Acute Postrenal Kidney Injury

Acute postrenal kidney injury is an abrupt fall in kidney function caused by obstruction of urine outflow from both kidneys, or one in a solitary functioning kidney.

• It accounts for 5 to 10 percent of acute kidney injury in hospital and more in community presentations; elderly men with benign prostatic hyperplasia and women with pelvic malignancy or after pelvic surgery are most affected.
• Causes: bladder outlet (prostatic enlargement, urethral stricture, clots, constipation, anticholinergic drugs, neurogenic bladder), bilateral ureteric (stones, tumours, retroperitoneal fibrosis, iatrogenic ligation, bladder tumour, cervical carcinoma), blocked catheter, papillary necrosis, fungal balls, and schistosomal strictures.
• Obstruction raises intratubular pressure and reduces glomerular filtration; prolonged obstruction causes tubular injury.

Anuria or abrupt fall in urine volume, variable urine output (intermittent flow), suprapubic pain, loin pain, lower urinary tract symptoms, palpable bladder, enlarged prostate on rectal examination.

• Features of uraemia: nausea, vomiting, drowsiness, hiccups, pruritus, pericarditis, fluid overload.
• Differential diagnosis: prerenal acute kidney injury, acute tubular necrosis, glomerulonephritis, acute interstitial nephritis, renal vascular occlusion, rhabdomyolysis, hepatorenal syndrome.
• Complications: hyperkalaemia and arrhythmia, metabolic acidosis, pulmonary oedema, uraemic encephalopathy, infection with sepsis, post-obstructive diuresis, haemorrhage after decompression, permanent loss of renal function.

Bladder scan for retention at the bedside (residual above 300 mL); urethral catheterisation gives both diagnosis and treatment.

• Ultrasound of kidneys and bladder (hydronephrosis in most; may be absent early within 24 to 48 hours, in dehydration or with encasement).
• Creatinine, urea, potassium, bicarbonate, calcium, phosphate, full blood count, urinalysis, culture, ECG for hyperkalaemia, PSA with prostate examination.
• CT urography or non-contrast CT for ureteric cause; renography and retrograde pyelography when uncertain.

Supportive: monitor urine output hourly, daily weights, cardiac monitoring if potassium is above 6.0 mmol/L; avoid nephrotoxins (NSAIDs, contrast, aminoglycosides); renally dose drugs.

Pharmacological

• Hyperkalaemia: calcium gluconate 10 percent 10 mL IV over 5 to 10 minutes (repeat if ECG changes persist), soluble insulin 10 units with 25 g glucose (50 mL of 50 percent) over 15 minutes, salbutamol 10 to 20 mg nebulised, sodium bicarbonate 50 mmol IV if acidotic, and potassium binders (sodium zirconium cyclosilicate 10 g three times daily for up to 48 hours, or calcium resonium 15 g three times daily).
• Pulmonary oedema: furosemide 80 to 120 mg IV and nitrates; dialysis if refractory.
• Sepsis with obstruction: ceftriaxone 2 g IV daily plus gentamicin (reduced dose) or piperacillin-tazobactam 4.5 g IV every 8 hours (adjust for eGFR) and urgent drainage.
• Cervical cancer obstruction: palliative stents and oncology.

Procedures

• Immediate bladder drainage: urethral catheter (14 to 16 Fr; coude catheter for prostatic obstruction) or suprapubic catheter; document the drained volume.
• Upper tract: ureteric stent or percutaneous nephrostomy under imaging, the latter preferred for infected obstruction and malignancy.
• Post-obstructive diuresis monitoring: urine above 200 mL/h for 2 hours or above 3 L daily; replace with 0.45 percent saline at 50 to 75 percent of output, adjusting to weight, electrolytes and blood pressure; haematuria after decompression usually settles.
• Dialysis for refractory hyperkalaemia (above 6.5 mmol/L), pH below 7.1, refractory pulmonary oedema, uraemic pericarditis or encephalopathy.
• Definitive surgery: transurethral resection of the prostate, urethrotomy, stone removal, ureteric repair.

Special groups: elderly men (alpha blocker, removal of the catheter after 2 to 3 days to test voiding), pregnancy (stent or nephrostomy), diabetes (autonomic bladder).

Red flags: potassium above 6.5 mmol/L, urine output below 100 mL daily, fever with obstruction, pH below 7.1, volume overload.

Follow-up: creatinine daily until it plateaus; ultrasound after decompression; outpatient nephrology and urology review; creatinine at 3 months to document recovery.