Perinephritis refers to purulent inflammation of the perinephric tissue, and when it results in the formation of an abscess, it is called a perinephric abscess. A perinephric abscess typically arises from the rupture of an acute renal cortical abscess into the perinephric space or through hematogenous spread from infections in other parts of the body. The most common causative pathogens are Staphylococcus aureus and Escherichia coli. The lesions are generally located between the renal capsule and the perinephric fascia, often resulting from direct spread of infection from renal abscesses or cortical surface abscesses. Due to the rich and loose fat tissue in the perinephric region, infections tend to spread easily. Pus may drain into the psoas space, leading to the formation of a psoas abscess. If the diaphragm is breached, empyema may develop.
Symptoms in patients are often insidious. Most individuals with a perinephric abscess do not exhibit symptoms until more than five days after its onset. The clinical signs mainly include chills, fever, flank pain, muscle rigidity, and pronounced localized tenderness. Laboratory findings may show increased white blood cell count and neutrophilia. Since perinephritis is often accompanied by parenchymal renal infection, routine urinalysis may reveal the presence of pus cells. In cases of isolated perinephritis, urinalysis may appear normal. When an abscess ruptures and extends along the psoas muscle, it may irritate the muscle, causing an inability to extend the sacroiliac joint and leading to spinal curvature toward the affected side. Chest X-rays may show elevation and restricted movement of the ipsilateral diaphragm. Abdominal plain radiographs might reveal spinal curvature toward the affected side and loss of the psoas muscle opacity. Ultrasound and CT imaging can aid in identifying perinephric abscesses. Aspiration of the perinephric space under ultrasonographic guidance, followed by smear microscopy and bacterial culture of the pus, can determine the causative organisms and guide antibiotic selection.
In cases where an abscess has not yet formed, treatment primarily involves sensitive antibiotic use, local warm compresses, and enhanced systemic supportive therapy. If an abscess has developed, it may require aspiration or surgical incision and drainage.