What is pyelonephritis?

Pyelonephritis: Definition and Clinical Overview

Pyelonephritis is a severe urinary tract infection characterized by inflammation of the renal pelvis and kidney parenchyma, which has the potential to cause sepsis, shock, and death if left untreated. 1

Epidemiology

• Approximately 250,000 cases occur annually in the United States, resulting in more than 100,000 hospitalizations 2
• The annual incidence is estimated at 459,000 to 1,128,000 cases in the United States and 10.5 to 25.9 million cases globally 1
• Pyelonephritis occurs more frequently in young women but affects older men and women at similar rates 3

Etiology

• Escherichia coli is the predominant causative organism:
  • Accounts for >90% of cases in young healthy women 1, 4
  • Responsible for 70-90% of all pyelonephritis infections 3
• In men, elderly women, and urologically compromised patients, other pathogens are more common:
  • Less-virulent E. coli strains 1, 4
  • Gram-negative bacilli (Klebsiella, Pseudomonas, Proteus) 4, 3
  • Gram-positive organisms (Enterococcus, Staphylococcus) 5, 3
  • Candida (especially in immunocompromised patients) 4, 6

Pathophysiology

• Most commonly occurs through ascending infection from the lower urinary tract 3
• Rarely results from hematogenous seeding of the kidney 6, 3
• The inflammatory process leads to the formation of microabscesses that may coalesce into larger abscesses 4
• In severe cases, microabscesses can coalesce to form acute renal abscesses that may rupture into the perinephric space 1
• Infection may be confined to an obstructed collecting system causing pyonephrosis (accumulation of purulent material in the upper urinary collecting system) 1

Risk Factors

• Factors that disrupt normal urinary flow:
  • Vesicoureteral reflux 1, 5
  • Congenital urinary tract anomalies 1, 5
  • Altered bladder function 1, 5
  • Pregnancy 1, 5
  • Renal calculi 1, 5, 3
  • Mechanical obstruction 1, 5
• Host factors:
  • Diabetes mellitus 1, 5, 3
  • Genetic predisposition 1, 5
  • Personal or maternal history of UTIs 1, 5
  • Postmenopausal status with atrophic vaginitis 5
• Behavioral factors:
  • Sexual activity and new sexual partner 1, 3
  • Spermicide exposure 1, 5

Clinical Presentation

• Systemic inflammation signs:
  • Fever (though may be absent early in illness) 1, 4, 7
  • Chills and fatigue 1
• Flank pain and tenderness (nearly universal in acute pyelonephritis) 1, 4, 7
• Bladder inflammation symptoms (in approximately 80% of patients):
  • Urgency 1
  • Dysuria 1
  • Urinary frequency 1
• Clinical presentation can range from mild flank pain with low-grade or no fever to septic shock 1
• Up to 20% of patients may lack typical bladder symptoms 1, 4

Diagnosis

• Clinical diagnosis based on compatible history and physical examination 7
• Urinalysis showing pyuria and/or bacteriuria supports the diagnosis 1
• Urine culture yielding >10,000 colony-forming units of a uropathogen per milliliter is the fundamental confirmatory test 1, 8
• Blood cultures may assist with diagnosis but should be reserved for patients with uncertain diagnosis, immunocompromised patients, or those suspected of having hematogenous infections 2

Complications

• Acute kidney injury 4
• Renal scarring (occurs in approximately 15% of patients after first episode) 4
• Papillary necrosis 3
• Renal or perinephric abscess 3
• Emphysematous pyelonephritis 3
• Long-term complications from renal scarring may include hypertension and chronic renal failure 4

Treatment

• Uncomplicated pyelonephritis in men and women:
  • Short-course therapy with fluoroquinolones (5-7 days) or trimethoprim-sulfamethoxazole (14 days) based on antibiotic susceptibility 1
  • When local resistance to chosen oral antibiotic likely exceeds 10%, one dose of a long-acting broad-spectrum parenteral antibiotic should be given while awaiting susceptibility data 8
• Hospitalization indications:
  • Complicated infections 2
  • Sepsis 2
  • Persistent vomiting 2
  • Failed outpatient treatment 2
  • Extremes of age 2
  • Pregnancy (due to significantly elevated risk of severe complications) 8

Clinical Pearls and Pitfalls

• Not all patients with pyelonephritis present with fever - absence of fever does not exclude the diagnosis 5
• Absence of flank pain should raise suspicion of an alternative diagnosis 7
• Imaging studies are not necessary in uncomplicated cases but should be considered if there is no improvement in symptoms or if symptoms recur after initial improvement 7, 8
• Treatment failure may be caused by resistant organisms, underlying anatomic/functional abnormalities, or immunosuppressed states 2