Intracranial Pressure (ICP) is a measure of the pressure exerted by the brain tissue, cerebrospinal fluid (CSF), and blood within the rigid confines of the skull. The cranial vault is a fixed, bony container, meaning its internal volume cannot change to accommodate expanding contents. Therefore, the pressure inside the skull directly reflects the volume and balance of these three components, which must be tightly regulated for the brain to function correctly. This pressure is not static, constantly fluctuating with heartbeats, breathing, and even everyday activities like coughing or sneezing.
The Mechanics of Pressure Regulation
The physiological basis for ICP regulation is formally described by the Monro-Kellie doctrine, which treats the skull as a non-expandable container. In a healthy adult, brain tissue accounts for roughly 80% of the intracranial volume, while blood and CSF each make up about 10% of the total volume. Because the total volume is fixed, an increase in the volume of any one component requires a corresponding decrease in one or both of the other components to keep the pressure stable.
For example, if a new mass or swelling increases brain tissue volume, the body attempts to compensate by shifting CSF out of the cranial vault into the spinal canal. The body’s primary compensatory mechanisms involve displacing CSF into the spinal subarachnoid space. This ability to compensate means that early increases in one component’s volume may not immediately cause a significant rise in ICP. However, this compensatory reserve is limited, and once it is exhausted, any small additional increase in volume leads to a sharp and dangerous spike in intracranial pressure.
Defining Normal Levels and Monitoring Methods
A normal ICP level typically ranges from 5 to 15 millimeters of mercury (mmHg) for a supine adult. Pressures that consistently remain above 20 mmHg are generally considered pathological and require prompt medical intervention. These values are highly dependent on body position; when a person is standing upright, gravity causes the pressure to become slightly negative in the head, often reading around -10 mmHg. In addition to position, ICP naturally fluctuates with respiration and heart rate, meaning a single, acute measurement is less important than a sustained elevation.
Monitoring Methods
The most accurate method for directly measuring ICP is the insertion of an External Ventricular Drain (EVD), also known as an intraventricular catheter, which is considered the gold standard. This invasive procedure involves drilling a small hole in the skull and placing a catheter directly into one of the brain’s fluid-filled spaces, the lateral ventricle. This device not only measures pressure but can also be used to drain excess CSF to therapeutically lower dangerously high pressure. Other invasive monitoring techniques include placing fiberoptic transducer systems into the brain tissue itself or the subarachnoid space, providing continuous pressure data. While these methods are less accurate than an EVD, they avoid the risk of ventricular puncture.
Consequences of Elevated Intracranial Pressure
Intracranial Hypertension (ICH) results from numerous causes, including traumatic brain injury, bleeding within the skull (hemorrhage), brain tumors, or an abnormal buildup of CSF, such as in hydrocephalus. The resulting pressure compresses brain tissue and blood vessels, leading to dangerous effects. One of the greatest dangers of elevated ICP is its effect on Cerebral Perfusion Pressure (CPP), which is the pressure gradient that drives blood flow to the brain.
CPP is calculated by subtracting the ICP from the mean arterial pressure (MAP). As ICP rises, CPP falls, which starves the brain of oxygen and nutrients, leading to ischemia and tissue death. Common symptoms of ICH in adults include a persistent, severe headache that is often worse in the morning or when lying down, along with nausea and vomiting. Patients may also exhibit vision problems, such as double vision, or a swelling of the optic disc, known as papilledema. If the pressure continues to rise unchecked, it can force brain tissue through the openings in the skull base, an event called brain herniation, which compresses brainstem structures and can be fatal.
Consequences of Reduced Intracranial Pressure
Intracranial Hypotension (IH) is a condition where the pressure within the skull is abnormally low, typically defined as a CSF pressure less than 60 mmH2O. This low pressure is most often caused by a leak of cerebrospinal fluid (CSF) from the protective dural layer surrounding the brain and spinal cord. A CSF leak can occur spontaneously due to a weakness in the dura or, more commonly, as a complication following medical procedures like a lumbar puncture.
The reduction in CSF volume causes the brain to sag slightly within the skull, leading to traction on pain-sensitive structures and blood vessels. The primary symptom of IH is a characteristic orthostatic headache, which is severe when the person is sitting or standing upright and dramatically improves when they lie flat. This positional headache can be accompanied by other non-specific symptoms, including nausea, neck stiffness, changes in hearing, and dizziness. Treatment typically focuses on sealing the CSF leak.