks, but in some patients, acetazolamide makes weakness attacks worse, not better. Thiazide diuretics have also been effective for some patients. The medication KEVEYIS (dichlorphenamide), which is the only FDA-approved treatment for periodic paralysis, has been shown to reduce muscle weakness attacks.
There are some types of medications that people with paramyotonia congenita should avoid: Beta2 agonists (such as fenoterol and ritodrine), monocarboxylic amino acids, and depolarizing muscle relaxants (such as suxamethonium/ succinylcholine/ diacetylcholine), which are often used in anesthesia.
Aside from medication, patients should be careful with their daily activities. They should try to stay out of cold environments and avoid overexerting themselves during exercise. For hyperkalemic patients, avoiding potassium-rich foods is necessary, and managing stress is important as well because stress can trigger weakness attacks. Ironically, some people who have both hyperkalemic periodic paralysis and paramyotonia congenita can actually benefit from taking potassium supplements. This method should be supervised by a doctor, but it is possible to take just the right amount of potassium that is enough to help prevent myotonia attacks without causing a hyperkalemic periodic paralysis attack.
Your Next Steps
If you think you may have paramyotonia congenita, please talk with your doctor. Speaking with a doctor about a rare disorder can be intimidating, and many doctors have little to no prior knowledge about periodic paralysis. The PPA has many resources to help you become a better advocate for yourself. Click here to learn about other types of periodic paralysis. You can also submit a question to our Ask the Experts panel. Remember, we are here to support you all the way as you take action to advocate for your health!
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.
At the 2009 PPA Conference, some patients with suspected HyperKPP/PMC complained of muscle pain during and after attacks.
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.
Mexiletine is very effective in preventing and reducing the degree of cold-induced stiffness and weakness. Some paramyotonia congenita patients have taken it for decades and still use it. Others take a single dose 2-3h prior to physical exercise in cold environment. The heart must be checked before administration.
Mexiletine is much more effective in sodium channel myotonia and paramyotonia than in chloride channel myotonia. The reason why is that it directly blocks the otherwise non-inactivating mutant sodium channels. Its use-dependent effect is of particular advantage as the non-inactivating channels high-frequently reopen.