Intracranial Pressure Monitoring

Introduction

The normal range for intracranial pressure (ICP) is 5 – 15mmHg. The average intracranial volume is 1700ml (composed of the brain 1400ml + CSF 150ml + blood 150ml), with Cerebrospinal Fluid (CSF) production around 500 – 600ml per day.

The skull is a rigid compartment, and the Monro-Kellie doctrine describes a pressure-volume relationship, a dynamic equilibrium among components within the rigid skull compartment:

V_Intracranial = V_Brain + V_CSF + V_Blood

In order to protect the brain, the blood and CSF volume can be comprised as the primary protection. However, this mechanism can be overcome with large increases to these volumes.

This article discusses the importance of ICP monitoring and how it can be performed.

Cerebral Perfusion Pressure

Cerebral perfusion pressure (CPP) drives oxygenation and metabolite transfer to cerebral tissues.

The brain can autoregulate blood flow, through cerebral vessel constriction or dilatation, in order to ensure constant flow isolated from fluctuations in systemic blood pressure*; the Cerebral Perfusion Pressure is calculated by Mean Arterial Pressure minus Intracranial Pressure.

CSF is produced by ependymal cells within the choroid plexus and circulates within the ventricular system, being reabsorbed via arachnoid granulations. There is normally an equilibrium between CSF production and absorption, however when this relationship is disrupted, a raised ICP can occur

*This is the mechanism that is often lost due to head trauma, leading to cerebral ischaemia and neuronal death (secondary brain injury)

By TeachMeSeries Ltd (2020)

Figure 1 – The anatomy of the ventricular system

Clinical Features

The clinical features of raised ICP will progress from initial mild non-specific symptoms to late significant symptoms.

Early onset symptoms include morning headache (worse upon coughing, exertion or moving head), vomiting (with no associated nausea), and lethargy or altered mental status.

On examination, there may be ocular palsies, papilloedema, or pupil irregularities, including unilateral dilation or pupillary light defects

Late features include persistent vomiting, Cushing’s triad*, ophthalmoplegia, and eventual coma and death.

*Cushing’s Triad is composed of irregular respiration, bradycardia, and systolic hypertension (with wide pulse pressure)

Indications for ICP Monitoring

Whilst ICP monitoring is most commonly used for the management of severe head trauma, its use also extends to CSF circulatory disorders, and can be either diagnostic or therapeutic (by removing CSF to reduce pressures).

Current indications for ICP monitoring include traumatic brain injury (TBI), hydrocephalus or conditions at high risk of developing hydrocephalus (e.g. space-occupying lesions or subarachnoid haemorrhage), idiopathic intracranial hypertension, or Reye’s syndrome

Contra-indications to ICP monitoring include coagulopathies or anti-coagulation medication, scalp infections, or brain abscess.

The main complications following ICP monitor insertion are infection (meningitis, ventriculitis, wound infection), intracranial haemorrhage, device malfunction or difficulty with placement, and ventricular collapse (potentially leading to tentorial herniation).

Types of ICP Monitors

There two basic ICP monitor types are via ICP data only (commonly known as ‘bolts’) or ICP data plus CSF drainage. The various types of ICP monitor, and the most common are an External Ventricular Drain (EVD) and an Intraparenchymal Monitor. Subarachnoid and Epidural bolts are used less frequently (Fig. 2).

These probes can often also be used to measure other physiological parameters, including temperature, lactate, and pH.

By TeachMeSurgery (2020)

Figure 2 – Schematic representing the anatomical insertion site for ICP monitoring

Intraparenchymal Monitor

The intraparenchymal monitor (or bolt) consists of a small wire that is inserted through a single burr hole, usually performed at the bedside with a twist drill, into the brain parenchyma. This is a relatively low risk procedure and allows for good waveform resolution. Additional monitoring can be performed with specific types of parenchymal monitors, including brain tissue oxygen and brain chemistry (metabolites).

This type of ICP monitoring is commonly used in intensive care units, it is relatively easy to install and is less invasive than EVDs. A key limitation is that it does not allow therapeutic drainage of CSF.

External Ventricular Drain

The External Ventricular Drain (EVD), also termed fluid-filled transduced ventriculostomy, is the gold standard intervention for raised ICP*

Its main benefits include additional uses for therapeutic aspiration of CSF, rarely will occlude, and inexpensive, albeit difficult to install. Its limitations include infection (ventriculitis) and potential for damage to the underlying brain parenchyma.

They are most commonly inserted into Kocher’s point, however alternative points for EVD insertion include Keen’s point, Frazier’s point, and Dandy’s point

*Global CSF pressure is better reflected by ventricular pressure as opposed to subdural, extradural, or subarachnoid pressure

The External Ventricular Drain Procedure

An incision should be made at Kocher’s point (Fig. 3) and a burr hole made at this site. Open the dura and insert a catheter into the anterior horn of lateral ventricle; CSF will appear via catheter.

Tunnel away from burr hole site posterolaterally and suture in place. Confirm drain placement using CT imaging.

Attach the catheter to non-pressurised tubing filled with saline solution and connected to non-flush strain-gauge transducer (this will measure the pressure transmitted from CSF in the ventricles). Level the transducer to zero.

By TeachMeSurgery Ltd (2020)

Figure 3 – Kocher’s point from the superior axial view (10–11cm posterior to nasion in the midline, 1cm anterior to coronal suture, and 3cm lateral to midline)

ICP Monitoring Waveforms

The waveforms produced from the ICP monitoring can be used to infer the underlying pathology present. The wave forms produced are summarised in Table 1.

Table 1 – Waveforms characteristics from ICP monitoring

The height order of the wave phases on monitoring should be P1 > P2 > P3. If P2 is highest, this indicates potential raised ICP (Fig. 4).

By TeachMeSurgery (2020)

Figure 4 – (A) showing normal ICP waveforms (B) showing ICP waveforms in keeping with raised ICP