Focal infections of the kidney can be divided into intrarenal and perirenal pathology (Table 66.1). The classification of intrarenal abscess encompasses renal cortical abscess and renal corticomedullary abscess; the latter includes acute focal bacterial nephritis, acute multifocal bacterial nephritis, and xanthogranulomatous pyelonephritis. Perirenal abscesses are found in the perinephric fascia external to the capsule of the kidney, generally occurring as a result of extension of an intrarenal abscess. Papillary necrosis is a clinicopathological syndrome that develops during the course of a variety of syndromes, including pyelonephritis, affecting the renal medullary vasculature that in turn leads to ischemic necrosis of the renal medulla.

RENAL CORTICAL ABSCESS

A renal cortical abscess results from hematogenous spread of bacteria from a primary focus of infection outside the kidney, often the skin. The most common causative agent is Staphylococcus aureus (90%). Predisposing conditions include entities associated with an increased risk for staphylococcal bacteremia, such as hemodialysis, diabetes mellitus, and injection drug use. The primary focus of infection may not be apparent in up to one-third of cases. Ascending infection is an infrequent cause of renal cortical abscess formation. Ten percent of renal cortical abscesses rupture through the renal capsule forming a perinephric abscess.

Patients present with chills, fever, and back or abdominal pain, with few or no localizing signs (Table 66.2). Most patients do not have urinary symptoms as the process is circumscribed in the cortex and does not generally communicate with the excretory passages.