A circular hole made in the SKULL using a drill with a rounded tip, called a burr. The operation is done to relieve pressure on the BRAIN. Raised intracranial pressure is usually the result of blood collecting between the skull and the brain after a head injury. The presence of PUS or an increase in the amount of CEREBROSPINAL FLUID as a result of infection or tumours in the brain can also cause a potentially fatal rise in intracranial pressure which can be relieved by drilling a burr hole if medical measures such as mannitol infusions and steroids do not lower it. A neurosurgeon may make several burr holes when doing a CRANIOTOMY, a procedure in which a section of the skull is removed to provide access to the brain and surrounding tissues.