Intracranial pressure monitoring

Intracranial pressure (ICP) data form the basis for guiding the management of comatose head injury patients whose clinical examination is usually compromised by sedation and/or chemoparalysis. The Guidelines for the Management of Severe Head Injury published by the American Association of Neurological Surgeons recommend that ICP monitoring be undertaken in patients with a head injury and an abnormal CT scan on admission. ICP monitoring may also be undertaken in patients with severe head injury and a normal CT scan if they have any two of three adverse characteristics including age over 40 years, unilateral or bilateral motor posturing, or systolic blood pressure less than 90 mmHg on admission, and are therefore at a high risk of developing raised ICP.

Once ICP monitoring is deemed necessary, a monitoring device is placed by the neurosurgeon. A variety of such devices are available. The oldest and most commonly used technique is a fluid-coupled catheter placed in the ventricular cavity (ventriculostomy). Newer devices that permit reliable pressure recording when placed in the brain parenchyma or lateral ventricle include fiber-optic catheters and strain gauges. In both these solid state systems, the transducer is at the catheter tip, eliminating the need for a fluid column to transmit pressure from the intracranial cavity to the transducer. Although convenient, these systems cannot be recalibrated once placed intracranially and are prone to measurement drift over time (days). Complications of ICP monitoring include malfunction, obstruction, malposition, infection, and hemorrhage. Bacterial colonization of the ICP monitoring device seems to increase after 5 days of placement. Frequent irrigation of fluid-coupled systems increases the risk of colonization. Although the risk of colonization increases over time, clinically significant intracranial infections are relatively uncommon. Infections are best treated by removing the infected catheter and treating the patient with intravenous antibiotics. Routine removal or changing of catheters at a fixed day after insertion has been debated, with no conclusive evidence as to the value of this policy. Measures to minimize infection include tunneling the catheter subcutaneously away from its insertion site and minimizing the invasion of fluid-coupled systems.

The upper limit of normal ICP is generally considered to be around 15 to 20 mmHg in adults. ICP is generally treated if it is above 20 to 25 mmHg. Treatment of elevated ICP is performed in a stepwise manner and continued until the pressure normalizes. ICP monitoring is continued as long as there is intracranial hypertension or ongoing therapy for intracranial hypertension. Post-traumatic edema is typically maximum at 48 to 72 h. However, it is not unusual for the ICP to remain elevated for days. Monitoring may generally be safely discontinued in patients who maintain a normal ICP for 24 h without requiring specific therapy.

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