Polyarteritis nodosa brain slices. Four slices through the brain showing haemorrhaged areas (black, best seen on the lowest slice) due to polyarteritis nodosa. This disease, also called periarteritis nodosa, causes weakening of the walls of arteries. It is thought to be caused by a disturbance of the immune system. Its severity depends on the arteries affected. In the brain, the bleeding causes tissue death (infarction) in the areas supplied by the weakened arteries, which can cause paralysis and death. If it is diagnosed before symptoms become severe, it can be treated with corticosteroid and immunosuppressant drugs. The prognosis is poor, however.