Hemophagocytic lymphohistiocytosis (HLH) is a rare, life-threatening hyperinflammatory syndrome in which the immune system becomes dangerously overactivated, causing widespread tissue and organ damage. Without prompt recognition and treatment, HLH can be rapidly fatal, making early diagnosis one of the most critical factors in improving patient outcomes.
Key Takeaways
- HLH is a severe immune dysregulation disorder that can occur in both children and adults.
- The condition is classified as primary (genetic) or secondary (acquired), each with distinct triggers.
- Diagnosis relies on a combination of clinical, laboratory, and genetic criteria, often using the HScore or HLH-2004 guidelines.
- Standard treatment includes immunosuppressive therapy and, in eligible patients, hematopoietic stem cell transplantation.
- Prognosis varies significantly based on underlying cause, speed of diagnosis, and response to therapy.
Hemophagocytic Lymphohistiocytosis (HLH): Causes and Symptoms
Hemophagocytic lymphohistiocytosis refers to a condition in which cytotoxic T cells and natural killer (NK) cells fail to regulate the immune response appropriately, leading to uncontrolled activation of macrophages and histiocytes. These activated immune cells begin destroying red blood cells, white blood cells, and platelets in a process called hemophagocytosis, which is responsible for many of the disorder’s most serious manifestations.
HLH is broadly divided into two categories. Primary HLH, also called familial HLH, arises from inherited mutations in genes such as PRF1, UNC13D, STX11, and STXBP2, which impair the perforin-mediated cytotoxic pathway. Secondary HLH, sometimes called reactive HLH, develops in the context of infections, autoimmune disorders, or malignancies. Epstein-Barr virus (EBV) is the most commonly identified infectious trigger, though other viruses, bacteria, and fungi can also precipitate the syndrome.
The clinical presentation of HLH overlaps with several other critical illnesses, which frequently delays diagnosis. The most prominent signs include:
- Prolonged high fever unresponsive to antibiotics
- Splenomegaly (enlarged spleen), often with hepatomegaly
- Cytopenias affecting two or more blood cell lineages
- Hyperferritinemia (markedly elevated serum ferritin)
- Elevated triglycerides and low or absent NK-cell activity
- Neurological symptoms including seizures, altered consciousness, and ataxia
Ferritin levels above 500 µg/L raise suspicion for HLH, while levels exceeding 10,000 µg/L are considered highly specific for the diagnosis in the appropriate clinical context. A study published in Arthritis & Rheumatology found that ferritin levels above 10,000 µg/L carry approximately 90% specificity for HLH when combined with compatible clinical findings. Recognizing this constellation of features early is essential, as the inflammatory cascade can escalate within days.
Diagnosing and Treating Hemophagocytic Lymphohistiocytosis
HLH disease diagnosis relies on established criteria from the HLH-2004 protocol developed by the Histiocyte Society, which requires five or more of eight defined clinical and laboratory findings to be present. These include fever, splenomegaly, cytopenias, hypertriglyceridemia or hypofibrinogenemia, hemophagocytosis on bone marrow or tissue biopsy, low or absent NK-cell activity, elevated ferritin, and elevated soluble CD25 (sIL-2 receptor). The HScore is an additional validated tool that calculates the probability of HLH based on weighted clinical variables.
Genetic testing plays a central role in distinguishing primary from secondary HLH, particularly in pediatric patients. Flow cytometry to assess perforin expression and degranulation assays also support the diagnostic workup. Bone marrow biopsy, while not always diagnostic on its own, can confirm hemophagocytosis and help exclude underlying malignancy as a trigger.
Regarding HLH disease treatment, the HLH-94 and HLH-2004 protocols remain foundational. Initial therapy typically involves dexamethasone and etoposide, which suppress the hyperinflammatory response and deplete activated T cells. Cyclosporine A is often added to further modulate T-cell activity. In cases triggered by EBV, rituximab may be incorporated. For patients with primary HLH or refractory secondary HLH, allogeneic hematopoietic stem cell transplantation (HSCT) offers the only potential cure by replacing the defective immune system with a functioning one.
Emapalumab, an anti-interferon-gamma monoclonal antibody, received FDA approval in 2018 for patients with primary HLH who have refractory, recurrent, or progressive disease or who cannot tolerate conventional therapy. This targeted biologic addresses the central role of interferon-gamma in driving the cytokine storm that characterizes HLH, representing a meaningful advance in disease management.
HLH in Adults: Complications and Clinical Considerations
Hemophagocytic lymphohistiocytosis in adults presents distinct clinical challenges compared to pediatric cases. In adults, secondary HLH is far more common than the familial form, and it is frequently associated with malignancy—particularly lymphomas, which account for a substantial proportion of adult HLH cases. Autoimmune conditions such as systemic lupus erythematosus and adult-onset Still’s disease (AOSD) are also recognized triggers, and in these settings, the syndrome is sometimes called macrophage activation syndrome (MAS), a closely related entity.
The HLH syndrome signs observed in adults often develop insidiously and can be attributed to the underlying trigger condition, contributing to diagnostic delays. Clinicians may initially attribute fever and cytopenias to sepsis, malignancy progression, or drug toxicity, all of which require different management. Adults also tend to have higher rates of comorbidities that can complicate both diagnosis and treatment tolerability.
The complications of HLH in adults are severe and multisystemic. Organ failure—particularly involving the liver, central nervous system, and kidneys—is a leading cause of death. Coagulopathy, including disseminated intravascular coagulation (DIC), adds further risk. Neurological involvement occurs in roughly 30–50% of patients and may manifest as encephalopathy, cranial nerve palsies, or meningism. Pulmonary complications, including respiratory failure requiring mechanical ventilation, have also been well documented, particularly in critically ill adults presenting to intensive care units.
| Complication | Estimated Frequency | Clinical Impact |
|---|---|---|
| Hepatic failure | Common | Elevated liver enzymes, jaundice, coagulopathy |
| Neurological involvement | 30–50% | Encephalopathy, seizures, cranial nerve deficits |
| Disseminated intravascular coagulation | Frequent in severe cases | Life-threatening bleeding and clotting |
| Pulmonary failure | Variable | May require mechanical ventilation |
| Renal failure | Variable | May require dialysis support |
Prognosis and Survival Outcomes in HLH Syndrome
The hemophagocytic lymphohistiocytosis prognosis depends heavily on the underlying etiology, the patient’s age, speed of diagnosis, and response to initial therapy. Without treatment, HLH is almost universally fatal, with median survival historically measured in weeks. Even with treatment, mortality remains high. Studies have reported overall mortality rates ranging from 20% to over 50% across different patient populations, with malignancy-associated HLH carrying particularly poor outcomes.
In pediatric patients with primary HLH, HSCT has markedly improved long-term survival. Data from the HLH-94 trial demonstrated an estimated 5-year probability of survival of approximately 54% for patients who underwent transplantation, compared to near-zero long-term survival without it. For secondary HLH in adults, survival is closely linked to successful identification and treatment of the underlying trigger. When a reversible cause such as a viral infection is identified and managed effectively alongside immunosuppressive therapy, outcomes can be considerably better.
The hemophagocytic lymphohistiocytosis survival rate in adults with malignancy-associated HLH remains under 20–30% in many series, reflecting both the severity of the immune dysregulation and the challenges of treating aggressive underlying cancers simultaneously. Prognostic scoring tools, including the HScore, provide probability estimates but are not substitutes for individualized clinical judgment.
Early multidisciplinary involvement—including hematology, rheumatology, infectious disease, and critical care—is strongly associated with better outcomes. Delays of even a few days in initiating appropriate therapy can allow the inflammatory cascade to progress to irreversible organ damage. Ongoing clinical trials continue to evaluate novel agents targeting specific cytokines in the HLH pathway, with the hope of improving survival across all patient subgroups.
Frequently Asked Questions
Can HLH occur without a genetic mutation?
Yes. Secondary HLH develops without an underlying genetic defect and is triggered by infections, malignancies, or autoimmune conditions. It is, in fact, the more common form in adults. In these cases, identifying and treating the precipitating cause is a central component of management alongside immunosuppressive therapy. The absence of a familial mutation does not reduce the severity of the syndrome, and patients with secondary HLH can deteriorate just as rapidly as those with primary disease.
Is HLH curable?
Primary HLH can be cured with allogeneic hematopoietic stem cell transplantation in eligible patients, replacing the defective immune system with healthy donor cells. Secondary HLH may resolve if the underlying trigger is successfully treated, though some patients experience recurrence. Complete remission is possible but not guaranteed. Outcomes depend on the cause, severity at presentation, and how quickly effective treatment is initiated. Emapalumab has expanded treatment options for refractory primary HLH cases.
How does HLH differ from sepsis?
Both HLH and sepsis present with fever, organ dysfunction, and abnormal laboratory values, making differentiation challenging. However, HLH is driven by immune dysregulation rather than direct pathogen toxicity. Key distinguishing features of HLH include markedly elevated ferritin, hemophagocytosis on biopsy, very low NK-cell activity, and elevated soluble CD25. Sepsis does not typically produce these findings to the same degree. Misidentification as sepsis is a common cause of diagnostic delay in HLH.
