Intracranial Pressure Monitoring

Intracranial pressure monitoring measures the pressure within the cranial vault continuously with a catheter or probe placed in the ventricle, brain parenchyma or other intracranial compartment. It directs management of raised pressure and allows calculation of cerebral perfusion pressure, which is mean arterial pressure minus intracranial pressure.

The intraventricular catheter connected to an external drain is the reference method, because it measures global pressure and allows cerebrospinal fluid drainage. Intraparenchymal fibreoptic or strain gauge probes are the most common alternative.

Usage. It is used in severe traumatic brain injury, intracranial haemorrhage, hydrocephalus and selected metabolic and infective encephalopathies in intensive care.

Indications. Severe traumatic brain injury (Glasgow Coma Scale 8 or less) with an abnormal CT head, or with a normal CT and risk factors; poor grade subarachnoid haemorrhage; large intracerebral haemorrhage with deterioration; hydrocephalus; acute liver failure with encephalopathy in selected cases; and meningitis with hydrocephalus.

Preparation and technique. Insertion is by a neurosurgeon in theatre or in the intensive care unit, with sterile technique and local or general anaesthesia. The transducer is zeroed at the external auditory meatus (the tragus) for external ventricular drains. Intraparenchymal probes are zeroed before insertion and cannot be re-zeroed afterwards.

Contraindications and precautions. Coagulopathy and thrombocytopenia are corrected before insertion where possible, and local scalp infection at the entry site is a contraindication. Non-invasive estimates, such as optic nerve sheath diameter and transcranial Doppler pulsatility, are used when invasive monitoring is not possible, and they are less accurate.

Reference values. Normal adult supine pressure is 5 to 15 mmHg. Treatment is generally started when pressure stays over 20 to 22 mmHg. Cerebral perfusion pressure is usually kept at 60 to 70 mmHg, with the target set by the individual patient and the monitoring available.

Interpretation.

• The waveform has three components (P1 percussion, P2 tidal, P3 dicrotic). When P2 exceeds P1, intracranial compliance is reduced.
• Lundberg A waves (plateau waves) are rises to 50 to 100 mmHg lasting 5 to 20 minutes, and they indicate critically low compliance.
• B waves are rhythmic oscillations of 1 to 2 each minute, linked to respiration or vasomotor changes.
• A rising baseline with a falling cerebral perfusion pressure indicates deterioration.

Causes of raised pressure. Cerebral oedema, haematoma, hydrocephalus, venous outflow obstruction, mass lesion, hypercapnia, hyperthermia, seizures, and infection.

Critical values and findings needing immediate action.

• Sustained pressure over 20 to 22 mmHg requires urgent treatment and re-imaging for a surgical lesion.
• A value over 40 mmHg is immediately life threatening.
• A rise in pressure with hypertension and bradycardia (Cushing response) or a new dilated pupil indicates impending herniation and needs immediate action.

Limitations and artefacts. Zero drift occurs in intraparenchymal probes over days. Incorrect transducer level, a damped trace from a blocked catheter, air bubbles, and cough or head position distort readings. A single parenchymal probe may not reflect pressure in another compartment.

Complications. Infection (ventriculitis) rises with duration of monitoring, haemorrhage at the insertion tract occurs in a small proportion, and catheter malfunction, misplacement and cerebrospinal fluid leak also occur.

Follow up tests. CT head for any sustained rise, cerebrospinal fluid cell count and culture when infection is suspected, coagulation profile, serum sodium and osmolality, and arterial blood gases.

Special groups. Normal pressure is lower in children, and treatment thresholds are age specific. Infants with open fontanelles may show lower pressures despite significant pathology.