PAVIA, ITALY--A report of a male infant found cyanotic, apneic, and pulseless by his parents provides compelling evidence of a link between the long-QT syndrome and sudden infant death syndrome (SIDS).[1]

The infant, 44 days old at the time of his SIDS-like episode, had ventricular fibrillation, recurrent torsade de pointes often degenerating into ventricular fibrillation, and a prolonged QT interval of 648 ms. He was later found to have a mutation on SCN5A, the cardiac sodium-channel gene responsible for the LQT3 subtype of long-QT syndrome.[2] The mutation was believed to be spontaneous because neither of his parents had it and both the mother and father had normal QT intervals (380 and 425 ms, respectively).

"This case has all the classic features of near-SIDS," stated the report's authors. Indeed, the infant appeared to be completely healthy before the episode and was within the age range of peak SIDS incidence (5 to 12 weeks). The infant--who appeared lifeless when his parents discovered him--was admitted to the emergency department, and ventricular fibrillation was documented.

Fortunately, the infant's caregivers were able to restore him to normal sinus rhythm with prompt defibrillation and mechanical ventilation. A prolonged QT interval was found, and the infant was treated with 4 mg/kg of propranolol and 10 mg/kg of mexiletine. He experienced no additional arrhythmias.

At age 3 years, the child's QT interval was still prolonged, though not as severely (510 ms). "At nearly 5 years of age, the child remains free of symptoms, and there have been no neurologic sequelae," the authors reported.

Had the infant died, he likely would have received a SIDS diagnosis, they suggested. Normal QT intervals in the parents and the loss of any chance to confirm QT prolongation in the infant would have eliminated the suspicion of the long-QT syndrome.

QT INTERVAL SCREENING

The infant's SCN5A mutation precipitated the long-QT syndrome by causing the cardiac sodium channel to fail to close almost completely during the plateau of the ventricular action potential. "That, of course, will prevent normal ventricular repolarization and delay it severely, causing prolongation of the action potential and therefore of the QT interval," Robert Dumaine, PhD, one of the report's authors, told PULMONARY REVIEWS.

This case suggests that there may be other infants in whom the long-QT syndrome goes unrecognized. Although the long-QT syndrome is not an element in every SIDS episode, Peter J. Schwartz, MD, another of the study authors, has estimated that it could be a factor in up to 35% of these cases.

Long-QT syndrome is easily treated by medication, noted Dr. Dumaine, a research scientist in the Department of Molecular Biology at the Masonic Medical Research Laboratory in Utica, NY. Thus, he said, routine electrocardiographic screening to identify this disease could save many infants' lives.

"Such screening would not be very expensive and could be done two or three weeks after birth, which is usually when babies are brought back to see their pediatrician, anyway," Dr. Dumaine reasoned. However, he acknowledged that "it may be hindered by socioeconomic and medico-legal issues."

--Timothy Begany

References
1. Schwartz PJ, Priori SG, Dumaine R, et al. A molecular link between the sudden infant death syndrome and the long-QT syndrome. N Engl J Med. 2000;343:262-267.
2. Priori SG, Barhanin J, Hauer RN, et al. Genetic and molecular basis of cardiac arrhythmias: impact on clinical management part III. Circulation. 1999;99:674-681.

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