Yasser Arafat is reported to have suffered a brain haemorrhage.
Haemorrhages can occur in any part of the brain
Details of Mr Arafat's condition have not been released.
But experts say there are two types of brain haemorrhage; subarachnoid - which occurs on the surface of the brain, and intracerebral - which occur within it.
Subarachnoid haemorrhages can be caused by a blood vessel in the brain bursting, but experts say there can be a number of causes for the condition including high blood pressure of clotting disorders.
Both types of haemorrhage can be fatal. The outcome depends on how much blood is lost and exactly where in the brain the haemorrhage happens.
'Range of outcomes'
Consultant neurosurgeon Laurence Watkins, who is based at the National Hospital for Neurology and Neurosurgery, said previous reports Mr Arafat had some form of blood disorder suggested his haemorrhage could have been caused by a clotting problem.
Mr Watkins said: "Brain haemorrhages can lead to a huge spectrum of outcomes."
But he added that they could be very dangerous for someone in Mr Arafat's condition : "The more elderly and frail you are, the less likely you are to stand up to it."