Post-renal Acute Kidney Injury (Post-renal AKI) occurs when obstruction to the normal flow of urine leads to increased pressure within the urinary tract and impaired kidney function. Unlike prerenal and intrinsic AKI, post-renal AKI results from mechanical blockage rather than reduced blood flow or direct tissue injury. When detected early, post-renal AKI is highly reversible; however, prolonged obstruction can result in permanent kidney damage.

Urinary obstruction may occur at any level of the urinary tract, including the renal pelvis, ureters, bladder, or urethra. Common causes in adults include prostate enlargement, kidney or ureteric stones, strictures, tumors, blood clots, and neurogenic bladder dysfunction. In hospitalized and elderly patients, chronic bladder outlet obstruction may go unrecognized until kidney function deteriorates.

The pathophysiology of post-renal AKI involves back pressure transmitted from the obstructed urinary tract to the kidneys. This increased pressure reduces glomerular filtration and disrupts tubular function. Initially, kidney blood flow may increase in an attempt to compensate, but sustained obstruction eventually leads to vasoconstriction, inflammation, and structural damage.

Clinical presentation varies depending on the acuity and location of obstruction. Patients may experience reduced or absent urine output, flank or lower abdominal pain, urinary retention, or lower urinary tract symptoms such as weak stream or incomplete emptying. In some cases, especially with bilateral obstruction, kidney injury may present silently and be detected only through abnormal blood tests showing rising creatinine levels.

Diagnosis relies on clinical assessment, laboratory testing, and imaging. Blood tests reveal impaired kidney function and electrolyte disturbances, while urine analysis may show minimal abnormalities. Renal ultrasound is the imaging modality of choice and often demonstrates hydronephrosis, bladder distension, or dilated ureters. Prompt imaging is critical to confirm obstruction and guide intervention.

Management focuses on urgent relief of obstruction. This may involve bladder catheterization, ureteric stenting, or percutaneous nephrostomy, depending on the level and cause of blockage. Supportive care includes careful fluid management, correction of electrolyte imbalances, and monitoring for post-obstructive diuresis, a condition characterized by excessive urine output following relief of obstruction.

When treated promptly, kidney function often improves significantly within days to weeks. However, prolonged or recurrent obstruction increases the risk of chronic kidney disease. Early collaboration between nephrology and urology teams is essential to optimize outcomes and prevent long-term renal impairment.