Pyonephrosis—An Updated Morphological Glimpse into the Contemporary Management of an Immemorial Pathology: A Narrative Review

Pyonephrosis—pus within an obstructed, infected renal collecting system, with suppurative parenchymal destruction—is a urological emergency and a potentially reversible cause of acute kidney injury (AKI); a contribution to chronic kidney disease (CKD) is biologically plausible but has not been demonstrated in pyonephrosis-specific longitudinal studies. This narrative review, reported with reference to the SANRA criteria and based on structured searches of PubMed/MEDLINE, Scopus, and Web of Science, synthesizes the epidemiology, pathophysiology, microbiology, morphological diagnosis, and contemporary management of pyonephrosis, distinguishing evidence generated in confirmed pyonephrosis from that in infected hydronephrosis, obstructive pyelonephritis, and stone-associated sepsis. Sonographic, computed tomographic, and magnetic resonance features are illustrated with original schematics and a case of perforated calculous pyonephrosis; reported Hounsfield-unit thresholds and apparent diffusion coefficients derive from small, single-center cohorts and are protocol- and population-dependent rather than universal cut-offs. Urgent decompression is strongly recommended by guidelines, but the supporting data are observational, so associations between drainage, its timing, and mortality remain vulnerable to confounding. Randomized comparisons of percutaneous nephrostomy and retrograde stenting are few, small, and inconsistent, and establish neither superiority nor equivalence in infected systems. We rate the certainty of each principal conclusion and identify research priorities.