Intracranial pressure

Intracranial pressure

Before considering coma and unconsciousness it is useful to understand intracranial pressure and brain herniation. The intracranial compartment is a fixed box containing brain, blood and CSF. The dynamics of CSF flow are illustrated in Figure 1. Pressure may be increased by:

intracranial masses, either intrinsic (e.g. glioma) or extrinsic (e.g. extradural haematoma)

diffuse brain swelling following severe hypoxia, liver failure or hypertensive encephalopathy

impaired circulation or absorption of CSF.

Fig. 1 CSF production and flow.

Hydrocephalus and ventricular dilatation occur if CSF is under increased pressure relative to the surrounding brain. This may be due to blocked flow (obstructive) or failure of reabsorption (communicating). Excess CSF production due to a tumour of the CSF-producing choroid plexus is a rare cause of communicating hydrocephalus.

There are two semi-rigid sheets, the falx cerebri and the tentorium cerebelli, which almost divide the brain into separate compartments with relatively small apertures between them (Fig. 2). When the pressure rises in one compartment, the brain may herniate through the apertures into adjacent compartments. The cerebellar tonsils and fourth ventricle may also be forced down through the foramen magnum. These are life-threatening complications that further block CSF flow, causing a vicious circle of rising intracranial pressure, resulting in coma (see below).

Fig. 2 Tentorial herniation.

Failure of CSF reabsorption may occur because of thrombosis in the sagittal sinus, increased venous pressure reducing the pressure gradient across the arachnoid villi, or because the function of the arachnoid villi is damaged by the contents of the CSF: excessive cellularity (chronic meningitis), subarachnoid haemorrhage or very high protein content (some tumours and inflammatory conditions).

In some circumstances, such as idiopathic intracranial hypertension (IIH), previously called benign intracranial hypertension (BIH), no cause may be found (p. 40).

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