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Acute Pyelonephritis: Definition and Causes

• Acute pyelonephritis (1/3): definition, causes and pathology
• Acute pyelonephritis (2/3): symptoms and diagnostic work-up
• Acute pyelonephritis (3/3): treatment

Review literature: (Fihn, 2003) (Nickel, 2001) (Roberts, 1999) (DGU S3-Guideline urinary tract infections).

Definition of Acute Pyelonephritis

Acute pyelonephritis is a bacterial infection of the renal pelvis and the renal parenchyma with fever, flank pain and bacteriuria. Since bacterial infection of the kidney cannot be proven, acute pyelonephritis is defined clinically as a urinary tract infection with flank pain, flank tenderness and/or fever.

Uncomplicated pyelonephritis:

Acute pyelonephritis limited to non-pregnant women before the menopause, without anatomical or functional abnormalities of the urinary tract or infection-promoting comorbidities.

Complicated pyelonephritis:

All patients with pyelonephritis, which cannot be classified as uncomplicated: in males or pregnant women, presence of anatomical deformities, bladder catheter, renal disease or infection-promoting comorbidities.

Etiology (Causes) of Pyelonephritis

Ascending Infection:

Ascending infection is the most common cause for acute pyelonephritis. Since the female urethra is short and intestinal bacteria tend to colonize the perineum and vulva, women have significantly more often bladder infection and pyelonephritis than men. See section "General principles of urinary tract infections".

Vesicoureteral Reflux:

Vesicourethral reflux (VUR) leads to refluxing urine, which becomes infected. Pyelonephritis in VUR tends to causes renal scarring, since intrarenal reflux is common. The anatomy of the papillae causes an intrarenal reflux especially at the renal poles. See also section "Vesicoureteral reflux".

Renal Scarring due to Pyelonephritis:

Recurrent relapses of acute pyelonephritis produce renal scarring in children. Chronic pyelonephritis and Ask-Upmark kidney may develop. New renal scarring is not common beyond the age of 5 years, but may occur until puberty.

The cause for the vulnerability of the child's kidney for renal scarring is explained with following mechanisms:

• intra-renal reflux happens with much lower vesical pressures than in adulthood
• decreased immunocompetence for bacterial infections in the first years of life
• clinical difficulties in the early diagnosis of pyelonephritis in the first years of life

Pyelonephritis in Pregnancy:

The incidence of bacteriuria in pregnancy is 4–7%, comparable to the rate of bacteriuria without pregnancy. Of these patients with bacteriuria, approximately 20–30% develop acute pyelonephritis (1–4% of all pregnant women). The most common manifestation of pyelonephritis is the second trimester.

Complications of pyelonephritis during pregnancy: anemia (23%), sepsis (17%), transient renal failure (2%), pulmonary complications (7%) and premature births (rare). See section urinary tract infections in pregnancy.

Emphysematous Pyelonephritis:

Emphysematous pyelonephritis is a serious complication of acute pyelonephritis with high mortality (43%). Risk factors for emphysematous pyelonephritis are diabetes mellitus or upper urinary tract obstruction. Most probably, E. coli causes gas formation by fermentation. The gas remains restricted within the renal fascia.

Emphysematous pyelonephritis must be distinguished from a renal abscess, which also may contain gas. Renal abscess has a favorable prognosis after drainage and antibiotic treatment.

Common Pathogens of Acute Pyeloneprhitis

Gram-negative pathogens

E. coli, Klebsiella, Enterobacter, Serratia, Pseudomonas, Proteus mirabilis and other gram-negative coliform bacteria in 80%.

Gram-positive pathogens

Staphylococcus aureus, Staph. saprophyticus, Enterococci.

Fungi

Candida (rare).

Pathology of Pyelonephritis

Gross Pathology

• Enlarged kidney with inflammatory edema
• Small subcapsular abscesses with hemorrhagic rim
• Pus-filled tubules
• The urothelium of the renal pelvis is thick and covered with exudate

Histology of Acute Pyelonephritis

Microscopy shows a dense granulocytic inflammation and tissue destruction with liquefaction in the renal parenchyma and renal pelvis. The inflammation affects especially the tubules, the glomeruli are less affected.

References

DGU S-3 Guideline Urinary tract infections

Epidemiologie, Diagnostik, Therapie und Management unkomplizierter bakterieller ambulant erworbener Harnwegsinfektionen bei erwachsenen Patienten
AWMF, 2010, Register-Nr. 043/044

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