Haematoma: A collection of blood forming a definite swelling which compresses and damages the brain around it.

Head Injury/ Mild: Causes brief loss of consciousness for 15 minutes or less with a period of post-traumatic amnesia of less than 1 hour.

Head Injury/ Moderate: Defined as being a condition where the patient has been in a coma for 6 hours, and a period of post-traumatic amnesia of up to 24 hours.

Head Injury/ Severe: Defined as being a condition where the patient has been in a coma for 6 hours or more, or a post-traumatic amnesia of 24 hours or more.

Hematoma: Heavy bleeding into or around the brain caused by damage to a major blood vessel in the head.

Hemianopia: Blindness in the same side of both eyes which can follow damage to the brain. This can cause an inability to see things on the left or right side.

Hemispheres: The two halves of the brain which are divided down the middle and called the cerebrum.

Hemorrhagic Stroke: A stroke caused by bleeding out of one of the major arteries leading to the brain.

Hydocephalus: Enlargement of the ventricles due to an increase of cerebrospinal fluid (CSF) on the brain.

Hypoxia: Decreased oxygen levels in an organ, such as the brain; less severe than anoxia.