Dr. med. Dirk Manski

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Acute Pyelonephritis: Definition and Causes

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

Definition of Acute Pyelonephritis

Acute pyelonephritis is a bacterial infection of the renal pelvis and the renal parenchyma with fever, flank pain, and bacteriuria. Standard laboratory tests cannot prove bacterial infection of the kidney; acute pyelonephritis is defined clinically as a urinary tract infection with flank pain, flank tenderness, or fever.

Uncomplicated pyelonephritis:

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

Complicated pyelonephritis:

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

Etiology (Causes) of Pyelonephritis

Ascending Infection:

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

Vesicoureteral Reflux:

Vesicourethral reflux (VUR) leads to refluxing urine, which becomes infected. Pyelonephritis in VUR tends to cause renal scarring, since intrarenal reflux is common. The anatomy of the papillae causes intrarenal reflux, especially at the renal poles. See also the 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 uncommon beyond five years but may occur until puberty.

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

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 may also contain gas. The renal abscess has a favorable prognosis after drainage and antibiotic treatment.

Emphysematous pyelonephritis (right side): CT without contrast media shows gas formation within the renal fascia, the patient present with severe sepsis. With kind permission Prof. Dr. K. Bohndorf, Augsburg.
figure CT scan of Emphysematous pyelonephritis

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.


Candida (rare).

Pathologic Findings

Gross Pathology

Histology of Acute Pyelonephritis

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

Index: 1–9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


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  Deutsche Version: Akute Pyelonephritis