Which neuromuscular disorders affect the heart?
The heart is the muscle controlled by the nervous system, so it is not surprising that in many neuromuscular diseases there are disorders in it. Heart disease has been observed in the majority of neuromuscular diseases, but in some cases it is much more common.
Most muscular dystrophies affect the heart, but one type of heart disease is particularly common in Duchenne and Becker dystrophy, while other types of dysfunction are common in myotonic dystrophy and Emery-Dreyfuss dystrophy.
Complications with the heart are not often found in limb-rim dystrophy, face and shoulder and scleral dystrophy, although they may occur in these types of diseases.
Diseases that mainly affect the nervous system or the neuromuscular interface are not likely to affect the heart. Cardiac disorders are therefore rare in spinal muscular atrophy, amyotrophic lateral sclerosis (ALS) and in myasthenia gravis. The exception is Friedrich’s ataxia, a disease that primarily affects the peripheral nerves, in which heart disease is frequent and can be serious.
Complications with the heart also occur in other types of neuromuscular disorders, such as muscle disorders on the metabolic background and are sometimes found in polymyositis and dermatomyositis. What exactly happens with the heart in neuromuscular disorders?
Most heart diseases that are read in the newspapers or heard on television are coronary heart disease. This disease is the result of a diet rich in fats and calories, overweight, sedentary lifestyle and smoking (so-called high-risk factors).
People with neuromuscular disorders are in no way immune to coronary heart disease and, unfortunately, to any other disease in modern society. However, coronary heart disease is not a problem too often associated with neuromuscular disorders.
There are two types of heart disease associated with neuromuscular disorders. These are cardiomyopathy and cardiac arrhythmia.
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Cardiomyopathy – the problem with myocardium is most often found in Duchenne and Becker dystrophy. Arrhythmia – arrhythmia – is more commonly found in Emery-Dreyfuss’s myotonia and dystrophy.
Both cardiomyopathy and arrhythmia are often found in metabolic muscle diseases. In Friedrich’s ataxia, both kinds of problems are commonplace.
Cardiomyopathy and arrhythmia are common disorders in people (although not as common as coronary heart disease) and cardiologists know them well. If the reason is muscular disease, they do not require different treatment. However, the cardiologist must take into account the following factors: the ability of the patient to exercise, his respiratory efficiency and the possible effect of cardiac drugs on other muscles (especially respiratory).
What exactly causes cardiomyopathy in muscular dystrophy?
The primary cause of cardiomyopathy in Duchenne and Becker dystrophy is probably the lack of dystrophin protein – the same protein that occurs in skeletal muscle and the lack of which leads to general weakness and respiratory problems. Dystrophin is also needed for cardiac muscle.
The heart muscle differs from skeletal muscle. The cardiac muscle has a slightly different structure and its proteins often have a slightly different form from their cousins in skeletal muscles. These slightly different proteins, known as isoforms, are derived from the same gene. (Each gene is a recipe for protein.) Some genetic mutations, such as in Duchenne, Becker and limb-rim dystrophy, probably prefer the skeletal or cardiac form of the protein, leading to minor or major heart problems.
Protein dystrophin deficiency is probably not the most important factor in heart dysfunction in neuromuscular diseases. For example, the flow of blood through tissues and the return of blood to the heart are difficult for a person who can not exercise. Thus, the degree of fitness to exercise can be one of the factors affecting the condition of the heart.
How are heart diseases detected and monitored?
Currently, there are many different ways to detect and monitor heart disease. The most popular is the electrocardiogram (ECG). Electrodes placed on the chest and on the limbs measure the current that flows through the heart. The ECG measures the electrical activity of the heart, not the functioning of its muscle, so it does not detect cardiomyopathy. ECG sometimes shows abnormalities in the functioning of the heart muscle, and sometimes not.
A little more complicated, but more accurate, method of diagnosing cardiomyopathy is an echocardiogram, an ultrasound image of a beating heart. The echocardiogram can be recorded on a cassette so that later the cardiologist can see the changes that have taken place between subsequent tests.
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Cardiomyopathy can occur, but it can be hidden, completely without any symptoms, in people who do not exercise or do not exercise at all. Many physicians now recommend frequent ECGs, and some recommend relatively regular, eg once a year, echocardiograms in patients at risk for this disease, even if there are no symptoms of cardiomyopathy.
The symptoms you need to pay attention to while exercising are fatigue and shortness of breath. In people with neuromuscular diseases, fatigue and shortness of breath may be caused by both weakened respiratory muscles and heart problems. Then the physician should determine the cause of the ailment.