Vasovagal syncope in humans and protective reactions in animals

Vasovagal syncope (VVS) is not known to occur in animals, although other similar reflex responses are common. This review examines the possible relation of these latter presumably protective reflexes in animals to VVS in humans. The goal is to provide practitioners, and ultimately their patients, a meaningful understanding of the origins and appropriate management of this unpredictable affliction. This report utilized review of computer databases (e.g. PubMed) addressing VVS pathophysiology and origins, spontaneous transient loss of consciousness in animals, and comparative physiology. We also examined articles cited in the publications obtained by computer search and others suggested by colleagues. Articles were chosen based on those providing original observations and/or suggestions of novel mechanisms. In animals self-preservation is directed towards protection of the body through an escalation of behaviours depending on severity and proximity to danger. In humans self-preservation is directed not only to protection of the body, but also to protection of the brain's functional integrity. By virtue of loss of postural tone, the faint causes the body to assume a gravitationally neutral position, thereby offering a better chance of restoring brain blood supply and preserving brain function. Vasovagal syncope may seem to be a disadvantageous evolutionary adaptation. However, it is a reversible condition, that while exposing risk of injury and embarrassment, ultimately favours brain self-preservation in potentially threatening circumstances.