Intracranial pressure (ICP) is the pressure exerted by the contents within the rigid skull, a measurement of high importance in neurological health. The skull contains three main components: brain tissue, blood, and cerebrospinal fluid (CSF). The Monro-Kellie doctrine states that because the adult skull is a fixed, non-expanding box, an increase in the volume of any one component must be offset by a corresponding decrease in one or both of the others to keep the overall pressure constant. When this delicate balance is disrupted, ICP can rise rapidly, leading to potentially devastating consequences for the brain.
Clinical Conditions Requiring Monitoring
A sudden or sustained elevation in ICP often signals a serious medical emergency, necessitating continuous monitoring to guide treatment. The most common situation requiring this intervention is severe Traumatic Brain Injury (TBI), particularly in patients who are unconscious or have concerning findings on brain imaging. Severe TBI can cause brain swelling or bleeding, which directly increases the volume inside the skull.
Other conditions requiring monitoring include large cerebral hemorrhages or strokes that cause significant tissue swelling. Hydrocephalus, characterized by an abnormal buildup of CSF, also raises ICP because the fluid cannot drain correctly. Monitoring may also be necessary for severe brain infections like meningitis or conditions such as hepatic encephalopathy, where generalized brain swelling occurs.
Direct Invasive Measurement Techniques
Direct measurement of ICP provides the most accurate and continuous pressure readings and is considered the gold standard for monitoring. These techniques require a neurosurgical procedure to place a sensor or catheter directly inside the brain or its fluid spaces. The two primary methods are the ventriculostomy catheter and the parenchymal microtransducer catheter.
External Ventricular Drain (EVD)
The External Ventricular Drain (EVD), or ventriculostomy catheter, is placed into the lateral ventricle through a small hole drilled in the skull. This is widely regarded as the most accurate method because it measures the pressure directly within the CSF system. A significant advantage of the EVD is its therapeutic capability, as it allows for the controlled drainage of CSF to immediately lower high pressures. Risks include infection (ventriculitis) and hemorrhage during placement.
Parenchymal Microtransducer Catheters
Parenchymal microtransducer catheters offer an alternative invasive method that is generally less traumatic to insert. These thin, fiber-optic or strain-gauge sensors are placed into the brain substance (parenchyma). The microtransducers provide continuous pressure data with high accuracy, comparable to the EVD. A key limitation is that the parenchymal probe does not allow for the therapeutic drainage of CSF. However, the infection and hemorrhage rates associated with these microtransducers are generally lower than those of the EVD.
Indirect Non-Invasive Assessment Methods
While invasive monitoring is highly accurate, it carries risks and is not always feasible, leading to the development of non-invasive screening methods. These techniques do not provide a direct, continuous pressure reading but offer a correlation that suggests the presence of elevated ICP. These methods are particularly useful for initial assessment in the emergency setting.
Transcranial Doppler (TCD) Ultrasonography
TCD ultrasonography uses sound waves to measure the velocity of blood flow in the major cerebral arteries. Increased ICP compresses the brain’s blood vessels, affecting the resistance to blood flow. TCD measures parameters like the pulsatility index, which provides an indirect indicator of rising pressure.
Optic Nerve Sheath Diameter (ONSD) Ultrasound
ONSD ultrasound relies on the anatomical connection between the brain and the eye. The optic nerve is surrounded by a sheath filled with CSF. When ICP rises, the CSF pushes this sheath outward, causing it to widen, a change measured using an ultrasound probe placed on the closed eyelid. An ONSD measurement greater than five millimeters is often considered a sign of increased ICP.
Understanding and Managing Elevated Pressure
ICP is measured in millimeters of mercury (mm Hg); a normal range for an adult lying down is typically between five and 15 mm Hg. Clinically, values consistently above 20 mm Hg are treated as elevated, particularly in the context of an acute brain injury. The ultimate goal in managing a patient is not just to control ICP, but to maintain a sufficient Cerebral Perfusion Pressure (CPP), which is the net pressure driving blood flow to the brain tissue.
CPP is calculated by subtracting the ICP from the Mean Arterial Pressure (MAP), following the formula: CPP = MAP – ICP. If ICP rises too high, it directly lowers the CPP, which can deprive the brain of oxygen and nutrients. Therefore, treatment aims to keep CPP within a healthy target range, typically between 60 and 70 mm Hg, to ensure adequate brain blood flow.
Initial management focuses on immediate interventions to lower the pressure. Elevating the head of the bed to about 30 degrees helps optimize venous drainage, reducing the blood volume component. If an EVD is in place, draining CSF is a rapid and effective way to lower the pressure. Medicines known as hyperosmolar agents, such as mannitol or hypertonic saline, are administered intravenously to draw excess fluid out of swollen brain cells. If these measures fail, more aggressive measures, including surgical decompression of the skull, may be considered.