24.5: Acute Pyelonephritis I: Introduction

Pyelonephritis is a bacterial infection that primarily affects the renal parenchyma and collecting system, including the renal pelvis, tubules, and interstitial tissue of one or both kidneys. It can be classified as either acute—a sudden, severe infection—or chronic, which refers to long-term or recurrent kidney infections.

The primary cause of acute pyelonephritis (APN) is bacterial infection, with Escherichia coli accounting for approximately 70-80% of cases. Other bacteria, such as Proteus, Klebsiella, Enterococcus, and Pseudomonas, can also be responsible. APN often results from an ascending infection from the bladder, particularly in the presence of urinary stasis or anatomical abnormalities.

Several factors can predispose individuals to develop APN:

• Untreated or recurrent lower urinary tract infections (UTIs): These can lead to ascending bacteria and an increased risk of kidney infection.
• Vesicoureteral reflux (VUR): This condition, where urine flows backward from the bladder to the kidneys, increases the risk of kidney infections.
• Obstructive uropathy: Conditions like kidney stones, tumors, or an enlarged prostate can obstruct urine flow, fostering bacterial growth.
• Immunosuppression: Individuals with weakened immune systems, such as those with diabetes, HIV, or those on immunosuppressive medications, are more susceptible to infections.
• Pregnancy: Hormonal changes and mechanical obstruction during pregnancy can increase the risk of APN.
• Indwelling catheters: Long-term catheter use can introduce bacteria into the urinary tract, leading to infection.

Pathophysiology:

The pathophysiology of acute pyelonephritis typically involves bacteria ascending from the lower urinary tract to the kidneys. The infection usually begins with bacterial colonization of the urethra, followed by migration to the bladder, resulting in cystitis (bladder infection). If untreated, bacteria can ascend through the ureters and reach the kidneys, particularly in individuals with urinary stasis or vesicoureteral reflux.

Once the bacteria reach the kidneys, they invade the renal interstitium and tubules, triggering an inflammatory response. The immune system reacts by releasing cytokines and other inflammatory mediators, leading to renal edema, neutrophil infiltration, and the destruction of renal tissue. If not treated promptly, this inflammation can progress to complications like renal abscess formation or, in severe cases, sepsis.

Clinical Manifestations:

Patients with acute pyelonephritis often present with a range of symptoms that reflect both localized infection and systemic inflammatory responses. Common symptoms include:

• Fever and chills: High fever and chills are typical, indicating the body’s systemic response to bacterial invasion.
• Leukocytosis: An elevated white blood cell count reflects the immune system’s activation in response to the infection.
• Bacteriuria and pyuria: The presence of bacteria and pus in the urine confirms the infection within the urinary tract.
• Flank and low back pain: Pain, often on the affected side, is due to inflammation and infection of the kidney.
• Nausea and vomiting: Gastrointestinal symptoms may accompany the infection due to the systemic inflammatory response.
• Malaise and headache: General feelings of fatigue and headache are common as part of the body’s response to infection.
• Dysuria (painful urination): Pain during urination indicates inflammation in the lower urinary tract.