New hope for an inherited form of heart disease
The term "heart disease" often refers to plaque-filled heart arteries that can lead to a heart attack. More accurately described as coronary artery disease, it's by far the most common and best known type of heart disease. Most people are far less familiar with the most common inherited form of heart disease, hypertrophic cardiomyopathy (HCM), which affects the heart muscle rather than its arteries.
HCM is thought to affect one in 500 people. It's far more common than better known diseases such as multiple sclerosis and cystic fibrosis. But HCM can be tricky to diagnose. "In the clinic, we suspect hypertrophic cardiomyopathy in people who have unexplained thickening of the heart muscle seen on an echocardiogram," says Harvard Medical School professor Dr. Christine Seidman. Other conditions, such as longstanding high blood pressure, can also cause the heart to enlarge. Because high blood pressure is so common, it's sometimes mistakenly blamed for the signs of HCM in older people, she says. In younger people, HCM is sometimes misdiagnosed as exercise-induced asthma or anxiety.
Mutations in heart muscle
HCM is passed from one generation to the next by way of dominant-acting mutations in genes that govern the structure of the heart muscle. That means that parents, siblings, and children of an affected person have a 50% chance of having inherited the same mutation. More than a dozen different genes are known to cause HCM, but just two are responsible for more than 80% of the cases. All the genes are involved in making properly functioning sarcomeres, the basic units of a muscle. The mutated genes cause muscle tissue to thicken (see illustration).
"The mutations actually tend to make the heart contract more effectively, which is unusual, as we rarely think of hearts as being sick' when they work better than normal," says Dr. Seidman. The problem is that the muscle fails to relax normally. That means the blood vessels feeding the heart can't effectively deliver oxygen and nutrients to the muscle, especially during high-intensity exercise. And when the heart's main pumping chamber (left ventricle) can't relax, it's like trying to fill up and squeeze a stiff bucket instead of a flexible balloon, she explains.
Diagnosis and treatment
In fact, before the first HCM genes were discovered in the 1990s, HCM was often recognized only after a person died from cardiac arrest. Genetic testing can now help people discover more about their prognosis and identify family members who may be at risk of developing the condition.
Many different treatments can address HCM symptoms and lower the risk of serious complications. These include medications, procedures to destroy excess heart tissue, and implanted devices such as pacemakers and defibrillators to correct abnormal heart rhythms.
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