[Fear paralysis--still a possible cause of crib death]

There is overwhelming evidence that prone sleeping entails more risk of sudden infant death than supine sleeping does. However, no generally accepted biological explanation of this phenomenon has been found. In a viewpoint article in Acta Paediatrica in 1994 the author has re-emphasized that the fear paralysis reflex (tonic immobility) could be a cause of sudden infant death. The response consists of immobility, apnea, profound bradycardia and vasoconstriction. The bradycardia may proceed to irreversible asystole and silent death within a few minutes. In infant monkeys, this innate, atavistic reflex, which is prompted by aversive environmental fear-producing events, has the same age distribution as sudden infant death syndrome between the 2nd and 5th month of life. Main triggering stimuli are: restraint of movement (preventing flight) and sudden exposure to unfamiliar environments and persons, events for which the organism appears to be unprepared. Movements of the extremities are more restrained in the prone than in the supine position with greater chance of eliciting fear paralysis responses and death in prone sleepers. This biological mechanism presumably also applies to the winter peak (restraint due to overwrapping), sleeping outdoors (overwrapping) and co-sleeping in the same bed as another (parent).