A perinephric abscess is a collection of suppurative material in the perinephric space. A perinephric abscess can pose a great diagnostic challenge, even to an astute clinician. This is very important because a delay in diagnosis increases the risk of morbidity and mortality. Diagnosis of a perinephric abscess should be considered in any patient with fever and abdominal or flank pain.
The increased use of CT scanning has allowed for earlier and accurate diagnoses of this condition, and newer antibiotics have been helpful in the appropriate treatment during the last 3 decades.
A perinephric abscess is a collection of purulent material around the kidneys, with a presentation that is insidious (>14 d). This abscess formation occurs secondary to urinary tract obstruction and/or hematogenous spread from infection sites.
Perinephric abscess is an uncommon complication of urinary tract infections. The incidence ranges from 1-10 cases for every 10,000 hospital admissions. Men and women are affected with equal frequency. Patients with diabetes account for one third of all perinephric abscess cases. 
Escherichia coli, Proteus species, and Staphylococcus aureus are the usual etiologic organisms. The use of antibiotics for skin and wound infections also has decreased the incidence of staphylococcal infection from 45% to 6% over the last 6 decades. However, this rate has increased from 8% to 30% for E coli infections and from 4% to 44% for Proteus mirabilis infection.
Other gram-negative bacteria that can cause this infection include Klebsiella, Enterobacter, Pseudomonas, Serratia, and Citrobacterspecies.
Occasionally, the infection can occur from enterococci infection. One case caused by Streptococcus pneumoniae infection has been reported.  Anaerobes such as Clostridium, Bacteroides, and Actinomyces may account for some of the culture-negative abscesses.
Other causes include fungi, especially Candida species, and Mycobacterium tuberculosis. Multiple bacteria can be present in as many as 25% of cases.
Perinephric abscess secondary to Candida infection usually occurs in patients with diabetes. Predisposing factors include surgery (including renal transplantation  ) and prolonged antibiotic therapy.
Perinephric abscesses are located between the capsule of the kidney and the Gerota fascia. The abscesses remain confined in this location because of the Gerota fascia. Perinephric abscesses usually occur because of disruption of a corticomedullary intranephric renal abscess, recurrent pyelonephritis, xanthogranulomatous pyelonephritis, or an obstructing renal pelvic stone causing pyonephrosis.  Approximately 30% of cases are attributed to hematogenous dissemination of organisms from sites of infection such as wound infection, furuncles, or pulmonary infections. Abscesses can also be caused by ascending urinary tract infection.
The most common mechanism for gram-negative bacterial abscess to develop is the rupture of a corticomedullary abscess, while the most common mechanism for the development of a staphylococcal infection is the rupture of a renal cortical abscess. This finding frequently is observed in association with a previous renal operation such as a partial nephrectomy or nephrolithiasis or, most commonly, as a complication of diabetes mellitus (60-90%).
Perforation of a ureter or a calyceal fornix may rarely result in perinephric abscess formation.
Occasionally, a perinephric abscess results from the spread of infection from extraperitoneal sites, such as in retroperitoneal appendicitis, diverticulitis, pancreatitis, and pelvic inflammatory conditions. In some instances, perinephric abscess is caused by bowel perforation, Crohn disease, or osteomyelitis from the spine.
Patients with polycystic renal disease who undergo hemodialysis may be particularly susceptible to developing perinephric abscess (62% of cases).
Predisposing factors for perinephric abscess include neurogenic bladder, vesicoureteral reflux, bladder outlet obstruction, renal papillary necrosis, obstructing calculus, genitourinary tuberculosis, trauma (eg, renal biopsy,  urinary instrumentation, urologic surgery), immunosuppression, and intravenous drug abuse.
When a perinephric infection ruptures through the Gerota fascia into the pararenal space, it leads to the formation of a paranephric abscess. Paranephric abscesses may also be caused by infectious disorders of the intestine, pancreas, liver, gall bladder, prostate, and pleural cavity, and they may be caused by osteomyelitis of adjacent ribs or vertebrae. Sometimes, with a superimposed infection, a perirenal hematoma can progress to a perinephric abscess.
Because of nonspecific findings, in many cases, diagnosing a perinephric abscess can be difficult. Typically, patients present with a history of skin infections or urinary tract infections. An infection may be followed in 1-2 weeks by fever and unilateral flank pain. However, this is an uncommon presentation.
Typically, the onset of symptoms is insidious, and 58% of patients have symptoms for more than 14 days.
Presenting symptoms are often nonspecific. Only occasionally, a patient presents with a syndrome suggestive of acute pyelonephritis, with fever and abdominal and flank pain (usually unilateral). One distinguishing feature to note is that most patients with uncomplicated pyelonephritis are symptomatic for less than 5 days before hospitalization, whereas most patients with perinephric abscesses are symptomatic for more than 5 days.
The most common symptoms include fever (66-90%), flank or abdominal pain (40-50%), chills (40%), dysuria (40%), weight loss, lethargy, and gastrointestinal symptoms (25%). Pleuritic pain may occur due to diaphragmatic irritation. If the abscess is pressing the adjacent nerves, the referred pain may be felt in the groin, thighs, or knees.
Physical findings include flank or costovertebral tenderness. When abdominal tenderness is present (60%), it may complicate the diagnosis. Patients may present with rigidity and fullness. A flank mass is palpable if the abscess is large or located in the inferior pole of the kidney space (9-47%). A renal malignancy must be ruled out in these patients with appropriate radiographic studies (eg, CT scanning, MRI). Splinting may be present, with resultant scoliosis. Patients may experience pain upon bending toward the contralateral side, upon active flexion of the ipsilateral thigh against resistance, and upon extension of the thigh while walking. Consider the diagnosis of perinephric abscess in patients with unilateral flank pain and fever, no response to treatment for acute pyelonephritis, pyrexia of unknown origin, unexplained peritonitis, pelvic abscess, and empyema.
Promptly treat all perinephric abscesses. Failure to treat can result in severe morbidity or even death. Certain conditions, such as renal cortical abscess or enteric fistulas, may require immediate surgical intervention (see Surgical therapy). 
Knowledge of the retroperitoneal structures is vital in understanding the development of perinephric abscesses.
Anterior and posterior layers of renal fascia divide the retroperitoneum into 3 extraperitoneal spaces. The first, the anterior paranephric space, extends from the posterior peritoneum to the anterior renal fascia (Gerota). The second, the perinephric space, lies between 2 layers of the renal fascia. The third, the posterior paranephric space, extends from the posterior renal fascia to the fascia that lies anterior to the psoas and quadratus lumborum muscles.
The renal fascia (Gerota) surrounds the kidney and adrenal gland. Perinephric fat is present between the renal capsule and this fascia. The perinephric space also contains some blood vessels and lymphatics, which facilitate the spread of infection. The 2 layers join above the adrenal glands and are attached to the diaphragmatic fascia. They join laterally to form the lateroconal fascia that is present posterior to the colon. The anterior fascia of Zuckerkandl extends anterolaterally and then blends with the parietal peritoneum. Posteriorly, the Gerota fascia joins the quadratus lumborum fascia medially, while the anterior fascia joins the root of the mesentery and lies behind the pancreas and the duodenum.
The perinephric space becomes cone-shaped as it narrows inferiorly and medially and then joins with the iliac fascia. The inferomedial angle of the space is the weakest point, accounting for the extension of fluid collection across the midline and into the pelvis.
The only contraindication to treatment of a perinephric abscess is bleeding dyscrasias. Correct this condition prior to percutaneous drainage.
A relative contraindication is patients who are at increased anesthetic risk who require nephrectomy for treatment. Optimize these individual medical conditions prior to surgery.