What is Pyelonephritis?
Pyelonephritis is a bacterial infection of the renal pelvis and kidney parenchyma, characterized by inflammation that can lead to microabscess formation, renal scarring, and potentially life-threatening complications including sepsis, acute kidney injury, and progression to chronic renal failure. 1
Definition and Pathophysiology
Pyelonephritis represents an upper urinary tract infection distinct from simple cystitis. The infection is formally defined as "acute nonobstructive pyelonephritis" - a renal infection characterized by costovertebral angle pain and tenderness, often with fever, occurring in patients who may also experience symptoms of bladder inflammation 2.
The pathophysiological process involves:
- Bacterial invasion of renal tissue leading to inflammatory microabscess formation that may coalesce into larger abscesses 1
- In severe cases, these microabscesses can form acute renal abscesses that may rupture into the perinephric space 1
- Infection may become confined to an obstructed collecting system, causing pyonephrosis (accumulation of purulent material in the upper urinary collecting system) 1
- Renal damage occurs when toxic oxygen radicals are released during ischemic episodes and prolonged phagocytic respiratory bursts, particularly when treatment is delayed 3
Epidemiology and Causative Organisms
The annual incidence in the United States is approximately 459,000 to 1,128,000 cases, with 10.5 to 25.9 million cases globally 1. Approximately 250,000 cases occur annually in the U.S., resulting in more than 100,000 hospitalizations 4.
Escherichia coli is overwhelmingly the predominant pathogen, accounting for:
- >90% of cases in young healthy women 1
- Approximately 75% of all recurrent UTIs and pyelonephritis cases 5
In men, elderly women, and urologically compromised patients, the microbial spectrum broadens to include less-virulent E. coli strains, other Gram-negative bacilli (Serratia marcescens), and Gram-positive organisms (Enterococcus faecalis, Staphylococcus saprophyticus) 1, 5.
Clinical Presentation
Flank pain and tenderness are nearly universal findings in acute pyelonephritis, and their absence should raise suspicion of an alternative diagnosis 1, 6.
The classic presentation includes:
- Systemic inflammation signs: fever, chills, and fatigue (though fever may be absent early in illness) 1, 6
- Costovertebral angle pain and tenderness 2, 1
- Bladder inflammation symptoms in approximately 80% of patients: urgency, dysuria, and urinary frequency 1
Critical Pitfall
Not all patients present with fever - absence of fever does not exclude the diagnosis 5. This atypical presentation can lead to delayed diagnosis and increased risk of cortical scar formation 7.
Risk Factors
Risk factors fall into two major categories:
Factors disrupting normal urinary flow:
- Vesicoureteral reflux (particularly significant in bilateral cases) 1, 5
- Congenital urinary tract anomalies 1, 5
- Renal calculi 1, 5
- Mechanical obstruction and altered bladder function 1, 5
- Pregnancy 1, 5
Host factors:
- Diabetes mellitus 1, 5
- Genetic predisposition 1, 5
- Personal or maternal history of UTIs 1, 5
- Postmenopausal status with atrophic vaginitis 1, 5
Complications and Long-Term Outcomes
The most serious complications directly impact mortality, morbidity, and quality of life:
- Acute kidney injury 1
- Renal scarring leading to hypertension and chronic renal failure 1
- Sepsis with potential for multi-organ failure 4
- Progression to end-stage renal disease when scarring from chronic pyelonephritis leads to loss of renal tissue and function 3
Bilateral pyelonephritis carries a higher risk of complications, including renal scarring that can lead to hypertension and chronic renal failure 5.
Diagnostic Approach
Urinalysis showing pyuria and/or bacteriuria supports the diagnosis 1. The combination of leukocyte esterase and nitrite tests (with either positive) has a sensitivity of 75-84% and specificity of 82-98% for urinary tract infection 4.
Urine culture yielding >10,000 colony-forming units of a uropathogen per milliliter is the fundamental confirmatory test 1. Urine cultures are positive in 90% of patients with acute pyelonephritis and should be obtained before initiating antibiotic therapy 4.
Blood cultures should be reserved for patients with uncertain diagnosis, immunocompromised status, or suspected hematogenous infections 4.