ABSTRACT
After the fontanelles have closed the intracranial contents – consisting of the brain (about 70% of the intracranial volume), cerebrospinal fluid (CSF, about 15%) and blood (about 15%) – are enclosed in a rigid bony container so that any increase in the volume of one or more of these compartments will, unless compensated for by a corresponding reduction in the volume of the other components, lead to an increase in intracranial pressure (ICP). The most common cause of raised ICP in clinical practice is an intracranial expanding lesion and, as a result of distortion and herniation of the brain, this leads frequently to impaction of brain tissue in the tentorial incisura or in the foramen magnum (Fig. 4.1): this results in a pressure gradient between the supratentorial and infratentorial compartments of the skull, or between the intracranial and the spinal subarachnoid spaces. Such a gradient leads to rapid deterioration of the patients’ clinical state (the patient is often said to have ‘coned’), and may inadvertently be produced by lumbar puncture in a patient with an intracranial expanding lesion. Removal of CSF from the lumbar subarachnoid space ‘opens’ a ‘closed’ space, thus allowing downward
Figure 4.1 Raised intracranial pressure, showing the intracranial compartments and the result of an extradural hematoma (H) on the right hand side. In addition to a shift of the midline structures and distortion of the ventricular system, there is (a) a supracallosal hernia, (b) a tentorial hernia and (c) a tonsillar hernia.
