Postrenal acute kidney injury: Clinical sciences

Acute kidney injury, or AKI, refers to a sudden decline in kidney function that results in electrolyte imbalances, extracellular dysregulation, and the accumulation of nitrogenous waste, such as ammonia and uric acid.

Based on the underlying cause, acute kidney injury can be subdivided into prerenal-, renal, and postrenal acute kidney injury. Postrenal acute kidney injury is further classified based on the location of the obstruction in the urinary tract as postrenal acute kidney injury due to upper and lower obstruction of the urinary tract.

Now, if your patient presents with chief concerns suggesting postrenal AKI, perform an ABCDE assessment to determine if they are unstable or stable.

If unstable, stabilize the airway, breathing, and circulation. Next, obtain IV access, which might include dialysis access, and put your patient on continuous vital sign monitoring and cardiac telemetry!

Finally, if you identify hyperkalemia, metabolic acidosis, volume overload, or symptomatic uremia, start emergent hemodialysis!

Now, let’s go back to the ABCDE assessment and take a look at stable patients.

In stable individuals, obtain a focused history and physical exam, which is going to help you differentiate different types of AKI. Patients with post-renal AKI typically report pelvic discomfort, often in combination with slow stream or dribbling. In acute cases, the patient could report sudden pelvic pain and urinary retention. Additionally, history might reveal sudden flank pain and bloody urine or conditions like prostate disease and nephrolithiasis.

Next, the physical exam might show palpable bladder and flank tenderness. Finally, on the digital rectal exam, you might notice prostate enlargement or palpable mass! With these findings, you should suspect postrenal causes of AKI. Next, order a basic metabolic panel and urinalysis with microscopy, measure the patient’s urine output over time, and check renal ultrasound!

In all types of AKI, labs will reveal a rise in serum creatinine of 0.3 milligrams per deciliter or more over 48 hours; a rise of serum creatinine 1.5 times the baseline or more in the last 7 days, or urine output less than 0.5 milliliters per kilogram per hour for six hours.

However, in postrenal AKI, the urinalysis with microscopy will show bland urine sediment and the renal ultrasound will reveal obstruction and hydronephrosis. At this point, diagnose postrenal AKI, so your next step is to order a post-void residual, or PVR, bladder scan.

Post-void residual volume below 100 milliliters suggests upper urinary tract obstruction, so order abdominal and pelvic CT to assess the underlying cause.

If the CT confirms hydronephrosis and shows bilateral ureteral stones, diagnose nephrolithiasis. Bilateral ureteral obstruction in healthy individuals is rare. However, conditions like hyperuricemia and hyperparathyroidism increase the risk of kidney stones, so these individuals have a higher chance of developing bilateral obstruction. Keep in mind that in patients with a single-functioning kidney, unilateral ureteral stone can result in AKI. Treatment includes surgical consultation for ureteral stenting, lithotripsy, or stone removal.