Haemorrhage in the space between the innermost and the middle cerebral membrane (the arachnoid mater), as a result of an abnormality of a blood vessel. This can be an aneurysm (balloon-like dilation of a blood vessel) or a cluster of dilated vessels. These are vulnerable conditions, and can occur in adulthood without particular cause. The symptoms are very sudden headache associated with nausea and vomiting as a result of irritation of the cerebral membrane, and sometimes a short period of unconsciousness followed by confusion. The vessels of the brain become narrower because of the loss of blood and this sometimes also causes cerebral infarction and paralysis on one side. In the case of a torn cluster of vessels aphasia or epilepsy can occur as a result of irritation of the cerebral cortex.