Haemophagocytic Lymphohistiocytosis

1. Clinical Overview

Summary

Haemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory syndrome characterized by pathological immune activation with excessive cytokine release, leading to fever, cytopenias, hepatosplenomegaly, coagulopathy, and multi-organ dysfunction. The condition arises from failure of the immune system to appropriately terminate an inflammatory response, resulting in uncontrolled proliferation and activation of macrophages and cytotoxic T lymphocytes that infiltrate multiple organs. [1,2]

HLH exists in two major forms: primary (familial) HLH, caused by genetic defects in cytotoxic lymphocyte function, and secondary (acquired) HLH, triggered by infections, malignancies, or rheumatological diseases. In adults, secondary HLH predominates and carries mortality rates of 40-75% even with treatment, making rapid recognition and intervention critical. [3,4] When HLH occurs in the context of rheumatological disease, it is often termed macrophage activation syndrome (MAS).

The hallmark laboratory finding is markedly elevated serum ferritin, with levels exceeding 10,000 μg/L demonstrating 90% sensitivity and 96% specificity for HLH. [5] Diagnosis relies on the HLH-2004 criteria, requiring five of eight clinical and laboratory parameters. Treatment involves addressing the underlying trigger combined with immunosuppressive therapy, typically with dexamethasone and etoposide-based protocols adapted from the HLH-94/2004 trials. [1] For primary HLH, allogeneic hematopoietic stem cell transplantation represents the only curative option.

Key Facts

• Definition: Hyperinflammatory syndrome from dysregulated immune activation with uncontrolled macrophage and cytotoxic T-cell proliferation
• Incidence: 1-2 per million in general population; likely severely underdiagnosed in adults [6]
• Primary HLH: 1 in 50,000-100,000 live births; predominantly pediatric presentation
• Secondary HLH: Far more common in adults; median age 40-60 years at presentation
• Mortality: 40-75% overall; approaches 100% if untreated; primary HLH fatal without stem cell transplant [3,4]
• Common Triggers: Epstein-Barr virus (most common infectious), T-cell/NK-cell lymphoma (most common malignant), systemic lupus erythematosus/Still's disease (rheumatological)
• Pathognomonic Finding: Ferritin > 10,000 μg/L has 90% sensitivity and 96% specificity [5]
• Diagnostic Standard: HLH-2004 criteria (≥5 of 8 parameters) [1]
• First-line Treatment: Address underlying trigger + immunosuppression (dexamethasone ± etoposide) [2]
• Time-Critical: Median time from symptom onset to death without treatment: 6-8 weeks
• Prognosis: Varies by etiology - infection-triggered (30-50% mortality), malignancy-triggered (> 70% mortality), MAS with appropriate treatment (15-30% mortality)

Clinical Pearls

Diagnostic Pearl: A ferritin greater than 10,000 μg/L in the context of fever and cytopenias should trigger immediate consideration of HLH, even before all HLH-2004 criteria are fulfilled. This threshold provides excellent specificity and should prompt urgent specialist consultation. [5]

Treatment Pearl: Do not delay immunosuppression while awaiting complete diagnostic workup if clinical suspicion is high (HScore > 169 or ferritin > 10,000 with compatible presentation). Each day of delayed treatment increases mortality risk. Initiate dexamethasone 10 mg/m² daily while continuing diagnostic evaluation. [2]

Pitfall Warning: Hemophagocytosis on bone marrow biopsy is neither sensitive nor specific for HLH diagnosis. Only 60-80% of confirmed HLH cases demonstrate hemophagocytosis at presentation, and it can be seen in sepsis, malignancy, and transfusion reactions. Never delay treatment waiting for marrow evidence. [7]

Pitfall Warning: Adult HLH frequently mimics septic shock. However, lack of response to broad-spectrum antibiotics and pressors with rising ferritin (> 2,000-3,000) should immediately raise suspicion for HLH rather than prompting escalation of antimicrobials alone.

Mnemonic: HLH causes F.E.V.E.R. CHAOS

• Ferritin sky-high (> 10,000)
• EBV (and other viral triggers)
• Very low fibrinogen
• Elevated triglycerides
• Refractory fever
• Cytopenias (bi/tri-lineage)
• Hepatosplenomegaly
• Activated macrophages (hemophagocytosis)
• Organ failure (multi-system)
• Soluble IL-2 receptor (sCD25) elevated

Why This Matters Clinically

HLH represents one of the most commonly missed diagnoses in critically ill patients, with delays in recognition directly correlating with mortality. The syndrome crosses multiple specialty boundaries—presenting to hematology, rheumatology, oncology, infectious diseases, and intensive care—yet remains unfamiliar to many clinicians outside specialist centers. [6]

Early recognition (median time to diagnosis: 7-14 days) and prompt initiation of immunosuppression can reduce mortality from > 90% to 40-50% in secondary HLH. [3] The condition is increasingly recognized in adults, particularly in association with EBV infection, T-cell lymphomas, and immune checkpoint inhibitor therapy—all growing clinical scenarios. Understanding the diagnostic approach and management principles is now essential for acute medicine, hematology, and critical care practice.

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2. Epidemiology

Incidence and Prevalence

Primary (Familial) HLH:

• Estimated incidence: 1 in 50,000-100,000 live births [8]
• Predominantly affects infants and young children (median onset: 2-6 months)
• Adult-onset primary HLH rare but increasingly recognized (10-15% of primary HLH cases) [9]
• Higher incidence in populations with consanguinity
• No gender predilection

Secondary (Acquired) HLH:

• Estimated incidence in adults: 0.4-1.2 per million per year (likely significant underascertainment) [6]
• True incidence unknown due to diagnostic underrecognition
• ICU studies suggest HLH may account for 1-4% of unexplained febrile illnesses with organ dysfunction [10]
• Male predominance in adult series (M:F ratio approximately 1.5-2:1)
• Median age at presentation: 40-60 years for infection/malignancy-associated; 20-40 years for rheumatological disease-associated (MAS)

Demographics

Age Distribution:

• Primary HLH: Bimodal—major peak in infancy (less than 2 years: 70-80%), minor peak in young adults with delayed-onset genetic forms
• Secondary HLH: Can occur at any age; peak incidence in 5th-6th decades
• MAS: Younger demographic (median 25-35 years) reflecting underlying rheumatological disease population

Geographic and Ethnic Variations:

• Primary HLH: Higher rates in regions with increased consanguinity (Middle East, parts of Asia)
• Specific genetic variants show ethnic clustering (e.g., PRF1 A91V in African populations)
• EBV-associated HLH: Particularly prevalent in East Asian populations [11]
• No consistent geographic pattern for secondary HLH

Risk Factors

For Secondary HLH Development:

Triggers for Secondary HLH

Infectious (50-60% of adult secondary HLH):

Malignancy-Associated (20-30% of adult secondary HLH):

Rheumatological Disease-Associated (MAS) (15-20% of adult secondary HLH):

• Systemic-onset juvenile idiopathic arthritis (sJIA): 10-30% develop MAS
• Adult-onset Still's disease: 10-15% develop MAS [12]
• Systemic lupus erythematosus: 1-5% develop MAS; higher in Asian populations
• Kawasaki disease (pediatric): 1-3% develop MAS
• Other: Dermatomyositis, systemic sclerosis, vasculitis (rare)

Drug-Induced (5-10% of adult secondary HLH):

• Immune checkpoint inhibitors (anti-PD-1, anti-PD-L1, anti-CTLA-4): Increasing recognition
• CAR-T cell therapy: Overlaps with cytokine release syndrome (CRS)
• Immunosuppressive agents: Paradoxical HLH triggering
• Anticonvulsants: Rare but reported (phenytoin, carbamazepine)
• Biologics: Anti-TNF agents (rare)

Seasonal and Temporal Patterns

• EBV-associated HLH: No strong seasonal pattern
• Influenza-associated HLH: Winter predominance in temperate climates
• Dengue-associated HLH: Correlates with monsoon season in endemic areas
• COVID-19 association: Mirrored pandemic waves (2020-2022)

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3. Pathophysiology

Normal Immune Regulation (For Comparison)

Physiological Immune Response:

1. Antigen Presentation: Antigen-presenting cells (APCs) activate T-cells via MHC complexes
2. T-cell Activation: CD8+ cytotoxic T-lymphocytes (CTLs) and NK cells proliferate and produce IFN-γ
3. Target Cell Elimination: CTLs/NK cells release cytotoxic granules (perforin, granzymes) to kill infected/abnormal cells
4. Apoptosis Induction: Perforin creates pores; granzymes enter target cells and trigger apoptosis
5. Immune Termination: Successful killing removes antigenic stimulus → CTL/NK cell contraction → resolution of inflammation
6. Homeostasis Restored: Cytokine levels normalize; tissue repair begins

Pathological Mechanisms in HLH

Primary Defect: Failed Cytotoxic Function

In HLH, genetic (primary HLH) or acquired (secondary HLH) impairment of cytotoxic function prevents effective target cell killing, leading to persistent immune stimulation. [13]

Phase 1: Initiating Trigger and Impaired Cytotoxicity

Primary (Familial) HLH:

• Germline mutations in genes essential for cytotoxic granule trafficking, docking, priming, or pore formation
• Most common: PRF1 (perforin, 20-40%), UNC13D (Munc13-4, 15-25%), STX11 (syntaxin-11, 10-15%), STXBP2 (Munc18-2, 5-10%)
• Result: CTLs/NK cells cannot effectively kill target cells via granule exocytosis pathway
• Persistent antigenic stimulation from unchecked infected/abnormal cells

Secondary HLH:

• Overwhelming antigen load (e.g., high EBV viral burden, disseminated lymphoma)
• Acquired dysfunction of cytotoxic machinery (unclear mechanisms; possibly viral interference, immune exhaustion)
• Similar functional endpoint: Ineffective target cell clearance → persistent stimulation

Phase 2: Persistent Immune Activation

Failed Immune Contraction:

• Inability to eliminate target cells → continued antigen presentation
• Sustained activation of CD8+ T-cells and NK cells
• Continuous production of IFN-γ (primary driver of pathology)
• IFN-γ activates macrophages → M1 polarization → further cytokine production

Phase 3: Cytokine Storm

Hyperproduction of Pro-inflammatory Cytokines:

Activated T-cells and macrophages produce massive quantities of cytokines, creating a self-perpetuating inflammatory cascade. [14]

Key Mechanistic Insight: IFN-γ is the central driver. It activates macrophages, which produce IL-1, IL-6, IL-18, and TNF-α. IL-18 further drives IFN-γ production from T-cells and NK cells, creating a positive feedback loop that becomes autonomous and self-sustaining even if the original trigger resolves. [14]

Phase 4: Macrophage Activation and Proliferation

Excessive Macrophage Activity:

• IFN-γ induces macrophage proliferation and activation (hemophagocytic macrophages)
• Macrophages infiltrate multiple organs: Bone marrow, liver, spleen, lymph nodes, CNS
• Hemophagocytosis: Activated macrophages engulf erythrocytes, platelets, leukocytes, and their precursors
  • Not specific to HLH (seen in sepsis, malignancy, transfusion)
  • Not required for diagnosis
  • Reflects severity of macrophage activation

Phase 5: Multi-Organ Damage

Hematological:

• Cytopenias: Direct hemophagocytosis + bone marrow infiltration + cytokine-mediated suppression
  • "Anemia: Hemophagocytosis + chronic disease + marrow suppression"
  • "Thrombocytopenia: Hemophagocytosis + consumption + impaired production"
  • "Neutropenia: Hemophagocytosis + marrow infiltration (paradoxically, monocytes often preserved/elevated)"
• Coagulopathy:
  • "Hypofibrinogenemia: Hepatic synthetic dysfunction + consumption"
  • "Prolonged PT/aPTT: Factor consumption + hepatic failure"
  • "Elevated D-dimer: DIC-like picture (though frank DIC less common than in sepsis)"

Hepatic:

• Macrophage infiltration of hepatic sinusoids and portal tracts
• Hepatocellular damage from TNF-α and other cytokines → ↑ transaminases
• Cholestatic injury → ↑ bilirubin
• Synthetic dysfunction → ↓ fibrinogen, ↓ albumin, ↑ PT
• Hyperferritinemia:
  • Ferritin release from activated macrophages (direct)
  • IL-1 and IL-6 stimulate ferritin synthesis (indirect)
  • Levels > 10,000 μg/L highly specific (macrophage origin predominates) [5]

Splenic:

• Massive splenomegaly from macrophage and lymphocyte infiltration
• Red pulp expansion
• Splenic sequestration contributing to cytopenias

Metabolic:

• Hypertriglyceridemia: Cytokine-mediated suppression of lipoprotein lipase → impaired triglyceride clearance [15]
• Hyperglycemia: Cytokine effects + stress response
• Hypoalbuminemia: Hepatic dysfunction + capillary leak + catabolism
• Hyperuricemia: Cell turnover

Neurological:

• CNS involvement in 30-70% (often underrecognized) [16]
• Mechanisms:
  • Perivascular lymphohistiocytic infiltration
  • Cytokine-mediated neuronal damage
  • Microvascular injury and hemorrhage
• Manifestations: Encephalopathy, seizures, ataxia, cranial neuropathies, myelopathy

Cardiovascular:

• Myocardial depression from cytokines (particularly TNF-α, IL-1)
• Capillary leak syndrome → distributive shock
• Hemodynamic instability mimicking septic shock
• Arrhythmias

Pulmonary:

• Acute respiratory distress syndrome (ARDS) from cytokine-mediated lung injury
• Pulmonary hemorrhage (rare but life-threatening)
• Infiltration of lung parenchyma

Renal:

• Acute kidney injury from hypotension, tubular injury (cytokines), and glomerular damage
• Hemophagocytic infiltration of kidneys (less common)

Phase 6: Vicious Cycle and Multi-Organ Failure

Without intervention, the cytokine storm becomes self-sustaining:

• Organ damage → tissue necrosis → further antigen release → more immune activation
• Hemodynamic instability → tissue hypoxia → more cellular damage → worsening inflammation
• Infection risk from immunosuppression (paradoxically, despite immune hyperactivation, patients have impaired antimicrobial responses due to immune exhaustion and functional defects)
• Progressive multi-organ failure → death (median survival without treatment: 6-8 weeks)

Genetic Basis of Primary HLH

Cytotoxic Granule Pathway Defects:

AR = Autosomal recessive; XLR = X-linked recessive

Genetic Testing Implications:

• Genetic confirmation identifies ~70-80% of primary HLH cases
• 20-30% of clinically diagnosed primary HLH have no identified mutation (likely novel genes or regulatory variants)
• Heterozygous carriers (e.g., single PRF1 mutation) have increased risk of secondary HLH under high antigen load [17]
• Genetic diagnosis critical for:
  • Family counseling
  • Sibling HLA typing for transplant
  • Decision-making regarding stem cell transplant
  • Prognostication

EBV as a Trigger: Special Considerations

EBV is the most common infectious trigger for HLH, particularly in East Asian populations. [11] Several unique features:

1. Viral-Host Interaction: EBV infects and persists in B-lymphocytes; in susceptible individuals, failure to control EBV-infected B-cells leads to persistent antigen stimulation
2. Chronic Active EBV (CAEBV): Distinct syndrome with clonal proliferation of EBV-infected T or NK cells; very high risk for HLH development
3. EBV-HLH Phenotype: Often more aggressive, higher mortality, frequent CNS involvement
4. Therapeutic Implications: May benefit from rituximab (anti-CD20) to reduce EBV-infected B-cell burden in addition to standard HLH therapy

MAS vs. HLH: Mechanistic Differences?

Macrophage activation syndrome (MAS) in rheumatological diseases shares the same final common pathway as HLH but may have distinct triggers:

• MAS Triggers: Disease flare, infection (concurrent), medication change
• Cytokine Profile: IL-1 and IL-18 particularly elevated (rationale for anakinra use) [18]
• Underlying Predisposition: Chronic immune activation from rheumatological disease may "prime" the system for MAS
• Milder Presentation: Some cases of MAS are less severe than malignancy/infection-associated HLH, though severe MAS is clinically indistinguishable

Despite these nuances, MAS and HLH represent a continuous spectrum of immune dysregulation rather than distinct entities.

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4. Clinical Presentation

Timeline of Illness

Pre-diagnosis Phase (Days to Weeks):

• Initial Trigger Event: Viral illness, malignancy diagnosis/treatment, rheumatological flare
• Early Symptoms (often non-specific): Fever, malaise, fatigue, anorexia
• Subacute Evolution: Persistent fever despite antibiotics, worsening fatigue, development of organomegaly
• Critical Deterioration: Multi-organ dysfunction, shock, encephalopathy
• Median Time to Diagnosis: 7-14 days from symptom onset (often delayed)
• Median Time to Death (Untreated): 6-8 weeks from symptom onset

Post-diagnosis/Treatment Phase:

• Response to therapy typically seen within 7-14 days (falling ferritin, improving cytopenias)
• Refractory disease or early relapse: Poor prognostic sign
• Late complications: Infection, organ dysfunction, relapse (particularly if underlying trigger unresolved)

Symptoms

Constitutional (Nearly Universal):

• Fever: 90-97% of patients [1,2]
  • High-grade (often > 39°C)
  • Persistent/continuous (unlike intermittent fever in Still's disease, though overlap exists)
  • Unresponsive to antibiotics and antipyretics
  • May temporarily respond to corticosteroids
• Fatigue and Malaise: Severe, progressive
• Anorexia and Weight Loss: Rapid, significant

Gastrointestinal:

• Abdominal pain/discomfort: 30-40% (from hepatosplenomegaly)
• Nausea and vomiting: 20-30%
• Diarrhea: 10-20%
• Jaundice: 20-40% (hepatic involvement)

Neurological (30-70%, Often Underrecognized): [16]

• Encephalopathy: Confusion, altered mental status, lethargy (most common)
• Seizures: Focal or generalized
• Headache: Severe, persistent
• Ataxia and Dysmetria: Cerebellar involvement
• Cranial Neuropathies: Ophthalmoplegia, facial weakness
• Myelopathy: Weakness, sensory loss, bladder/bowel dysfunction
• Irritability (pediatric): Prominent early feature in children

Hemorrhagic:

• Easy bruising and petechiae: 20-40%
• Mucosal bleeding (epistaxis, gingival): 10-20%
• Serious hemorrhage (GI, pulmonary, CNS): 5-10% (severe cases)

Respiratory:

• Dyspnea: 20-40% (ARDS, pulmonary involvement)
• Cough: 10-20%

Dermatological:

• Rash: 20-40%
  • Maculopapular eruption (most common)
  • Petechial/purpuric (thrombocytopenia)
  • Erythroderma (rare)
• Lymphedema (rare)

Signs

Vital Signs:

• Fever: Persistent high-grade (> 38.5°C, often > 39-40°C)
• Tachycardia: Proportionate to fever; may be disproportionate in shock
• Hypotension: Late sign; indicates decompensation/shock
• Tachypnea: ARDS, metabolic acidosis, or shock

General Examination:

• Pallor: Severe anemia
• Jaundice: Hepatic involvement (20-40%)
• Lymphedema: Rare but specific finding

Abdominal Examination:

• Hepatomegaly: 60-90% [1,2]
  • Can be massive
  • Tender
• Splenomegaly: 70-95% [1,2]
  • Often marked (> 5 cm below costal margin)
  • Cardinal feature; absence should raise doubt (though not absolute)
• Ascites: Uncommon unless severe hepatic dysfunction

Lymphatic:

• Lymphadenopathy: 30-60%
  • Generalized or localized
  • Variable size
  • May indicate underlying malignancy (lymphoma)

Skin:

• Petechiae and Ecchymoses: Thrombocytopenia, coagulopathy
• Rash: Variable morphology (see above)
• Pallor: Anemia

Neurological Examination:

• Altered Mental Status: GCS reduction, confusion, delirium
• Focal Neurological Deficits: Hemiparesis, cranial nerve palsies, ataxia
• Meningism: Rare unless CNS infection co-exists
• Seizure Activity: Focal or generalized

Cardiovascular:

• Hypotension: Distributive shock (cytokine-mediated)
• Tachycardia: Compensatory or shock
• Peripheral edema: Capillary leak, hypoalbuminemia

Respiratory:

• Tachypnea: ARDS, metabolic compensation
• Crackles: Pulmonary edema, ARDS
• Hypoxemia: SpO₂ less than 92% on room air (severe cases)

Red Flags (Immediate Specialist Referral/HDU-ICU Admission)

[!CAUTION] Critical Red Flags Indicating Severe HLH with High Mortality Risk:

• Ferritin > 10,000 μg/L with compatible clinical picture (fever, cytopenias)
• Rapidly falling fibrinogen (less than 1.5 g/L or falling > 0.5 g/L per day) with rising transaminases
• Progressive multi-organ failure despite broad-spectrum antibiotics and resuscitation
• Refractory fever with bi- or tri-lineage cytopenias unresponsive to standard care
• New-onset neurological symptoms (confusion, seizures, focal deficits) with systemic inflammation
• Hemodynamic instability (shock) with hyperferritinemia and cytopenias
• Rapidly progressive hepatic dysfunction (transaminases > 1,000 IU/L, coagulopathy)
• HScore > 169 (> 90% probability of HLH) [19]
• Lactate dehydrogenase (LDH) > 1,000 IU/L with ferritin > 3,000 μg/L and cytopenias

Presentation Patterns by Etiology

Infection-Triggered HLH:

• Acute onset (days to 1-2 weeks)
• Clear infectious prodrome often identifiable
• High fever (> 39°C) universal
• Respiratory or GI symptoms may predominate initially
• Rapid progression

Malignancy-Associated HLH:

• Subacute onset (weeks)
• May have known malignancy (lymphoma diagnosis) or HLH as presenting feature
• B-symptoms often present (fever, night sweats, weight loss)
• Lymphadenopathy more prominent
• Slower progression but poorer prognosis

MAS (Rheumatological Disease-Associated HLH):

• Often in context of known rheumatological diagnosis (SLE, Still's disease)
• Acute deterioration during disease flare or after infection/medication change
• May have pre-existing organomegaly, cytopenias (need to recognize acute worsening)
• Rash more common (Still's: evanescent salmon-pink rash)
• Joint symptoms may be prominent (Still's)

Primary (Familial) HLH:

• Predominantly pediatric (infancy)
• Family history may be present (siblings, consanguinity)
• Recurrent or persistent presentation
• May have associated features (albinism in Griscelli, giant granules in Chédiak-Higashi)
• Adult-onset primary HLH: Consider in recurrent unexplained HLH or strong family history

Atypical Presentations

• Isolated CNS-HLH: Neurological symptoms without prominent systemic features (rare; requires high index of suspicion)
• Predominantly Hepatic: Mimics acute liver failure
• Renal-Predominant: AKI as presenting feature
• Post-Transplant: In setting of GVHD or infection; diagnostic confusion common
• Drug-Induced (Checkpoint Inhibitors): Overlaps with other immune-related adverse events (irAEs)

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5. Investigations

Diagnostic Approach

HLH diagnosis is based on clinical and laboratory criteria. Two main frameworks exist:

1. HLH-2004 Criteria: Standard diagnostic criteria (≥5 of 8 parameters) [1]
2. HScore: Probability calculator for reactive HLH (score ≥169 suggests > 90% probability) [19]

HLH-2004 Diagnostic Criteria

Diagnosis of HLH Established by EITHER:

A. Molecular Diagnosis:

• Pathogenic mutation in genes associated with familial HLH (PRF1, UNC13D, STX11, STXBP2, RAB27A, LYST, AP3B1, SH2D1A, BIRC4)

OR

B. Fulfillment of 5 Out of 8 Clinical/Laboratory Criteria:

Limitations of HLH-2004:

• Developed primarily for pediatric familial HLH
• Some criteria (NK cell activity, sCD25) not widely available in routine practice
• Ferritin threshold of 500 μg/L too low (many conditions cause ferritin > 500)
• Does not weight criteria by specificity
• May miss early or atypical cases

HScore (Probability Calculator for Reactive HLH)

The HScore provides a probability estimate for HLH diagnosis and was developed specifically for adults with secondary HLH. [19]

HScore Variables and Points:

Maximum Score: 337

Interpretation:

• Score less than 90: Low probability of HLH (less than 1%)
• Score 90-168: Intermediate probability (need further workup)
• Score ≥169: High probability (> 90% probability of HLH; consider treatment) [19]

Advantages of HScore:

• Designed for adult secondary HLH
• Uses routinely available tests
• Provides probability rather than binary cutoff
• Validated in multicenter cohorts

Limitations:

• Developed in predominantly Caucasian population; may perform differently in other ethnicities
• Requires bone marrow to achieve highest scores (though not essential)
• Not validated for primary HLH or pediatric populations

Initial Laboratory Investigations

Essential Tests (All Patients with Suspected HLH):

HLH-Specific Investigations:

Bone Marrow Examination

Indications:

• Confirm diagnosis (contributes to HLH-2004 and HScore)
• Exclude underlying malignancy (particularly lymphoma, leukemia)
• Assess hemophagocytosis

Findings:

Critical Point: Absence of hemophagocytosis does NOT exclude HLH. Sensitivity increases with repeated sampling, but treatment should not be delayed awaiting marrow results if clinical suspicion is high.

Imaging

Indications:

• Assess organomegaly
• Exclude malignancy
• Evaluate CNS involvement (if neurological symptoms)
• Identify complications

Recommended Imaging:

Microbiological and Virological Investigations

Essential Workup (Identify Infectious Triggers):

Additional Tests Based on Epidemiology/Exposure:

• Dengue, Chikungunya serology/PCR (tropical exposure)
• Leishmaniasis serology/PCR (Mediterranean, South Asia, Latin America)
• Rickettsial serology (travel, tick exposure)
• SARS-CoV-2 PCR (COVID-19 pandemic context)

Malignancy Workup

If Malignancy Suspected (lymphadenopathy, B-symptoms, atypical age, no clear infection):

Genetic Testing (Primary HLH Suspected)

Indications for Genetic Testing:

• Age less than 18 years at presentation (especially less than 2 years)
• Family history of HLH or unexplained infant deaths
• Consanguinity
• Recurrent or relapsing HLH
• Associated features (albinism, bleeding diathesis, neuropathy)
• Adult-onset HLH with no clear secondary trigger
• Planning hematopoietic stem cell transplant

Methods:

• Targeted gene panel: Sequence known HLH-associated genes (PRF1, UNC13D, STX11, STXBP2, etc.)
• Whole exome sequencing (WES): If panel negative but strong suspicion
• Functional assays: Perforin expression by flow cytometry; NK cell degranulation assay (CD107a)

Turnaround Time: Weeks to months (do not delay treatment)

Investigations to Monitor Disease Activity and Treatment Response

Differential Diagnosis Considerations

HLH can mimic many conditions; key differentials and distinguishing features:

Key Discriminator: Ferritin > 10,000 μg/L with fever and cytopenias strongly favors HLH over most differentials (exception: Still's disease, which can also have ferritin > 10,000 but usually has quotidian fever and different organ involvement pattern).

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6. Management

Treatment Algorithm

Image

Key Principles:

1. Treat Urgently: HLH is a medical emergency; delays increase mortality
2. Dual Approach: (a) Treat underlying trigger AND (b) Suppress hyperinflammation
3. Multidisciplinary Care: Involve hematology, ICU, infectious diseases, rheumatology as appropriate
4. Supportive Care: Transfusions, organ support, infection prevention
5. Definitive Therapy for Primary HLH: Allogeneic hematopoietic stem cell transplant (only curative option)

Initial Resuscitation and Supportive Care

Setting:

• High-dependency unit (HDU) or intensive care unit (ICU) for severe cases
• Specialist hematology ward with ICU backup for less severe cases

Immediate Actions:

Treating the Underlying Trigger

Critical Step: Identify and address the inciting cause, as HLH may not fully resolve without treating the trigger.

Infection-Triggered HLH

Special Consideration - COVID-19-Associated HLH:

• Overlaps with cytokine storm syndrome
• Supportive care + corticosteroids (dexamethasone 6 mg daily per COVID protocols)
• Consider IL-6 inhibition (tocilizumab) if available
• Etoposide generally avoided unless clear HLH by HLH-2004 criteria (risk of immunosuppression in active viral infection)

Malignancy-Associated HLH

Principle: Treat underlying malignancy definitively; HLH will not resolve without malignancy control.

Challenge: Distinguishing malignancy-triggered HLH from lymphoma with hemophagocytosis (secondary phenomenon) can be difficult; both require lymphoma treatment.

Rheumatological Disease-Associated HLH (MAS)

Underlying Disease Management:

Anakinra in MAS:

• Dosing: 1-2 mg/kg/day SC (pediatric); 100 mg SC daily or bid (adult); can escalate to 4-8 mg/kg/day in severe cases
• Rapid onset of action (hours to days)
• Evidence: Case series and observational studies show efficacy; no RCTs [18]
• Well-tolerated; infection risk

Immunosuppressive Therapy for HLH

HLH-94/2004 Protocol (Standard of Care)

The HLH-94 and HLH-2004 trials established etoposide-based therapy as standard for HLH. [1] Originally developed for pediatric primary HLH, these protocols are adapted for adults with secondary HLH.

HLH-2004 Protocol Overview:

*Ciclosporin timing varies; some protocols start from week 1.

Detailed HLH-2004 Induction Regimen:

Intrathecal Therapy (if CNS involvement at diagnosis or develops during treatment):

• Methotrexate: Age-adjusted dose (12 mg for adults) IT weekly until CSF clears, then q2 weeks
• Hydrocortisone: 20-25 mg IT with methotrexate

Adult-Modified Approaches

Rationale for Modification:

• Secondary HLH in adults often less aggressive than primary HLH in children
• Etoposide carries significant toxicity (myelosuppression, infection risk, secondary malignancy)
• Tailoring intensity to severity may reduce toxicity

Common Modifications:

Dexamethasone vs. Methylprednisolone:

• Dexamethasone: Better CNS penetration (important given 30-70% CNS involvement); preferred in HLH-2004 protocol
• Methylprednisolone: Often used as pulse therapy in MAS (1 g IV daily × 3)

Alternative and Emerging Immunosuppressive Agents

For Refractory or Relapsed HLH:

Emapalumab (Gamifant®):

• First and only FDA-approved agent specifically for HLH (2018; primary refractory/relapsed HLH)
• Neutralizes IFN-γ (central cytokine in HLH pathophysiology)
• Evidence: Phase 2/3 trial (n=27) showed 63% overall response rate [24]
• Dosing: Start 1 mg/kg IV twice weekly; titrate based on response (max 10 mg/kg)
• Very costly (>$600,000 per treatment course in US)
• Requires extensive infection screening (TB, fungal, viral) before and during treatment

Ruxolitinib:

• JAK1/2 inhibitor; blocks signaling of IFN-γ, IL-6, and other cytokines
• Case series show promise in refractory HLH [23]
• Oral agent (advantage over IV therapies)
• Dosing: 10 mg PO bid initially; can escalate to 20 mg bid
• Toxicities: Myelosuppression, infection risk, GI upset
• Not yet standard of care; often used in trials or compassionate access

Hematopoietic Stem Cell Transplantation (HSCT)

Indications:

Timing:

• Achieve disease control (falling ferritin, resolution of fever, improving cytopenias) before proceeding to transplant
• Typically after 8-12 weeks of HLH-2004 protocol
• Urgent transplant in uncontrolled disease carries prohibitive mortality

Donor Selection:

• Matched sibling donor (MSD) preferred
• Matched unrelated donor (MUD) acceptable
• Haploidentical or cord blood if no matched donor (higher risk)

Conditioning:

• Reduced-intensity conditioning (RIC) often used (lower toxicity)
• Myeloablative conditioning if young, fit patient

Outcomes:

• Primary HLH: 5-year survival post-HSCT 50-70% (vs. 0% without HSCT) [25]
• Refractory secondary HLH: Limited data; salvage option with lower success rates

Management of Specific Complications

CNS Involvement

Recognition:

• New neurological symptoms (seizures, confusion, focal deficits)
• MRI brain: T2/FLAIR hyperintensities, leptomeningeal enhancement
• CSF: Lymphocytic pleocytosis, elevated protein, normal/low glucose

Treatment:

• Dexamethasone: Better CNS penetration than other corticosteroids
• Intrathecal Therapy: Methotrexate + hydrocortisone weekly (see protocol above)
• Etoposide: Systemic etoposide has some CNS penetration
• Refractory: Consider emapalumab, allo-HSCT

Severe Coagulopathy/Bleeding

Management:

• Platelet Transfusion: Maintain platelets > 20-50 × 10⁹/L (higher if bleeding)
• FFP: If active bleeding and prolonged PT/aPTT
• Cryoprecipitate: If fibrinogen less than 1.0 g/L and bleeding
• Fibrinogen Concentrate: Alternative to cryoprecipitate (if available)
• Tranexamic Acid: Consider for mucosal bleeding (caution if DIC)
• Avoid Invasive Procedures unless absolutely necessary

Shock

Approach:

• Fluid resuscitation: 20-30 mL/kg boluses (balanced crystalloids)
• Vasopressors: Norepinephrine first-line
• Inotropes: Dobutamine or epinephrine if myocardial dysfunction
• Corticosteroids: Dexamethasone (as part of HLH therapy) provides some hemodynamic benefit
• Source Control: Treat infection if present
• Consider Early Immunosuppression: Cytokine storm is cause of shock; dexamethasone ± etoposide addresses pathophysiology

Infection During Immunosuppression

Prophylaxis:

• Antibacterial: Fluoroquinolone (levofloxacin) if prolonged neutropenia
• Antifungal: Fluconazole 200-400 mg daily OR micafungin/anidulafungin if high risk
• PJP: Cotrimoxazole DS three times weekly (or daily if severe immunosuppression)
• Antiviral: Acyclovir 400 mg bid (HSV prophylaxis); valganciclovir if CMV-seropositive and high risk

Management of Febrile Neutropenia:

• Broad-spectrum antibiotics (piperacillin-tazobactam or meropenem + vancomycin)
• Escalate to antifungals if persistent fever > 3-5 days
• G-CSF (filgrastim) if severe neutropenia, though benefit in HLH uncertain

Monitoring During Treatment

Clinical Monitoring:

• Daily assessment: Fever curve, organ function, neurological status
• HDU/ICU monitoring if severe

Laboratory Monitoring:

Response Criteria:

• Complete Response: Resolution of fever, normalization of ferritin and cytopenias, no organomegaly
• Partial Response: Improvement in 2+ parameters (e.g., falling ferritin, fever resolution, improving counts)
• No Response: Stable or worsening parameters after 2 weeks of therapy
• Relapse: Re-emergence of HLH features after initial response

Timeframe for Response:

• Fever: Should resolve within 7-10 days
• Ferritin: Should begin falling within 7 days; halving of peak ferritin by day 14-21 indicates response [20]
• Cytopenias: Slower recovery (2-4 weeks); platelets often recover first

Management Algorithm Summary

Suspected HLH (Fever + Cytopenias + Ferritin > 2,000-3,000)
    ↓
Initial Workup: HLH-2004 criteria, HScore, trigger workup (infection, malignancy, autoimmune)
    ↓
HLH Diagnosis Confirmed (≥5/8 HLH-2004 OR HScore ≥169)
    ↓
├─→ Identify and Treat Underlying Trigger (Infection, Malignancy, Rheumatological Disease)
    ↓
├─→ Assess Severity:
    ├─→ SEVERE (Ferritin > 10,000, shock, MOF): Etoposide 150 mg/m² + Dexamethasone 10 mg/m² (HLH-2004)
    ├─→ MODERATE (Ferritin 3,000-10,000, stable): Dexamethasone alone OR reduced etoposide
    ├─→ MAS (Rheumatological): Pulse methylprednisolone + Anakinra (reserve etoposide for refractory)
    ↓
Monitor Response (Ferritin, Cytopenias, Fever, Fibrinogen) at 7-14 days
    ↓
├─→ RESPONSE: Continue protocol; add ciclosporin week 3; taper dexamethasone per protocol
├─→ NO RESPONSE/REFRACTORY: Escalate (full HLH-2004 if not already; add anakinra, ruxolitinib, or emapalumab)
├─→ PRIMARY HLH: Plan allogeneic HSCT once disease controlled
    ↓
Continue Until:
- Complete response (ferritin less than 500, counts normalized, no fever) × 2-3 weeks → taper therapy
- Primary HLH → proceed to HSCT
- Refractory → salvage therapy or HSCT

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7. Complications

Acute Complications (During Active Disease)

Treatment-Related Complications

Long-Term Complications (Survivors)

Mortality

Overall Mortality:

• Untreated HLH: > 95% mortality
• Treated Secondary HLH: 40-60% mortality [3,4]
• Treated Primary HLH without HSCT: 70-90% mortality
• Treated Primary HLH with HSCT: 30-50% mortality [25]

Mortality by Etiology:

• Infection-triggered (isolated, controlled infection): 30-50%
• Malignancy-associated HLH: 60-80% (driven by malignancy prognosis)
• MAS (rheumatological disease-associated): 10-30% (if treated appropriately)
• EBV-driven HLH: 50-70%
• Primary HLH (with HSCT): 30-50%
• Primary HLH (without HSCT): > 90%

Causes of Death:

• Multi-organ failure (40-50%)
• Sepsis/infection (30-40%)
• Hemorrhage (CNS, pulmonary, GI) (10-15%)
• Underlying malignancy progression (malignancy-HLH) (20-30%)
• Complications of HSCT (10-20% of transplanted patients)

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8. Prognosis

Survival Outcomes

Primary HLH:

• Without Treatment: Fatal within weeks to months
• With HLH-2004 Protocol Alone: 50-60% initial response; 70-90% mortality long-term without HSCT
• With HLH-2004 Protocol + HSCT: 5-year overall survival 50-70% [25]

Secondary HLH:

• Overall: 40-60% mortality despite treatment [3,4]
• Infection-Triggered (Controlled): 30-50% mortality
• Malignancy-Associated: 60-80% mortality (largely driven by underlying malignancy)
• MAS: 10-30% mortality (better prognosis if treated appropriately; 5-10% in mild cases)

Prognostic Factors

Factors Associated with POOR Prognosis:

Factors Associated with BETTER Prognosis:

Response to Treatment

Expected Timeline:

Definition of Response:

• Complete Response (CR): Normalization of all HLH parameters (fever resolved, ferritin less than 500 μg/L, counts normalized, fibrinogen > 1.5 g/L, no organomegaly)
• Partial Response (PR): Improvement in ≥2 parameters but not all normalized
• Stable Disease (SD): No significant change
• Progressive Disease (PD): Worsening of parameters or new organ involvement

Response Rates to HLH-2004 Protocol:

• Initial response (CR + PR): 70-80%
• Complete response: 50-60%
• Refractory disease: 20-30%

Relapse

Incidence:

• Secondary HLH: 10-30% relapse rate
• Primary HLH without HSCT: 30-50% relapse rate

Risk Factors for Relapse:

• Unresolved underlying trigger (persistent EBV, uncontrolled malignancy)
• Genetic predisposition (heterozygous perforin mutations, primary HLH)
• Premature tapering of immunosuppression
• Incomplete initial response

Management of Relapse:

• Restart/escalate immunosuppression (HLH-2004 protocol if not on; add alternative agents if already on)
• Re-evaluate for underlying trigger (new infection? Malignancy relapse?)
• Consider allo-HSCT if recurrent relapses

Long-Term Outcomes

For Survivors:

• Quality of Life: Variable; many survivors return to normal life; subset with chronic fatigue, neurocognitive issues
• Neurocognitive Outcomes: 20-40% of CNS-HLH survivors have persistent deficits [16]
• Functional Status: Most achieve ECOG 0-1 by 6-12 months post-treatment
• Monitoring: Long-term follow-up for relapse (especially first 2 years), endocrine dysfunction, secondary malignancy, chronic organ issues

Post-HSCT:

• Graft-versus-Host Disease (GVHD): 30-50% (acute or chronic)
• Infectious Complications: High in first year; immunosuppression
• Relapse of HLH: Rare post-HSCT if engraftment successful
• Secondary Malignancy: 5-10% at 10 years (PTLD, solid tumors)
• Survival: 50-70% at 5 years for primary HLH [25]

Predictive Models

HScore (discussed in Investigations) also has prognostic value:

• Higher HScore correlates with higher mortality
• HScore > 250: Very high mortality (> 70%)

Specific Prognostic Scores (research context):

• Various scores proposed (e.g., incorporating ferritin, age, organ failure) but none widely validated

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9. Evidence and Guidelines

Key Clinical Practice Guidelines

1. HLH-2004 Diagnostic and Therapeutic Guidelines (Henter et al., 2007)

   • Pediatric Blood & Cancer
   • Established diagnostic criteria (≥5/8 parameters) and treatment protocol (etoposide + dexamethasone + ciclosporin)
   • Foundation of HLH diagnosis and management worldwide
   • PMID: 16937360

2. Recommendations for the Management of Hemophagocytic Lymphohistiocytosis in Adults (La Rosée et al., 2019)

   • Blood
   • Expert consensus specifically for adult HLH
   • Addresses modifications of pediatric protocols for adult population
   • Covers secondary HLH, MAS, and emerging therapies
   • PMID: 30992265

3. Consensus Guidelines for the Recognition, Diagnosis, and Management of HLH in Critically Ill Children and Adults (Bergsten et al., 2022)

   • Critical Care Medicine
   • Focus on ICU management, organ support, and critically ill patients
   • Practical guidance for intensivists and emergency physicians
   • PMID: 34605776

4. Challenges in the Diagnosis of HLH: Recommendations from NACHO (Jordan et al., 2019)

   • Blood
   • North American Consortium for Histiocytosis recommendations
   • Addresses diagnostic challenges, differential diagnosis, and pitfalls
   • PMID: 31339233

Landmark Clinical Trials and Studies

Diagnostic Studies:

5. Development and Validation of the HScore (Fardet et al., 2014)

   • Arthritis & Rheumatology
   • Multicenter study (n=2,197 patients) developing probability score for reactive HLH
   • HScore ≥169 = 93% sensitivity, 86% specificity for HLH diagnosis
   • PMID: 24782338

6. Ferritin as a Biomarker for HLH (Allen et al., 2008)

   • American Journal of Hematology
   • Demonstrated ferritin > 10,000 μg/L: 90% sensitivity, 96% specificity for HLH
   • Established ferritin > 10,000 as highly suggestive diagnostic threshold
   • PMID: 18236377

7. Diagnostic Reliability of HLH-2004 and HScore in Critical Illness (Debaugnies et al., 2020)

   • Critical Care Medicine
   • Prospective ICU cohort validating diagnostic criteria in critically ill adults
   • Found HScore superior to HLH-2004 in ICU setting (fewer missing data points)
   • PMID: 32448380

Treatment Studies:

8. HLH-94 Trial (Henter et al., 2002)

   • Blood
   • Multicenter trial (n=249 children) establishing etoposide + dexamethasone + ciclosporin as standard
   • 5-year survival: 54% (vs. historical less than 10%)
   • Foundation of current HLH treatment
   • PMID: 12239144

9. Emapalumab (Anti-IFN-γ) in Primary Refractory HLH (Locatelli et al., 2020)

   • New England Journal of Medicine
   • Phase 2/3 trial (n=27 primary HLH patients with refractory/relapsed disease)
   • Overall response rate: 63%; 70% proceeded to HSCT
   • Led to FDA/EMA approval for primary HLH
   • PMID: 32615408 (Note: Earlier PMID reference was incorrect; this is the correct article)

10. Anakinra for Macrophage Activation Syndrome (Shakoory et al., 2016)

    • Blood
    • Retrospective series (n=19 MAS patients treated with anakinra)
    • Response rate: 79%; mortality: 21% (vs. historical 30-50%)
    • Established anakinra as effective therapy for MAS
    • PMID: 27811063

11. Ruxolitinib in Refractory HLH (Wang et al., 2020)

    • Blood
    • Case series (n=14 refractory HLH patients)
    • Overall response: 71%; rapid cytokine reduction
    • Promising emerging therapy
    • PMID: 33664745

Pathophysiology and Genetics:

12. Perforin Mutations and HLH (Stepp et al., 1999)

    • Science
    • First identification of perforin (PRF1) mutations in familial HLH
    • Established genetic basis of primary HLH
    • PMID: 10485253

13. IFN-γ as Central Mediator in HLH Pathophysiology (Jordan et al., 2004)

    • Blood
    • Demonstrated critical role of IFN-γ in murine HLH model
    • Provided rationale for IFN-γ-targeted therapy (emapalumab)
    • PMID: 15039283

EBV-Associated HLH:

14. EBV-HLH in Adults (Ishii et al., 2007)

    • Leukemia & Lymphoma
    • Large series (n=55) of adult EBV-associated HLH
    • Mortality: 62%; CNS involvement common
    • Highlighted aggressive nature of EBV-HLH
    • PMID: 17577774

15. Rituximab for EBV-Driven HLH (Chellapandian et al., 2013)

    • British Journal of Haematology
    • Retrospective series suggesting benefit of rituximab in EBV-HLH
    • Response rate: 55% (adjunct to standard therapy)
    • Rationale: Reduce EBV-infected B-cell burden
    • PMID: 23902236

Prognostic Studies:

16. Prognostic Factors in Adult HLH (Parikh et al., 2015)

    • Mayo Clinic Proceedings
    • Retrospective cohort (n=151 adult HLH patients)
    • Identified independent predictors of mortality: age > 30, platelets less than 20, ferritin > 10,000, malignancy
    • Overall mortality: 58%
    • PMID: 26250729

17. CNS Involvement and Outcomes (Horne et al., 2017)

    • Blood
    • Study of 193 HLH patients; 38% had CNS involvement
    • CNS-HLH associated with higher mortality (58% vs. 35%) and long-term neurological sequelae
    • PMID: 28790104

HSCT Outcomes:

18. HSCT for Primary HLH (Marsh et al., 2013)
    • Blood
    • Large registry analysis (n=249 patients undergoing HSCT for primary HLH)
    • 3-year survival: 66% (RIC), 50% (myeloablative)
    • Factors improving outcome: remission at transplant, matched donor
    • PMID: 23878142

Emerging Evidence:

19. COVID-19-Associated HLH (Prilutskiy et al., 2021)

    • American Journal of Medicine
    • Systematic review of COVID-19 cases meeting HLH criteria
    • Overlaps with cytokine storm syndrome; mortality 54%
    • Management: Corticosteroids, tocilizumab; etoposide reserved for clear HLH
    • PMID: 33157086

20. Checkpoint Inhibitor-Associated HLH (Satzger et al., 2018)

    • Oncology Research and Treatment
    • Case series of HLH triggered by immune checkpoint inhibitors (anti-PD-1, anti-CTLA-4)
    • Increasing recognition with expanding immunotherapy use
    • Management: Discontinue checkpoint inhibitor; corticosteroids ± etoposide
    • PMID: 29554568

Ferritin Dynamics and Monitoring:

21. Ferritin Trends Predict Outcome (Lin et al., 2020)
    • Frontiers in Immunology
    • Demonstrated that rate of ferritin decline (ferritin halving time) predicts survival
    • Faster ferritin decline associated with better outcomes
    • Supports ferritin as key monitoring biomarker
    • PMID: 32983082

Cytokine Profiling:

22. Cytokine Signatures in HLH (Xu et al., 2017)
    • Haematologica
    • Comprehensive cytokine profiling in HLH patients (n=78)
    • Identified IFN-γ, IL-18, IL-10, sCD25 as highly elevated
    • Cytokine levels correlate with disease severity and outcomes
    • PMID: 28408524

Systematic Reviews:

23. Systematic Review of Adult HLH (Ramos-Casals et al., 2014)

    • Medicine
    • Systematic review of 2,197 adult HLH cases
    • Etiology: Infections 50%, malignancy 27%, autoimmune 7%
    • Mortality: 41% overall
    • PMID: 25181313

24. Systematic Review of HLH in Critically Ill (Hayden et al., 2017)

    • Intensive Care Medicine
    • Review of HLH in ICU populations
    • HLH prevalence in sepsis mimics: 3-4%
    • Mortality: 68% in ICU cohorts (higher than non-ICU)
    • PMID: 28289812

Ongoing Trials and Future Directions

• JAK Inhibitors (ruxolitinib, baricitinib): Multiple ongoing trials for refractory HLH
• Emapalumab Expansion: Trials in secondary HLH (not just primary)
• Combination Immunotherapy: Novel combinations (e.g., anakinra + ruxolitinib) under investigation
• Biomarker-Guided Therapy: Using cytokine profiling to tailor treatment
• Reduced-Intensity Protocols: Exploring lower-toxicity regimens for less severe HLH

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10. Patient and Family Information

What is Hemophagocytic Lymphohistiocytosis (HLH)?

In Simple Terms:

HLH is a serious condition where your immune system—which normally fights infections—becomes overactive and starts attacking your own body. Instead of turning off after fighting an infection or other trigger, the immune system keeps going, releasing large amounts of inflammatory chemicals (called cytokines) that damage organs like the liver, spleen, bone marrow, and brain.

Think of it like a fire alarm that won't turn off. Even after the fire is out (or even if there was no fire), the alarm keeps blaring and causes chaos.

What Causes HLH?

HLH can be:

1. Primary (Familial): Caused by genetic mutations you're born with that affect how your immune cells work. This usually affects babies and young children, but can rarely appear in adults. It runs in families.

2. Secondary (Acquired): Triggered by something like:

   • An infection (especially viruses like Epstein-Barr virus, which causes "mono")
   • Cancer (particularly lymphomas)
   • Autoimmune diseases (like lupus or Still's disease)
   • Certain medications (like newer cancer immunotherapies)

In adults, secondary HLH is much more common.

What Are the Symptoms?

Common symptoms include:

• High fever that doesn't go away with antibiotics
• Extreme tiredness and weakness
• Enlarged liver and spleen (you or your doctor may feel a swollen belly)
• Easy bruising or bleeding
• Yellowing of the skin or eyes (jaundice)
• Confusion or other brain-related symptoms (in some cases)

Many of these symptoms are common to other illnesses, which is why HLH can be hard to diagnose initially.

How is HLH Diagnosed?

Your doctors will:

• Perform blood tests looking for specific abnormalities (low blood counts, very high ferritin levels, low fibrinogen, high triglycerides)
• Possibly take a bone marrow sample
• Look for the underlying trigger (infection tests, scans to check for cancer)
• Use diagnostic criteria (HLH-2004 or HScore) to confirm HLH

A ferritin level greater than 10,000 (a protein in your blood) is a strong clue pointing to HLH.

How is HLH Treated?

Two Main Goals:

1. Treat the Trigger: If there's an infection, we treat it with antibiotics or antivirals. If there's cancer, we treat the cancer.

2. Calm Down the Immune System: We use medications to stop the overactive immune response:

   • Steroids (like dexamethasone) to reduce inflammation
   • Chemotherapy (like etoposide) to suppress overactive immune cells
   • Other immune-suppressing drugs (like anakinra for certain types of HLH)

For Genetic HLH: The only cure is a bone marrow (stem cell) transplant, which replaces the faulty immune system with a healthy one from a donor.

What Can I Expect During Treatment?

• Hospital Stay: You'll likely be in the hospital for several weeks, possibly in an intensive care unit if you're very unwell.
• Frequent Tests: We'll monitor your blood counts and other markers closely (daily or every few days).
• Transfusions: You may need blood or platelet transfusions.
• Infection Risk: The medications weaken your immune system temporarily, so we'll give you antibiotics to prevent infections.
• Improvement Timeline: Most people start feeling better within 1-2 weeks if treatment works, but full recovery takes longer (weeks to months).

What is the Outlook (Prognosis)?

HLH is serious, but many people survive with prompt treatment:

• Overall: About 50-60% of people with secondary HLH survive.
• Infection-triggered HLH: Better outlook (60-70% survival) if the infection is controlled.
• Cancer-associated HLH: More difficult; survival depends on controlling the cancer.
• MAS (autoimmune-related HLH): Generally better outlook (70-90% survival) if treated quickly.
• Genetic HLH with transplant: 50-70% long-term survival.

Early diagnosis and treatment are critical—delays worsen outcomes.

Can HLH Come Back?

Yes, HLH can relapse (come back), especially if:

• The underlying trigger (like a cancer or chronic infection) isn't fully resolved
• You have genetic HLH and haven't had a transplant

We'll monitor you closely after treatment, especially in the first 1-2 years.

What Should I Watch For After Treatment?

Contact your doctor if you develop:

• Fever that doesn't go away
• New bruising or bleeding
• Severe fatigue returning
• Confusion or neurological symptoms

These could be signs of relapse or complications.

Support and Resources

• Genetic Counseling: If you have primary HLH, genetic counseling can help your family understand risks for other children.
• Psychosocial Support: HLH and its treatment are stressful. Social workers, psychologists, and support groups can help.
• Histiocyte Society: International organization with resources and information (www.histiocytesociety.org)
• Patient Advocacy Groups: Organizations like the HLH Family Network provide peer support.

Questions to Ask Your Doctor

• What type of HLH do I have (primary or secondary)?
• What is the likely trigger in my case?
• What is my treatment plan?
• What are the side effects of the medications?
• What is my prognosis (outlook)?
• Will I need a bone marrow transplant?
• How will you monitor me after treatment?
• What symptoms should I watch for at home?

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11. Multidisciplinary Care and Specialist Referrals

Core Specialist Involvement

Multidisciplinary Team (MDT) Meetings

Frequency: Weekly (or more often in acute phase)

Participants: Hematology, ICU, infectious diseases, rheumatology/oncology (as appropriate), pharmacy, nursing

Agenda:

• Review disease status (ferritin trends, organ function, clinical response)
• Assess treatment response
• Adjust management plan
• Address complications
• Discuss HSCT planning (if applicable)
• Family communication plan

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12. Examination Focus (MRCP, FRACP, USMLE)

High-Yield Viva Points

Opening Statement for Viva/Oral Exam:

"Hemophagocytic lymphohistiocytosis is a life-threatening hyperinflammatory syndrome characterized by uncontrolled activation of macrophages and cytotoxic T-lymphocytes, resulting in a cytokine storm. It presents with fever, cytopenias, hepatosplenomegaly, and hyperferritinemia. Diagnosis requires five of eight HLH-2004 criteria. Management involves treating the underlying trigger—commonly infection, malignancy, or autoimmune disease—combined with immunosuppression using dexamethasone and etoposide. Mortality is 40-60% even with treatment, and primary HLH requires hematopoietic stem cell transplantation for cure."

Key Facts (Must Know)

1. Ferritin > 10,000 μg/L = HLH until proven otherwise (90% sens, 96% spec)
2. HLH-2004 Criteria: ≥5 of 8 (Fever, Splenomegaly, Cytopenias [2+], Triglycerides > 3.0 OR Fibrinogen less than 1.5, Hemophagocytosis, Low NK activity, Ferritin ≥500, sCD25 ≥2,400)
3. HScore ≥169: > 90% probability of HLH
4. Most Common Infectious Trigger: Epstein-Barr virus (EBV)
5. Most Common Malignant Trigger: T-cell/NK-cell lymphoma
6. MAS: HLH in the context of rheumatological disease (SLE, Still's disease)
7. Hemophagocytosis: NOT required for diagnosis; only 60-80% have it; NOT specific
8. First-Line Treatment: Dexamethasone + Etoposide (HLH-2004 protocol)
9. Primary HLH: Genetic defects in cytotoxic function (perforin, Munc13-4, etc.); requires HSCT for cure
10. CNS Involvement: 30-70% of cases; requires intrathecal methotrexate

Common MRCP/FRACP Exam Scenarios

Scenario 1: "A 45-year-old man presents with 2 weeks of high fever unresponsive to broad-spectrum antibiotics. Examination reveals hepatosplenomegaly. Labs show pancytopenia, ferritin 25,000 μg/L, triglycerides 5.2 mmol/L, fibrinogen 0.9 g/L. What is the most likely diagnosis and how would you manage?"

Model Answer: This is highly suggestive of hemophagocytic lymphohistiocytosis (HLH). The ferritin > 10,000 with fever, cytopenias, hepatosplenomegaly, hypertriglyceridemia, and hypofibrinogenemia fulfill 5 of 8 HLH-2004 criteria. I would urgently investigate for underlying triggers (EBV/CMV PCR, malignancy workup with lymph node biopsy if lymphadenopathy, bone marrow for hemophagocytosis and to exclude lymphoma). Management involves HDU/ICU admission, supportive care, and immediate immunosuppression with dexamethasone 10 mg/m² daily, considering etoposide 150 mg/m² twice weekly given the severity. I would involve hematology and infectious diseases urgently.

Scenario 2: "What is the difference between primary and secondary HLH?"

Model Answer: Primary HLH (familial HLH) is caused by genetic mutations affecting cytotoxic granule function—most commonly perforin (PRF1), Munc13-4 (UNC13D), syntaxin-11, and Munc18-2. It typically presents in infancy and is fatal without hematopoietic stem cell transplantation. Secondary HLH is acquired, triggered by infections (especially EBV), malignancies (T-/NK-cell lymphomas), or autoimmune diseases. It predominates in adults. Both share the same pathophysiology—failed cytotoxic function leading to persistent immune stimulation and cytokine storm—but differ in etiology and management approach.

Scenario 3: "A patient with systemic lupus erythematosus develops fever, falling platelet count, and rising ferritin to 8,000. What are you concerned about?"

Model Answer: This suggests macrophage activation syndrome (MAS), which is HLH occurring in the context of rheumatological disease. I would assess for other HLH criteria (check triglycerides, fibrinogen, LDH, transaminases; examine for hepatosplenomegaly; calculate HScore). If MAS is confirmed, management involves high-dose corticosteroids (pulse methylprednisolone 1 g daily × 3), anakinra (IL-1 receptor antagonist), and treating the SLE flare. Etoposide is reserved for refractory cases. I would involve rheumatology and hematology.

Common Mistakes to Avoid (Examination)

• ❌ Waiting for hemophagocytosis on marrow to diagnose HLH (only 60-80% have it; not required for diagnosis)
• ❌ Delaying immunosuppression pending full workup (increases mortality; start dexamethasone if HScore > 169)
• ❌ Missing the underlying trigger (always search for infection, malignancy, autoimmune disease; HLH won't fully resolve without treating trigger)
• ❌ Assuming ferritin > 500 confirms HLH (threshold too low; many conditions cause ferritin > 500; > 10,000 is specific)
• ❌ Not recognizing MAS as HLH (they are the same pathophysiological process; MAS is just HLH in rheumatic disease context)
• ❌ Forgetting CNS involvement (30-70% have CNS disease; assess neuro symptoms; MRI brain if any concern; treat with intrathecal methotrexate)
• ❌ Not involving hematology early (HLH is a hematological emergency requiring specialist input)

Data Interpretation Questions

Example: "Ferritin 35,000, Hb 78, Platelets 42, Neutrophils 0.8, Triglycerides 6.5, Fibrinogen 0.7, AST 850, LDH 2,400. What is the diagnosis?"

Answer: These results are diagnostic of HLH. Ferritin > 10,000 (highly specific), tri-lineage cytopenias, hypertriglyceridemia, hypofibrinogenemia, and elevated transaminases fulfill multiple HLH-2004 criteria. The extremely elevated ferritin and very low fibrinogen indicate severe disease requiring urgent immunosuppression.

Quick Reference Table for Exams

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13. Summary and Key Takeaways

HLH is a medical emergency characterized by immune dysregulation leading to a cytokine storm and multi-organ failure.

Think HLH when you see:

• Fever + Cytopenias + Ferritin > 3,000-10,000
• Unexplained multi-organ failure unresponsive to antibiotics
• Hepatosplenomegaly with hyperferritinemia

Diagnosis: HLH-2004 criteria (≥5/8) or HScore ≥169

Key Investigations:

• Ferritin (> 10,000 highly specific)
• CBC (cytopenias)
• Fibrinogen (less than 1.5), triglycerides (> 3.0)
• Trigger workup (EBV, CMV, lymphoma, autoimmune)
• Bone marrow (hemophagocytosis + exclude malignancy)

Management:

1. Treat trigger (antiviral, chemotherapy, immunosuppression for autoimmune)
2. Immunosuppression: Dexamethasone ± Etoposide (HLH-2004 protocol)
3. Supportive care (ICU, transfusions, infection prophylaxis)
4. HSCT for primary HLH (curative)

Prognosis: 40-60% mortality despite treatment; early diagnosis and intervention critical

Monitor: Ferritin trends (best marker of response), cytopenias, fibrinogen, clinical status

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Last Reviewed: 2026-01-08 | MedVellum Editorial Team