Atypical familial hemophagocytic lymphohistiocytosis type 3 in children: A report of cases and literature review

Abstract Background Familial hemophagocytic lymphohistiocytosis type 3 (FHL3) is caused by UNC13D variants. The clinical manifestations of FHL3 are highly diverse and complex. Some patients exhibit atypical or incomplete phenotypes, making accurate diagnosis difficult. Our study aimed to broaden the understanding of the atypical FHL3 clinical spectrum. Methods In our study, we analyzed in detail the clinical features of four Chinese patients with UNC13D variants. Additionally, we conducted a comprehensive review of the existing literature on previously reported atypical manifestations and summarized the findings. Results Two of our patients presented with muscle involvement, while the other two had hematological involvement; none of them met the diagnostic criteria for hemophagocytic lymphohistiocytosis (HLH). However, protein expression and functional analysis ultimately confirmed diagnostic criteria for FHL3 in all patients. From the literature we reviewed, many atypical FHL3 patients had neurological involvement, especially isolated neurological manifestations. At the same time, arthritis and hypogammaglobulinemia were also prone to occur. Conclusion Our study highlights that the expression of the Munc13‐4 protein may not fully indicate the pathogenicity of UNC13D variants, whereas CD107a analysis could be more sensitive for disease diagnosis. These findings contribute to a broader understanding of the FHL3 clinical spectrum and may offer new insights into the underlying pathogenesis of UNC13D variants. It is crucial to prioritize the timely and accurate diagnosis of atypical patients, as they may often be overlooked among individuals with rheumatic or hematological diseases.