Acute Kidney Injury

Three anatomical compartments, three mechanisms of injury

Acute kidney injury is not a single disease but a final common pathway reached by three distinct routes, and the anatomical location of the insult is the organising principle for everything that follows. Pre-renal AKI is a perfusion problem: the kidney itself is structurally normal but is not receiving enough blood, from hypovolaemia, hypotension or reduced cardiac output, so glomerular filtration falls even though the nephron is intact. Intrinsic (renal) AKI is a problem within the kidney tissue itself: most often acute tubular necrosis from prolonged ischaemia or direct nephrotoxicity, but also glomerulonephritis or interstitial nephritis. Post-renal AKI is obstruction to urine outflow anywhere from the renal pelvis to the urethra, with back-pressure transmitted upstream to the glomerulus. Sorting a case into one of these three categories before reaching for a differential list is the entire skill.