Polyarteritis nodosa. Close-up of the hands of a patient affected by polyarteritis nodosa (PAN). PAN is an uncommon condition that causes vasculitis (blood vessel swelling) of medium & small-sized arteries, which become swollen and damaged from attack by rogue immune cells. Here, the condition has caused tissue death (necrosis, black) due to a lack of blood supply. In this case, the arteries are being replaced with obstructing fibrous tissue and associated inflammation. The cause of PAN is not well understood but may be associated with hepatitis B virus.