Acute Pyelonephritis

Slide 1 · Acute pyelonephritis

Routes of infection · suppurative morphology · complications · clinical correlation

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Distinguish ascending infection from hematogenous renal seeding.

Explain the anatomic basis of reflux-associated renal infection.

Recognize the characteristic neutrophilic tubular and interstitial lesions.

Differentiate papillary necrosis, pyonephrosis, and perinephric abscess.

Integrate symptoms, pyuria, urine casts, culture, and clinical course.

Slide 3 · Acute pyelonephritis is a tubulointerstitial infection

Tubules

Tubular lumina provide a route for neutrophils and infection through the nephron.

Interstitium

Neutrophil-rich inflammation produces focal suppuration and abscesses.

Renal pelvis

Infection of the collecting system links urinary infection with renal parenchymal injury.

Slide 4 · The affected renal compartments

Pyelonephritis involves tubules, interstitium, and the renal pelvis.

Injury is not initially a primary glomerular disease.

In severe cases, surrounding structures may also be destroyed.

Simplified renal anatomy

Cortex

Medulla

Pelvis

Slide 5 · Definition: Suppurative inflammation of the kidney

Core lesion

Acute pyelonephritis is an infectious, neutrophil-rich inflammation involving renal tubules and interstitium, usually linked to ascending urinary infection.

Slide 6 · Two entry routes reach the kidney

Ascending route — common

Lower urinary infection can ascend via the ureters, particularly with vesicoureteral and intrarenal reflux.

Hematogenous route — less common

Organisms reaching the bloodstream can seed the kidney during bacteremia.

Slide 7 · Ascending infection follows a urinary pathway

Bladder

Infected urine

Ureter

Retrograde movement

Renal pelvis

Collecting system reached

Parenchyma

Intrarenal extension

Vesicoureteral reflux and intrarenal reflux promote delivery of infected urine to renal tissue.

Slide 8 · Hematogenous spread bypasses the ascending route

Bacteremia

Organisms circulate

Renal blood supply

Bloodborne arrival

Renal tissue

Local infection and suppuration

Hematogenous seeding as an alternative, less common route.

Slide 9 · Enteric gram-negative bacilli predominate in UTIs

Microbial context

More than 85% of urinary tract infections are attributable to enteric gram-negative bacilli, which commonly originate from intestinal flora.

Slide 10 · Escherichia coli is the leading organism

Most common

E. Coli is identified by Robbins as the leading urinary tract pathogen.

Clinical relevance

Persistent or recurrent renal infection can involve E. Coli or other organisms.

Slide 11 · Other important enteric urinary pathogens

Proteus

Enteric gram-negative organism listed in the pyelonephritis pathogenesis section.

Klebsiella

Enteric gram-negative organism associated with urinary infection.

Enterobacter

Another enteric genus included in the source organism list.

Slide 12 · Fungal and mycobacterial infection are possible

Typical bacterial disease

Neutrophilic, suppurative tubulointerstitial inflammation.

Fungal infection

Candida-associated infection may elicit a granulomatous interstitial response.

Mycobacterial infection

Less common infectious cause; Granulomatous responses merit consideration.

Slide 13 · Vesicoureteral reflux promotes upward spread

Normally, urine passes from ureters into the bladder.

Vesicoureteral reflux permits retrograde passage of bladder urine toward the renal pelvis.

Reflux provides a route for infected urine to reach the kidney.

Renal pelvis

Ureter

Bladder

Infected urine

Retrograde flow

Slide 14 · Intrarenal reflux drives bacteria into parenchyma

Vesicoureteral reflux

Infected urine ascends into the renal pelvis.

Open papillary ducts

Reflux may pass through papillary collecting ducts.

Renal tissue exposure

Organisms gain access to tubules and adjacent interstitium.

Slide 15 · Upper and lower poles are vulnerable to intrarenal reflux

Reflux is particularly frequent at the renal poles.

Polar papillae commonly have flattened or concave tips.

Open ducts at these papillae permit retrograde entry into renal parenchyma.

Simplified renal anatomy

Polar reflux-prone sites

Slide 16 · Lower urinary infection does not imply pyelonephritis

Without significant upward reflux

With reflux and intrarenal spread

Infection may remain in the bladder.

Bacteria can reach the renal pelvis and papillary ducts.

Cystitis or urethritis may predominate.

Tubulointerstitial inflammation may develop.

Persistent bacteriuria alone does not establish renal involvement.

Renal signs and nephron-derived casts support upper tract involvement.

Slide 17 · Urinary obstruction is a major predisposition

Outflow resistance

Congenital or acquired lesions impair drainage.

Urinary stasis

Residual urine favors continued bacterial growth.

Clinical consequence

Unrelieved obstruction increases the risk of severe or recurrent pyelonephritis.

Slide 18 · Residual urine favors persistence of infection

Poor emptying

Urinary retention

Residual urine

Reduced clearance

Bacterial growth

Persistent burden

Ascending risk

Upper tract exposure

Urinary stasis with obstruction and with bladder atony due to neurologic injury.

Slide 19 · Instrumentation increases susceptibility

Urinary catheterization is a common form of instrumentation.

Instrumentation can facilitate urinary infection.

Patients with repeated instrumentation may have additional predisposing conditions.

Instrumentation

Urinary catheterization

Bacterial access

Urinary infection

Additional risks

Predisposing conditions

Slide 20 · Pregnancy is a recognized risk context

Urinary infection risk

Bacteriuria may arise during pregnancy.

Clinical significance

Without treatment, some pregnant patients with bacteriuria develop symptomatic urinary infection.

Interpretation

Pregnancy is a predisposing clinical context, not a distinct morphologic subtype.

Slide 21 · Sex and age modify susceptibility

Life stage

Predisposing pattern

Infancy

Congenital urinary tract abnormalities may be detected, including in male infants.

After infancy to approximately 40 years

Urinary infections are more frequent in females.

Older age

Male incidence rises with prostatic enlargement and instrumentation.

Slide 22 · Diabetes creates multiple susceptibility mechanisms

Host susceptibility

Greater propensity for infection.

Bladder dysfunction

Neurogenic bladder can promote impaired emptying.

Instrumentation

More frequent urinary procedures may add exposure.

Slide 23 · Immunodeficiency increases clinical risk

Acquisition

Immunosuppression or immunodeficiency is listed among predisposing conditions.

Disease severity

Complicated infection may lead to repeated episodes of septicemia.

Slide 24 · Preexisting renal lesions can amplify risk

Prior renal abnormality

Renal scarring or anatomic distortion may obstruct urinary flow.

Urinary stasis

Impaired drainage facilitates persistence of bacteria.

Subsequent renal infection

The predisposition can sustain recurrent upper urinary infection.

Slide 25 · Risk factors converge on impaired urinary defense

Predisposing factor

Principal link to renal infection

Reflux

Ascending passage of infected urine

Obstruction / residual urine

Stasis and bacterial persistence

Instrumentation

Urinary tract exposure

Diabetes / immunodeficiency

Greater susceptibility or severe infection

Pregnancy / age-related changes

Clinical context favoring urinary infection

Slide 26 · Gross lesions are patchy and suppurative

Gray-white cortical foci may represent inflammation and abscess formation.

Lesions may be discrete or confluent.

Morphologic distribution reflects locally destructive infection.

Simplified renal anatomy

Slide 27 · Abscesses may coalesce into wedge-like areas

Discrete microabscesses

Focal neutrophil collections initially involve tubules and interstitium.

Larger wedge-like lesions

Coalescent suppuration may extend through a sector of renal tissue.

Slide 28 · Either one kidney or both may be involved

Unilateral disease

A single kidney may display localized focal suppuration.

Bilateral disease

Infection may involve both kidneys.

Distribution is variable

Neither laterality nor gross extent alone defines the underlying route.

Slide 29 · Four histologic hallmarks of acute pyelonephritis

Hallmark

Microscopic implication

Interstitial suppuration

Neutrophil-rich inflammation within renal interstitium

Intratubular neutrophils

Collections of neutrophils in tubular lumina

Neutrophilic tubulitis

Neutrophils infiltrating tubular epithelium

Tubular injury

Epithelial injury associated with acute inflammation

Slide 30 · Early inflammation may be confined to tubules

The earliest neutrophilic infiltrate may be limited to tubular lumina.

Tubular lumina can act as pathways for infection.

Interstitial spread follows when infection extends beyond affected tubules.

Schematic — not a histologic specimen

Slide 31 · Neutrophils can aggregate inside tubular lumina

Intratubular aggregates are a core feature of acute pyelonephritis.

Collections reflect neutrophil recruitment to infected tubular spaces.

Urinary leukocyte casts may derive from this intrarenal inflammatory compartment.

Schematic — not a histologic specimen

Slide 32 · Neutrophilic tubulitis indicates epithelial invasion

Finding

Neutrophils are present in or traverse the tubular epithelium.

Pathologic meaning

The tubule is directly involved, not merely surrounded by interstitial inflammation.

Slide 33 · Tubular injury accompanies neutrophilic infiltration

Infection

Microbial invasion

Tubular inflammation

Neutrophils in tubules

Epithelial damage

Tubular injury

Tubular injury is one of the four defining microscopic hallmarks emphasized by Robbins.

Slide 34 · Interstitial spread produces suppurative inflammation

Neutrophils extend from involved tubules into surrounding interstitium.

Interstitial collections alter and destroy normal tubular architecture.

Focal suppuration can form abscesses.

Schematic — not a histologic specimen

Slide 35 · Abscess formation can destroy involved tubules

Focal abscesses comprise collections of suppurative inflammatory cells.

Progressive infection causes loss of tubules within the abscess.

This differs from mere luminal neutrophils in early disease.

Schematic — not a histologic specimen

Slide 36 · Glomeruli are relatively resistant early in disease

Early or moderate disease

Predominant damage involves renal tubules and interstitium.

Glomeruli

Glomerular structures are often spared initially.

Interpretation

The characteristic morphologic emphasis is tubulointerstitial, not glomerulonephritic.

Slide 37 · Extensive infection can eventually destroy glomeruli

Severity-dependent extension

Relative glomerular sparing is not absolute. With sufficiently extensive renal destruction, glomeruli may also be involved.

Slide 38 · Fungal pyelonephritis may be granulomatous

Typical acute bacterial pattern

Fungal-associated pattern

Suppurative interstitial inflammation

Granulomatous interstitial inflammation may occur

Tubular neutrophils and tubulitis

Inflammatory morphology may depart from the typical neutrophilic pattern

E. Coli and other enteric organisms predominate

Candida is a fungal example provided

Slide 39 · Morphologic progression: From tubules to abscess

Tubular neutrophils

Inflammatory cells are initially concentrated in renal tubules.

Interstitial extension

Suppuration spreads into surrounding renal interstitium.

Focal destruction

Abscesses destroy tubules; Severe disease may extend further.

Slide 40 · Morphology must be interpreted at the correct scale

Examination

Characteristic observation

Gross kidney

Patchy gray-white suppurative cortical areas, sometimes wedge-like

Microscopy — tubules

Intraluminal neutrophil aggregates, tubulitis, tubular injury

Microscopy — interstitium

Suppurative inflammation and focal abscesses

Severe disease

Extensive tubular destruction and possible glomerular involvement

Slide 41 · Three major complications

Papillary necrosis

Ischemic coagulative necrosis of renal papillae.

Pyonephrosis

Pus accumulates in an obstructed renal collecting system.

Perinephric abscess

Suppuration extends through the capsule into perinephric tissue.

Slide 42 · Papillary necrosis has characteristic risk factors

Diabetes

A major setting for papillary necrosis complicating infection.

Sickle cell disease

Another important predisposing condition.

Urinary obstruction

Outflow impairment increases susceptibility.

Slide 43 · Papillary necrosis: Gross location of tissue death

One or several renal pyramids may be affected.

Necrosis may involve papillary tips or distal portions of pyramids.

The necrotic tissue is gray-white to yellow on cut section.

Simplified renal anatomy

Slide 44 · Papillary necrosis shows ischemic coagulative necrosis

Architecture

Outlines of necrotic tubules remain recognizable.

Inflammation

Leukocytic response is concentrated at the boundary of viable and dead tissue.

Interpretation

Coagulative papillary injury differs from diffuse intratubular neutrophilic suppuration.

Slide 45 · Papillary necrosis may precipitate acute renal failure

Clinical consequence

Superimposed papillary necrosis can produce acute renal failure, particularly in vulnerable patients with severe pyelonephritis.

Slide 46 · Pyonephrosis requires severe outflow obstruction

High-grade obstruction

Drainage becomes nearly or completely blocked.

Suppurative exudate persists

Purulent material cannot leave the collecting system.

Pyonephrosis

Pelvis, calyces, and sometimes ureter fill with pus.

Slide 47 · Pyonephrosis is pus within the collecting system

The primary accumulation is within pelvis and calyces.

The ureter may also be filled with pus.

Retention results from near-total or total obstruction, often high in the tract.

Simplified renal anatomy

Slide 48 · Perinephric abscess crosses the renal capsule

Intrarenal suppuration

Renal abscess

Capsular extension

Spread beyond capsule

Perinephric abscess

Infection of surrounding tissue

Anatomic location distinguishes this complication from pus confined to the collecting system.

Slide 49 · Complications are anatomically distinct

Complication

Compartment

Defining process

Papillary necrosis

Medullary papilla

Ischemic coagulative necrosis

Pyonephrosis

Pelvis, calyces ± ureter

Pus trapped by severe obstruction

Perinephric abscess

Beyond renal capsule

Extension of suppuration into perinephric tissue

Slide 50 · Resolution begins with a change in inflammatory cells

Active infection

Prominent neutrophilic infiltrate.

Postacute healing

Macrophages, plasma cells, and lymphocytes replace neutrophils.

Fibrous repair

Irregular cortical scars may eventually develop.

Slide 51 · Healed lesions may produce patchy fibrous scars

Gross change

Cortical surface may show irregular fibrous depressions.

Microscopic change

Tubular atrophy and interstitial fibrosis.

Residual inflammatory pattern

Lymphocytes may remain in a patchy, jigsaw-like pattern around preserved areas.

Slide 52 · Pyelonephritic scars often deform the calyces

Healing-related fibrosis may involve the underlying pelvis and calyces.

Pelvocalyceal deformation is an important clue to reflux-associated renal injury.

This links a parenchymal scar to the urinary route of infection.

Simplified renal anatomy

Slide 53 · Acute inflammation and chronic scarring are different

Acute infection

After healing / recurrent injury

Neutrophil-rich suppuration

Lymphocyte-rich chronic inflammation may persist

Tubulitis and acute tubular damage

Tubular atrophy and interstitial fibrosis

Focal renal abscesses

Irregular cortical fibrous depressions

Potentially reversible acute disease

Permanent architectural distortion can remain

Slide 54 · The clinical syndrome often begins abruptly

Flank or costovertebral angle pain

Reflects acute renal-region involvement.

Fever and malaise

Systemic manifestations of infection.

Sudden presentation

An abrupt course is typical of uncomplicated acute disease.

Slide 55 · Lower tract irritation may coexist

Dysuria

Painful urination may indicate concurrent lower tract involvement.

Frequency

Repeated need to void can accompany infection.

Urgency

Strong or sudden urge to urinate may occur.

Slide 56 · Pyuria confirms inflammation, not renal localization

Diagnostic limitation

Many urinary leukocytes are compatible with urinary tract infection, but pyuria by itself cannot distinguish lower urinary tract infection from pyelonephritis.

Slide 57 · Leukocyte casts indicate renal involvement

Renal tubules

Urinary casts are formed within tubular lumina.

Neutrophils within casts

Pus casts may contain many neutrophils.

Interpretation

Leukocyte casts support an intrarenal inflammatory process.

Slide 58 · Urine culture establishes urinary infection

Clinical suspicion

Symptoms and urine findings suggest urinary infection.

Culture

Urine culture is used to establish the infection.

Anatomic interpretation

Clinical features and renal-derived casts help localize involvement.

Slide 59 · Clinical and laboratory data answer different questions

Observation

Diagnostic contribution

Limitation

Fever + costovertebral pain

Raises suspicion of upper tract infection

Not an organism-specific result

Pyuria

Shows leukocytes in urine

Does not localize upper vs lower tract

Leukocyte casts

Supports intrarenal involvement

Interpret with clinical context

Urine culture

Establishes urinary infection

Does not alone show renal histology

Slide 60 · Uncomplicated disease usually responds well

Appropriate antibiotics

Symptoms commonly subside within days after treatment begins.

Course

Uncomplicated infection generally has a favorable clinical course.

Slide 61 · Persistence and recurrence are distinct possibilities

Persistence

Bacteria may remain in urine despite improvement of symptoms.

Recurrence

Later infections may involve new E. Coli serologic types or other organisms.

Slide 62 · Complicated disease can cause systemic deterioration

Persistent predisposition

Obstruction, diabetes, or immunodeficiency remains.

Severe infection

Episodes may be persistent or recurrent.

Systemic consequences

Repeated septicemic episodes; Papillary necrosis may cause acute renal failure.

Slide 63 · Clinical case 1: Localizing urinary inflammation

Vignette

An adult presents with fever, costovertebral angle pain, dysuria, and marked pyuria. Urine microscopy also shows leukocyte casts.

Question

Which finding best indicates that the kidney is involved rather than only the bladder?

Slide 64 · Case 1 interpretation: A nephron-derived finding

Leukocyte casts

Casts form in renal tubules; Neutrophil-rich leukocyte casts therefore provide evidence of renal involvement. Pyuria alone does not localize the infection.

Slide 65 · Clinical case 2: Obstructed suppurative infection

Vignette

A patient with severe urinary outflow obstruction develops pyelonephritis. Suppurative material accumulates within the renal pelvis and calyces because drainage is blocked.

Question

What complication is described, and what is its defining anatomic location?

Slide 66 · Case 2 interpretation: Pyonephrosis

Pyonephrosis

Near-complete or complete obstruction prevents drainage of suppurative exudate. Pus fills the pelvocalyceal system and may extend into the ureter.

Slide 67 · Clinical case 3: Diabetic renal papillary injury

Vignette

A patient with diabetes and severe acute pyelonephritis develops renal papillary injury. Microscopy shows necrotic papillary tissue retaining outlines of renal tubules.

Question

Which complication explains this characteristic pattern of tissue necrosis?

Slide 68 · Case 3 interpretation: Papillary necrosis

Ischemic coagulative necrosis

Papillary necrosis is a recognized complication in diabetes, sickle cell disease, and obstruction. Preserved ghost outlines of tubules typify the necrotic papilla.

Slide 69 · Core pathology-to-clinic synthesis

Structural finding

Interpretive or clinical link

Ascending infection with reflux

Urinary source of intrarenal infection

Tubular + interstitial neutrophils

Acute suppurative kidney infection

Leukocyte casts

Renal localization of urinary inflammation

Papillary necrosis

Risk of acute renal failure

Obstructed pus-filled collecting system

Pyonephrosis

Capsular spread

Perinephric abscess

Slide 70 · Five central teaching points

Ascending infection is common; Hematogenous seeding is an alternative.

Vesicoureteral and intrarenal reflux allow access to renal tissue.

Neutrophilic tubulitis, tubular injury, and interstitial suppuration define the lesion.

Papillary necrosis, pyonephrosis, and perinephric abscess are distinct complications.

Leukocyte casts indicate intrarenal involvement; Urine culture establishes infection.

Slide 71 · Source and recommended reading

Primary source

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026). Chapter 20, The Kidney: Pyelonephritis and Urinary Tract Infection; Acute Pyelonephritis. Printed pp. 841–844.

Scope of this lecture

Source-grounded description of routes, risks, morphology, complications, healing, and diagnostic interpretation. Original editable schematics replace unavailable textbook figures; they are not diagnostic specimens.