Perinephric Abscess

Perinephric abscess is a collection of pus in the perirenal space, between the renal capsule and Gerota fascia. Renal (intrarenal) abscess is within the renal parenchyma, and they often coexist. Both are uncommon, serious complications of upper urinary tract infection and carry significant mortality (up to 10 to 20% with delayed diagnosis).

Anatomy: the perirenal space contains the kidney, adrenal gland, perirenal fat and vessels, enclosed by Gerota fascia, which is open inferiorly and medially (infection can spread to the psoas, retroperitoneum and subphrenic spaces). Rupture of intrarenal abscess through the capsule creates the perinephric collection.

Pathogenesis:

• Ascending (about 75%): obstruction and infection with urinary stasis, from calculi (20 to 60%), strictures, ureteropelvic junction obstruction, neurogenic bladder, vesicoureteral reflux, prior instrumentation or surgery, and pyonephrosis. Gram-negative organisms predominate (E. coli, Proteus, Klebsiella, Pseudomonas, Enterococcus). Struvite stones from urease-producers are associated.
• Haematogenous seeding (about 25%): from distant sources (skin infection, endocarditis, injection drug use, bone infection, dental infection), usually Staphylococcus aureus, forming cortical abscess and carbuncles.
• Contiguous spread from adjacent organs (appendicitis, diverticulitis, Crohn disease with fistula, liver abscess, vertebral osteomyelitis, psoas abscess, pancreatitis) and haematoma infection after trauma or surgery.
• Others: Candida (diabetes, prolonged antibiotics, catheters), Mycobacterium tuberculosis (cold abscess), anaerobes, polymicrobial infection.

Risk factors: diabetes mellitus (in 30 to 50%), urinary tract obstruction or stones, instrumentation, immunosuppression, injection drug use, trauma, pregnancy, polycystic kidney disease, renal transplant, neurogenic bladder, and previous upper tract surgery.

Complications: sepsis and septic shock, retroperitoneal extension (psoas abscess, empyema thorax), renocolic and nephrocutaneous fistula, renal loss, and chronic infection.

Presentation is often insidious, with a delay in diagnosis of 2 weeks or more:

• Persistent fever and chills (despite 4 or more days of antibiotics for presumed pyelonephritis), flank, abdominal or back pain, weight loss, malaise.
• Features suggesting a persistent source: failure to respond to appropriate antibiotics, bacteriuria persisting, and fever for over 5 days.
• Lower tract symptoms are present in less than half. Urine may be normal if the abscess is closed off from the collecting system (obstructed pyonephrosis).

Signs:

• Flank or costovertebral angle tenderness, flank mass, flank swelling or erythema, abdominal tenderness, and psoas sign (pain on hip extension, with flexion of hip, due to psoas irritation).
• Pleural effusion, basal atelectasis or pneumonia on the affected side (diaphragmatic irritation).
• Scoliosis away from the lesion, and hip flexion contracture.

Differential diagnosis: acute pyelonephritis, renal carbuncle, infected renal cyst, xanthogranulomatous pyelonephritis, renal cell carcinoma, retroperitoneal tumour, psoas abscess, subphrenic abscess, appendiceal or diverticular abscess, hepatic abscess, pancreatitis, lower lobe pneumonia, and tuberculosis.

Red flags: sepsis, persistent fever after 72 hours of antibiotics, diabetes with poorly controlled glucose, obstruction, unusual organisms, gas on imaging, and signs of extension into the thorax or abdomen.

Laboratory:

• CBC (leukocytosis with left shift), CRP and ESR (markedly elevated), procalcitonin, creatinine, electrolytes, glucose and HbA1c, blood cultures (positive in about 40 to 50%), and lactate.
• Urinalysis (pyuria, bacteriuria in 50 to 70% of cases, may be sterile) and urine culture. Culture of pus from drainage is the definitive microbiological test, with Gram stain, aerobic and anaerobic cultures, fungal and mycobacterial cultures if indicated.

Imaging (the key to diagnosis):

• CT abdomen and pelvis with IV contrast (preferred, sensitivity over 95%): a low attenuation collection with rim enhancement in the perirenal space, thickened Gerota fascia, perinephric fat stranding, gas bubbles (anaerobic organisms), and the underlying cause (stones, obstruction, gas in the parenchyma). Assess extension into the psoas or other compartments.
• Ultrasound: first-line bedside tool for hydronephrosis, perinephric fluid collection, calculi, and guiding drainage (less sensitive).
• MRI in pregnancy or contrast allergy.
• Radiograph (abdominal): may show renal outline obscuring, psoas shadow loss, scoliosis, calculi, and gas.
• Gallium or labelled leukocyte scans are rarely used.

Aspiration: image-guided needle aspiration for diagnosis and culture.

Further evaluation: investigate the underlying cause (urography, CT urogram, cystoscopy), tuberculosis tests (interferon gamma release assay, urine AFB) where suspected, and echocardiography for staphylococcal bacteraemia.

Combine antibiotics, drainage, and treatment of the cause.

Antibiotics

• Start broad-spectrum IV antibiotics after cultures, covering Gram-negative bacilli, Enterococcus, and Staphylococcus, with anaerobic cover when gas is present or contiguous source:
• Piperacillin-tazobactam 4.5 g IV every 6 hours, or ceftriaxone 2 g daily plus metronidazole 500 mg every 8 hours.
• Add vancomycin 15 to 20 mg/kg every 8 to 12 hours for S. aureus (haematogenous source, MRSA risk) or flucloxacillin 2 g every 4 to 6 hours for methicillin-susceptible S. aureus.
• Meropenem 1 g IV every 8 hours for ESBL or healthcare-associated infection.
• Fluconazole 400 to 800 mg daily or liposomal amphotericin B 3 mg/kg daily for Candida.
• Narrow therapy by culture results. Total duration is 2 to 6 weeks, guided by clinical response and imaging (abscess resolution), with step-down to oral therapy (ciprofloxacin 500 mg twice daily, amoxicillin-clavulanate, trimethoprim-sulfamethoxazole, per susceptibility) when drained, afebrile, and tolerating oral intake.
• Small abscesses (under 3 cm) may be managed with antibiotics alone (4 to 6 weeks) with close monitoring.

Drainage (source control)

• Percutaneous drainage: CT- or ultrasound-guided catheter (8 to 14 French pigtail) for perinephric abscess over 3 to 5 cm, with success about 70 to 90%. Irrigate with saline daily, remove the catheter when output is below 10 to 20 mL/day and imaging shows resolution. Multiloculated or viscous collections may need larger catheters or fibrinolytics (alteplase 2 to 4 mg intracavitary).
• Open surgical drainage (flank incision with debridement and irrigation) for failure of percutaneous treatment, multiloculated or complex abscess, and extension to other spaces.
• Nephrectomy: for non-functioning, destroyed kidney with xanthogranulomatous pyelonephritis, emphysematous infection, or life-threatening sepsis unresponsive to other measures. Partial nephrectomy occasionally.

Relief of obstruction

• Ureteric stent or percutaneous nephrostomy for obstructing stones or other obstruction, as an emergency in infected systems. Definitive stone treatment (ureteroscopy, percutaneous nephrolithotomy) after infection resolves, because stones are a reservoir for recurrence.

Supportive care

• Fluid resuscitation, sepsis management, glucose control (insulin infusion in hyperglycaemia, target 140 to 180 mg/dL), analgesia, and nutritional support.
• Thromboprophylaxis with enoxaparin 40 mg daily.

Specific causes

• Tuberculosis: standard anti-tuberculosis therapy (isoniazid, rifampicin, pyrazinamide, ethambutol) with drainage.
• Contiguous disease: treat the source (appendectomy, bowel resection, vertebral disease).
• Haematogenous: search for and treat the primary focus (echocardiography, skin and bone infections, dental).

Follow-up

• Repeat imaging (CT) at 1 to 2 weeks after drainage or if there is no improvement, and at the end of therapy to confirm resolution. Follow creatinine and CRP. Monitor for recurrence and renal scarring.
• Address predisposing factors (diabetes control, stone clearance, urological abnormalities).