Renal Abscess

A renal (intrarenal) abscess is a localised collection of pus within the renal parenchyma. It results from liquefaction of a focus of acute bacterial nephritis (lobar nephronia) or, less commonly, seeding of the kidney.

Persistent fever despite 72 hours of appropriate antibiotics for pyelonephritis is a classic clue. Mortality is about 5 to 15 percent with delayed diagnosis.

There are two main pathways:

• Ascending infection from the urinary tract is the most common today. Causes are E. coli, Klebsiella, Proteus, Enterobacter, and Pseudomonas, especially with obstruction (stone, reflux, neurogenic bladder), diabetes mellitus, and immunosuppression.
• Haematogenous spread from a distant source (skin and soft tissue infections, endocarditis, injection drug use, osteomyelitis) is typically caused by Staphylococcus aureus and forms a renal cortical abscess (carbuncle).

Perinephric extension may produce a perinephric abscess. Fungal (Candida) abscesses occur in immunocompromised hosts and neonates.

• Fever and chills, flank or abdominal pain, nausea, vomiting, malaise and weight loss; the course is often subacute (symptoms for over 5 days) with persistent fever despite antibiotics.
• Costovertebral angle tenderness, and a palpable flank mass in some.
• Urinary symptoms (dysuria, frequency) in ascending infection; the urine may be normal in haematogenous abscess when the abscess does not communicate with the collecting system.
• Septic shock in severe cases.
• History of skin infection, injection drug use, diabetes, stones, or catheterisation.

• Laboratory: leucocytosis with neutrophilia, raised C-reactive protein and erythrocyte sedimentation rate, urinalysis (pyuria, bacteriuria, or normal), blood glucose, and creatinine. Send urine and blood cultures: blood cultures are positive in about 40 to 60 percent, and urine cultures in 70 percent in ascending disease.
• Contrast enhanced CT is the preferred imaging. It shows a low attenuation, round or wedge shaped lesion with a thick enhancing rim in the renal parenchyma, with gas in some, and with perinephric stranding and fascial thickening. It excludes tumour and perinephric extension.
• Ultrasound shows a hypoechoic or complex mass with debris, and no internal vascularity.
• MRI when contrast is contraindicated (diffusion weighted images show restricted diffusion).
• Image guided aspiration for culture (with antibiotics) when blood and urine cultures are negative or if the diagnosis is uncertain.
• Echocardiography when staphylococcal bacteraemia is present (to exclude endocarditis); search for a primary source.
• Differential diagnosis: acute pyelonephritis, renal cell carcinoma, infected renal cyst, xanthogranulomatous pyelonephritis, tuberculosis, and lymphoma.

• Antibiotics: initiate promptly, intravenously, and guided by culture and the likely route.
  • Ascending (Gram negative) infection: ceftriaxone 2 g daily, piperacillin tazobactam 4.5 g every 6 hours, or ciprofloxacin 400 mg intravenously every 12 hours, adding gentamicin 5 mg/kg daily initially in severe cases.
  • Haematogenous (staphylococcal) disease: cloxacillin or flucloxacillin 2 g every 4 to 6 hours (or cefazolin 2 g every 8 hours) for methicillin sensitive S. aureus. For MRSA risk, use vancomycin 15 to 20 mg/kg every 8 to 12 hours (with trough or area-under-the-curve monitoring).
  • Candida: fluconazole 400 to 800 mg daily (or an echinocandin, caspofungin 70 mg loading dose and 50 mg daily, or liposomal amphotericin B 3 mg/kg daily) for 2 to 6 weeks.
  • Duration is 2 to 4 weeks, often 4 to 6 weeks for large abscesses.
  • After clinical improvement, step down orally according to sensitivity: ciprofloxacin 500 mg twice daily; trimethoprim-sulfamethoxazole 160/800 mg twice daily; or amoxicillin clavulanate 625 mg three times daily.
• Abscess size guides drainage.
  • Abscesses under 3 cm are treated with antibiotics alone (about 80 to 90 percent success).
  • Abscesses of 3 to 5 cm may be treated with antibiotics alone if the patient is stable, or with drainage.
  • Abscesses above 5 cm (some use above 3 cm), or those not improving within 48 to 72 hours, need percutaneous drainage. Use an ultrasound or CT-guided pigtail catheter, 8 to 12 Fr, left until output is below 10 mL per day and the cavity has collapsed on imaging. This is successful in 70 to 90 percent.
  • Open or laparoscopic surgical drainage or partial nephrectomy is used for failure, multiloculated abscess, or necrotic tissue.
• Treat the cause: relieve obstruction (ureteral stent or nephrostomy), remove stones, control diabetes, treat the primary source of bacteraemia (drain skin abscess, treat endocarditis for 4 to 6 weeks), and remove infected catheters.
• Nephrectomy for destroyed, non-functioning kidney with ongoing sepsis.
• Monitoring: daily clinical review, inflammatory markers, repeat imaging at 1 to 2 weeks or if no improvement, and imaging at the end of therapy to confirm resolution; further follow up at 3 months.
• Complications: perinephric abscess, rupture into the collecting system or peritoneum, sepsis, renal scarring and loss of function, and metastatic infection.
• Prognosis is good with early diagnosis and combined drainage and antibiotics.