Pathophysiology of head injury

The management of head injury has been based on the concept of primary and secondary brain injury. The primary brain injury was defined as the irreversible pathology sustained at the time of the trauma, whereas the secondary brain injury has been considered the subsequent or progressive brain damage that occurs due to an evolving pathology following the injury. It has been the general contention that the primary injury is irreversible, and management should be directed at preventing or treating secondary pathology (such as cerebral swelling, hydrocephalus and intracerebral haematoma).

However, it is now clear that some of the biochemical events associated with what was considered irreversible brain injury are potentially preventable, or even reversible if treatment is instituted early enough. The distinction between primary and secondary injury has become blurred, and the terms, whilst useful concepts, are now becoming obsolete.

Most head injuries result from blunt trauma, as distinct from a penetrating wound of the skull and brain caused by missiles or sharp objects. The pathological processes involved in a head injury are:

• direct trauma

• cerebral contusion

• intracerebral shearing

• cerebral swelling (oedema)

• intracranial haemorrhage

• hydrocephalus.

Direct trauma. Although penetrating injuries produce most of their damage by direct trauma to the brain this is not the case with blunt injuries, in which the energy from the impact has a more widespread effect.

Cerebral contusion. This may occur locally under the position of the impact (coup injury) although it usually occurs more severely at a distance from the area of impact as a result of a ‘contre-coup’ injury. As the brain is mobile within the cranial cavity the sudden acceleration/deceleration force will result in the opposite ‘poles’ of the brain being jammed against the cranial vault. A sudden blow to the back of the head will cause the temporal lobes to slide across the floor of the middle cranial fossa and the frontal lobes across the floor of the anterior cranial fossa, causing contusion on the undersurface of those lobes and to the temporal and frontal poles of the brain as they strike the sphenoid ridge and frontal bones, respectively.

Cerebral contusion consists of lacerated haemorrhagic brain, and a ‘burst temporal lobe’ may result when the temporal pole has been severely injured.

Shearing forces. Intracerebral shearing forces occur as a result of the differential brain movement following blunt trauma, frequently in conjunction with a contre-coup type of injury. The rotational acceleration following injury will cause shear forces that result in petechial haemorrhages (particularly in the upper brainstem, cerebrum and corpus callosum), and tearing of axons and myelin sheaths (Diffuse Axonal Injury-DAI). The early pathological changes consist of retraction balls or microglial stars, and if the patient lives for a number of months before death then widespread degeneration of myelin will be apparent at postmortem.

Cerebral swelling. This occurs following trauma, either in a focal pattern around an intracerebral haematoma or diffusely throughout the cerebrum and/or cerebellum. The nature of the pathological processes are not clearly understood but involve a disturbance of vasomotor tone causing vasodilatation and cerebral oedema. In addition, cerebral contusion and petechial haemorrhages will contribute to the brain swelling.

Intracranial haemorrhage. Intracranial haemorrhage may be:

.intraventicular

.subarachnoid

• intracerebral

• subdural

• extradural.

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Hydrocephalus. This occurs infrequently in the early stages after a head injury. It may be due to obstruction of the 4th ventricle by blood, swelling in the posterior fossa, or the result of a traumatic subarachnoid haemorrhage causing obstruction to the absorption of CSF and resulting in a communicating hydrocephalus. This latter type of hydrocephalus is an uncommon, but important, cause of delayed neurological deterioration either in the weeks following the head injury or some years later.

Concussion

Concussion involves an instantaneous loss of consciousness as a result of trauma. The term ‘concussion’ was introduced by Pare and is derived from the Latin ‘concutere’ which means to shake. In 1941 Denny-Brown and Russell showed that concussion was produced by a blow on the cranium when it was free to move and subsequent studies showed that the acceleration/deceleration of the head resulted in shear strains, contre-coup injury, petechial and punctate haemorrhages throughout the brainstem, cerebral hemispheres and corpus callosum, and neuronal injury, the extent depending on the force of the impact. The term concussion is not strictly defined with respect to the severity of the injury.

However, a minimum criterion is that the patient will have had a period of amnesia. The retrograde amnesia of most cerebral concussion is usually short term, lasting less than 1 day. The initial retrograde amnesia may extend over a much longer period but it gradually shrinks down. A more reliable assessment of the severity of the head injury is the post-traumatic amnesia. If the amnesia following the head injury lasts more than 1 day then the concussion is regarded as being severe.

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