Cranial Trauma.

In the cinema and on TV, there is a convention that when the hero corners the bad guys in an early scene, the hero is knocked unconscious by a blow on the head, which allows the criminals to escape. The hero gets-up unharmed in the next frame. This allows the weak plot to be continued over several more chase scenes. In real life this doesn't happen...

The brain comprises neurons, vascular and supporting tissue, all fragile. It lies in a stiff box that conforms to its boundaries. Newtonian kinetics, in sudden deceleration, or inertia, in sudden acceleration of the skull by blunt instrument trauma, produce shear effects inside the skull. There is a fixed strong membrane between the cerebral hemispheres and another membrane separating the cerebellar hemispheres from the cerebral. The dural partitions can function rather like the baffles in the petrol tank of your car and spread the load of any shear force to other parts of the brain. Their presence may also induce temporary deformities in the soft brain.

A blow or sudden deceleration will force the brain against the vault margin. Sudden frontal or occipital movement of the brain will stretch the long tracts going into the cerebral hemispheres from the (relatively fixed) brain-stem and cerebellum. Shear forces can also damage vessels within the brain to cause local haematomas. As with infarcts, early scans may not reveal the degree to which the brain is damaged. Changes in the basal ganglia with lacunar low density occur fairly early in severe trauma and this area is worth a second look.

Sudden sideways displacement under the rigid falx will also distort the white matter, including corpus callosum and internal capsule. Small tears of the nerves occur, but may not be visible on the CT scan. Small cortical contusions can be seen adjacent to the falx in boxers. The accumulated loss of brain substance can often be seen in late scans as local widening of the inter-hemispherical fissure between the frontal lobes.

Trauma has four effects:

• A Vault fracture will usually damage the underlying brain and accumulating (extradural) haematoma around the fracture may compress the brain.
• Even without fracture, there is direct injury (contusion) to those parts of the brain that impinge upon fixed boundaries within or at the margins of the cranium.
• There is tearing of the long tracts in the brain as the brain 'tries to flow' around fixed regions in the skull.
• The brain is pulled away from the vault opposite to the site of trauma as it tries to obey Newton's 1st law. This tears the delicate meninges and can result in sub-dural haematoma.

When the cerebral cortex hits an obstruction, the result can be seen as an increase in density of the cortex, due to contusion, petechial haemorrhages. This may be missed against the artifact that sometimes occurs near the vault. Inspection of the sulci will usually distinguish pathology. Often blood will fill them, adjacent to a contusion. Free sub-arachnoid blood will give dense sulci. The displacing mass effect is inconsistently low for the size of the contusion density. Review areas are the underside of the temporal lobes after fist fights.

If the patient is deeply unconscious and the brain looks symmetrical, there are three things that you must consider:

• There may be symmetrical sub-dural haematomas.
• The haematoma may be sub-acute and isodense with normal brain.
• The patient may be brain dead. In this situation there is loss of vascular tone and no blood flow. The full toneless vessels fill with blood, which gives a general increase in density above normal values. You can see this by window techniques on the monitor or by direct measurement of the Hounsfield numbers. There will be no change in the brain density, after contrast.

Remember that the presence of iron, methaemoglobin, in the maturing haematoma, can cause paradoxical effects in MRI scans.

A cerebral contusion may confuse. It can cause extra density from bleeding into the brain-substance and is usally cortical. There may be ischaemia or oedema that defines its margin. Because the density is in brain, the density will 'infiltrate' rather than displace so that the distance from the density to landmarks within the brain will be decreased. On the other hand, a collection will displace brain with less reduction in the distance between its inner margin and a landmark, e.g. lateral ventricle. If the patient has been in a fight, look for contusions below the temporal lobe.

The skull base is in the plane of the cuts in a CT scan of the brain. The dense bone around the posterior fossa means that scans can under-read the consequences of fractures of the base of skull. Any clinical suspicion with bleeding from the ear often means serious brain-stem injury. Contrast enhancement or recourse to MRI are indicated, depending on the ongoing needs of clinical management.
Always review the cervical spine in all cases of cerebral trauma.

from Canada

Fractures

Extra-dural Collections.

Sub-dural Collections.