Injury by Blow
A blow inflicts brain injuries (Figs. 9.1, 9.5) mainly at the impact side (impact pole). If the skull remains intact, injury of the brain is due to local deformation (depression and recoil) of the bone. In very violent blows, the energy is transformed and delivered by deformation and fracture along a relatively long pathway. Acceleration is lower at the contralateral pole and the resulting injuries - mainly caused by negative pressure - are minimal or absent altogether. In contrast to falls, under these conditions, massive hemorrhages develop at the impact site and are limited not merely to the gyral crests, but can extend deep into the white matter.
Manually induced forceful impact of blunt instruments with small surface areas, but relatively low accelerated masses, generally do not create the critical negative pressure at the contralateral pole. Most of the energy is delivered locally to the impact site, where it causes contusion and/or laceration of the scalp and depressed fractures of the skull, sometimes even bursting fractures. At the impact pole, the injury is largely confined to a depressed skull fracture. The global effect on the skull, in the form of transient high acceleration, is too small to generate the necessary negative pressures at the contralateral pole to cause cavitation, even if the head is unsupported when it is struck.
The most common impact site is the parietal region. Most of the other sites of impact occur when the victim, who will be injured by a blow, is lying on the ground and his head is therefore supported. If the head is resting on a hard surface when struck, it cannot be accelerated, so contralateral cortical hemorrhages are uncommon under such circumstances.
Cortical hemorrhages located on the side of impact are a direct result of the forces delivered by the blow. Light force induces discrete cortical hemorrhages morphologically indistinguishable from con-
tralateral hemorrhages. The greater the force, the more extensive the focal brain injury, with slight or severe depression fracturing of the skull. The fractured bone causes lacerations of the dura or leptome-ninx as well as of the surface of the cortex. Depending on the force of the impact, destruction of brain tissue occurs with massive, space-occupying hemorrhaging. The brain injury can involve an entire hemisphere and lead to breaching of the blood-brain barrier and bleeding into the ventricular system.
There have been repeated attempts to establish morphological criteria for the differentiation between brain injuries induced by falls and by blows. Among the criteria discussed has been the formation of DAI (Adams et al. 1982; Geddes et al. 1997, 2000), which is more common after falls, i.e., after high acceleration loading (see also Oehmichen et al. 1997). In any given case, however, this criterion alone is not sufficient for reliable discrimination.
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