Acute Pyelonephritis

Overview
  • Acute Pyelonephritis refers to an inflammation of the renal interstitium caused by bacterial infection of the renal parenchyma.
Pathogenesis
  • Overview
    • Infections of the kiendy typically occurs due to ascending extension of bacteria from the lower urinary tract. Direct hematogenous seeding of the kidneys can occur but because this is extremely rare it will be only briefly detailed below and the remainder of our discussion will focus solely on ascending infections.
  • Hematogenous Spread
    • Involves seeding of the kidneys within the context of bacteremia frequently because of infective endocarditis or sepsis. Hematogenous spread is an extremely rare cause of acute pyelonephritis and only occurs with highly virulent organisms such as Staphylococcus aureus and in fairly debilitated patients.
  • Ascending Spread
    • Involves of ascent of bacteria up the urinary tract]] from a previous urinary tract infection that includes a component of cystitis in which the bladder has become infected. This is by far the most common route of acute pyelonephritis and can be caused by relatively non-virulent GI bacterial flora.
  • Vesicouretral Reflux
    • Normally a one-way valve exists at the junction of the ureters with the bladder that prevents or at least reduces retrograde flow of urine into the ureters during voiding, when pressure within the bladder increases substantially. It is thought that this one-way valve malfunctions in those who develop acute pyelonephritis, allowing for "Vesicouretral Reflux" of bacteria from an infected bladder. Refluxed urine can ascend the ureters, travel through the renal pelvis and calyces and into the interstitium, thus infecting the kidneys themselves.
Etiology
Morphology
  • Grossly:
    • Visibly apparent abscesses may be apparent on the surface of the kidneys.
  • Histologically:
    • Characteristically, presence of supperative inflammation and abscesses within the renal interstitium. Given the acute nature of the infection, large numbers of neutrophils are present. Inflammation and infection can eventually break into the nephron, particularly filling the collecting ducts with neutrophils. Interestingly, glomeruli appear to be somewhat resistant to infection.
Clinical Consequences
  • Signs and Symptomology
    • Acute pyelonephritis is a deep infection and patients often display consitutional symptoms such as fever, chills, and malaise. They often complain of a deep flank pain that can be elicited by tapping on the costovertebral angle (CVA), hence termed CVA tenderness. Urinalysis will show bacteria, pyuria, and often hematuria. In those with diabetes mellitus pyelonephritis may become complicated by necrotizing papillitis
Treatment
  • Patients will often require inpatient intravenous antibiotics with coverage of gram negative rods.