Fact-checked by Grok 2 weeks ago

Pyonephrosis

Pyonephrosis is a severe complication of urinary tract infection characterized by the accumulation of pus and purulent debris in the renal pelvis and collecting system due to obstruction in the setting of pyelonephritis, forming an abscess within a hydronephrotic kidney. It occurs in approximately 10% of cases of acute pyelonephritis. This condition represents a urological emergency, as the stagnant pus isolates the infection from systemic immune responses and antibiotics, potentially leading to rapid progression toward urosepsis, renal parenchymal destruction, or even kidney rupture. The primary etiology involves an obstructive process in the upper urinary tract, most commonly urinary stones (accounting for approximately 75% of cases), combined with ascending bacterial infection.

Background

Definition

Pyonephrosis is defined as the accumulation of in the and calyces due to urinary tract obstruction superimposed on an ascending infection, such as . This condition arises when purulent , consisting of inflammatory cells, infectious organisms, and necrotic urothelium, fills the hydronephrotic collecting system, creating an abscess-like environment. Anatomically, pyonephrosis transforms the renal collecting system into a closed, pus-filled sac, which pressures the surrounding renal and can lead to suppurative destruction if not promptly addressed. The term derives from the Greek words pyon () and nephros (). A key distinction from lies in the infectious component: while involves sterile fluid buildup from obstruction alone, pyonephrosis features infected and debris within the dilated system, often confirmed by imaging showing higher attenuation values. It typically develops as a severe progression of when obstruction prevents drainage of infected material.

Epidemiology

Pyonephrosis is an uncommon complication of , with a prevalence of approximately 10.9% among hospitalized pyelonephritis patients. In a descriptive of 550 patients admitted to a tertiary care center's and department, 60 cases of pyonephrosis were identified, underscoring its relative rarity within acute upper urinary tract infections. Demographically, pyonephrosis predominantly affects adults, with a mean age at of 55 years and a notable male predominance of 68%. It can occur across all age groups, though it remains exceptionally rare in neonates, documented primarily through isolated case reports rather than population-level data. The condition is more frequent in adults with underlying comorbidities, contributing to its higher incidence in this subgroup. In the United States, where approximately 250,000 cases of are diagnosed annually, pyonephrosis represents a small fraction. Among risk populations, pyonephrosis is more prevalent in immunosuppressed individuals, including those with diabetes mellitus or , as well as patients with structural urinary tract anomalies such as obstructions. These factors elevate susceptibility by facilitating bacterial stasis and infection progression within the .

Pathophysiology

Causes and Risk Factors

Pyonephrosis arises from the combination of urinary tract obstruction and superimposed bacterial , leading to pus accumulation in the . The most common cause of obstruction is urolithiasis, accounting for 50-70% of cases in adults, often involving staghorn calculi that promote and bacterial overgrowth. Other obstructive etiologies include tumors, ureteral strictures, congenital anomalies such as ureteropelvic obstruction, and complications from prior urologic surgeries. In children, congenital obstructing lesions predominate. Bacterial ascent from a lower is the typical infectious route, though hematogenous spread is less common. The predominant pathogens are , with as the most frequent isolate, responsible for the majority of cases due to its ability to colonize the urinary tract. Other common gram-negative organisms include and , while such as species and are more prevalent in nosocomial or catheter-related infections. Fungal pathogens like species occur occasionally, particularly in immunocompromised patients. Key risk factors include immunosuppression from conditions like diabetes mellitus or diseases such as AIDS, as well as iatrogenic factors like or . Indwelling urinary catheters and prior urinary tract instrumentation increase susceptibility by facilitating bacterial introduction and obstruction. predisposes women due to urinary from hormonal and mechanical changes. In men, prostatic or contributes to obstruction and infection risk. Anatomic variations, such as horseshoe or pelvic kidneys, and advanced age further elevate vulnerability. Spontaneous pyonephrosis is rare and typically occurs in patients with underlying vulnerabilities, often iatrogenically triggered post-procedure.

Mechanisms of Disease

Pyonephrosis develops when an ascending bacterial infection of the urinary tract, originating from the bladder and progressing to acute pyelonephritis, encounters an obstruction in the upper urinary tract, leading to urine stasis that facilitates bacterial proliferation within the renal pelvis and calyces. This stasis impairs the natural flushing mechanism of the urinary system, allowing pathogens to multiply unchecked and initiate a localized inflammatory cascade. Common obstructing agents, such as calculi or strictures, exacerbate this by preventing drainage and promoting a closed-loop environment conducive to infection escalation. The inflammatory response to the infection results in the production of purulent exudate, consisting primarily of neutrophils, cellular debris, and necrotic tissue, which accumulates in the dilated collecting system to form . As worsens due to the obstruction, intrarenal pressure rises, compressing the renal and vasculature; this reduces blood flow to the kidney tissue, further limiting oxygen delivery and impairing the penetration of systemic antibiotics into the infected area. The itself acts as a physical barrier, shielding from host immune defenses and agents, thereby enabling persistent and immune evasion. In early stages, the disease manifests as suppurative confined to the , but progression leads to advanced complications including the formation of cortical abscesses, papillary necrosis, and potential rupture of the collecting system into the or retroperitoneum. The sustained back-pressure from accumulated and induces of the nephrons through ischemic damage and compression, progressively impairing renal function. Additionally, elevated intrarenal pressures can trigger pyelovenous backflow, where infected material refluxes into the renal veins, facilitating bacteremia and systemic dissemination.

Clinical Presentation

Signs and Symptoms

Pyonephrosis often manifests with a combination of systemic and local symptoms, reflecting the underlying bacterial and urinary tract obstruction. Systemic signs typically include high fever exceeding 38.5°C, chills, and rigors, which arise from the inflammatory response to pus accumulation in the . accompanies these in many patients, while and occur frequently, contributing to and overall debility. In severe or delayed presentations, symptoms can rapidly progress to urosepsis, characterized by , , and , necessitating urgent intervention. Local symptoms primarily involve the affected and may include unilateral flank pain or , ranging from dull ache to severe discomfort, reported in up to 77% of cases. , , or may occur if the obstruction extends to the lower urinary tract, though these are less consistent. A palpable abdominal or flank mass due to the distended hydronephrotic is occasionally noted on . These features overlap with those of uncomplicated but tend to be more persistent or intense owing to the suppurative process. The clinical presentation varies widely, with approximately 15% of cases being and discovered incidentally through for unrelated issues, while others range from low-grade fever and dull pain to overt . Up to 30% of patients may lack classic signs such as fever, pain, or significant , underscoring the need for vigilance in at-risk individuals. In pediatric and neonatal patients, symptoms are often nonspecific and less localized, such as , poor feeding, , , high fever, and a palpable renal mass in severe cases; obstruction from congenital anomalies is a common precipitant. These manifestations highlight the condition's potential for subtlety in younger populations.

Differential Diagnosis

Pyonephrosis, characterized by pus accumulation in an obstructed , often presents with flank , fever, and systemic signs of , necessitating differentiation from other renal, obstructive, and systemic conditions to guide urgent . The presence of both and obstruction distinguishes it from sterile processes, while clinical suspicion prompts to exclude mimics.

Renal Conditions

Acute pyelonephritis without obstruction mimics pyonephrosis through similar fever, flank pain, and urinary symptoms but lacks the obstructive element, often responding to antibiotics alone without drainage. Renal abscess presents as a focal collection of pus, potentially complicating untreated pyelonephritis, with localized tenderness and imaging evidence of a discrete mass rather than pelvic distension. Xanthogranulomatous pyelonephritis, a chronic destructive form associated with staghorn calculi, shares obstructive features and chronic inflammation but typically involves parenchymal replacement by lipid-laden macrophages, leading to a non-functioning kidney over time.

Obstructive/Non-Infectious Conditions

Simple due to ureteral stones or strictures causes flank pain and renal enlargement without infectious signs like fever or , highlighting the need to assess for superimposed in pyonephrosis. from nephrolithiasis presents with acute, colicky pain radiating to the groin, often without systemic symptoms, differing from the constant ache and in pyonephrosis. Urinary tract tumors, such as , may cause chronic obstruction mimicking pyonephrosis but typically lack acute infectious features and present with or gradual renal failure.

Systemic Mimics

Sepsis from non-renal sources, such as or intra-abdominal infections, can produce fever and with referred flank pain, but lacks urinary findings and requires source identification to differentiate. conditions like or may cause lower quadrant pain radiating to the flank, accompanied by gastrointestinal symptoms rather than . Musculoskeletal flank pain, from conditions like or muscle strain, is typically mechanical, exacerbated by movement, and absent fever or laboratory evidence of infection. Key differentiators include the combination of fever indicating with evidence of obstruction, which favors pyonephrosis over sterile or non-obstructive , underscoring the role of clinical correlation in narrowing possibilities.

Diagnosis

Laboratory Investigations

Laboratory investigations play a crucial role in supporting the clinical suspicion of pyonephrosis by identifying markers of , , and renal impairment, while guiding initial selection. Blood tests typically reveal with a left shift indicating acute bacterial , though this finding is nonspecific and can occur in uncomplicated urinary tract infections or . Inflammatory markers such as (CRP) and (ESR) are elevated, with CRP levels greater than 28 mg/L serving as a reliable indicator for the need of emergent in suspected cases. Renal function assessments, including serum creatinine and , may show elevations in bilateral or severe unilateral disease due to post-renal obstruction. Blood cultures are recommended in patients with systemic signs, yielding positive results in approximately 20% of hospitalized cases with acute complicated by obstruction, helping to detect bacteremia. Urinalysis commonly demonstrates pyuria with more than 10 per high-power field, , and positive tests for nitrites and , reflecting active infection in the upper urinary tract. Microscopic is frequently observed, occurring in a significant proportion of patients with obstructive . Urine culture is essential to identify the causative pathogen—most often or other gram-negative enteric bacteria—and determine antibiotic sensitivities, though results from voided urine may be less representative in complete obstruction. Microbiological evaluation is enhanced by obtaining samples directly from the upper tract when feasible; pus aspirated via tube under guidance should undergo for rapid preliminary identification of the organism and (including aerobic, , and fungal) for definitive and testing. In patients presenting with , adjunctive tests include to assess for tissue hypoperfusion and severity of systemic involvement, as well as panel to evaluate for or imbalances secondary to fever and reduced intake.

Imaging Studies

Ultrasound serves as the initial imaging modality of choice for suspected pyonephrosis due to its accessibility, lack of radiation, and high diagnostic accuracy, achieving a sensitivity of 90% and specificity of 97% in distinguishing pyonephrosis from uncomplicated . Key ultrasonographic features include marked with echogenic debris layering within the dilated collecting system, often forming fluid-fluid levels that shift with patient position, representing accumulation. This "dirty" appearance contrasts with the anechoic, clear fluid seen in simple , while additional signs such as renal pelvic wall thickening exceeding 2 mm or hyperechoic lines from gas in emphysematous variants further support the diagnosis. Persistent internal echoes, either dispersed throughout or dependent in the pelvocaliceal system, provide a reliable sonographic criterion when clinical suspicion of is present. Computed tomography (CT) is considered the gold standard for confirming pyonephrosis and evaluating its extent, particularly in complex cases, with non-contrast phases ideal for detecting calculi and contrast-enhanced phases for assessing parenchymal and . demonstrates a dilated containing low-attenuation, pus-filled fluid with higher Hounsfield units than simple urine, accompanied by pelvic wall thickening greater than 2 mm, perinephric fat stranding, and inflammatory changes in the . In cases of gas-forming , intraluminal gas bubbles or fluid-gas levels may be evident, and the modality excels at identifying obstructing lesions or extrarenal extension, though its sensitivity for specific signs like wall thickening is approximately 76%. Emerging tools, such as deep learning-based automated detection models using CT images (as of 2025), show promise in improving diagnostic efficiency. Magnetic resonance imaging (MRI) is reserved for equivocal ultrasound or CT findings, offering superior soft tissue contrast to differentiate pus from hemorrhage or tumor within the collecting system, without exposure. Intravenous pyelography (IVP) is infrequently employed owing to contrast-related risks and its poor in acute inflammatory settings. Nuclear , such as with dimercaptosuccinic acid, aids in functional assessment of the affected when structural suggests non-viability, helping guide decisions.

Management

Treatment

The primary treatment for pyonephrosis involves urgent decompression of the obstructed, infected urinary tract to prevent further renal damage and sepsis, combined with systemic antimicrobial therapy, in line with the 2024 European Association of Urology (EAU) Guidelines on Urological Infections. Emergency drainage is prioritized, with percutaneous nephrostomy (PCN) as the preferred method due to its high technical success rate of over 96-99% and ability to rapidly resolve infection within 24-48 hours. PCN is performed under ultrasound or CT guidance, typically accessing the posterior calyx to place a drainage catheter, and is indicated especially for hemodynamically unstable patients with sepsis. For stable patients without signs of systemic instability, retrograde ureteral stenting serves as an alternative, achieving a technical success rate of approximately 98%, though it requires prior intravenous antibiotics and may limit direct antibiotic delivery to the kidney. Antimicrobial therapy is initiated empirically with broad-spectrum intravenous agents to cover common gram-negative pathogens such as , prior to or concurrent with drainage procedures. Recommended initial regimens include piperacillin-tazobactam or a like , with alternatives such as plus ampicillin-sulbactam for polymicrobial coverage or cephalosporins like or per EAU guidelines; therapy is tailored based on or cultures obtained during drainage, typically lasting 7-14 days intravenously before step-down to oral agents once the patient stabilizes. For carbapenem-resistant organisms, ceftazidime-avibactam or newer agents like cefepime-enmetazobactam (approved 2024) may be used. Once the acute is controlled, the underlying obstruction is addressed to prevent recurrence, usually 1-2 weeks after initial . Interventions include endoscopic stone removal via ureteroscopy or for calculi, surgical resection for tumors, or and stenting for strictures, guided by follow-up such as antegrade nephrostography. Supportive care is essential and follows management protocols, including aggressive intravenous fluid resuscitation, analgesia for pain control, and vasopressor support (e.g., ) if persists. is reserved rarely for cases of a non-functioning after infection control, as and antibiotics often preserve renal function. Since the early 2000s, management has shifted toward minimally invasive interventional radiology techniques like PCN over open surgical approaches, significantly reducing morbidity and mortality from historical rates of up to 19% to near 0% with prompt intervention.

Prognosis and Complications

With prompt diagnosis and appropriate drainage, the prognosis for pyonephrosis is generally favorable, with infectious processes resolving within 24-48 hours in most cases and preservation of renal function achievable through timely intervention. In a series of 53 patients, split renal function improved in 56.6% following nephrostomy drainage, particularly among younger adults and children where recovery rates are higher. Mortality remains low at approximately 3-10% when treatment is initiated early, though rates can escalate significantly with delays leading to overwhelming sepsis. Several factors influence , including the rapidity of diagnosis and underlying patient characteristics such as , , or advanced age, which worsen outcomes and increase the risk of irreversible damage. Bilateral involvement or pre-existing renal insufficiency further complicates , while early can lead to full without scarring in a majority of uncomplicated cases. Complications arise primarily from delayed management and include or as the most frequent adverse event, occurring in up to 25% of cases and potentially progressing to multi-organ failure. Other notable risks encompass perinephric or psoas abscess formation, , and generalized from rupture, particularly in the presence of obstructing ureteral stones. Rare sequelae involve the development of fistulas, such as renocolonic or renoduodenal types. Long-term renal health depends on of the underlying obstruction and prevention of recurrence; follow-up is essential to confirm drainage efficacy and monitor for scarring that could precipitate . If the predisposing cause, such as urolithiasis, remains unaddressed, the risk of recurrent pyonephrosis persists, potentially necessitating in up to 70% of advanced cases.

References

  1. [1]
    Pyonephrosis: Practice Essentials, Etiology, Prognosis
    Apr 15, 2024 · Pyonephrosis—pus in the renal pelvis—results from urinary tract obstruction in the presence of pyelonephritis.
  2. [2]
    Pyonephrosis Ultrasound and Computed Tomography Features - NIH
    Feb 17, 2021 · Pyonephrosis (PN) refers to infected hydronephrosis with associated suppurative destruction of the kidney parenchyma, with partial or total loss ...<|control11|><|separator|>
  3. [3]
    Pyonephrosis among Patients with Pyelonephritis Admitted in ...
    Pyonephrosis is a serious infective condition of kidneys characterised by the presence of pus in the renal collecting system. It is associated with obstruction ...Missing: causes | Show results with:causes
  4. [4]
    Pyonephrosis | Radiology Reference Article - Radiopaedia.org
    Pyonephrosis is a term given to an infection of the kidney with pus in the upper collecting system which can progress to obstruction.
  5. [5]
    Giant Pyonephrosis Due to Ureteropelvic Junction Obstruction - NIH
    Pyonephrosis is a suppurative infection of upper urinary tract due to obstruction of the ureter. It is usually associated with suppurative damage of renal ...Missing: symptoms | Show results with:symptoms
  6. [6]
    Pyonephrosis - American Journal of Roentgenology
    Pyonephrosis is defined as the presence of pus in a dilated renal collecting system. The therapy of pyonephrosis has evolved from primary nephrectomy [1] to ...
  7. [7]
    Pyonephrosis - an overview | ScienceDirect Topics
    Pyonephrosis is defined as “pus under pressure” and signifies the presence of infected urine within an obstructed urinary collecting system. It commonly ...Missing: history | Show results with:history
  8. [8]
    Pyonephrosis: is the kidney always doomed? - Urology News
    Mar 9, 2020 · Pyonephrosis (Greek pyon 'pus' + nephros 'kidney') is defined in Campbell-Walsh Urology [1] as an infected hydro-nephrosis associated with ...Missing: etymology | Show results with:etymology
  9. [9]
    Neonatal pyonephrosis--a case report - PubMed
    Pyonephrosis is uncommon in adults as well as children and rarely reported in neonates. Candidial pyonephrosis in a neonate has been first reported in 1988 ...
  10. [10]
    Acute Pyelonephritis - StatPearls - NCBI Bookshelf - NIH
    Acute pyelonephritis is a bacterial infection causing inflammation of the kidneys. Pyelonephritis is generally a complication of an ascending urinary tract ...
  11. [11]
    Acute pyelonephritis - Symptoms, diagnosis and treatment
    Complicated infections can result from underlying medical problems (e.g., diabetes mellitus, HIV), genitourinary anatomic abnormalities, obstruction (e.g., ...
  12. [12]
    Emergent Percutaneous Nephrostomy for Pyonephrosis: A Primer ...
    Risk factors include advanced age, diabetes, bladder dysfunction, neurogenic ... This step is preferred in nonemergencies, but in pyonephrosis the additional ...Patient Preparation · Catheter Placement · Complications And Management
  13. [13]
    A Giant Case of Pyonephrosis Resulting from Nephrolithiasis - NIH
    Pyonephrosis is a disease causing suppurative destruction of the renal parenchyma. If it is not diagnosed early, it can worsen rapidly and cause the death of ...Missing: definition | Show results with:definition
  14. [14]
    Predictive Factors Of Pyonephrosis In Renal Calculus Patients
    Sep 29, 2024 · ... male and female patients with renal calculus presenting with pyonephrosis. With regards to age, there was also no significant discrepancy ...
  15. [15]
    Urinary Obstruction, Stone Disease, and Infection - NCBI - NIH
    Mar 21, 2018 · Pyonephrosis is an acute infection with pus within an obstructed collecting system. Renal destruction is rapid, and urgent drainage of the ...20.1. Urinary Infection · 20.1. 4. Acute Renal... · 20.2. Urinary Stones And...Missing: mechanisms | Show results with:mechanisms
  16. [16]
    Mechanism of urosepsis: relationship between intrarenal pressures ...
    Jun 12, 2023 · The present study demonstrates that pyelotubular and pyelovenous backflow may potentially contribute to the ascending pathway for uropathogens.
  17. [17]
    Pyonephrosis Ultrasound and Computed Tomography Features
    Patients with pyonephrosis may present with signs and symptoms of acute infection, or with low-grade fever, weight loss and dull pain, although as many as 15% ...<|control11|><|separator|>
  18. [18]
    A Rare Case of Pyonephrosis in an Infant Induced by Extended ...
    Apr 17, 2020 · Pyonephrosis is an infection that leads to obstructed hydronephrotic kidney, with suppurative destruction of parenchyma and pus accumulation, ...
  19. [19]
    Pyonephrosis - an overview | ScienceDirect Topics
    Pyonephrosis is characterized by accumulation of purulent debris and sediment in the renal pelvis and urinary collecting system.Missing: reliable | Show results with:reliable
  20. [20]
    Pyonephrosis | Radiology Reference Article - Radiopaedia.org
    Pyonephrosis is a term given to an infection of the kidney with pus in the upper collecting system which can progress to obstruction.
  21. [21]
    Xanthogranulomatous pyelonephritis | Radiology Reference Article
    Jun 1, 2010 · Xanthogranulomatous pyelonephritis (XGP) is a rare form of chronic pyelonephritis and represents a chronic granulomatous disease resulting in a non-functioning ...
  22. [22]
    Abdominal Pain and Flank Pain - Urology Textbook
    Abdominal pain or flank pain due to urological disorders: · Ureteral stone · Pyelonephritis · Renal abscess · Retroperitoneal bleeding or hematoma · Kidney ...Missing: pyonephrosis | Show results with:pyonephrosis
  23. [23]
    Diagnosis and Management of Acute Pyelonephritis in Adults - AAFP
    Mar 1, 2005 · The differential diagnosis of acute pyelonephritis includes pelvic inflammatory disease, cholecystitis, appendicitis, lower lobe pneumonia ...Abstract · Pathogenesis · Clinical Presentation · Diagnostic TestingMissing: pyonephrosis | Show results with:pyonephrosis
  24. [24]
    The Role of C-Reactive Protein and Erythrocyte Sedimentation Rate ...
    The Role of C-Reactive Protein and Erythrocyte Sedimentation Rate in the Diagnosis of Infected Hydronephrosis and Pyonephrosis ... leukocytosis, and 86.8% for CRP ...
  25. [25]
    Pyelonephritis: It's not always so straightforward… - emDocs
    Jan 19, 2016 · Though nausea and vomiting is common, it is non-specific. In 86% of patients, flank pain or CVA tenderness is found.2,3,10. As in many other ...Missing: pyonephrosis | Show results with:pyonephrosis
  26. [26]
  27. [27]
  28. [28]
    Pyonephrosis: epidemiological, clinical, and therapeutic aspects at ...
    Intraoperative complications were noted in 25 patients. Peritoneal and pleural ... The prognosis of pyonephrosis is improved by the use of septic urine ...
  29. [29]
    Managing of Pyonephrosis: About 42 Cases - Mathews
    May 21, 2024 · Results: Among 42 patients who have been diagnosed with pyonephrosis, the mean age was 42 years, and 67% were males. The most common clinical ...
  30. [30]
    [PDF] Percutaneous Nephrostomy in Complicated Urinary Tract Infections
    Jul 9, 2022 · In patients with pyonephrosis, the septic complications may reach 25%. ... Complications following PCN may increase morbidity and mortality; new ...