What is HLH (Hemophagocytic Lymphohistiocytosis)?
HLH is a life-threatening hyperinflammatory syndrome caused by uncontrolled activation of macrophages and cytotoxic T cells, leading to excessive cytokine release, end-organ damage, and death without immunosuppressive treatment. 1
Core Pathophysiology
HLH represents an excessive but ineffective immune response characterized by:
- Aberrant activation of CD8+ T cells and natural killer (NK) cells that fail to properly eliminate target cells
- Rampant cytokine secretion creating a self-sustaining inflammatory cascade, particularly driven by interferon-γ 2
- Macrophage hyperactivation leading to hemophagocytosis (macrophages engulfing blood cells)
- Multi-organ dysfunction from unchecked inflammation that rapidly progresses to terminal organ failure 1, 2
Two Major Forms
Primary (Genetic) HLH
- Most common in children but can occur in adolescents and young adults 1
- Caused by bi-allelic germline mutations in genes essential for cytotoxic granule function: PRF1, UNC13D, STX11, STXBP2 2
- Results in markedly reduced or absent CD8 T-cell and NK-cell killing ability 2
- These genetic defects prevent proper formation and release of cytolytic granules needed to eliminate infected or abnormal cells
Secondary (Acquired) HLH
- By far the most common form in adults 1
- Triggered by strong immunological stimuli including:
- Patients generally lack bi-allelic mutations in primary HLH genes 2
Clinical Presentation
Patients present with a sepsis-like syndrome that can be indistinguishable from sepsis or multiple organ dysfunction syndrome 1. Key features include:
- Persistent high fever (recurrent, unremitting)
- Cytopenias (low blood cell counts affecting multiple cell lines)
- Hepatosplenomegaly (enlarged liver and spleen)
- Liver dysfunction with elevated aminotransferases
- Coagulopathy (bleeding disorders)
- Neurological impairment (can occur) 3
- Rapid progression to multi-organ failure if untreated 1, 4
Laboratory Hallmarks
- Marked hyperferritinemia (often >10,000 µg/L) 5
- Hypertriglyceridemia
- Hypofibrinogenemia
- Elevated lactate dehydrogenase
- Bicytopenia or pancytopenia 1, 5
Critical Clinical Pitfall
The most dangerous pitfall is mistaking HLH for sepsis alone. Both present with fever, organ dysfunction, and laboratory derangements. However, treating HLH as sepsis without addressing the hyperinflammation leads to continued deterioration and death 1. Early recognition is critical because irreversible organ damage occurs rapidly - this is a medical emergency requiring immediate immunosuppressive intervention 1, 4.
Diagnostic Challenge in Adults
The HLH-2004 diagnostic criteria were developed for children and are not validated for adults 1. The HScore is a validated tool specifically for estimating probability of secondary HLH in adults, with scores >220 indicating >93% probability 3. However, maintaining a high index of suspicion is more important than waiting for diagnostic criteria to be fully met when clinical presentation is consistent 1.
Prognosis Without Treatment
HLH is uniformly fatal without therapy directed at suppressing the overactive immune system 2, 6. Mortality remains high even with treatment, particularly in adults, making prompt recognition and aggressive management essential 4, 6.
Why Early Recognition Matters for Outcomes
Early intervention prevents irreversible organ damage and death 1. The hyperinflammatory cascade, once established, creates a positive feedback loop that becomes increasingly difficult to control. Delayed recognition allows progression to: