The Heart Is a Muscle, Too: Part One

Frequently asked questions about cardiac problems in neuromuscular disease

by Margaret Wahl on April 1, 1999 - 5:00pm

Cardiac problems are common in several neuromuscular disorders. They can be quite serious, particularly in Duchenne and Becker muscular dystrophy (DMD and BMD). In this, the first of a two-part series, we'll explore cardiomyopathy, the type of heart problem that's found most often in DMD and BMD and also occurs in some other neuromuscular conditions. (Read Part 2 of this series.)


[heart]
The heart has an electrical system and a mechanical (muscular) system. When electrical impulses travel from A to B and around the bottom of the heart via C, they cause heart muscle cells to contract rhythmically. When something goes wrong with these electrical impulses, the result is an arrhythmia, an irregularity in the heart's pacing system. when something goes wrong with the muscle layer of the heart, the result is a cardiomyopathy, a problem with the heart's mechanical pump.

Q: Which neuromuscular diseases involve the heart?

A: The heart is a muscle that responds to the nervous system and contains special nervelike tissue, so it isn't surprising that it's affected in many neuromuscular disorders. What actually is surprising is that there are some disorders in which the heart isn't affected, and that there's a lot of variation in the type and degree of cardiac involvement in the same disease.

Heart problems have been reported in nearly all the neuromuscular disorders, but they're much more common and severe in some of these than in others.

Most muscular dystrophies can involve the heart, but heart problems of one type are particularly common and serious in Duchenne and Becker MD, while problems of another type are common and serious in myotonic (MMD) and Emery-Dreifuss (EDMD) MD.

Heart complications are not common in limb-girdle MD (LGMD), facioscapulohumeral MD or congenital MD, although there are reports of them in all of these conditions in some patients.

Diseases that primarily involve the nervous system or the neuromuscular junction aren't likely to involve the heart. Heart complications are rare, for example, in spinal muscular atrophy and amyotrophic lateral sclerosis and in myasthenias.

However, Friedreich's ataxia (FA), a disease that mostly affects peripheral nerves, is a notable exception, where heart problems are common and can be serious.

Heart complications occur in various other types of neuromuscular disorders, such as the metabolic muscle disorders, and they're sometimes seen in inflammatory myopathies (polymyositis and dermatomyositis). They've been reported in central core disease and occasionally in periodic paralysis.

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Q: What actually happens to the heart in neuromuscular disorders? Are the problems the same as heart problems in general?

A: Most of the "heart disease" we hear about in the newspaper or on television is a type of heart problem known as coronary artery disease, a "plugging" of the blood vessels that supply the heart. This is the kind of problem that appears to result from a diet rich in fat and calories, being overweight, being sedentary and smoking (although it's possible to have coronary artery disease in the absence of these risk factors). It's the type of heart disease most often associated with "heart attack," a total blocking of one or more of these vessels.

People with neuromuscular disorders are in no way immune to coronary artery disease or, unfortunately, to any other common ailment of modern life. However, coronary artery disease isn't the problem most often associated with neuromuscular disorders.

Two types of heart problems are closely associated with various neuromuscular disorders. They are cardiomyopathy, a problem with the muscle layer of the heart, and cardiac arrhythmias, abnormalities in the electrical pacing system of the heart. (It may sound strange to talk about the heart having an electrical system, but it does. Electricity is a form of energy that results from the interaction of charged particles. It's the same energy, whether in a biological or man-made system.)

Cardiomyopathy, the heart muscle problem, is the kind of abnormality most often seen in DMD and BMD, while arrhythmias, the pacing system problems, are the ones more commonly seen in myotonic and Emery-Dreifuss MD. When there are heart complications in other forms of MD, the main problem is usually cardiomyopathy, although arrhythmias can also occur. Both cardiomyopathy and arrhythmias occur often in metabolic muscle disorders. In FRDA, both types of problem are also common.

Cardiomyopathy and cardiac arrhythmias are common disorders in the general population (though not as common as coronary artery disease), and cardiologists are familiar with them. Their diagnosis and treatment aren't markedly different when the underlying cause is a neuromuscular disease. However, the patient's ability to exercise, his respiratory status and the possible effects of cardiac drugs on other muscles (particularly the respiratory ones) must be taken into account by the cardiologist when there is a neuromuscular disorder in addition to the cardiac problem.

In this article, we'll concentrate on cardiomyopathy, the main problem in DMD and BMD. Its treatment is quite different from the treatment for arrhythmias, which we'll consider in part 2.

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Q: What actually causes cardiomyopathy in MD?

A: The primary cause of cardiomyopathy in DMD and BMD is probably a lack of the protein dystrophin, the same protein deficiency that exists in the skeletal muscles and leads to generalized weakness, wasting and respiratory complications. Dystrophin is also needed by cardiac muscle, and its lack (complete in DMD and partial in BMD) probably leads to the loss of cardiac muscle cells under the stress of constant contraction.

When heart problems do occur in other forms of MD, such as LGMD or congenital, it's likely that the underlying cause is also a lack of a muscle protein. In four forms of LGMD, for example, the sarcoglycan proteins are known to lie near dystrophin in the muscle cell membrane, and their absence could be a factor in any heart problem in those disorders.

But the story is a little more complicated. For instance, most people with DMD and BMD have heart involvement, but some don't. And, in the sarcoglycan-deficient forms of LGMD, heart involvement is unusual, despite the probable role of the sarcoglycans in heart muscle.

The answers to these puzzles may lie in the heart muscle's differences from skeletal muscle. Heart muscle has a slightly different structure, and its proteins often come in a slightly different form from their "cousins" in skeletal muscle. These slightly differing proteins, known as isoforms, come from the same gene. (Each gene is a "recipe" for a protein.) Some genetic mutations, such as those in Duchenne, Becker and limb-girdle MD, probably have a preference for the skeletal or cardiac form of the protein, leading to more or less heart involvement.

Then, too, protein deficiencies are probably not the whole story in cardiac dysfunction in neuromuscular disease. For example, blood flow through the tissues and the return of blood to the heart are compromised in a person who can't exercise, so varying exercise capabilities may also be a factor in the degree of cardiac involvement.

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Q: What happens to someone with cardiomyopathy?

A: Cardiomyopathy (something wrong with the muscle layer, or myocardium, of the heart), usually takes one of two forms.

[heart diagram - cardiomyopathy]

In one form, a portion of the heart muscle enlarges, a condition known as hypertrophic cardiomyopathy. As the heart muscle wall enlarges and overgrows, the heart itself may be slightly enlarged, but the main pumping chambers — the ventricles — have less room to fill with blood because of the thickened muscle layer. If the thickened part includes the divider between the ventricles (septum), ejection of blood from the heart to the rest of the body can be impaired.

In the second form, dilated cardiomyopathy, the myocardium dilates and expands. The ventricles thin out and become floppy. The heart is enlarged, sometimes markedly, but its function is compromised by the abnormal, thinned muscle layer.

In some people, hypertrophic cardiomyopathy becomes dilated cardiomyopathy after a time.

In either type of cardiomyopathy, if the problem is bad enough, a condition known as heart failure (also sometimes called congestive heart failure) is the end result. Heart failure can develop quickly, but more often, it develops gradually, over several years.

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Q: What is heart failure?

A: The term heart failure sounds scary, and it is in fact a very serious disorder. However, heart failure doesn't mean that the heart stops. It means that the heart can no longer meet the demands of the body's tissues for blood and its nutrients — mainly, oxygen. Heart failure can be treated and its progress slowed with medications and other approaches. In some people with severe heart failure, however, only a heart transplant can permit survival.

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Q: Are there degrees of heart failure?

A: Yes. Heart failure can be mild, moderate or severe, and can change over time. There are many different causes of heart failure, muscular dystrophy being only one of many, but the results and the progression are generally the same.

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Q: What does a person experience as the heart 'fails'?

A: The heart has enormous reserves of energy, so that, even under adverse conditions, such as a weakening muscle structure, it can usually adapt and keep up an adequate blood supply to the body for some time, often years. By the time symptoms start to show up, usually quite a bit of damage has already occurred.

What happens in heart failure

In a person who's not exercising, such as an older boy with DMD, there may be no symptoms that would indicate heart failure until the problem is quite far advanced. The heart problems can, however, be detected with diagnostic tests.

In someone who's able to exercise, symptoms of heart failure are likely to occur as he exerts himself. He's likely to feel short of breath and fatigued with exertion. (An often used medical term for this shortness of breath is dyspnea.) These symptoms arise because, when the blood's pump begins to fail, the body's tissues don't receive enough blood to support the demands of exercise, particularly with respect to oxygen.

Later, as the heart fails further, dyspnea will occur even at rest, because the blood supply to the tissues becomes inadequate even under these circumstances. Usually, heart failure is quite advanced by the time "dyspnea at rest" is detected.

As heart failure progresses, blood "backs up" behind the failing ventricle, just as fluid backs up behind any pump that's not moving it forward (see illustration). Usually, the left ventricle fails first. Blood normally returns to the left ventricle after picking up oxygen and dropping off carbon dioxide in the lungs. If the left ventricle isn't pumping normally, blood begins to accumulate in the large veins that lead back from the lungs to the heart (pulmonary veins).

Blood pressure rises in the pulmonary blood vessels, and fluid (mostly water) starts to leak out across the small capillaries that supply the lung tissue.

Fluid begins to build up in the air sacs of the lung, a serious condition known as pulmonary edema. Breathing can become difficult, especially when the person lies down. He may develop a cough, sometimes tinged with slightly bloody (pink) sputum.

The right ventricle can also fail, especially if the left ventricle is already weakened, causing more stress on the right side.

Blood returning to the right ventricle comes from the general circulation via two major veins that return blood to the right side of the heart. If the right ventricle is too weak to pump the blood forward into the lungs, or if pressures in the pulmonary vessels are too high for it to push against, blood will start to back up as it enters the right side of the heart. As this blood accumulates, it raises blood pressure in the general circulation. Eventually, pressures build in the capillaries of the general circulation, and fluid starts to leak out across the capillary walls into the tissues.

The person with right heart failure starts to show edema (swelling, or fluid retention) in the parts of the body that are most affected by gravity. So, if he's standing or sitting most of the time, the ankles and feet will show the most edema. The first inkling of the problem may be that the shoes or socks are too tight. If he's lying down, the area around the lower spine may be most affected.

As right heart failure progresses, generalized edema may occur, including fluid retention in the abdominal organs, with abdominal distention and possibly nausea and vomiting.

The person with generalized edema gains weight and finds his clothes and rings are too tight.

At night, if he tries to lie flat, fluid from the periphery of the body will move by gravity to the lungs, compounding any pulmonary edema that's already there and causing an urgent sensation of needing to sit upright.

Fortunately, all these symptoms can be controlled, at least for a long time, with medications and other treatments.

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Q: Why does heart failure get worse?

A: Heart failure almost always worsens over time, partly because of progressive deterioration in the myocardium (particularly true in muscular dystrophy), but also, ironically, because of the body's misguided attempts to remedy the situation.

As the myocardium weakens and blood flow to the tissues decreases, the body's strategically placed pressure sensors send out urgent signals to the nervous system and kidneys indicating that "pressure is down." The nervous system sends out chemicals that constrict blood vessels, raise blood pressure and make the heart beat harder and faster. At the same time, the kidneys hold onto sodium and water to increase the amount of fluid in the blood.

If the reason for the pressure drop sensed by the body were a "leak" — say, from a stab wound — it's easy to see that all these measures would probably help. In a person who's hemorrhaging, raising pressure, increasing the heart's rate and force, and holding onto as much fluid as possible would help preserve blood flow to vital organs.

Unfortunately, if a failing pump is the reason for a pressure drop, then all these measures are actually harmful. They only worsen matters by placing more stress on the already weakened myocardium, which now has to work against higher resistance and a higher fluid load.

So far, medicine has little to offer that directly repairs the failing myocardium. However, there are many treatments that reduce the strain on the heart and preserve its function for a long time. Most of them are aimed at counteracting the body's misguided response to cardiomyopathy and heart failure.

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Q: How can heart problems be detected and monitored?

A: There are many good tests in use today for detecting and following the progress of cardiac problems. The familiar EKG — the electrocardiogram — is perhaps the most often used. In this test, surface electrodes placed on the chest and limbs indirectly measure the current that flows through the heart. But the power of the EKG to reveal cardiomyopathies isn't very great. An EKG measures electrical activity of the heart, not its muscle function. Abnormalities in muscle function are sometimes reflected in the EKG and sometimes not.

A somewhat more elaborate but much more accurate test for cardiomyopathy is the echocardiogram, an ultrasound picture of the heart in action that resembles the kind of imaging study often done in pregnant women to check on the fetus. The technician can videotape the echocardiogram so the cardiologist can compare changes between tests.

More invasive tests, such as cardiac catheterization studies, can be done under specific circumstances, if the situation warrants them. Cardiac catheterization studies involve putting probes into the cardiac blood vessels to directly measure pressures in different parts of the circulation.

Cardiomyopathy can be "silent," completely without symptoms, in someone who's not exercising or exercising minimally. Many doctors today recommend frequent EKGs and some recommend frequent (for example, yearly) echocardiograms for patients at risk for cardiomyopathy, even if there are no symptoms.

Symptoms to watch for in the exercising person are, early on, fatigue and shortness of breath. Later, fluid buildup in the lungs (in left heart failure) or feet (right heart failure) comes into the picture, with overall edema and cough following if treatment isn't sought.

In the person with neuromuscular disease, fatigue and shortness of breath are just as likely to come from weakened respiratory muscles as they are from weakened cardiac muscles. A doctor should determine the origin of the problem and refer you to the appropriate specialist.

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A: Today, doctors have a wide range of treatments to choose from. The precise combination of drug and nondrug therapies will depend on the symptoms, the underlying cause of the symptoms and the stage of heart failure being treated. In the past, much therapy relied on resting the patient (advising against exercise) and boosting the heart's activity with medication.

Nowadays, the thinking has nearly reversed. There's less emphasis on resting the whole patient and more on specifically allowing the heart to "rest," or at least to work with greater ease.

Mild, regular exercise is usually encouraged, if it's possible (which it may not be in some types of neuromuscular disease). And instead of giving drugs that increase the heart's contractile force, doctors are more likely to prescribe those that reduce the heart's work and allow it to rest.

Here are some common treatments.

Nondrug treatments

Low-sodium diet. If fluid overload is a problem, a diet low in sodium may be prescribed. The usual recommendation is a daily intake of 2 to 4 grams of sodium (2,000 to 4,000 milligrams). You'll have to read labels on prepared foods and avoid adding salt in cooking or at the table.

Mild exercise. Exercise isn't always possible in neuromuscular disease, but if it is, it's often recommended. Walking, swimming or using a stationary bicycle may be recommended by your doctor. If active exercise isn't possible, your doctor may suggest passive exercise in the form of physical therapy. Strenuous exercise is usually best avoided, and swimming shouldn't be done alone.

Assisted ventilation. When blood oxygen levels are low, the heart has to work harder to get the same amount of oxygen to the tissues. (This is why the heart works harder at high altitudes, where atmospheric oxygen is low and leads to low blood oxygen levels.) Low blood oxygen levels also lead to high blood pressure in the pulmonary circulation.

In many types of advanced neuromuscular disease, respiratory muscle weakness leads to low blood oxygen, placing an additional strain on the heart. Assisted ventilation — the use of mechanical devices to help failing respiratory muscles move air in and out of the lungs — can be a crucial intervention that can boost blood oxygen levels, ease breathing and help the heart work better. Ask your doctor about these devices.

Oxygen therapy. In some cases, the doctor may prescribe supplemental oxygen to ease the heart's workload. This has to be done with extreme care in patients with neuromuscular diseases in whom the respiratory system is impaired, because giving oxygen can complicate matters. Good communication between the pulmonary specialist and the cardiologist is important here.

Drug treatments

Easing the heart's burden. The drugs most often used today are those that reduce blood pressure (and therefore the resistance against which the heart has to pump), reduce the fluid content of the blood (reducing volume and pressure) and counteract those influences of the nervous system that raise blood pressure and increase the heart's rate and force.

Increasing force. Drugs that increase the heart's contractile force are also sometimes used, but with extreme care, as they can have dangerous effects.

Drug interactions and anesthesia pose risks. All these drugs can have their actions interfered with or occasionally increased by other medications, including many over-the-counter remedies for such things as colds and diarrhea, so the cardiologist should be kept aware of any other medications being used or considered. Anesthesia can also be a problem, because of potential chemical interactions and also because of the underlying heart problem itself.

Be sure your cardiologist and surgeon communicate with each other if any surgery is planned. You may wish to wear a MedicAlert tag or bracelet explaining your condition. These become especially important in case of accident or unplanned hospital admission. (MedicAlert's phone number is (800) 825-3785; the Web site is www.medicalert.org.)

The drugs listed below are among those often prescribed for heart failure. They can be used singly or in combination, and different drugs are selected depending on the patient's overall condition and stage of heart failure.

Medications Used for Heart Failure
Type of Medication How it works
Example(s); Generic (Brand Name)
ACE (Angiotensin-Converting Enzyme) Inhibitor Blocks actions of ACE, a molecule that normally leads to fluid retention and raised blood pressure through activating the chemical angiotensin fosinopril (Monopril); lisinopril (Prinivil, Zestril)
Vasodilator Dilates veins, reducing amount of blood returning to the heart as it relaxes between beats isosorbide dinitrate (ISDN)
Loop Diuretic Increases urine output by acting on kidneys (in part of kidney known as loop of Henle); reduces fluid overload furosemide (Lasix)
Potassium-Sparing Diuretic Acts on kidneys to increase urine production and reduce fluid overload; decrease excretion of potassium, counteracting excess potassium loss caused by some other diuretics spironolactone (Aldactone); triamterene (Dyrenium)
Beta Blocker Reduces heart rate, force of heart's contractions and speed of nerve impulses through heart by blocking beta receptors (docking sites) that receive signals from nervous system propranalol (Inderal); metoprolol (Lopressor, Toprol XL)
Alpha-Beta Blocker Same as beta blockers but also blocks alpha receptors (docking sites), stopping nerve signals that constrict blood vessels and raise blood pressure carvedilol (Coreg)
Inotrope (Influencer of Muscle Force) Increases force of muscle contractions of heart; high rate of complications, including abnormal heart rhythms digoxin (Lanoxin, Lanoxicaps)

Surgical treatments

Heart transplants. Heart transplants are by no means routine, but they're becoming increasingly common as a treatment for heart failure when other measures have failed. Transplants have saved the lives of several young men with Becker MD, in whom the skeletal muscle weakness (including respiratory weakness) was minor compared with the heart failure. Donor hearts are usually given to people who have a good overall prognosis.

Reduction surgery. There's a new procedure to reduce the size of floppy, dilated ventricles that's reported to be of some benefit in some types of heart failure. This could provide an alternative to transplant for some people with advanced cardiomyopathy.

Septal myotomy. In some people with severe hypertrophic cardiomyopathy, a surgeon can relieve some of the obstruction caused by the overgrown muscle layer by cutting the septum — the part of the muscle between the two ventricles. This procedure can partially relieve the constriction of the ventricles.


Consultants for this article were:

  • William Groh, a cardiologist specializing in electrophysiology of the heart at Indiana University, Indianapolis;
  • Stanley Goldberg, a pediatric cardiologist specializing in echocardiography at University Medical Center, Tucson, Ariz.;
  • and Kevin Campbell, a specialist in muscle membrane proteins at the University of Iowa, Iowa City.
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