| disease | Acute Pyelonephritis |
Acute pyelonephritis is an infectious disease involving the kidney parenchyma and renal pelvis, usually affecting one side, and occasionally involving both kidneys.
bubble_chart Etiology
Aerobic Gram-negative bacteria are the primary pathogens, with the most common being large intestine bacilli and Proteus. Proteus has a strong ability to synthesize urease, which breaks down urea, alkalizing the urine and leading to the precipitation of phosphates, forming magnesium ammonium phosphate and calcium phosphate stones. Klebsiella has a weaker ability to synthesize urease but can produce other substances that facilitate stone formation.
Gram-positive cocci, especially coagulase-positive staphylococci (Staphylococcus epidermidis, Staphylococcus saprophyticus), Staphylococcus aureus, and Group D streptococci (enterococci), occasionally cause acute pyelonephritis. Staphylococci can invade the kidneys via the bloodstream, leading to bacteriuria and renal abscesses. Anaerobic bacteria rarely cause pyelonephritis.
Kidney infections are usually caused by ascending infections from the lower urinary tract, while hematogenous infections are uncommon, and lymphatic infections are very rare.
The female urethra is shorter and closer to the anus, so pathogenic bacteria around the urethral opening can easily enter the bladder through sexual intercourse or vaginal examinations. Female patients with biological, anatomical, or other abnormal factors that impair local defense mechanisms often have pathogenic bacteria proliferating around the vaginal and urethral openings, making them prone to ascending urinary tract infections.
Men are less susceptible to ascending infections because the male urethra is longer, and the urethral opening is farther from the anus. Additionally, the bactericidal substances secreted by the normal prostate help prevent the invasion of pathogenic bacteria.Once pathogenic bacteria enter the bladder through the urethra, whether an infection occurs depends on the bladder's defense mechanisms, such as the coordinated action of bladder smooth muscles and urination function, the bactericidal properties of urine, and various factors that promote or inhibit bacterial adhesion to bladder surface cells.
Once a bladder infection occurs, whether bacteria ascend the ureters to invade the kidneys is influenced by factors such as bacterial virulence, vesicoureteral reflux, ureteral peristalsis, and the susceptibility of the renal medulla to bacteria.
bubble_chart Pathological Changes
(1) Naked eye: The kidneys may enlarge due to inflammatory edema, with small, raised yellow microabscesses forming beneath the membrane, surrounded by petechial hemorrhages. Sectioning the kidney reveals that the abscesses are mainly located in the cortex, where these round, tiny abscesses are distributed in a wedge-shaped pattern locally. Yellow linear streaks, consisting of pus-filled collecting ducts, extend from the cortex through the medulla and terminate at the renal papillae. The renal pelvis and calyces show congested and thickened mucous membranes, covered with exudates.
(2) Microscopic: The kidney parenchyma, especially the cortex, exhibits extensive inflammatory tissue changes. The interstitium and renal tubules are infiltrated by polymorphonuclear leukocytes, along with frequent infiltration of lymphocytes, plasma cells, and eosinophils. Similar pathological changes can also be observed in the renal medulla. Likewise, the epithelium of the renal pelvis and calyces shows acute inflammatory changes. The glomeruli are generally unaffected unless the inflammation is severe.
(1) Symptoms: Common symptoms of acute pyelonephritis include marked shivering, grade II or grade III fever, persistent lumbago (unilateral or bilateral), and symptoms of cystitis (frequency, urgency, dysuria). It is often accompanied by general malaise, prostration, nausea, vomiting, and even diarrhea.
(2) Signs: The patient generally presents with an acutely ill appearance, intermittent shivering, fever (38.5–40°C), and tachycardia (90–140 beats/min). The affected costovertebral angle is tender to percussion. Due to tenderness and local muscular rigidity, the kidney is often not palpable. Abdominal muscle guarding may be present, and rebound tenderness suggests peritoneal infection, in which case bowel sounds are diminished.
(3) Laboratory findings: Typical blood tests show a marked increase in white blood cells (polymorphonuclear neutrophils and band cells) and an elevated erythrocyte sedimentation rate. The urine is cloudy, with possible pyuria, bacteriuria, grade II proteinuria, and commonly microscopic or gross hematuria. Occasionally, white cell casts and glitter cells may be seen. Urine culture colony counts are ≥105/ml. Antibiotic sensitivity testing is crucial for guiding treatment and managing concurrent bacteremia. Since acute pyelonephritis is often accompanied by bacteremia, serial blood cultures are necessary. Uncomplicated acute pyelonephritis usually does not alter renal function.
(4) X-ray findings: Due to renal and perirenal edema, plain abdominal films may show indistinct renal outlines. Since pyelonephritis complicated by stones or obstructive calculi requires specific treatment, suspicious calcifications must be carefully evaluated. In uncomplicated acute pyelonephritis, excretory urography typically shows no significant abnormalities. In severe cases, the kidney may be enlarged, with delayed contrast excretion and poor or absent calyceal visualization. With appropriate treatment, urographic findings usually return to normal.
(5) Radionuclide imaging: 67Ga-citrate renal imaging or 131I-labeled white blood cell scans can localize the infection but cannot differentiate between acute pyelonephritis and renal abscess.
bubble_chart Treatment Measures
(1) Special Treatment: For patients with severe infections or acute pyelonephritis complicated by kidney disease or urinary tract abnormalities, hospitalization is required. Immediately collect blood and urine samples for culture, and perform antibiotic sensitivity tests after identifying the causative bacteria. Before the results of the sensitivity tests are available, antibiotics may be selected empirically. Although the choice of antibiotics varies among physicians, a combination of an aminoglycoside and ampicillin administered intravenously is commonly used. If the causative bacteria are sensitive to the drugs and clinical efficacy is good, continue treatment for 1 week, then switch to an appropriate oral antibiotic for 2 weeks. Factors contributing to complications, such as urinary tract obstruction or infected stones, must be identified and treated as early as possible to prevent complications.
(2) General Treatment: Bed rest is recommended until symptoms subside. Pain, fever, and nausea can be managed symptomatically with medication. Encourage increased fluid intake or intravenous hydration to maintain adequate body fluids and urine output.
(3) Treatment for Poor Responders: If there is no improvement after 48–72 hours of treatment, it may be due to inappropriate antibiotic selection or the presence of adverse factors (e.g., urinary tract obstruction). In such cases, perform excretory urography; if contraindicated, retrograde urography should be used. Unless treated promptly and effectively, acute pyelonephritis complicated by urinary tract obstruction can lead to bacteremia and irreversible kidney damage.
(4) Follow-up: Clinical improvement does not equate to complete eradication of the infection. One-third of patients still harbor pathogens even after symptoms have fully resolved. Therefore, repeated urine cultures must be performed during and after treatment, with follow-up for at least 6 months.
Patients with acute pyelonephritis without various adverse factors such as urinary tract obstruction, if diagnosed promptly and treated appropriately, have a good prognosis and can be cured quickly without any sequelae. Those with severe adverse factors and elderly patients have a poorer prognosis and may develop serious sequelae.
Due to the immature development of the kidneys, acute pyelonephritis can easily lead to scarring, renal atrophy, and impaired renal function. Therefore, urinary tract infections in infants and young children should be taken seriously and treated adequately. Children with urinary system malformations are prone to urinary tract infections and other complications. For these patients, a thorough urological examination is necessary, and urinary tract malformations should be corrected. Patients with a tendency to develop severe infections must be carefully followed up and given long-term oral antibiotics to prevent infections.
Although adult patients without urinary tract malformations or kidney disease rarely experience persistent renal damage due to acute pyelonephritis, the condition can still be severe or even fatal. It is essential to carefully evaluate and eliminate factors that induce infection or complicate the condition. For patients with a tendency for prolonged infection or rapid progression to severe infection, long-term antibiotic treatment is required.
If acute pyelonephritis is diagnosed and treated promptly, complications are rare. The prognosis of acute pyelonephritis complicated by underlying kidney disease or urinary tract abnormalities is relatively poor, and the pathogenic bacteria often have medicinal property. Unless kidney stones, especially infected stones, are removed, pyelonephritis is difficult to control. Infections complicated by urinary tract obstruction are also difficult to cure, often progressing to a chronic course and potentially leading to bacteremia.
The most severe complication of acute pyelonephritis is toxic shock. Emphysematous pyelonephritis is a rare but potentially fatal form of pyelonephritis, typically seen in diabetic patients, caused by gas released into the infected tissue by pathogenic bacteria (often a strain of large intestine bacilli).
With adequate treatment and in the absence of other kidney diseases or urinary tract abnormalities, acute pyelonephritis usually resolves completely without causing kidney scarring or persistent kidney damage. However, in infants and young children with incompletely developed kidneys, especially when acute pyelonephritis is complicated by kidney disease or urinary tract abnormalities, it often leads to persistent kidney damage and scarring.
Because the location and nature of pain in pancreatitis and acute pyelonephritis are similar, acute pyelonephritis can sometimes be misdiagnosed as pancreatitis. Elevated serum amylase and normal urine tests help confirm pancreatitis and exclude acute pyelonephritis.
Basal pneumonia is a febrile disease that causes subcostal pain, but the pain has pleuritic characteristics, and chest X-ray shows abnormal findings.
Sometimes acute appendicitis, cholecystitis, diverticulitis, and other acute abdominal conditions must be differentiated from acute pyelonephritis. Although the early symptoms and signs are similar, urinalysis and other laboratory tests aid in differential diagnosis.
Female patients with acute pelvic inflammatory disease must be differentiated from acute pyelonephritis. Physical examination revealing characteristic signs and negative urine culture suggest acute pelvic inflammatory disease.
In male patients, acute pyelonephritis needs to be differentiated from acute prostatitis and acute epididymo-orchitis.
Acute pyelonephritis also needs to be differentiated from renal abscess and perinephric abscess.
