Chronic pyelonephritis is characterized by renal inflammation and fibrosis induced by recurrent or persistent renal infection, vesicoureteral reflux, or other causes of urinary tract obstruction. The diagnosis of chronic pyelonephritis is made based on imaging studies such as ultrasound or CT scanning. It occurs almost exclusively in patients with major anatomic anomalies, most commonly in young children with vesicoureteral reflux (VUR). 
VUR is a congenital condition that results from incompetence of the ureterovesical valve due to a short intramural segment. VUR is present in 30-40% of young children with symptomatic UTIs and in almost all children with renal scars. It may also be acquired by patients with a flaccid bladder due to spinal cord injury. VUR is classified into 5 grades (I-V), according to the increasing degree of reflux. (See Treatment.) The diagnosis of VUR is frequently established on the basis of radiologic evidence obtained during an evaluation for recurrent urinary tract infection (UTI) in young children.
For patient education information, see Urinary Tract Infections (UTIs).
Etiology and Pathophysiology
Chronic pyelonephritis is associated with progressive renal scarring, which can lead to end-stage renal disease (ESRD). For example, in reflux nephropathy, intrarenal reflux of infected urine is suggested to induce renal injury, which heals with scar formation.  In some cases, scars may form in utero in patients with renal dysplasia with perfusion defects. Infection without reflux is less likely to produce injury. Dysplasia may also be acquired from obstruction. Scars of high-pressure reflux can occur in persons of any age. In some cases, normal growth may lead to spontaneous cessation of reflux by age 6 years.
Factors that may affect the pathogenesis of chronic pyelonephritis are as follows: (1) the sex of the patient and his or her sexual activity; (2) pregnancy, which may lead to progression of renal injury with loss of renal function; (3) genetic factors; (4) bacterial virulence factors; and (5) neurogenic bladder dysfunction. In cases with obstruction, the kidney may become filled with abscess cavities (see Pyonephrosis).
In the United States, VUR may be present in 30-40% of children with UTIs. The prevalence rate of VUR in siblings of patients with chronic pyelonephritis is approximately 35%. VUR and chronic pyelonephritis are less common in African American children than in white children, with chronic pyelonephritis occurring 3 times more often in white children.  Chronic pyelonephritis is also twice as common in females as it is in males.
Chronic pyelonephritis occurs more often in infants and young children (younger than 2 y) than it does in older children and adults.
The Birmingham Reflux Study clearly showed that medical and surgical management are equally effective in preventing renal damage from VUR.  Almost all children should receive a trial of medical management.
Although most children with chronic pyelonephritis due to VUR may experience spontaneous resolution of reflux, approximately 2% can still progress to renal failure, and 5-6% can have long-term complications, including hypertension. 
Hypertension contributes to the accelerated loss of renal function in persons with chronic pyelonephritis. Reflux nephropathy is the most common cause of hypertension in children, occurring in 10-20% of children with VUR and renal scars. The resolution of reflux does not appear to correct hypertension.
Complications of chronic pyelonephritis can also include the following:
Progressive renal scarring leading to end-stage renal disease 
Pyonephrosis - May occur in cases of obstruction
Progressive renal scarring (reflux nephropathy)