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Acute pyelonephritisry

Image: “Very high magnification micrograph of acute pyelonephritis. H&E stain. Kidney biopsy. ” by Nephron. License: CC BY-SA 3.0


Pyelonephritis is a life-threatening infection, caused by a bacterial infection of the kidney (involving the tubules, renal pelvis, and interstitium), resulting in pyelonephritis. The presentation is either acute or chronic. Acute pyelonephritis is caused by an ascending infection of the urinary tract or from the hematogenous spread of systemic infections. Generally, chronic pyelonephritis is due to chronic recurrent infections secondary to urinary reflux or an obstruction in the genitor-urinary tract.


The prevalence of acute pyelonephritis is to be 15–17 cases per 10,000 females and 3–4 cases per 10,000 males in the United States. It is more common in pregnant females diagnosed with asymptomatic bacteriuria.

Chronic pyelonephritis is common in children and is associated with vesicoureteral reflux disease.

Pyelonephritis has no racial predilection, and its distribution indicates that it is more common in the extremes of age with 2 peaks; 1 at 0–4 years and the other at > 65 years.

Women Men Either Gender
Risk factors similar to acute cystitis

  • Sexual intercourse
  • History of previous UTI
  • Post-menopausal state
  • Pregnancy
  • Anatomic or functional abnormality of urinary tract
  • UTI often complicated
  • Frequently associated with urologic abnormalities
  • Anal-insertive sexual intercourse
  • Urinary tract instrumentation
  • Diabetes mellitus
  • Immunosuppression


Acute pyelonephritis: is a medical emergency. If untreated, it can progress to an abscess, sepsis, and organ failure. Urinary tract infections (UTIs) due to Escherichia coli, Proteus mirabilis, or Klebsiella are associated with an increased risk by ascending the ureters. Hematogenous infection is usually due to staphylococcus aureus.

Chronic pyelonephritis: is due to recurrent bacterial infections and occurs predominantly in children with genitor-urinary tract anomalies. Vesicoureteric reflux in these patients results in chronic infections. Chronic pyelonephritis eventually leads to scarring of the kidney.


Acute Pyelonephritis

Acute pyelonephritisry

Image: “Very high magnification micrograph of acute pyelonephritis. H&E stain. Kidney biopsy” by Nephron. License: CC BY-SA 3.0

E. coli, Proteus, and Pseudomonas are the typical organisms associated with urinary tract infections. E. coli is the most common organism involved in acute pyelonephritis. The routes of infection are either hematogenous or an ascending infection.

Ascending Infection

Ascending infection with E. coli is by far the most common cause of acute pyelonephritis.

Favorable urothelium helps the bacteria attach. Instrumentation, especially with cystoscopy and catheterization, predisposes ascending infection to the bladder and the renal pelvis. A short urethra and the close proximity of the urethra to the rectum increases the risk of infection. Urine present in the bladder is usually sterile unless an ascending infection contaminates it.  The infection gradually ascends from the bladder to the renal pelvis and kidney, resulting in acute pyelonephritis.

Hematogenous Spread

Hematogenous spread is usually uncommon. It occurs due to staphylococcus and E. coli infection. It results in the seeding of the bacteria in the kidney, which can result in pyelonephritis.

Chronic Pyelonephritis

The causes of pyelonephritis include:

Vesicoureteral reflux

Primary vesicoureteral reflux is seen in patients where the pathology is primarily in the urethrovesical junction. Usually, there is normal closure of the intravesical part of the ureter. However, a defect in the urethrovesical junction leads to reflux into the ureters and renal pelvis, resulting in vesicoureteral reflux.

Secondary vesicoureteric reflux develops in patients with neurogenic bladder.

Urinary Tract Obstruction

Lower urinary tract obstruction is the predominant cause of chronic pyelonephritis, which can be due to benign prostatic hyperplasia and renal calculus.

Renal Papillary Necrosis

This variant of pyelonephritis occurs predominantly in diabetics. It is characterized by the involvement of the renal papilla, called papillary necrosis. It is characterized by the presence of ischemic and suppurative necrosis at the tip of the renal papilla.

Conditions showing renal papillary necrosis:

Emphysematous Pyelonephritis

This is a rare, acute, and necrotizing form of pyelonephritis that occurs in diabetic or immunocompromised patients. EP is also associated with urinary tract obstructions. It is typically caused by E. coli or K. pneumoniae, and is characterized by an accumulation of gas in the renal parenchyma, collecting system, or perinephric tissue viewed using a CT scan or ultrasound. The patient will present with acute renal failure, micro- or macroscopic hematuria, and severe proteinuria. Prognosis is poor in emphysematous pyelonephritis because of septic complications.

Xanthogranulomatous Pyelonephritis

This is a chronic inflammatory disorder of the kidney characterized by the destruction of the renal parenchyma by the growth of granulomatous mass. Chronic infection associated with urinary tract obstruction (usually by staghorn calculi) results in suppurative infection of the kidney. It is typically caused by Proteus, E. coli, or Pseudomonas. Histopathological changes show infiltration by lipid-laden macrophages and the presence of large, irregular, yellow-orange masses which can be confused for true renal neoplasms.

Xanthogranulomatous pyelonephritis

Image: “CD68 immunohistochemical staining of macrophages and giant cells in a case of xanthogranulomatous pyelonephritis.” by Nephron. License: CC BY-SA 3.0

Clinical Examination and Symptoms

Acute Uncomplicated Pyelonephritis

The classic triad of symptoms of acute uncomplicated pyelonephritis is:

  • Fever
  • Costovertebral tenderness
  • Nausea/vomiting

It is usually associated with symptoms of cystitis, which include increased frequency, urgency, dysuria, and suprapubic tenderness.

Children present with additional features of difficulty in feeding and failure to thrive, while elderly patients present with additional features of altered mental status and general disorientation.

Acute Complicated Pyelonephritis

Uncomplicated pyelonephritis, in the presence of any of the following associations, is considered complicated pyelonephritis:

  • A recent history of urinary tract instrumentation
  • Diabetes
  • Pregnancy
  • Multi-drug resistant pathogens
  • Urinary tract obstruction
  • Recent history of hospitalization

Renal abscess, emphysematous pyelonephritis, and papillary necrosis are also associated with complicated pyelonephritis.

Chronic Pyelonephritis

A past history of acute pyelonephritis is usually present. Chronic and recurrent infections can predispose children to hypertension. In children, symptoms such as fever, lethargy, flank pain, and nausea may be present.

Overview of sign and symptoms of pyelonephritis and perinephric abscess

Children Women Men
  • Non-specific
  • Fever
  • Failure-to-thrive
  • Vomiting
  • Fever (a sign of sepsis)
  • Flank pain
  • Frequent, painful urination of small amounts of turbid urine
  • Suprapubic heaviness or pain (occasional)
  • Costovertebral angle tenderness
  • Presence of fever may indicate prostatitis or pyelonephritis
  • All cases of pyelonephritis or perinephric abscess need urologic evaluation

Diagnosis and Laboratory Investigations

Blood and Urine Examinations

  • Acute pyelonephritis classically shows the presence of WBC casts and hematuria along with pyuria and bacteriuria.
  • Blood investigations show increased serum creatinine levels. Complete blood count results show leukocytosis and the characteristic increase in the number of neutrophils. Inflammatory markers, such as C-reactive protein and erythrocyte sedimentation rate, are also elevated.

Gross and Histopathology Findings

Acute Pyelonephritis

This condition is characterized by a focal abscess formation in the cortex and medulla. The lower pole of the kidney is relatively spared; renal tubules show characteristic microabscess.

Chronic Pyelonephritis

Scarring of the glomeruli and tubular atrophy are characteristic of chronic pyelonephritis. There is an increased eosinophilic substance in the tubules (due to atrophy). This characteristically resembles the thyroid tissue on histopathological examination. This process of depositing an eosinophilic substance in the tubule is called thyroidization.

Radiological Investigations

  • A voiding cystourethrogram helps diagnose vesicoureteral reflex.
  • CT scanning is the investigation of choice whenever an obstruction or congenital anomalies are suspected.
  • On intravenous pyelogram, typical cortical scars with blunt calyces are seen.


Acute Pyelonephritis

Acute complicated pyelonephritis requires treatment with intravenous cephalosporins, followed by oral fluoroquinolones. Oral fluoroquinolones alone are sufficient for treating uncomplicated pyelonephritis.

Chronic Pyelonephritis

Preventing reflux uropathy in childhood forms the cornerstone of managing chronic pyelonephritis. It might require surgical correction depending on the grade of the vesicoureteric reflex. Almost all VUR cases should begin treatment with medical therapy. Hypertension should be treated with angiotensin receptor blockers.


Acute uncomplicated pyelonephritis has an excellent prognosis, and complete recovery is seen. Emphysematous pyelonephritis in diabetics shows a very poor prognosis irrespective of the treatment. Chronic pyelonephritis in children, due to vesicoureteral reflux, usually resolves spontaneously and only a few need surgical correction. Complications of chronic pyelonephritis include focal glomerulosclerosis and renal scarring, which progress gradually to end-stage renal failure.

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