H-syndrome: Proposed diagnostic criteria for a clinical challenge

H-syndrome is a disorder of histiocytes with a notable reduction in nucleoside transport activity of hENT3 protein.Cutaneous involvement is a hallmark characteristic of H-syndrome with bilateral and symmetrical hyperpigmented hypertrichotic indurated plaques being the hallmark of the disease.A variety of other cutaneous lesions in addition to systemic features have been reported.In this review, we address H-syndrome as a diagnostic challenge due to its wide intrafamilial and interfamilial phenotypic variability, its evolving nature, and the overlapping features with other SLC29A3 related disorders.The main highlight is to propose diagnostic criteria to help physicians identify cases in clinical settings avoiding delay in diagnosis, unnecessary testing, and inappropriate treatment.In addition, the differentiating features from other skin disorders that share H-syndrome cutaneous features are detailed.The advances in the disease pathogenesis and their relation to its cutaneous presentation are highlighted.H-syndrome still represents a therapeutic challenge, the evidence from different treatment regimens adopted, and their effectiveness is summarized.( JAAD Reviews 2025;