The cause remains unknown; however, the variety of
pathologic features suggests multiple pathogenic mechanisms. Spontaneously occurring
polyarteritis nodosa occurs in hyperimmunized human volunteers, in animals with
experimental serum sickness, and in patients developing hypersensitivity reactions.
Drugs (e.g., sulfonamides, penicillin, iodide, thiouracil, bismuth, thiazides,
guanethidine, methamphetamine), vaccines, bacterial infections (e.g., streptococcal,
staphylococcal), and viral infections (e.g., serum hepatitis, influenza, HIV) have been
associated with disease onset. Segmental, necrotizing inflammation of media and
adventitia characterizes the lesion. The pathologic process most commonly occurs at
points of vessel bifurcation, beginning in the media and extending into the intima and
adventitia of medium-sized arteries, often disrupting the internal elastic lamellae.
Early endothelial lesions contain polymorphonuclears and occasionally eosinophils; later
lymphocytes and plasma cells can be seen. Immunoglobulin, complement components, and
fibrinogen are deposited in the lesions, but their significance is unclear. Intimal
proliferation with secondary thrombosis and occlusion leads to organ and tissue
infarction. Weakening of the muscular vessel wall may cause small aneurysms and arterial
dissection. Healing can result in nodular fibrosis of the adventitia.