Purpose or Case Report: Evaluate prevalence of clinical occult pyelonephritis in children younger than 7 years with CT diagnosis of pyelonephritis.
Methods & Materials: From the radiology information system, we retrieved all children < 7 years that presented with acute symptoms and underwent abdominal CT from 2011 to 2021 with a diagnosis of pyelonephritis in the radiologist report. From the electronic medical records, we reviewed demographic, lab, and clinical information. We excluded children with neurogenic bladder, chromosomal abnormalities, systemic disease, severe developmental delay, history of abdominal trauma in the last 4 weeks, or urologic abnormalities except for vesicoureteral reflux (VUR). A pediatric radiologist reviewed all CT scans and recorded the findings. Chi-Square statistics were used to compare the outcomes of renal atrophy and dilated (grades 3-5) VUR between a single focus of pyelonephritis in any of the kidneys and more extensive involvement.
Results: The study group included 69 children (59 females) with a mean age of 4.7 years. 54/69 (77.1%) had no prior history of UTI. 60 children (85.7%) had fever and 54 (77.1%) had abdominal pain. In 29/69 children (42.0%), a diagnosis of pyelonephritis was not in the differential diagnosis before the CT scan. In 19 of these 29 (65.5%), r/o appendicitis was the primary indication. Urinalysis was negative in 7/69 children (10%). Urine culture was negative in 18 of 66 children (27.3%). 10 children received antibiotics before the urinalysis and urine culture. Of those, 8 had positive urinalysis and 7 had positive urine culture.

Follow-up US at least 3 months after the CT diagnosis of pyelonephritis was performed in 20/69 patients (29.0%). 4 of these 20 children (25.0%) had new renal atrophy. VCUG was performed in 38/69 patients (55.1%); of these, 23/38 had VUR (60.5%), and 14 of these had dilating VUR (60.9%). There was no statistical difference between extent of pyelonephritis in CT and renal atrophy (p=0.129) or dilated VUR (p=0.692). 10 children were treated with ureteral reimplantation and one child with Deflux injection.
Conclusions: In about 40% of the children younger than 7 years, the diagnosis of pyelonephritis was not suspected before the CT scan. In 10% of the children urinalysis was negative, and about a quarter of the children had negative urine culture. Appendicitis was the most common mimicker of pyelonephritis. CT diagnosis of pyelonephritis should be followed with US and cystography, due to risk of renal scarring and high incidence of VUR.