Nervous tics in the heart.

Nerve growth run amok may explain why, after surviving a heart attack, some people begin to suffer from irregular heartbeats and may die suddenly when their heart stops beating.

Scientists have known that heart attacks damage both the heart muscle and the nerves that infiltrate the muscle. The electrical signals that trigger the heart to beat are impaired when sent through damaged muscle.

In some people, the disparity between electrical signaling in normal and damaged tissue may be exacerbated when nerves grow back more densely than before, says Peng-Sheng Chen of the Cedars-Sinai Medical Center in Los Angeles. Chen initially looked at the effect of nerve growth on electrical function in the heart because his wife, a neurologist, pointed out that epilepsy can be triggered by abnormal nerve growth. Epileptic seizures are caused by bursts of irregular nerve activity in the brain.

Chen and his colleagues looked at tissue samples from the dysfunctional hearts removed from 53 people who received heart transplants and 12 hearts studied after autopsy. Some in each group had had irregular heartbeats. Nerve density was significantly higher in the 30 hearts from people who had experienced abnormal rhythms, Chen reports in the April 25 CIRCULATION.

The study doesn't prove that the dense nerve growth caused the abnormal heartbeats, says Chen. His team induced heart attacks in 15 dogs and gave 9 of them injections of a hormone that stimulates nerve-cell growth. Four of the treated dogs--but none of the others--died suddenly of rapid but ineffective contractions of the heart, he says in the April 14 CIRCULATION RESEARCH.

"The results of these studies may explain the efficacy of beta-blockers, drugs that block the activity of sympathetic nerves, in the prevention of sudden cardiac death," Chen says.