The "lost autoregulation hypothesis" and brain lesions in the newborn--an update.

Autoregulation of cerebral blood flow is an essential homeostatic mechanism which is easily disturbed during the stresses in the birth process. In its absence, moderate hypotension, a frequent complication in neonatal asphyxia, may induce cerebral ischemia, the arterial watershed zones in periventricular regions being particularly prone. The initial ischemia in perinatal stress produces functional disturbance (EEG depression), which is not readily reversible as the circulation improves. Hypotension, cerebral hypoperfusion and EEG depression precedes severe periventricular hemorrhage, which seems to be triggered by fluctuations in arterial blood pressure and flow, with fluctuations in the transmural pressure gradient across the capillary wall. The penetration of the hemorrhaging into surrounding brain tissue is related to the magnitude of the preceding ischemic insult.