The nature of shaken baby syndrome injuries and the significance of a “Lucid Interval”

SummaryBackgroundThe possibility of shaking an infant sufficiently violently to cause brain damage by head rotational acceleration remains controversial and the nature of the anticipated injuries poorly defined. Animal studies of whiplash injuries have revealed two modes of injury; subdural haemorrhage (SDH) and cortex surface injury (CSI). CSI can be induced without SDH with a suitable whiplash impulse, suggesting independent mechanisms. Experimental whiplash injury has been found to bring an immediate cessation of cerebral function (coma) with a threshold approximating to peak velocity (applied acceleration × applied time).Tangential forces producing brain acceleration are transferred across the subarachnoid space by trabeculae. Their total cross-sectional area is much less than that of the arachnoid and pia surfaces, producing high stress concentrations where trabeculae bond with the pia mater.HypothesisThat CSI occurs within the pia–glia formation when astrocytes linking the pia below trabeculae to the cortex fail under the high local stress concentrations. The pia may be locally pulled off the neural tissue, dragging neural tissue with it. Axons will be stretched or broken, thereby immediately disrupting interneuron communications (coma).ConclusionIn contrast to such immediate mechanical injuries, chemical (e.g. hypoxic) insults take time to develop, described as a “lucid interval”. Crying requires recognition of a noxious stimulus, and coordination of respiratory and vocal muscles, showing that the infant is not in a coma but in a lucid interval. Therefore, when crying can be heard on 999 or 911 tape recordings the initial event could not have been shaking.