A brain herniation is the displacement of brain tissue, cerebrospinal fluid, and blood vessels outside the compartments in the head that they normally occupy. A herniation can occur through a natural opening at the base of the skull (called the foramen occipitalis) or through surgical openings created by a craniotomy procedure.

Herniation can also occur between compartments inside the skull, such as those separated by a rigid membrane called the "tentorium".

A brain herniation occurs when pressure inside the skull (intracranial pressure) increases and displaces brain tissues. This is commonly the result of brain swelling from a head injury. It can also be caused by space-occupying lesions such as primary brain tumor, metastatic brain tumor, and hemorrhages or strokes that produce swelling within the brain. Hydrocephalus (accumulation of fluid in the brain) can also lead to brain herniation.

A brain herniation itself often causes massive stroke. This results from poor blood supply to some areas of the brain and compression of vital structures that regulate your breathing and circulation. This can rapidly lead to death or brain death.

Brain herniations are the most common secondary effect of expanding masses in the brain.

• Progressive loss of consciousness
• Coma
• Irregular breathing
• Respiratory arrest (no breathing)
• Irregular pulse
• Cardiac arrest (no pulse)
• Loss of all brainstem reflexes (blink, gag, pupillary reaction to light)

A neurologic examination would show an impaired level of consciousness. Depending on the severity of the herniation, one or several brainstem reflexes and cranial nerve functions will be impaired. The patient would show an inability to breathe consistently, and heart rhythms would be irregular.