Renal Abscesses

• Intrarenal abscesses are classified into renal cortical abscess and renal corticomedullary abscess.
• The treatment of renal abscesses is still under debate, perhaps because of the failure of many recent studies to distinguish between renal cortical abscesses and renal corticomedullary abscesses, which are different in their pathogenesis, prognosis, and therapies.
• Similar to perirenal abscesses, patients with intrarenal abscesses can present to the
  Emergency Department acutely with fever, flank pain, nausea, and vomiting, masking as a classic pyelonephritis. However, mortality rate for intrarenal abscess has been positively correlated with the timeliness of diagnosis.

• Renal carbuncles result from hematogenous spread of bacteria from primary focus of infection elsewhere in the body, usually skin lesions such as cutaneous carbuncles, furuncles, paronychia, cellulitis, osteomyelitis, and endovascular infections.
• The most common cause is S. aureas infection, and conditions associated with an increased risk for staphylococcal bacteremia suchas diabetes mellitus and intravenous drug use are predisposing factors.
• Most carbuncles are unilateral, single lesions occurring in the right kidney. Only 10% of these eventually rupture through the renal capsule to form a perinephric abscess.
• In contrast, renal corticomedullary infections occur most commonly as a complication of ascending urinary tract infections with or without accompanying urinary tract abnormalities, which include most commonly obstructive problems such as scarring from previous infections or renal stones, or genitourinary abnormalities associated with diabetes mellitus.
• Enteric aerobic Gram-negative bacilli, including Escherichia coli, Klebsiella species, and Proteus species are commonly responsible for renal corticomedullary infections.
• Another contrast between renal cortical abscesses and renal corticomedullary abscesses is that the Gram-negative corticomedullary infection frequently causes a severe parenchymal infection that may extend to and perforate the renal capsule, thus more commonly forming a perinephric abscess.

Clinical Features

• Unlike other intrarenal abscesses, renal carbuncles are approximately three times more common in men than in women.
• They occur most commonly between the second and fourth decades of life in patients presenting with chills, fever, back or abdominal pain, and few localizing signs.
• Although 95% of patients present with elevated white blood cell counts, most patients do not have bacteruria or dysuria as the infectious process is circumscribed in the cortex and generally does not communicate with the excretory passages.
• Consequently, urinalysis is usually normal. Likewise, blood cultures are usually negative.
• Although dysuria may not necessarily be present in renal corticomedullary patients, they may have a previous history of recurrent urinary tract infections, renal calculi, or a history of prior genitourinary emergencies instrumentation.
• Again, leukocytosis is generally present, but urinalysis is often abnormal in renal corticomedullary abscesses (70% of the time) with bacteriuria, pyuria, proteinuria, or hematuria because of drainage into the collecting system.

Diagnosis

• The nonspecific clinical presentation of fever, chills, and back pain may be seen with a variety of renal processes.
• Renal cortical abscesses can mimic renal tumors, cysts, renal corticomedullary abscesses, and perirenal abscesses. Furthermore, renal cortical abscesses are difficult to distinguish from renal medullary abscesses.
• Ultrasonography is useful in the diagnosis of cortical abscesses because it provides information about renal morphology and characterizes an intrarenal lesion as cystic, tumorous, or suppurative.
• Furthermore, the ultrasound can provide information about the presence of an obstructive uropathy, retroperitoneal or intra-abdominal processes, and suppurative renal complications.
• Although the ED ultrasound is used often to diagnose patients with intrarenal abscess, there are no current studies that describe the sensitivity and specificity of its use in the ED.
• To date, the CT scan provides the most anatomic information and is able to detect abscesses < 2 cm in size.
• Particularly if the ultrasound is equivocal or negative, CT scan may be of benefit in definitive diagnosis.
• On CT, most abscesses appear as low-density masses with vascular enhancement of the wall. Gas within a low-density mass is pathognomonic for an abscess.

Prognostic Factors

• Bamberger et al demonstrated that poor prognostic factors were abscesses of diameter >5 cm, involvement by more than one organism, presence of Gram-negative bacilli, duration of therapy < 4 wk, and use of aminoglycoside as the only antibiotic.
• Factors that bear resistance to antibiotic therapy alone include large abscesses, renal obstruction, advanced age, and urosepsis.

Treatment

• Unlike emphysematous pyelonephritis, renal abscesses are managed medically as firstline treatment.
• There is mounting evidence that the success of renal abscesses treated with antibiotics \alone can be as high as 86% in large studies. Because S. aureus is usually the cause of the renal carbuncle, it responds to antistaphylococcal antibiotics, and surgical intervention is not required.
• If urinalysis shows no bacteria or Gram-positive cocci, oxacillin or nafcillin, 1-2 g every 4-6 h, is the therapy of choice.
• For penicillin allergic patients, first generation cephalosporins provide adequate Gram-positive coverage.
• Parenteral antibiotics should be continued for 10 days to 2 wk, and subsequent oral antistaphylococcal therapy for 2-4 wk.
• The course of resolution includes defervescence after 5-6 days of IV antibiotics, and improvement of flank pain in <24 h.
• For renal corticomedullary abscesses, medical therapy is successful in most cases; however, smaller renal abscesses are more successfully treated with antibiotic treatment alone than larger abscesses, >5 cm in diameter.
• In most cases, an intensive trial of appropriate antibiotic therapy should be attempted before considering surgical drainage for lesions localized to the renal parenchyma.
• Antimicrobial therapy should target the most common bacterial organisms, including E.coli, Klebsiella, and Proteus species.
• Monotherapy can be given with an extended spectrum penicillin, and extended spectrum cephalosporin or ciprofloxacin. Combination therapy has not been proven to be any more effective.
• Considerations for the ED physician in determining whether further surgical intervention is needed include failure of antibiotic therapy, large abscess >5 cm diameter, multifocal abscesses, obstructive uropathy, advanced age, deteriorating patient, and immunocompromised patient.
• Percutaneous drainage of the abscess combined with full course of parenteral antibiotics have been shown to be successful in those requiring drainage, offering the advantages of minimal invasiveness, favorable nephron-sparing, and minimal morbidity.
• If open drainage is required, incision and drainage, not total nephrectomy, are recommended when possible.
• Nephrectomy is reserved for elderly, septic patients with diffuse renal parenchymal injury requiring urgent intervention for survival.