Urinary tract obstruction in the presence of pyelonephritis may lead to the collection of white blood cells (WBCs), bacteria, and debris in the collecting system, which may subsequently result in pyonephrosis. In this situation, with the accompanying "pus under pressure," patients may deteriorate rapidly and become septic. Thus, early recognition and treatment of acute infections of the kidney, especially in patients with suspected urinary tract obstruction, are of paramount importance.
Similar to an abscess, pyonephrosis is typically associated with fever, chills, and flank pain, although some patients may be asymptomatic. Pyonephrosis may be caused by a broad spectrum of pathologic conditions involving either an ascending infection of the urinary tract or the hematogenous spread of a bacterial pathogen. 
Risk factors for pyonephrosis include immunosuppression due to medications (eg, steroids), disease (eg, diabetes mellitus, acquired immunodeficiency syndrome [AIDS]), and any anatomic urinary tract obstruction (eg, stones, tumors, ureteropelvic junction [UPJ] obstruction, pelvic kidney, horseshoe kidney).
Pyonephrosis is uncommon in adults and rare in children, and it is thought to be extremely rare in neonates. However, pyonephrosis has been reported in several neonates  and adults, making it clear that the condition may develop in any age group.
The true incidence of development of pyonephrosis with other renal infections is not reported. However, the risk of pyonephrosis is increased in patients with upper urinary tract obstruction secondary to various causes (eg, stones, tumors, ureteropelvic junction [UPJ] obstruction). [3, 4]
In pyonephrosis, purulent exudate—consisting of inflammatory cells, infectious organisms, and a necrotic, sloughed urothelium—collects in a hydronephrotic collecting system and forms an abscess. The exudate becomes walled off and protected from the body's natural immune system and from antibiotics. If not recognized and treated promptly, this infectious process may progress, often resulting in clinical deterioration of the patient with urosepsis, which can occur swiftly.
With the advent of ultrasonography and computed tomography (CT) scanning, drainage—either percutaneously or retrograde with a ureteral stent—has become the mainstay of treatment. [5, 6, 7] Drainage has low morbidity and mortality rates and an excellent outcome. Drainage guided by CT scan or ultrasonography significantly decreases the need for nephrectomy, resulting in renal preservation.
In selected healthy, stable patients, consider retrograde decompression with a stent as an option. This avoids placement of a percutaneous nephrostomy tube and allows internalization of the drainage catheter. However, it does not allow for the antegrade medication infusion or the treatment of obstruction that is sometimes needed with funguria and infected stones. [8, 9]
When indicated, laparoscopic nephrectomy for inflammatory kidney disorders such as pyonephrosis has been shown to be safe and effective. The largest risk to the patient in these cases is typically the need to convert to an open procedure (28%) and wound infections. [10, 11]
For patient education information, see Urinary Tract Infections (UTIs).
Immunocompromised patients and those who are treated with long-term antibiotics are at an increased risk for fungal infections. When fungus balls are present, they may obstruct the renal pelvis or the ureter, resulting in pyonephrosis. Xanthogranulomatous pyelonephritis, a clinical condition consisting of upper renal calculus and infection, has been reported to progress to pyonephrosis when obstruction is present. Several case reports indicate that obstruction from tumors, such as in transitional cell carcinoma, may also cause pyonephrosis. [15, 16] The disease process of pyonephrosis consists of 2 parts: infection and infection.
As reported in the current literature, multiple infectious agents have been isolated in patients with pyonephrosis. These include the following, in decreasing order of incidence:
Candida species and other fungal infections
Methicillin-resistant Staphylococcus aureus (MRSA) 
Tuberculosis (causes both infection and strictures)
The etiology of the obstruction may relate to any of the following factors:
Stones and staghorn calculi - In as many as 75% of patients
Metastatic retroperitoneal fibrosis - Eg, renal tumors, testicular cancer, colon cancer
Mucinous adenocarcinoma of the renal pelvis 
Ureteropelvic junction (UPJ) obstruction
Ureterovesical junction obstruction
Chronic stasis of urine and hydronephrosis secondary to neurogenic bladder
Duplicated kidneys with obstructive components
Ectopic ureter with ureterocele 
Other, rare causes, such as sciatic hernias that cause ureteral obstruction
The prognosis of pyonephrosis is good in most patients who receive prompt diagnosis and treatment. Most infectious processes resolve within 24-48 hours and significantly improve after either nephrostomy or retrograde stent drainage of the infection. If pyonephrosis is recognized and treated promptly, recovery of the affected renal unit is rapid. Long-term complications are rare when pyonephrosis is managed promptly; however, injury to the functional renal unit, abscesses, fistulas, and scarring may occur when definitive therapy is delayed.
Patients with pyonephrosis that is not recognized early may rapidly deteriorate and develop septic shock. In addition to the morbidity and mortality associated with septic shock, potential complications of delayed diagnosis and treatment of pyonephrosis include irreversible damage to the kidneys, with the possible need for nephrectomy. Even in the modern era of antibiotics, adequately controlling an overwhelming infection in an obstructed renal unit without surgical intervention may be impossible. If the diagnosis is delayed, unduly prolonged illness and death may result.
Sepsis is the most common complication in the perioperative period when treatment is delayed.
Generalized peritonitis can result from a rupture of the pyonephrotic kidney. In 1996, Hendaoui et al reported the first case of a splenic abscess that developed from a ruptured pyonephrosis after the development of generalized peritonitis.  Other occurrences of intraperitoneal and retroperitoneal spontaneous rupture have been reported by Sugiura et al, in 2004;  Chung et al, in 2009;  and Quaresima et al, in 2011,  making it possibly much more common than originally thought.
Fistulas may develop and can be associated with peritonitis.  Renocolonic, renoduodenal,  and renocutaneous fistulas are the most common; therefore, suspect these in patients who manifest continued electrolyte disorders, diarrhea, and recurrent urinary tract infections after resolution of pyonephrosis.
Other rare complications include pneumoperitoneum from lithogenic pyonephrosis, nephrobronchial fistula, renal vein thrombosis, psoas abscess and/or perinephric abscess, and rhabdomyolysis.
Delay in diagnosis and treatment may result in a loss of renal function from parenchymal damage.
Perinephric hematomas, blood transfusions, and the need for nephrostomy tube revision are complications of percutaneous drainage. If a nephrectomy must be performed in the future, long-term nephrostomy tubes are reported to increase the risk of a postoperative wound infection. 
Patients with pyonephrosis may present with clinical symptoms ranging from asymptomatic bacteruria (15%) to frank sepsis. Maintain a high index of suspicion when examining a patient with a history of fever, flank pain, evidence of a urinary tract infection, and obstruction or hydronephrosis. On physical examination, a palpable abdominal mass may be associated with the hydronephrotic kidney. Rarely, the infected hydronephrotic kidney may rupture spontaneously into the peritoneal cavity, causing some patients to present with diffuse peritonitis and sepsis. 
A complete blood cell count (CBC) with a manual differential, serum chemistry with blood urea nitrogen (BUN) and creatinine, a urinalysis with culture, and blood cultures are indicated in the initial workup of a patient with suspected pyonephrosis. 
A urine culture of the fluid above the obstruction must be obtained in order to guide antibiotic therapy. A culture specimen may be obtained from an open-ended catheter that has been advanced above the obstruction during stent placement. Cultures should also be obtained from the percutaneous tube at the time of nephrostomy placement if this course of action is chosen.
Leukocytosis and bacteruria may be present; however, they are not specific for pyonephrosis and may be due to other causes (eg, pyelonephritis, uncomplicated urinary tract infections).
Pyuria, while often present in pyonephrosis, is nonspecific. Bacteruria, fever, pain, and leukocytosis can be absent in 30% of patients with pyonephrosis.
C-reactive protein (CRP) value has been shown to help in the diagnosis of pyonephrosis. In a study of 110 patients with renal colic, CRP levels greater than 28 mg/L indicated the need for emergent drainage, with good reliability. [25, 26] However, in a systematic review the evidence for use of CRP testing to differentiate pyelonephritis from cystitis in children with urinary tract infections did not support routine use in clinical practice. 
Lertdumrongluk et al investigated the use of urine heparin-binding protein (UHBP), a cytokine released from activated neutrophils, for the diagnosis of acute pyelonephritis (APN) and concluded that UHBP is a valid adjunctive diagnostic tool for aiding clinicians in making rapid treatment decisions for APN. 
Aspiration of the collecting system with CT scan or ultrasonographic guidance, with Gram stain and culture of the fluid, provides a definitive diagnosis of pyonephrosis. Sending the culture for aerobic, anaerobic, and fungal pathogens is important. If clinically indicated, perform acid-fast stain and send cultures for tuberculosis testing.
Routine radiographic imaging of patients with uncomplicated urinary tract infections is not generally advocated. However, appropriate radiographic studies are beneficial in diagnosing pyonephrosis, emphysematous pyelonephritis, and renal and/or perirenal abscess when patients do not improve rapidly with appropriate antibiotics. [29, 30]
Ultrasonographic findings suggestive of pyonephrosis include the presence of hydronephrosis in conjunction with hyperechoic debris in the collecting system (see image below). The presence of debris and layering of low-amplitude echoes in the hydronephrotic kidney indicate pyonephrosis. These low-level echogenic foci in the collecting system remain the most consistent finding in pyonephrosis. These findings are specific enough that their absence excludes pyonephrosis with a high degree of accuracy. [31, 34, 20, 35, 36, 37]
Echogenic gas is rarely demonstrated. Intrarenal gas appears as "dirty shadows." If echogenic gas is present, assume that the patient has a severe infection and possible renal injury suggestive of emphysematous pyelonephritis.
Ultrasonography does have drawbacks. [37, 38] For example, it may not always differentiate hydronephrosis from early pyonephrosis. In these cases, consider conducting an ultrasonographically guided aspiration of the hydronephrotic fluid for microscopic examination to establish the diagnosis.
CT scanning is extremely helpful in diagnosing pyonephrosis (see the image below). Advantages of CT scanning include definitive delineation of the obstruction, the function of the kidney, and the severity of hydronephrosis, as well as the presence of other abdominal pathologies, including metastatic cancer, retroperitoneal fibrosis, and renal stones that are not visible on the sonogram. 
Increased wall thickness of the renal pelvis of ≥2 mm
The presence of renal pelvic contents and debris
Parenchymal and perirenal findings, such as perirenal fat stranding.
The presence of pyonephrosis during episodes of acute pyelonephritis has been described as a negative prognostic indicator for patients who do not respond appropriately to treatment for the pyelonephritis. Kim et al developed a scoring system for CT scanning of patients admitted with acute pyelonephritis that includes the presence or absence of pyonephrosis.  Increased scores on admission indicate more severe illness, thus possibly requiring surgical treatment or other intervention for the infected hydronephrosis.
This study is seldom helpful in the diagnosis of pyonephrosis. The decreased function of the affected kidney that accompanies active infection typically prevents visualization on contrast studies.
Magnetic resonance imaging (MRI) has come to be used extensively for imaging of inflammatory conditions of the genitourinary tract.  Findings in pyelonephritis include renal enlargement with a heterogeneous appearance on T2-weighted images. Perirenal edema can also be seen on T2-weighted images, as areas of high signal intensity. 
A method to distinguish pyonephrosis from noninfected hydronephrosis has been proposed based on diffusion MRI.  In this method, pyonephrosis is correlated with marked, hyperintense signals in the collecting system (which correspond to the pus in the system), while the hydronephrotic kidney without pus is hypointense.
Renal nuclear scanning
Renal nuclear scanning is not particularly helpful in the immediate diagnostic workup of pyonephrosis. Acutely, scans may exhibit prolonged cortical uptake with delayed excretion of radionuclide similar to that observed in acute obstruction. The image may also be similar to that observed in acute pyelonephritis, with defects in uptake of the radiopharmaceutical that can be unifocal, multifocal, or diffuse. These defects often resolve with resolution of the infection; however, persistence in follow-up renal nuclear scans may indicate permanent damage to the renal cortex. 
Renal nuclear scanning may be helpful when a kidney is believed to be nonfunctional on any imaging study during the acute phase of pyonephrosis. Performing follow-up nuclear imaging studies is prudent after resolution of the infection to establish the function of the involved kidney. If a kidney is proven to be nonfunctional after resolution of infection and treatment of the etiology of the obstruction, then nephrectomy may be indicated to prevent further episodes of pyonephrosis.
This test may be extremely helpful in determining the etiology of the obstruction associated with pyonephrosis and in planning further treatment strategies.
As with any invasive procedure, nephrostography should be delayed until the patient is stable, on antibiotics, and afebrile for 1-2 weeks after placement of a nephrostomy tube.
Further imaging tests
When a definitive anatomic abnormality, such as the presence of a stone or tumor, cannot be determined, further imaging studies and tests may be needed to establish the etiology of the pyonephrosis.
These tests may include voiding cystourethrography, to exclude vesicoureteral reflux; multichannel urodynamics, to establish a possible neurogenic bladder with urine stasis; and serial renal ultrasonography, to document resolution of hydronephrosis after treatment.
The treatment of pyonephrosis has changed dramatically over the years. Prior to the 1980s, emergency surgical excision with nephrectomy was the standard of care. However, this was associated with a high morbidity and complication rate, including sepsis, wound infections, peritonitis, and fistulas.
Initially, treat patients with appropriate intravenous antibiotics consisting of an aminoglycoside (gentamicin) and gram-positive coverage (ampicillin), prior to instrumentation. Depending on the clinical situation, additional anaerobic coverage with clindamycin may be needed. Be cognizant of the fact that patients may have fungal infection or tuberculosis. The use of antifungal or antibacterial agents is predicated on culture results.
Many patients are septic and may require aggressive fluid resuscitation with crystalloids. Pressor support (with dopamine) may be needed to maintain adequate blood pressure and hemodynamics.
With the global emergence of carbapenem-resistant enterobacteriaceae, ceftazidime-avibactam offers a highly effective alternative for the treatment of patients with complicated urinary tract infection, including acute pyelonephritis. 
Decompression, Drainage, and Blockage Removal
Pyonephrosis is a surgical emergency and needs immediate intervention. Pyonephrosis may be treated with either an antegrade or a retrograde decompression. Retrograde decompression, or placement of a ureteral stent, is indicated in stable patients with no signs of hemodynamic instability. Intravenous antibiotics must be given prior to stent placement in stable patients; after this is done, retrograde decompression may be safely undertaken. New evidence indicates that retrograde stent placement may be safe in some cases and does not increase the risk of sepsis and bacteremia. 
Disadvantages of retrograde decompression include lack of antegrade access for radiologic studies, a smaller-caliber urinary drainage catheter than with percutaneous access, increased irritative urinary symptoms, an inability to administer medications such as antibiotics via nephrostomy tube, and limitation of percutaneous chemolysis that may cause dissolution of any stones. To maximize drainage, a urethral catheter should be left in place after stent placement.
A retrograde approach usually requires a general anesthetic, and bypassing the obstruction may not be possible in some patients. In addition, pyelovenous, pyelolymphatic, and pyelosinus backflow of infected urine into the systemic circulatory system is always a risk with retrograde manipulation. This may result in iatrogenic sepsis and patient decompensation.
Definitive management of stones and obstruction with ureteroscopy, lithotripsy, or endopyelotomy is contraindicated in the immediate treatment of patients with pyonephrosis. If retrograde stent placement is chosen, the surgeon should attempt to minimize instrumentation and retrograde pyelography as much as possible and should decompress the obstruction with minimal trauma to the urinary tract.
Reports exist of ureteroscopic instrumentation involving stone and obstruction removal during active infection. Although performed at some institutions, the authors do not recommend this practice, because it may result in sepsis and worsening infection.
Antegrade treatment with percutaneous nephrostomy tube placement is indicated in any patient with hemodynamic instability or sepsis and when retrograde instrumentation may cause an inappropriate delay in treatment or unnecessary trauma to the genitourinary tract.
While some believe this technique is more invasive, placement of a nephrostomy tube has certain advantages, as follows:
It allows administration of medication directly to the collecting system and ureter to treat difficult infections
Stones may sometimes be dissolved chemically with antegrade irrigation
It allows antegrade treatment of obstructing stones
Antegrade radiographic studies often help with treatment planning once the patient is stable
Most importantly, it allows drainage of an infected renal unit with minimal trauma or risk to the patient, and it avoids the additional risks of a general anesthetic
Disadvantages to nephrostomy tube placement include the possibility of renal trauma and difficulties in placing the tube in some patients because of body habitus or mild hydronephrosis that makes localization with ultrasonography difficult.
In the management of pyonephrosis, nephrostomy tubes should never be placed transpleurally. This avoids pneumothorax, pleural infections, and empyema formation.
Percutaneous suprapubic tube placement guided by ultrasonography or radiography can be helpful in selected patients with urosepsis due to bladder outlet obstruction when a Foley catheter cannot be easily placed.
Consider the following 2-stage treatment for patients with pyonephrosis.
Stage 1 (decompression and drainage)
Perform retrograde stent placement. Use percutaneous CT scan- or ultrasonography-guided nephrostomy.  The posterior calyx should be entered from an oblique posterolateral approach in the posterior axillary line, 2-4cm below the 12th rib. This avoids the pleura, colon, liver, and spleen and is least likely to result in hemorrhage. Initial entry is made with a 20-gauge Chiba or 18-gauge sheath needle. The tract is then dilated using the Seldinger technique, and an 8F-14F nephrostomy tube is placed and connected to a closed-system drainage bag.
The infectious process often resolves within 24-48 hours following drainage, and the patient may improve significantly once this occurs.
Eliminate the obstruction by removing the stone, fungus ball, or tumor 1-2 weeks after percutaneous drainage or stent placement. Accomplish this with the use of electrohydraulic lithotripsy, laser lithotripsy,  percutaneous nephrolithotomy, extracorporeal shockwave lithotripsy, endopyelotomy, transurethral resection, or open surgical procedures. All of these are based on the type of obstruction and clinical situation.
In patients with uric acid stones and fungus balls, antegrade irrigation with alkaline fluids and antifungals through the nephrostomy tube may be needed prior to surgical intervention.
Contraindications to stent placement
While retrograde placement of a stent will not likely increase the risk of septicemia,  it is still contraindicated in unstable patients with preexisting sepsis. In these situations, proceeding directly with percutaneous placement of a nephrostomy tube is best for maximal decompression of the infected system.
Retrograde stent placement is relatively contraindicated in patients with known impacted and obstructing upper tract stones that may ultimately need percutaneous treatment or in those with fungus balls that may need additional therapy with antegrade irrigation and instrumentation.
Treatment Controversies and Considerations
Treatment of infections occurring from pyelonephritis  and pyonephrosis is changing rapidly and dramatically. The persistent use of broad-spectrum antibiotics, an increase in the population of immunocompromised patients (eg, patients with AIDS, patients undergoing chemotherapy), and the evolution of multiple drug–resistant bacteria complicate the picture.
Rare organisms, multiple-organism infections, and Candida species are now commonly associated with infected calculi. Antegrade percutaneous nephrostomy placement allows the drainage of purulent material and the antegrade infusion of antifungal medication and antibiotics to adequately treat these infections. Retrograde stent placement does not allow this form of therapy; therefore, many experts stress the importance of nephrostomy drainage rather than retrograde transurethral drainage for pyonephrosis. The authors currently prefer retrograde stent drainage after loading the patient with broad-spectrum antibiotics, reserving antegrade drainage for patients who may require further intervention, as previously discussed.
The need for nephrectomy after percutaneous nephrostomy drainage and antibiotic treatment is debated. Some advocate the need for removal if the source of obstruction is not clearly identified. This can help to exclude the presence of a malignant etiology for the obstruction, such as transitional cell carcinoma of the renal pelvis. Nonresponsiveness to therapy and progression of disease after percutaneous drainage are additional indications for nephrectomy; however, current technology reflects that preserving the maximal number of renal units is prudent.