Descriptive Study of Hemophagocytic Lymphohistiocytosis (HLH) Admissions in Adult Population: Data from Nationwide Inpatient Sample (NIS) Database
Authors: Lingamaneni Prasanth , Vohra Ishaan , Farooq Muhammad Zain , Malapati Sindhu , Singh Sunny R K 
Published: November 2019

Background: Hemophagocytic lymphohistiocytosis (HLH) is a syndrome of severe immune activation and dysregulation, characterized by hyperactive macrophages and lymphocytes, resulting in multi-organ damage and high mortality. Epidemiologic data of HLH in the adult population is limited. The aim of our study is to characterize HLH admissions in the US adult population.
Methods: We performed a retrospective study using the Nationwide Inpatient Database for the year 2016. 330 admissions with principal diagnosis of HLH in adults were identified, using ICD code specific for HLH. Using ICD 10 codes for malignancies and rheumatologic conditions, we subdivided HLH admissions into malignancy-associated HLH, autoimmune disease-related HLH and non-malignancy, non-autoimmune HLH. Statistical analyses were performed using STATA. One-way ANOVA was used to compare means of continuous variables and chi-square test was used to compare proportions of categorical variables.
Results: Mean age of our study group was 50.1, with women making up 44% of the population. 15% of the patients had an underlying malignancy, most common type being mature T/NK cell lymphoma. Also, 15% of the patients had a rheumatologic diagnosis, SLE constituting half of this sub-group. 47% of the total population under study underwent either bone marrow, liver or splenic biopsy. Mortality rate was 21.2% and average length of stay was 17 days. 25.8% received systemic chemotherapy during the admission.
Within the subgroups, we did not find a statistically significant difference in the age, gender, race, percentage undergoing biopsy or inpatient chemotherapy. There was also no statistically significant difference in mortality or length of stay between the three groups.
Discussion: HLH is a life-threatening condition of immune hyper-activation and may be divided into primary HLH, which is an autosomal recessive condition, caused by mutations impairing the cytotoxic function of NK cells and cytotoxic T lymphocytes. Secondary HLH is the more frequent presentation in adults, and frequently identified triggers include hematologic malignancies, infections and autoimmune conditions. Autoimmune-associated HLH is usually termed as macrophage activation syndrome (MAS). Mortality in secondary HLH varies based on underlying condition, around 8% when associated with JIA, compared to more than 80% in malignancy associated HLH in some studies. Our study sample was underpowered to detect notable differences between the study groups. This can be attributed to the rarity of this condition. HLH is primarily a pediatric disease. Incidence of HLH in the pediatric age group has been estimated to be around 1 in 100,000 children. Incidence in the adult population is less clear. In our study, we were able to represent the epidemiologic aspects of adults with HLH in the real world. We need large observational studies to further characterize HLH in the adult population.

Disclosures
No relevant conflicts of interest to declare.