Papillary necrosis is usually bilateral but may be unilateral.One or all of the pyramids of the affected kidney may be involved.
GROSS : On cut section, the tips or distal two thirds of the pyramids have areas of gray-white to yellow necrosis.
HPE : On microscopic examination the necrotic tissue shows characteristic ischemic coagulative necrosis, with preservation of outlines of tubules.The leukocytic response is limited to the junctions between preserved and destroyed tissue.
SPECIAL CLINICAL CORRELATE : On occasion, necrotic papillae are excreted, and may cause gross hematuria or renal colic due to ureteric obstruction.
Papillary necrosis is seen in :-
 Analgesic nephropathy
 Diabetes mellitus
 Urinary tract obstruction
 Sickle cell disease or trait
 Focally in renal tuberculosis.
BASIC CAUSE :- In all cases it is caused by ischemia resulting from compression or obstruction of small blood vessels in the medulla.