Raised intracranial pressure

Normal supine intracranial pressure is 7-17 mmHg and is frequently measured on neurointensive care units. Cerebral perfusion pressure can then be calculated and manipulated. A reasonable estimate of ICP can be made in patients with brain injury who are not sedated:

• drowsy and confused with GCS 13-15: ICP 20 mmHg

Clinical features of raised ICP are nausea and vomiting, confusion and a reduced conscious level. Raised ICP can occur in cerebral haemorrhage, infarction with surrounding oedema, tumours, encephalitis, global ischaemia, or after TBI. As the skull is a rigid box, its contents are incompressible so ICP depends on the volume of intracranial contents (normally 5% blood, 5-10% CSF, and 85% brain). The Monro-Kellie doctrine is named after two Scottish anatomists (Figure 9.2). It states that, as the cranial cavity is a closed box, any change in intracranial blood volume is accompanied by an opposite change in CSF volume, if intracranial pressure is to be maintained.

Intracranial volume

Figure 9.3 Effect of increasing intracranial volume on intracranial pressure (ICP)

Intracranial volume

Figure 9.3 Effect of increasing intracranial volume on intracranial pressure (ICP)

When ICP is raised the following occurs:

• CSF moves into the spinal canal and there is increased reabsorption into the venous circulation;

• compensatory mechanisms are eventually overwhelmed, so further small changes in volume lead to large changes in pressure (Figure 9.3);

• as ICP rises further, CPP and CBF decrease;

• eventually brainstem herniation (coning) occurs.

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