Increased intracranial pressure: early and late signs

The skull is a closed box holding brain, blood and cerebrospinal fluid, so any added volume -- a bleed, a swelling brain, a tumor, cerebrospinal fluid that cannot drain -- is first absorbed by shifting fluid out and squeezing the venous blood down, and while that compensation lasts the pressure barely moves, but once it is spent a small further addition produces a large rise in pressure that cuts the perfusion of the brain and finally pushes brain tissue through the openings of the skull onto the brainstem; the cerebral cortex and the reticular activating system that keep a person awake, attending and answering are the first tissue to suffer as perfusion falls, so the earliest sign of rising pressure is a change in the level of consciousness -- new restlessness or confusion, slowed speech and a delay in answering, increasing drowsiness -- joined by the early companions of a headache that is worst on waking and with coughing or straining, vomiting that arrives without nausea, a pupil on the side of the lesion that becomes sluggish and then large as the oculomotor nerve is compressed, and a weakness on the opposite side, every one of them reported at once because this is the stage at which the cause can still be treated; the brainstem centers that drive the pulse, the blood pressure and the breathing are the last to be reached, so the rising systolic pressure with a widening pulse pressure, the slowing pulse and the irregular breathing of Cushing's response, together with a rising temperature, abnormal posturing, a fixed dilated pupil and the loss of the corneal, gag and swallowing reflexes, are late signs that mean the brainstem is already being compressed and herniation is close, and the interval between the first drowsiness and the first change in the vital signs is the whole of the time in which the pressure can be relieved.