Temporal trends in the incidence of haemophagocytic lymphohistiocytosis: a nationwide cohort study from England 2003-2018

BackgroundHaemophagocytic lymphohistiocytosis (HLH) is rare, results in high mortality and is increasingly being diagnosed. Little is known about what is driving the apparent rise in the incidence of this disease. MethodsUsing national linked electronic health data from hospital admissions and death certification cases of HLH that were diagnosed in England between 1/1/2003 and 31/12/2018 were identified using a previously validated approach. We calculated incidence rates of diagnosed HLH per million population using mid-year population estimates by calendar year, age group, sex and associated comorbidity (haematological malignancy, inflammatory rheumatological or bowel diseases (IBD)) associated with the diagnosis of HLH. We modelled trends in incidence and the interactions between calendar year, age and associated comorbidity using Poisson regression. FindingsThere were 1674 people with HLH diagnosed in England between 2003 and 2018. The incidence rate quadrupled (Incidence Rate Ratio (IRR) 2018 compared to 2003: 3.88 95% Confidence Interval (CI) 2.91 to 5.28), increasing 11% annually (adjusted IRR 1.11 95% CI 1.09 to 1.12). There was a rising trend in all age groups except those aged less than 5 years. There was a transition across the age groups with greater increases in those aged 5 to 14 years of HLH associated with rheumatological disease/IBD compared to HLH associated with haematological malignancy, with similar increases in HLH associated with both co-morbidities for those 15-54, and greater increases in associated haematological malignancies for those 55 years and older. InterpretationThe incidence of HLH in England has quadrupled between 2003 and 2018, increasing 11% annually. Substantial variation in the incidence occurred by age group and by HLH associated comorbidities with inflammatory rheumatological diseases or IBD associated HLH increasing more among the young and middle age groups, whereas in older age groups the largest increase was seen with haematological malignancy-associated HLH. Evidence before this studyThere is a paucity of population-based data on the epidemiology of HLH worldwide. The available evidence relies mostly upon a collection of cases series published in The Lancet in 2014 which described 2197 cases of HLH in adults reported in the literature to that point. Almost all of these were from tertiary referral specialist centres and/or described in small case series. The incidence of HLH has only been described in a few reports - and mainly this has focused on children with primary HLH. No previous studies have been large enough to examine trends in incidence by age, sex, underlying risk factors and calendar time. Added value of this studyThis study quantifies the incidence of diagnosed HLH for the first time in a nationwide manner for all age groups. It reports on 1674 patients with HLH from England and shows that there is substantial variation in the incidence by age group, associated disease and calendar time. The results imply reasons for the increase in HLH could be related to the increasing occurrence of haematological cancer, inflammatory rheumatological or bowel diseases and the treatments given for these conditions. This study has been carried out in partnership with the National Congenital Anomalies and Rare Diseases Registration Service and the methodology described can in future be applied to many rare diseases that as yet lack a way of quantifying crucial epidemiological metrics. Implications of all the available evidenceThe incidence of HLH is rising rapidly in people older than 5 years of age. This could be due to an increase in the biologic, immunomodulation or immunosuppressive therapy being used in people with haematological cancer and inflammatory rheumatological and bowel diseases. Further work should focus on how to minimise the risk of HLH occurring, or to improve treatment of this often fatal disease among those who need treatment for an associated comorbidity.