Hemophagocytic lymphohistiocytosis (HLH), also known as histiocytic medullary reticulosis, macrophage activation syndrome, and familial hemophagocytic lymphohistiocytosis (for inherited forms), is a relatively uncommon hemophagocytic syndrome. HLH affects all age groups, but the majority of cases are in infants (often younger than 1 year old) and young children.
HLH tends to affect the bone marrow, spleen, lymph nodes, liver, central nervous system, and skin. Up to 65% of patients have nonspecific cutaneous involvement ranging from a transient generalized morbilliform rash to petechiae, purpuric macules, or papules to erythroderma. Without treatment, HLH can be rapidly fatal.
It is classified into primary (familial) and secondary (acquired, sporadic) forms:
Familial hemophagocytic lymphohistiocytosis (FHL) is inherited in an autosomal recessive pattern; approximately 24% of 122 cases reviewed by the FHL Study Group of the Histiocyte Society had a history of parental consanguinity. Estimated incidence of FHL is 1.2 per million in children under the age of 15; 70%-80% of FHL cases present before 1 year of age. Up to 80% of FHL cases are caused by mutations in 1 of 3 genes (encoded protein in parentheses): PRF-1 (perforin, most common), UNC13D (Munc13-14), and STX-11 (syntaxin-11). Both perforin and Munc13-14 proteins are involved in host response to infection, while the exact role of syntaxin-11 has yet to be determined.
The pathophysiology behind both primary and secondary HLH is T-cell immune dysregulation. While the exact mechanism is unknown, the dysregulatory process results in an overproduction of inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha), interferon gamma (IFN-gamma), and various interleukins (IL). These molecules activate T-cells, macrophages, and histiocytes. This disrupts the mechanisms that regulate apoptosis. Natural killer (NK) cells, particularly in FHL, are absent or reduced. Under normal circumstances, NK cells (which have granules containing perforin and granzymes that disrupt target cell membrane integrity) participate in natural cell death.
Proposed HLH 2004 diagnostic criteria include the following (at least 5 of 8 required):
Fever (> 7 days)
Splenomegaly (> 3 cm below costal margin)
Blood cell dyscrasia (> 2 of 3 lineages – hemoglobin < 9.0 g/dL, platelets < 100 000/L, absolute neutrophils < 1000/μL – in absence of hypocellular marrow)
Tissue sample (from bone marrow, liver, or lymph nodes) demonstrating macrophage phagocytosis of erythrocytes, platelets, leukocytes, and precursor cells
Low/absent NK cell activity
Hyperferritinemia (> 500 μg/L)
High levels of soluble IL-2 receptor
Other common clinical signs include hepatomegaly, lymphadenopathy, nonspecific cutaneous eruptions, and neurological abnormalities.
Left untreated, HLH is fatal, especially in the familial forms.
Immunocompromised Patient Considerations: HLH can be triggered by immunodeficient states such as HIV infection.
Myelofibrosis – Look for pancytopenia, pallor (anemia), ecchymoses (thrombocytopenia), susceptibility to infection (leukopenia), splenomegaly, and arthralgias. Bone marrow biopsy shows fibrosis, replacement of marrow with collagenous connective tissue.
Langerhans cell histiocytosis (histiocytosis X) – Look for osteolytic bone lesions (especially in skull, arms), diffuse cutaneous eruption of red-brown papules (especially scalp), lung involvement, pancytopenia, and endocrine (anterior pituitary) abnormalities; immunohistochemical stains positive for CD1.
Leukemia cutis – Look for red-violaceous-brown papules, plaques, or nodules; T-cell immunohistochemical stains positive for CD45 / leukocyte common antigen (LCA), CD45RO, and CD3, while B-cell immunohistochemical stains positive for CD20; associated with myelogenous leukemia or congenital leukemia.
X-linked lymphoproliferative syndrome – Rare; most patients develop near-fatal fulminant EBV infection with bone marrow and hepatic failure; high risk for HLH.
Chediak-Higashi syndrome – Rare, autosomal recessive immunodeficiency disorder of lysosome dysfunction; look for hypopigmentation, neutropenia, recurrent infections, progressive neurological deficits, and death in childhood; higher risk for HLH.
Griscelli syndrome – Rare, autosomal recessive disorder; look for generalized hypopigmentation, severe but stable neurological deficits from birth; higher risk for HLH.
Below is a list of drugs with literature evidence indicating an adverse association with this diagnosis. The list is continually updated through ongoing research and new medication approvals. Click on Citations to sort by number of citations or click on Medication to sort the medications alphabetically.
