Depressed skull fractures are of significance only if the underlying dura is lacerated or the brain is compressed by indentation of bone. They then are surgically elevated, preferably within the first 24 to 48 h.
Much has been written about the mechanisms of concussion in closed head injury and its definition has undergone serial revision. In the past, a transient loss of consciousness and amnesia after a blow to the head had been considered necessary to qualify as concussion but lesser degrees of mild confusion, incoordination, or even symptoms such as headache and fatigue that follow mild head injury are now encompassed under the term. Whether all these problems derive from the same mechanism cannot be stated with confidence.
History of concepts of concussion The mechanism of concussive “cerebral paralysis” has been interpreted in various ways throughout medical history in light of the state of knowledge at a particular period of time. The favored hypotheses for the better part of a century were “vasoparalysis” (suggested by Fischer in 1870) or an arrest of circulation by an instantaneous rise in intracranial pressure (ICP) (proposed by Strohmeyer in 1864 and popularized by Trotter in 1932). Jefferson, in his essay on the nature of concussion (1944), convincingly refuted these vascular hypotheses. Later, Shatsky and coworkers, by the use of high-speed cineangiography, showed displacement of vessels but no arrest of circulation immediately after impact.
Beginning with the work of Denny-Brown and Russell in 1941, the physical factors involved in head and brain injuries were subjected to careful analysis. These investigators demonstrated that in monkey and cat the concussion resulted when the freely moving head was struck by a heavy mass. If the head was prevented from moving at the moment of impact, the same degree of force invariably failed to produce concussion. The importance of head motion was verified by Gennarelli and colleagues, who were able to induce concussion in primates by rapid acceleration of the freely moving head without impact, a condition that rarely occurs in humans.
Holbourn, a Cambridge physicist, from a study of gelatin models under conditions simulating head trauma, deduced that when the head is struck, movement of the partly tethered but suspended brain always lags (because of inertia), but inevitably the brain moves also, and when it does it must rotate in an arc because of attachment to the neck. Ommaya and Gennarelli (1974) proved the correctness of this assumption by photographing the brain through a transparent calvarium at the moment of impact. The brain was thus subjected to stresses set up by rotational forces mainly in the sagittal plane, centered at its point of tethering in the high midbrain. The torque at the level of the upper reticular formation would explain the immediate loss of consciousness, as described later. An extensive and scholarly review of the pathophysiology of concussion was done by Shaw (although we are uncertain of the validity of his view of a seizure-concussive mechanism).