Intracranial pressure (ICP) is the pressure of the cerebrospinal fluid within the central nervous system. It is measured in millimeters of water (mm H20). Intracranial pressure can be measured by means of a lumbar puncture or continuously with intracranial transducers (only used in neurosurgical intensive care).

There are many symptoms associated with changes in intracranial pressure, including headaches, transient visual obscurations , nausea and/or vomiting, decreased consciousness and Cushing's triad (hypertension, bradycardia and irregular respirations). However, these symptoms may be vague or absent. The most reliable sign of increased intracranial pressure is decreased consciousness. Papilledema, while being a classic sign of elevated intracranial pressure, takes several hours to develop and is often not present in an acute setting.

While increased intracranial pressure is a far more common (and far more serious) sign, it is possible for the intracranial pressure to drop below normal levels as well. The symptoms for both conditions are often the same, leading many medical experts to believe that it is the change in pressure rather than the pressure itself causing the above symptoms.

Causes of increased intracranial pressure include:

• Pseudotumor cerebri
• Arnold-Chiari malformation
• A brain tumor or other mass lesion
• Severe Hypertension
• Lyme disease
• Hydrocephalus