The bacterial infection is the most common cause of pyelonephritis. Acute complicated pyelonephritis is an acute emergency and needs immediate treatment with intravenous antibiotics. Infection in acute pyelonephritis is usually due to ascending infection, or due to hematogenous infection. The presence of recurrent infections, usually due to structural and functional anomalies in the genitourinary tract is the hallmark of chronic pyelonephritis. Clinical features include fever and abdominal flank pain with or without symptoms of cystitis. WBC casts are the characteristic findings seen in the urine analysis. Fluoroquinolones and cephalosporins are the mainstays in the treatment of pyelonephritis. Prognosis is excellent in uncomplicated pyelonephritis, while it is poor in emphysematous pyelonephritis.
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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

Definition of Kidney Infection

Bacterial infection of the kidney (involving tubules, renal pelvis, and interstitium) results in pyelonephritis. Presentation is usually either acute or chronic.

Acute pyelonephritis results either from ascending infection of the urinary tract or from hematogenous spread of systemic infections.

Chronic pyelonephritis generally is due to chronic recurrent infections secondary to urinary reflux, or obstruction in the genitor-urinary tract.

Pyelonephritis is a life-threatening infection that can lead to scarring of the kidneys.

Epidemiology of Kidney Infection

Acute pyelonephritis is estimated at a prevalence of 15-17 cases per 10000 among females and 3-4 cases per 10000 among males in the population. It is more common in pregnant females who are diagnosed with asymptomatic bacteriuria.

Incidence of chronic pyelonephritis is common in children as they are associated with vesicoureteral reflux disease.

Pyelonephritis has no racial predilection and its distribution indicates that it is more common in the extremes of ages with two peaks; one at 0-4 years and the other >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

Classification/Types of Kidney Infection

Acute pyelonephritis: It is an acute medical emergency. If untreated, it can progress to abscess, sepsis and organ failure. Urinary tract infections (UTIs) due to Escherichia coli, Proteus mirabilis, Klebsiella most commonly predispose through ascending the route. Hematogenous infection is usually due to Staphylococcus aureus.

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

Pathophysiology of Kidney Infection

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 organisms associated with urinary tract infections. E. coli is the most common organism involved in acute pyelonephritis. The routes of infection are either a hematogenous route or ascending infection route.

Ascending Infection

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

Favourable urothelium aids in the attachment of the bacteria. Instrumentation, especially with cystoscopy and catheterization, predisposes ascending infection to the bladder followed by the renal pelvis. Short urethra and the close proximity of the urethra to the rectum favors the infection. Urine present in the bladder is usually sterile unless ascending infection contaminates it.  The infection gradually ascends from the bladder to the renal pelvis and kidney and results 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:

Vesicoureteric reflux

Primary vesicoureteric reflux is seen in patients where the pathology is primarily in the urethrovesical junction. In normal individuals, usually, there is normal closure of the intravesical part of the ureter. Incompetent urethrovesical junction leads to the reflux of the urine into the ureters and renal pelvis resulting in the vesicoureteric 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

It is a variant of pyelonephritis, which is predominantly seen 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

In some diabetic patients, there is a severe form of pyelonephritis. It shows an accumulation of gas in the renal and perinephric patients. Prognosis is poor in emphysematous pyelonephritis.

Xanthogranulomatous Pyelonephritis

Chronic infection associated with urinary tract obstruction (usually by staghorn calculi) results in suppurative infection of the kidney. Histopathological changes show infiltration by lipid-laden macrophages.

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 of Kidney Infection

Acute Uncomplicated Pyelonephritis

The classic triad of symptoms of acute uncomplicated pyelonephritis includes:

  • 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 as 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 results due to complicated pyelonephritis.

Chronic Pyelonephritis

A past history of acute pyelonephritis is usually present. Chronic and recurrent infections can predispose to hypertension in children. 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 & Laboratory Investigations of Kidney Infection

Blood and Urine Examination in Pyelonephritis

  • Acute pyelonephritis classically shows the presence of WBC casts, hematuria along with pyuria and bacteriuria.
  • Blood investigations show increased serum creatinine levels. A complete blood picture shows the characteristic increase in the number of eosinophils.

Gross and Histopathology Findings in Pyelonephritis

Acute Pyelonephritis

It is characterized by the presence of 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. Presence of increased eosinophilic substance in the tubules (due to atrophy) is seen. This characteristically resembles the thyroid tissue on histopathological examination. This process of deposition of eosinophilic substance in the tubule is called as thyroidisation.

Radiological Investigations

  • Voiding cystourethrogram helps in the diagnosis of the vesicoureteric reflex.
  • CT scanning is the investigation of choice whenever there is suspicion of any obstruction or the presence of any congenital anomalies.
  • On intravenous pyelogram, typical cortical scars with blunt calyces are seen.

Treatment of Kidney Infection

Treatment of Acute Pyelonephritis

Acute complicated pyelonephritis requires treatment with intravenous cephalosporins, followed by oral fluoroquinolones. Oral fluoroquinolones would, alone, be sufficient in uncomplicated pyelonephritis.

Treatment of Chronic Pyelonephritis

Prevention of the reflux uropathy in childhood forms the cornerstone in the management of chronic pyelonephritis. It might require surgical correction, based on the grade of the vesicoureteric reflex. Almost all cases of VUR should be tried on medical therapy. Hypertension should be treated by the administration of angiotensin receptor blockers.

Prognosis of Kidney Infection

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

Review Questions

The answers are below the references.

1. The most common cause of acute pyelonephritis is?

  1. Ascending infection with E. coli
  2. Pseudomonas
  3. Listeria
  4. Clostridium perfringens
  5. Salmonella

2. Renal papillary necrosis is predominantly seen in?

  1. Alcoholics
  2. Diabetics
  3. Tuberculosis patients
  4. Melanoma patients
  5. Hypertensive patients

3. Which of the following class of drug is most appropriate in uncomplicated pyelonephritis?

  1. Fluoroquinolones
  2. NSAIDs
  3. Tetracyclines
  4. Beta-blockers
  5. Penicillins
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