Pyelonephritis

Overview – Pyelonephritis

Pyelonephritis refers to inflammation of the renal pelvis and kidney parenchyma, typically due to a bacterial urinary tract infection (UTI) that ascends from the bladder. It is a common and clinically significant condition that can lead to systemic infection or long-term renal scarring if untreated. The most common causative organism is Escherichia coli. Pyelonephritis can present with both lower urinary tract symptoms and systemic signs such as fever and flank pain, and it is especially important to recognise in vulnerable populations such as diabetics and those with urinary tract abnormalities.

Definition

Pyelonephritis is a bacterial infection and inflammation of the renal pelvis, tubules, and interstitium, most commonly caused by an ascending UTI.

Aetiology

Infectious Sources

• Ascending UTI (most common route)
  • E. coli is the most common pathogen
• Haematogenous spread (from systemic sepsis or bacteraemia)
  • Bacteria seed the kidneys via bloodstream

Risk Factors

• Anatomical abnormalities of the urinary tract
• Vesicoureteric reflux (VUR)
• Urinary catheters
• Diabetes mellitus
• Immunocompromised states
• Benign prostatic hyperplasia (BPH)

Morphology / Pathophysiology

• Bacteria travel via the ureters to the renal pelvis and infiltrate renal parenchyma
• Intense inflammatory reaction in the tubules and interstitium
• Histological features:
  • Lymphocyte infiltration
  • Interstitial abscesses containing pus
• In severe or recurrent cases, can lead to fibrosis and scarring of renal tissue

Clinical Features

• Fever
• Nausea and vomiting
• Flank pain (often radiating to the groin)
• Renal angle tenderness (positive Murphy’s kidney punch)
• Lower urinary tract symptoms (LUTS):
  • Dysuria, frequency, urgency
• Pyuria ± haematuria

Investigations

• Clinical diagnosis is often sufficient in typical presentations
• Urine dipstick and microscopy:
  • Pyuria, haematuria, bacteriuria
• Urine culture and sensitivity:
  • Essential for guiding targeted antibiotic therapy

Management

Outpatient (mild cases)

• Oral antibiotics (e.g.):
  • Ciprofloxacin
  • Amoxicillin–clavulanate (Augmentin)
  • Trimethoprim–sulfamethoxazole (Bactrim)

Inpatient (moderate to severe cases)

• IV antibiotics, such as:
  • Fluoroquinolones
  • Aminoglycosides
  • Third-generation cephalosporins

Complications

• Chronic pyelonephritis
  • Repeated episodes may result in renal fibrosis, atrophy, and long-term renal impairment
• Sepsis – may arise from ascending infection or haematogenous spread
• Acute renal failure, particularly in high-risk or immunocompromised patients

Differential Diagnosis

• Lower urinary tract infection (UTI)
• Renal colic / nephrolithiasis
• Appendicitis (especially in right-sided pain)
• Pelvic inflammatory disease (PID)
• Glomerulonephritis

Summary – Pyelonephritis

Pyelonephritis is a bacterial infection of the renal pelvis and interstitium, most often caused by ascending E. coli infection. It presents with fever, flank pain, and lower urinary tract symptoms, and must be promptly treated with appropriate antibiotics to prevent complications such as renal scarring or sepsis. For a broader context, see our Renal Overview page.