Hemophagocytic Lymphohistiocytosis (HLH)
(See also Overview of Histiocytic Syndromes.)
Hemophagocytic lymphohistiocytosis (HLH) is uncommon. It affects mostly infants < 18 months. It involves a defect in targeted killing and the inhibitory controls of natural killer and cytotoxic T cells, resulting in excessive cytokine production and accumulation of activated T cells and macrophages in various organs. Cells in the bone marrow and/or spleen may attack red blood cells, white blood cells, and/or platelets.
HLH can be
HLH is diagnosed when patients fulfill at least 5 of the criteria described below or have a mutation in a known HLH-associated gene.
Acquired HLH can be associated with infections (eg, Epstein-Barr virus, cytomegalovirus, or others), cancer (eg, leukemias, lymphomas), immune disorders (eg, systemic lupus erythematosus, rheumatoid arthritis, polyarteritis nodosa, sarcoidosis, progressive systemic sclerosis, Sjögren syndrome, Kawasaki disease) and can occur in kidney or liver transplant recipients.
In both familial and acquired forms, genetic abnormalities, clinical manifestations, and outcomes tend to be similar.
Hemophagocytic lymphohistiocytosis can be diagnosed if there is a mutation in a known causative gene or if at least 5 of 8 diagnostic criteria are met:
Fever (peak temperature of > 38.5° C for > 7 days)
Splenomegaly (spleen palpable > 3 cm below costal margin)
Cytopenia involving > 2 cell lines (hemoglobin < 9 g/dL [90 g/L], absolute neutrophil count < 100/mcL [0.10 × 109/L], platelets < 100,000/mcL [100 × 109/L])
Hypertriglyceridemia (fasting triglycerides > 177 mg/dL [2.0 mmol/L] or > 3 standard deviations [SD] more than normal value for age) or hypofibrinogenemia (fibrinogen < 150 mg/dL [1.5 g/L] or > 3 SD less than normal value for age)
Hemophagocytosis (in biopsy samples of bone marrow, spleen, or lymph nodes)
Low or absent natural killer cell activity
Serum ferritin > 500 ng/mL (> 1123.5 pmol/Lng/mL)
Elevated soluble interleukin-2 (CD25) levels (>2400 U/mL or very high for age)
Genetic mutations associated with HLH include
Because some of these tests may not be widely available and HLH is uncommon, patients are usually referred to specialized centers for evaluation.
Treatment for hemophagocytic lymphohistiocytosis should be started if the disorder is suspected, even if not all diagnostic criteria are fulfilled. Patients are usually treated by a pediatric hematologist and in a referral center experienced in treating patients with HLH. Treatment depends on the presence of factors such as a family history of HLH, coexisting infections, and demonstrated immune system defects. Treatment for HLH may include cytokine inhibitors, immune therapy, chemotherapy, some combination of these, and possibly stem cell transplantation.
In 2018, emapalumab, an interferon gamma–blocking monoclonal antibody, became available for patients with refractory, recurrent, or progressive HLH or with an intolerance to conventional therapy.
Hemophagocytic lymphohistiocytosis (HLH) is an uncommon rare disorder and usually affects infants < 18 months.
HLH can be familial (inherited) or acquired.
Diagnose HLH if the patient has at least 5 of 8 published diagnostic criteria or if the patient has a known mutation associated with HLH.
Treat with chemotherapy, cytokine inhibitors, immune suppression, and sometimes hematopoietic stem cell transplantation.
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