Pyogenic sacroiliitis in children: don't forget the very young.

Pyogenic sacroiliitis (PS) is rare with less than 100 pediatric cases reported in the medical literature. To better characterize PS in the pediatric population, we investigated a series of children presenting with PS. Retrospective data analysis was done at an academic tertiary center between the years of 2000 and 2017. All hospitalized children ≤ 16 years of age with PS were evaluated. Of the 894 children hospitalized with osteoarticular infections, 18 were diagnosed with PS (2%) and are included in the review. Two clinically distinct groups were identified. PS in infants (n = 13, 72.2%, mean age 1.1 years) had an indolent course and a faster recovery without any bacterial source identified. In contrast, the group of older children (n = 5, 27.8%, mean age 11.6 years) had a more complicated course and a higher rate of identified bacterial infections.Conclusion: We describe an under-recognized entity of PS in infants with a mild clinical course and fast recovery that differ from the "classical" septic sacroiliitis. Infants with PS did not suffer from invasive complications, and pathogen characteristics of older children were not identified. Infants with fever, irritability, decreased range of motion in the pelvic area, and pain during diapering should alert the clinician to this diagnosis. What is Known: • Pediatric pyogenic sacroiliitis is an extremely rare condition usually caused by Staphylococcus aureus with highest incidence in adolescents. • The diagnosis of PS is challenging due to its rarity and difficulty in assessing the sacroiliac joint. What is New: • We describe an under-recognized entity of PS in infants with a mild clinical course, without invasive complications and with fast recovery that differ from "classical" septic sacroiliitis. • Infants with fever, irritability, decreased range of motion in the pelvic area and pain during diapering should raise clinical suspicion of this diagnosis.