Hemophagocytic Lymphohistiocytosis/Macrophage Activation Syndrome in pediatric Inflammatory Bowel Disease: clinical characteristics and outcomes.

Hemophagocytic Lymphohistiocytosis (HLH)/Macrophage Activation Syndrome (MAS) in children with inflammatory bowel disease (IBD) has been reported only anecdotally. This study aimed at describing the clinical features and outcomes of children diagnosed with both IBD and HLH/MAS. Data on IBD and HLH/MAS characteristics, biochemical, microbiological and genetic assessments, treatments, and outcomes were collected from the Italian Pediatric IBD Registry and presented using descriptive statistics. Out of 4643 patients with IBD, 18 (0.4%) were diagnosed with HLH/MAS, including 12 with ulcerative colitis and 6 with Crohn disease. Among the 18 patients, 7 (39%) had early-onset IBD, but the median age at HLH/MAS diagnosis was 14.0 years (IQR 11.9-16.0). Half of the patients had active IBD at HLH/MAS diagnosis, 11 (61%) patients were on thiopurines, and 6 (33%) were on anti-TNF biologics. An infectious trigger was identified in 15 (83%) patients. One (5%) patients was diagnosed with XIAP deficiency. All patients discontinued thiopurines and 5 (83.3%) discontinued anti-TNF biologics; 16 (80%) patients received steroids for HLH/MAS. Three (17%) patients had a relapse of HLH/MAS. No patient developed lymphoma or died during a median follow-up of 2.7 years (IQR 0.8-4.4). Conclusions: HLH/MAS mainly affects children with early-onset IBD but primarily develops during adolescence, following an infection while on immunosuppressant treatment. Although the prognosis is generally favorable, it is crucial to investigate an underlying immune deficiency.