Family ties heighten the risk of enlarged heart.

For years, heart researchers have speculated about the cause of cardiomyopathy, a mysterious and sometimes deadly disease of the heart muscle. Now, a study suggests that genetic heritage may play a stronger role inthe development of cardiomyopathy than previously believed.

In some cases, alcohol abuse or a viral infection damages the heart muscle so that the lower chambers, or ventricles, enlarge and fail to pump blood effectively. However, most cases of cardiomyopathy occur for no known reason.

Although researchers had dentified several large families with a tendency to the disease, they considered such "familial" cardiomyopathy uncommon. But when cardiologists at the Mayo Clinic in Rochester, Minn., noticed a number of families prone to the disease, they decided to take a closer look at the frequency of familial cardiomyopathy.

Mayo geneticist Virginia V. Michels and her colleagues tudied 59 unrelated individuals with cardiomyopathy of unknown origin, as well as 315 of their relatives. Using echocardiography, a sound-wave method of determining the heart's size, the researchers discovered that 18 relatives of 12 cardiomyopathy patients also suffered from the heart ailment.

Thus, the team concludes that 12 of the 59 cardiomyopathy patients (20 percent) had a familial form of the disease. This suggests that cardiomyopathy may run in families more frequently than believed, says James H. Chesebro, a coauthor of the study. The researchers describe their findings in the Jan. 9 NEW ENGLAND JOURNAL OF MEDICINE.

Of the 18 relatives identified, 15 first learned that they had cardiomyopathy during the study, Michels says, and eight of these had no idea they had any heart problem at all.

Healthy people with a family history of cardiomyopathy may run a risk of eventually developing the disease, the researchers note. The team identified 22 healthy relatives who had slightly enlarged ventricles. Although their hearts still pumped normally, Michels says she wonders whether the slight swelling of the ventricles represents an early sign of a diseased heart.

"It could mean nothing, or it could mean that these are people who have a genetic predisposition to developing full-blown disease," Michels says. Her team will monitor these 22 people to see whether they develop cardiomyopathy later in life.

Scientists still don't know how cardiomyopathy originates. Indeed, Michels says, the new findings don't rule out the possibility of other factors playing a role in the disease. The observation that cardiomyopathy runs in families doesn't necessarily mean the cause is genetic. Michels notes that family members often share the same environment, which may harbor some unidentified, heart-damaging factor -- a virus, perhaps, or a dietary deficiency (SN: 9/27/86, p.201).

However, she suspects that one or more genes play a hefty role in making some people vulnerable to the disease. The Mayo team is now searching for a specific genetic culprit. Identifying the defective gene or genes might enable researchers to pinpoint the biochemical defect causing the muscle damage, and perhaps to devise a treatment for preventing the heart's decline, Michels says.

The new evidence suggests that people with a family history of cardiomyopathy should ask their doctors about getting an echocardiogram, Chesebro adds. This painless test can reveal swollen ventricles and impaired pumping ability.

Shortness of breath, weakness and other symptoms of cardiomyopathy might seem harmless, yet the disease, if left untreated, can leave some people prone to a type of erratic heartbeat that can cause sudden death, Chesebro notes. With early diagnosis and treatment, doctors hope to prevent such lethal heartbeats and forestall further weakening of the body's powerhouse pump, he adds.