The normal intracranial pressure (ICP) ranges between 5 and 15 mm Hg (7 to 20 cm H₂O). ICP can be monitored invasively as well as noninvasively. One of the most sensitive indicators of elevated ICP is the neurologic exam—as the ICP rises above normal, the neurologic exam declines. A drop in the level of consciousness serves as one of the first signs of elevated ICP. It is well known that as ICP rises, certain neurologic signs develop. For example, pressure on the third cranial nerve causes a fixed, dilated pupil, and diffusely increased ICP can cause bilateral sixth cranial nerve palsies, with impaired lateral gaze. When ICP rises high enough to cause translocation of brain tissues from their normal neuroanatomic locations, classic herniation syndromes can develop. Neurologic signs, however, lack sufficient accuracy and specificity. Most importantly, the goal of neurologic monitoring is to identify and treat elevated ICP before neurologic damage, such as herniation occurs. In addition, in patients with traumatic brain injury, a deleterious rise in ICP is observed shortly after the injury for an extended period of time. As ICP rises, cerebral perfusion pressure (CPP) drops, since CPP is the difference between mean arterial pressure (MAP) and ICP. When CPP declines sufficiently, ischemia can result (secondary brain injury). Management in this area is controversial, but some guidelines suggest maintaining a CPP between 60 and 70 mm Hg in order to avoid secondary brain injury as a result of decreased cerebral blood flow.

Noninvasive methods of monitoring ICP include neuroimaging (especially computed tomography and magnetic resonance imaging) and neurovascular ultrasound. Neither of these methods provides good accuracy. Transcranial Doppler studies have not been found to provide precise predictions of when an ICP will become acutely and dangerously elevated. Therefore, direct (i.e., invasive) methods remain the preferred technique. These include intraventricular catheters, parenchymal catheters, subarachnoid bolts, epidural catheters, and
lumbar drains (to measure the cerebrospinal fluid [CSF] pressure in the lumbar spinal space).

When a patient has an external intraventricular catheter placed into either the lateral ventricles or another part of the ventricular system of the brain, a pressure transducer can be connected to measure the fluid pressure of the CSF that drains. Intraventricular catheters are considered the gold standard of direct ICP monitoring, because fluid conveys pressure so well. Placement of intraventricular catheters carries with it up to a 6% risk of hemorrhage and up to about a 20% risk of infection, with the lowest reported infection rates of about 5%. Clinicians commonly use prophylactic antibiotics in conjunction with the placement of intraventricular catheters, but data are lacking about whether this practice reduces the infectious complications. Of the infections that do occur, most occur after at least 5 days of catheter placement. Some clinicians have recommended on this basis to replace catheters that are more than 5 days old.