Primary Brain Stem Hemorrhage
According to Bratzke (1981) and Krauland (1982), mechanically induced (primary) hemorrhages at the base of the cerebral peduncles, at the base of the fourth ventricle, in the interpeduncular fossa, and/ or in the rostral portion of the pons are primary loading-induced (non-contact) hemorrhages. They are thought to result from rotational acceleration-induced ruptures of small arteries and roots of Galen's vein. If there is an increase in supratentorial pres sure, however, axial displacement of the brain stem can induce venous congestion and extravasation from the great cerebral vein, with a consequent overlapping of the primary and secondary hemorrhages (cf. Bratzke 1981).
Primary (contact) hemorrhages tend to be located at the margin of the midbrain (Bratzke 1981) (Fig. 9.23a) and are often associated with fractures of the base of the skull (Dirnhofer and Patscheider 1977). Most victims survive only briefly (Krauland 1982), although a few cases with long survival times have been reported.
In addition to hemorrhages, which are generally also macroscopically apparent, necroses and axonal ruptures can occur in the brain stem as well - in the absence of local hemorrhage in the brain stem. Acceleration loadings subject the brain stem to massive shearing forces, which upon rotation can lead - as described above - to tears of axons resulting in DAI as well as tears at the branching of the large basal arteries and thus to traumatic subarachnoid hemorrhage (SAH).
A distinction must be made between primary and secondary mechanically induced brain stem hemorrhages known under the terms of Duret-Ber-ner hemorrhage: this is a microscopic hemorrhage near the third and fourth ventricles. Duret assumed that they are the result of an acute increase in intracranial pressure and of a contusion of the CSF with the ventricle wall. Today it is thought this represents a non-specific finding not attributable to mechanical processes (Unterharnscheidt 1993a, b). This is to be distinguished from the bleeding termed "Duret hemorrhage" in the American literature (Alexander et al. 1982; Knight 1996), which is a hemorrhage secondary to herniation of the midbrain and pons (see below).
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