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Acute Pyelonephritis

Pyelonephritis acuta

For medical students2 min readUpdated 2026-10-10

Acute pyelonephritis (pyelonephritis acuta) is an infectious-inflammatory disease of the kidneys accompanied by destructive changes in the renal parenchyma and tubular apparatus. A hallmark of this pathology is the formation of purulent foci within the renal tissue and the risk of severe complications when urinary flow is obstructed.

Parenchymal involvementTubular epithelial necrosis with high resistance of glomeruli

Gross pathologyFormation of multiple small abscesses in the renal tissue

Three complicationsPapillary necrosis, pyonephrosis, and perinephric abscess

Papillary necrosisCoagulative necrosis of the papillae or distal 2/3 of the pyramids

Morphological Characteristics of Renal Tissue Injury

In acute pyelonephritis, histological changes primarily affect the tubular system and renal parenchyma.

Glomeruli: Typically demonstrate resistance to the infection. Their damage is observed only when extensive areas of necrosis form. However, in fungal pyelonephritis (specifically caused by Candida species), the glomeruli are frequently involved.
Renal tubules: The primary morphological sign of acute inflammation is necrosis of the renal tubular epithelium. Histological examination of hematoxylin and eosin-stained slides reveals alteration of the tubular epithelium.
Gross appearance: The renal parenchyma shows multiple small abscesses, representing foci of purulent tissue liquefaction.

Pathomorphology of Renal Pyramid Necrosis

Papillary necrosis (necrosis of the renal papillae) is one of the three major complications of the disease.

Prevalence: Most commonly, the pathological process is localized within a single renal pyramid, but in rarer cases, absolutely all renal pyramids undergo necrotic changes.
Gross changes: The apices or distal two-thirds of the renal pyramids are affected. The necrotic areas acquire a gray-white or yellow color, visually resembling changes seen in a renal infarction.
Microscopic appearance: The morphology corresponds to infarct-like coagulative necrosis. Tubular contours in the damaged areas remain preserved. A distinct leukocyte infiltrate is clearly defined at the border between necrotic and viable tissue.

Severe Destructive Complications of Obstruction

The development of urinary tract obstruction against the background of inflammation leads to dangerous destructive processes.

Complication	Conditions and Mechanisms of Development	Morphological Essence
Pyonephrosis	Caused by complete or near-complete urinary tract obstruction (primarily high ureteral blockage).	Due to the inability to drain, purulent exudate accumulates in the renal pelvis, calyces, and ureters, leading to pyonephrosis.
Perinephric abscess	Develops when purulent inflammation breaches or spreads beyond the renal capsule.	The perirenal (perinephric) adipose tissue becomes involved in the inflammatory destructive process.

Thus, the triad of major complications (papillary necrosis, pyonephrosis, perinephric abscess) reflects the progressive purulent liquefaction of renal and perirenal structures.

Mnemonic

The three main "P"s of acute pyelonephritis complications: Papillary necrosis, Pyonephrosis, Perinephric abscess.

Frequently asked questions

Which microorganisms are the most common causative agents of acute pyelonephritis?

The most common causative agents of acute pyelonephritis are gram-negative bacteria belonging to the normal gastrointestinal flora, dominated by Escherichia coli. Other enterobacteria such as Proteus, Klebsiella, and Enterococcus also cause the disease.

Escherichia coli (E. coli) — the most frequent cause of pyelonephritis.
Proteus (Proteus) — a gram-negative microorganism.
Klebsiella (Klebsiella) — a member of the intestinal flora.
Enterobacter (Enterobacter) — a frequent cause of urinary tract infections.

Through which routes does infection enter the kidney in acute pyelonephritis?

Infection enters the kidney in pyelonephritis primarily via three main routes, with the ascending route predominating. The ascending route is realized by retrograde transport of microorganisms from the lower urinary tract. The hematogenous route is relatively rare and involves blood-borne dissemination during bacteremia from distant purulent foci or sepsis. A lymphatic route is also noted as a possible retrograde spread along lymphatic vessels.

Ascending (urogenogenous) route — the main route of infection from the lower urinary tract.
Hematogenous route — pathogen spread via the bloodstream during bacteremia.
Lymphatic route — retrograde spread via lymphatic vessels.

What histological changes develop in the renal interstitium in acute pyelonephritis?

Acute pyelonephritis is based on interstitial purulent inflammation accompanied by prominent exudative and destructive changes in the an organ's parenchyma.

Interstitial edema — an early manifestation of exudative changes.
Neutrophilic infiltration — marked accumulation of neutrophils in the medullary interstitium and perivascular zones.
Destructive changes — dystrophy and necrosis of the renal tubular epithelium with destruction of their structure.

What are the possible gross outcomes of acute pyelonephritis on the renal surface?

The gross appearance of acute pyelonephritis on the surface and within the kidney tissue is characterized by the formation of purulent foci of varying size and shape depending on the nature of suppuration.

Multiple small abscesses — identified throughout the renal tissue during generalized progression.
Localized abscesses — may affect one or both kidneys.
Carbuncle — large wedge-shaped areas of suppuration in the renal parenchyma.

Are the glomeruli involved in acute pyelonephritis?

Glomeruli are typically resistant to infection. They are damaged only in the presence of large parenchymal necrosis zones or in fungal (candidal) pyelonephritis, where their involvement is frequent.

What are the gross and histological signs of papillary necrosis?

Grossly, the tips or distal 2/3 of the pyramids are necrotic (gray-white or yellow, resembling an infarct). Histologically, infarct-like coagulative necrosis is revealed with preserved tubular outlines and a leukocyte infiltrate at the tissue border.

What is the difference between pyonephrosis and perinephric abscess?

Pyonephrosis occurs with complete urinary obstruction and accumulation of pus inside the renal pelvis, calyces, and ureters. A perinephric abscess forms when pus spreads through the renal capsule into the surrounding perinephric adipose tissue.

Go deeper

Features of glomerular involvement in candidal (fungal) pyelonephritis
Microscopic differential diagnosis of tubular epithelial necrosis using hematoxylin and eosin staining
Comparative morphological characteristics of renal infarction and papillary necrosis
Pathogenesis of pyonephrosis in high ureteral obstruction
Routes of spread of the purulent process into the perinephric tissue in perinephric abscess