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Acute Pyelonephritis: Definition and Causes

• Acute pyelonephritis: definition, causes, and pathology
• Acute pyelonephritis: symptoms and diagnostic workup
• Acute pyelonephritis: treatment

Guidelines: EAU Guidelines Urological Infections, (German S3 Guidelines UTI).

Definition of Acute Pyelonephritis

Acute pyelonephritis is a bacterial infection of the renal pelvis and renal parenchyma associated with fever, flank pain, and bacteriuria. Clinically, acute pyelonephritis is defined as a urinary tract infection accompanied by flank pain, costovertebral angle tenderness, and/or fever (>38°C).

Uncomplicated pyelonephritis

Acute upper urinary tract infection in patients without relevant functional or anatomical abnormalities, renal dysfunction, or comorbidities that predispose to urinary tract infection or serious complications.

Complicated pyelonephritis

Any pyelonephritis that cannot be classified as uncomplicated, including cases in patients with anatomical abnormalities, pregnant women, urinary tract obstruction, an indwelling bladder catheter, renal disease, or comorbidities that promote infection.

Epidemiology

The annual incidence is approximately 12–17 per 10,000 women and 3–4 per 10,000 men. See also the epidemiology of urinary tract infections.

Etiology and Pathogenesis of Pyelonephritis

Ascending infection

Ascending infection is the most common cause. Intestinal bacteria colonize the periurethral area and ascend through the urethra and bladder into the upper urinary tract. Women are affected substantially more often than men because of their shorter urethra. See also the general principles of urinary tract infections.

Pathogens

The most common pathogens in uncomplicated urinary tract infections are E. coli and enterococci, followed by Proteus mirabilis, staphylococci, and Klebsiella pneumoniae [see also Table 1].

Hematogenous infection

Hematogenous infection is rare and occurs through septic dissemination in Staphylococcus aureus bacteremia.

Vesicoureteral reflux

Vesicoureteral reflux (VUR) causes refluxing urine, which may become infected and trigger pyelonephritis through intrarenal reflux. Papillary anatomy predisposes to intrarenal reflux, particularly at the renal poles. See also the section on vesicoureteral reflux.

Renal scarring after pyelonephritis

Recurrent episodes of acute pyelonephritis during childhood cause renal parenchymal scarring. Chronic pyelonephritis (Ask-Upmark kidney) may develop. New renal parenchymal scars rarely develop after the age of five years, but may occur until puberty.

The following mechanisms may explain why the kidneys of young children are susceptible to renal parenchymal scarring: intrarenal reflux occurs at substantially lower pressures than in adults; immune competence against bacterial infections is reduced during the first years of life; and early diagnosis of febrile urinary tract infections may be difficult.

Pyelonephritis in pregnancy

The prevalence of bacteriuria during pregnancy is 4–7%, similar to that in nonpregnant women. Approximately 20–30% of pregnant patients with bacteriuria develop acute pyelonephritis (1–4% of all pregnant women), most often during the second trimester. Possible complications include anemia, sepsis, transient renal failure, pulmonary complications, and preterm birth. See also urinary tract infections in pregnancy.

Emphysematous pyelonephritis

Emphysematous pyelonephritis is a rare, severe complication of acute pyelonephritis with high mortality. It occurs primarily in patients with diabetes mellitus and/or urinary tract obstruction. The precise mechanism of this gram-negative infection is unclear. E. coli fermentation is thought to produce gas, which remains confined within the renal fascia.

Emphysematous pyelonephritis should be distinguished from renal abscesses containing gas, which have a more favorable prognosis after drainage and antibiotic treatment.

Right-sided emphysematous pyelonephritis: CT without contrast medium shows gas within the renal fascia; the patient had severe sepsis. With kind permission, Prof. Dr. K. Bohndorf, Augsburg.

Pathology

Gross pathology

• Enlarged kidneys due to inflammatory edema
• Small, raised, subcapsular abscesses with hemorrhagic rims, occasionally coalescing
• Yellow streaks of pus-filled tubules extending from the cortex toward the papillae
• Thickened renal pelvic mucosa covered with exudate
• Healing with scarring, particularly in children

Histology

Predominantly neutrophilic tubulointerstitial inflammation with microabscess formation, tubular injury, and interstitial edema. The glomeruli are usually less affected.

References

G. Bonkat, R. Bartoletti, F. Bruyère, S. E. Geerlings, F. Wagenlehner, and B. Wullt, “EAU Guideline: Urological Infections.” [Online]. Available: https://uroweb.org/guidelines/urological-infections/

DGU, DEGAM, and PEG, “S3 Leitlinie Epidemiologie, Diagnostik, Therapie, Prävention und Management unkomplizierter, bakterieller, ambulant erworbener Harnwegsinfektionen bei erwachsenen Patienten Aktualisierung 2024.” [Online]. Available: https://register.awmf.org/assets/guidelines/043-044l_S3_Epidemiologie-Diagnostik-Therapie-Praevention-Management-Harnwegsinfektione-Erwachsene-HWI_2024-09.pdf

Dalla-Palma und Pozzi-Mucelli 2000 DALLA-PALMA, L. ; POZZI-MUCELLI, F.: [The imaging of chronic renal infections].
In: Radiologe
40 (2000), Nr. 6, S. 537–46

Fihn 2003 FIHN, S. D.: Clinical practice. Acute uncomplicated urinary tract infection in women.
In: N Engl J Med
349 (2003), Nr. 3, S. 259–66

Kawashima u.a. 2000 KAWASHIMA, A. ; SANDLER, C. M. ; GOLDMAN, S. M.: Imaging in acute renal infection.
In: BJU Int
86 Suppl 1 (2000), S. 70–9

Nickel 2001 NICKEL, J. C.: The management of acute pyelonephritis in adults.
In: Can J Urol
8 Suppl 1 (2001), S. 29–38

Roberts 1999 ROBERTS, J. A.: Management of pyelonephritis and upper urinary tract infections.
In: Urol Clin North Am
26 (1999), Nr. 4, S. 753–63

  Deutsche Version: Akute Pyelonephritis

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