Chronic pyelonephritis is a long-standing renal disorder characterized by tubulointerstitial inflammation with cortical scarring that eventually may result in end-stage renal disease. It includes variants like xanthogranulomatous pyelonephritis and emphysematous pyelonephritis. Diagnosis is based on renal function tests, urinalysis, urine culture, histopathology and radiological investigations.
Chronic pyelonephritis (CP) is marked by chronic renal tubulointerstitial inflammation which leads to scarring within the renal cortex and calyceal clubbing. It may lead to end-stage renal disease (ESRD). The disease is also known as reflux nephropathy, interstitial nephritis, and chronic atrophic pyelonephritis. CP also includes rare variants like xanthogranulomatous pyelonephritis (XGP) and emphysematous pyelonephritis (EPN) . The clinical presentation of CP is variable and non-specific. Some patients may be asymptomatic while others may present with malaise, fatigue, loss of appetite, loss of weight, cloudy urine, fever, backache, flank pain, or abdominal discomfort. In advanced stages of the disease, patients will have hypertension. XGP may be characterized by the presence of a unilateral palpable abdominal mass, while patients with EPN may present with signs of sepsis.
Chronic pyelonephritis may follow inadequate treatment or recurrence of acute pyelonephritis, composed of a localized immune response to bacteria that have been eradicated . In children, the chief cause of CP is vesicoureteral reflux (VUR) and recurrent urinary tract infections (UTIs) associated with renal scarring  . In adults, renal scarring is rarely due to urinary tract infections  .
Emphysematous pyelonephritis (EPN) is another rare, potentially life-threatening variant of CP. The disease develops secondary to renal parenchymal infection with E.coli, Klebsiella, or Proteus species. It is characterized by a sudden onset of acute necrotizing pyelonephritis and the presence of a renal parenchymal gas, usually in poorly controlled diabetics .
The diagnosis of CP can be difficult due to its variable and often vague clinical presentation. It can be suspected in patients who provide a history of renal calculi, recurrent UTIs, acute pyelonephritis or VUR. Occasionally the diagnosis is suspected based on incidental radiological findings or in patients presenting with hypertension. Laboratory tests like a complete blood count may reveal normochromic, normocytic anemia with leukocytosis. C-reactive protein (CRP) may be elevated in severe CP . Urinalysis is likely to show hematuria, proteinuria, leukocytes with occasional epithelial cells, and rarely WBC casts. The severity of renal dysfunction can be estimated by renal function tests like serum creatinine, blood urea nitrogen, and estimated glomerular filtration rate . Depending on the extent of renal damage there will be electrolyte imbalances like hyperkalemia, hyponatremia, and acidosis. Renal imaging is performed to detect the etiology of CP. Currently, magnetic resonance imaging (MRI) and computed tomography (CT) provide detailed information and are preferred over older modalities like plain X-rays, urography, and Tc-99m-DMSA nuclear scintigraphy, as well as histology . In children, an ultrasound and a cystourethrogram are performed to exclude VUR .
Xanthogranulomatous pyelonephritis (XGP): Proteus, and less commonly Escherichia coli, Klebsiella, and Staphylococcus aureus, are found in urine cultures. Imaging studies like CT and MRI may detect a renal mass resembling a tumor . However, the final diagnosis of XGP is based on histology of the nephrectomy specimen  .
Emphysematous pyelonephritis (EPN): Laboratory tests can show leukocytosis with elevated CRP , elevated blood glucose, and HbA1C levels. Blood and urine cultures may grow Escherichia coli, Klebsiella, or Proteus species. Gas may be noticed within the renal parenchyma on plain X-rays and ultrasound, while CT or MRI scans are useful in diagnosing the disease extent.    .