Hemophagocytic lymphohistiocytosis (HLH) is a syndrome of pathologic immune activation, occurring as either a familial disorder or a sporadic condition, in association with a variety of triggers. This immune dysregulatory disorder is prominently associated with cytopenias and a unique combination of clinical signs and symptoms of extreme inflammation. Prompt initiation of immunochemotherapy is essential for survival, but timely diagnosis may be challenging because of the rarity of HLH, its variable presentation, and the time required to perform diagnostic testing. Therapy is complicated by dynamic clinical course, high risk of treatment-related morbidity, and disease recurrence. Here, we review the clinical manifestations and patterns of HLH and describe our approach to the diagnosis and therapy for this elusive and potentially lethal condition.

The lesions of Langerhans'-cell histiocytosis (histiocytosis X), a proliferative histiocytic disorder of unknown cause, contain histiocytes similar in phenotype to dendritic Langerhans' cells. The disease ranges in severity from a fatal leukemia-like disorder to an isolated lytic lesion of bone. Intermediate forms of the disease are usually characterized by multiorgan involvement, diabetes insipidus, and a chronic course.

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory syndrome comprising familial/genetic HLH (FHL) and secondary HLH. In the HLH-94 study, with an estimated 5-year probability of survival (pSu) of 54% (95% CI, 48-60%), systemic therapy included etoposide, dexamethasone and, from week nine, cyclosporine A (CSA). HSCT was indicated in patients with familial/genetic, relapsing, or severe and persistent disease. In HLH-2004, CSA was instead administered upfront, aiming to reduce pre-HSCT mortality and morbidity. During 2004-2011, 369 children aged <18-years fulfilled the HLH-2004 inclusion criteria (5/8 diagnostic criteria, affected siblings, and/or molecular diagnosis in FHL-causative genes). At a median follow-up of 5.2 years, 230/369 (62%) patients were alive (5-year pSu 61%, 56-67%). The 5-year pSu in children with (n=168) and without (n=201) family history/genetically verified FHL was 59% (52-67%) and 64% (57-71%), respectively [familial occurrence (n=47): 58% (45-75%)]. Comparing with historical data (HLH-94), using HLH-94 inclusion criteria, pre-HSCT mortality was non-significantly reduced from 27% to 19% (P=.064 adjusted for age and gender). Time from start of therapy to HSCT was shorter compared to HLH-94 (P=.020 adjusted for age and gender) and reported neurological alterations at HSCT were 22% in HLH-94 and 17% in HLH-2004 (using HLH-94 inclusion criteria). Five-year pSu post-HSCT overall was 66% [verified FHL 70% (63-78%)]. Additional analyses provided specific suggestions on potential pre-HSCT treatment improvements. HLH-2004 confirms that a majority of patients may be rescued by the etoposide/dexamethasone combination but intensification with CSA upfront, adding corticosteroids to intrathecal therapy, and reduced time to HSCT did not improve outcome significantly. Key points Early introduction of cyclosporine did not improve HLH outcome in patients treated with the HLH-94 etoposide-dexamethasone backbone (P=.06) HLH-2004 may possibly be improved by risk group stratification, less therapy reduction weeks 7-8 for verified FHL patients, and earlier HSCT

Rosai-Dorfman-Destombes disease (RDD) is a rare non–Langerhans cell histiocytosis characterized by accumulation of activated histiocytes within affected tissues. RDD, which now belongs to the R group of the 2016 revised histiocytosis classification, is a widely heterogeneous entity with a range of clinical phenotypes occurring in isolation or in association with autoimmune or malignant diseases. Recent studies have found NRAS, KRAS, MAP2K1, and ARAF mutations in lesional tissues, raising the possibility of a clonal origin in some forms of RDD. More than 1000 reports have been published in the English literature; however, there is a lack of consensus regarding approach for the clinical management of RDD. Although in most cases RDD can be observed or treated with local therapies, some patients with refractory or multifocal disease experience morbidity and mortality. Here we provide the first consensus multidisciplinary recommendations for the diagnosis and management of RDD. These recommendations were discussed at the 32nd Histiocyte Society Meeting by an international group of academic clinicians and pathologists with expertise in RDD. We include guidelines for clinical, laboratory, pathologic, and radiographic evaluation of patients with RDD together with treatment recommendations based on clinical experience and review of the literature.