Macrophages Activation Syndrome

Educational clinical summary of Macrophages Activation Syndrome for doctors and students: definition, workup, management, and key caveats. Not a substitute for guidelines.

Maintained by Dr. Vivek Karn Review process: editorial policy

MEDICAL DISCLAIMER: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Do not disregard professional medical advice or delay in seeking it because of something you have read here. Clinical practice is dynamic, and recommendations may change. Clinicians must always exercise independent medical judgment in the context of individual patient circumstances, and current evidence and guidelines.

Macrophage Activation Syndrome

Overview

Definition: Macrophage Activation Syndrome (MAS) is a severe, life-threatening form of secondary hemophagocytic lymphohistiocytosis (HLH) that occurs predominantly in the setting of systemic inflammatory conditions, most commonly systemic juvenile idiopathic arthritis (sJIA) and, less frequently, systemic lupus erythematosus (SLE) and adult-onset Still's disease (AOSD). It is characterized by uncontrolled activation and proliferation of macrophages and T lymphocytes, leading to excessive cytokine production ("cytokine storm") and multi-organ dysfunction.
Epidemiology:
- Incidence: Exact incidence is unknown but estimated to affect 10-15% of children with sJIA, with subclinical forms potentially much higher (up to 30-40%). Less common in adults with AOSD (5-10%) or other rheumatic diseases.
- Demographics: Predominantly affects pediatric patients, particularly those with sJIA. No significant gender predilection. Can occur at any age.
Classification: MAS is considered a reactive form of HLH, falling under the umbrella of secondary HLH.
- Primary HLH (Genetic HLH): Caused by inherited genetic defects in cytotoxic pathways (e.g., mutations in PRF1, UNC13D, STXBP2, LYST, SH2D1A, BIRC4), leading to impaired perforin-mediated cytotoxicity. Typically presents in infancy or early childhood.
- Secondary HLH (Acquired HLH): Occurs in the context of strong immunological triggers in individuals without primary genetic defects. MAS is a specific type of secondary HLH triggered by systemic inflammatory conditions, distinct from HLH secondary to infections (e.g., viral HLH, especially Epstein-Barr Virus), malignancies (e.g., lymphoma-associated HLH), or immunodeficiency states.

Pathophysiology

Mechanism: The fundamental defect in MAS/HLH is a failure of activated cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells to properly kill target cells (e.g., antigen-presenting cells, virus-infected cells) and downregulate immune responses. This leads to persistent immune activation, resulting in unchecked proliferation and activation of macrophages and T cells.
- Impaired Cytotoxicity: Dysfunction of perforin-mediated cytotoxic granule release. While genetic defects underlie primary HLH, in MAS, inflammation or specific disease mechanisms (e.g., sJIA-associated pathways) transiently impair this function.
- Cytokine Storm: The persistently activated and dysregulated immune cells produce massive amounts of pro-inflammatory cytokines, including interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), IL-18, and IL-1β.
- Macrophage Activation: These cytokines, especially IFN-γ, activate macrophages (histiocytes) that then infiltrate various organs (bone marrow, liver, spleen, lymph nodes, CNS). These activated macrophages phagocytose hematopoietic cells (erythrocytes, leukocytes, platelets), leading to cytopenias, and contribute to tissue damage.
- Organ Damage: The systemic inflammatory response and direct tissue infiltration by activated macrophages cause widespread organ dysfunction, including liver failure, coagulopathy, bone marrow suppression, and central nervous system involvement.
Risk Factors:
- Underlying Rheumatic Disease: Systemic juvenile idiopathic arthritis (sJIA) is the most significant risk factor. Other rheumatic diseases include systemic lupus erythematosus (SLE), adult-onset Still's disease (AOSD), Kawasaki disease, and juvenile dermatomyositis.
- Infections: Viral (especially Epstein-Barr virus, cytomegalovirus, parvovirus B19, influenza, COVID-19), bacterial, fungal, or parasitic infections can trigger MAS in susceptible individuals.
- Malignancies: Lymphomas, leukemias.
- Drug Exposure: Certain drugs, including NSAIDs, gold salts, and some biologics (rarely). Withdrawal of immunosuppressive therapy.
- Genetic Predisposition: While MAS is secondary, individuals with heterozygous mutations in HLH-associated genes or single nucleotide polymorphisms (SNPs) may be more susceptible.
Protective Factors: No specific protective factors are known, but prompt diagnosis and aggressive treatment of the underlying inflammatory disease and the MAS itself are crucial for improving outcomes. Early recognition of triggers and signs can mitigate severity.

Clinical Presentation

Signs and Symptoms

MAS presents with a rapid, fulminant onset of systemic inflammation that often mimics or exacerbates the underlying rheumatic disease, but with unique and severe features.

Cardinal Features:
- Persistent High Fever: Refractory to antibiotics and antipyretics, often spiking (>39-40°C).
- Hepatosplenomegaly: Enlargement of the liver and spleen, often tender.
- Cytopenias: Bicytopenia or pancytopenia (anemia, leukopenia, thrombocytopenia). Thrombocytopenia is often the earliest and most consistent cytopenia.
- Coagulopathy: Bleeding tendencies (purpura, ecchymoses, epistaxis, GI bleeding) due to consumption coagulopathy/DIC.
- Neurological Dysfunction: Headache, irritability, lethargy, seizures, confusion, coma (in severe cases).
Early Signs:
- Persistent high fever despite appropriate therapy for the underlying rheumatic disease.
- Unexpected decrease in platelet count (thrombocytopenia).
- Abnormal liver function tests (LFTs): rising AST/ALT.
- Subtle changes in mental status or increased irritability.
- Paradoxical decrease in ESR (due to low fibrinogen) despite active inflammation (high CRP, ferritin).
Advanced Signs:
- Overt bleeding (petechiae, purpura, GI hemorrhage).
- Deepening coma, focal neurological deficits.
- Severe multi-organ failure: acute kidney injury, respiratory distress syndrome.
- Profound pancytopenia.
- Jaundice.
Atypical Presentations:
- MAS can sometimes be the presenting manifestation of an underlying rheumatic disease (e.g., sJIA).
- Mild presentations mimicking severe disease flare, especially in patients already on immunosuppression.
- Predominant neurological involvement without overt systemic signs.
- Localized MAS (rare), e.g., only affecting the lung.

Physical Examination

Inspection:
- Skin: Macular rash (non-specific, can be part of sJIA), petechiae, purpura, ecchymoses (indicating coagulopathy). Jaundice.
- General: Pallor (anemia), lethargy, altered mental status, signs of systemic illness.
Palpation:
- Abdomen: Hepatosplenomegaly (tender or non-tender), distension.
- Lymph Nodes: Generalized lymphadenopathy (though sometimes less prominent than in primary HLH).
Special Tests:
- Neurological Exam: Assess for signs of encephalitis (meningeal irritation, seizures, focal deficits, altered consciousness). Fundoscopy for retinal hemorrhages.

Diagnostic Approach

MAS is a diagnosis of exclusion and requires a high index of suspicion in patients with underlying rheumatic diseases who experience a sudden worsening of their clinical condition.

History Taking

Key Questions:
- Underlying Disease: History of sJIA, SLE, AOSD, or other systemic inflammatory conditions? Current disease activity and treatment?
- Fever Pattern: Onset, duration, and height of fever. Response to antipyretics.
- Recent Infections: Any preceding viral, bacterial, or other infections? Travel history?
- Medication Changes: Recent changes in immunosuppressants, withdrawal of therapy, or new medications?
- Bleeding Symptoms: Any new bruises, petechiae, nosebleeds, gum bleeding, or gastrointestinal bleeding?
- Neurological Symptoms: Any new headache, confusion, seizures, or changes in behavior?
- Family History: Any family history of recurrent fevers, immunodeficiencies, or early unexplained deaths (suggestive of primary HLH)?
Red Flags:
- Sudden, unexplained drop in platelet count in a patient with a known rheumatic disease.
- Paradoxical decrease in ESR despite increasing CRP and ferritin.
- New onset coagulopathy or bleeding.
- Rapid deterioration with signs of multi-organ dysfunction.
- Unexplained neurological symptoms.

Laboratory Tests

Laboratory findings are crucial for diagnosis and monitoring.

First-Line:
- Complete Blood Count (CBC) with Differential: Often shows bicytopenia (anemia, thrombocytopenia, leukopenia) or pancytopenia. Thrombocytopenia (<100 x 10^9/L) is common and often the earliest finding.
- Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP): Often divergent; ESR may drop or be normal due to hypofibrinogenemia, while CRP remains high or increases.
- Liver Function Tests (LFTs): Elevated AST, ALT, LDH; hyperbilirubinemia.
- Coagulation Profile: Prolonged PT/aPTT, decreased fibrinogen (<1.5 g/L), elevated D-dimer.
- Ferritin: Extremely high levels (>500-10,000 ng/mL, often >10,000 ng/mL) are a hallmark. Highly sensitive, but not specific on its own.
- Triglycerides: Elevated (>265 mg/dL or 3 mmol/L).
Confirmatory (and for differentiation from primary HLH):
- Soluble CD25 (sIL-2R): Elevated levels (>2400 U/mL or >2 standard deviations above age-adjusted normal range) reflect T-cell activation and are highly sensitive for HLH/MAS.
- NK Cell Activity: Decreased or absent NK cell cytotoxic activity (functional assay). This differentiates HLH from other inflammatory states.
- Soluble CD163 (sCD163): Marker of macrophage activation, highly elevated in MAS.
- Bone Marrow Biopsy: Hemophagocytosis (activated macrophages engulfing hematopoietic cells) is characteristic but not always found, and its absence does not rule out MAS. It is often non-diagnostic in early stages. It may be necessary to rule out malignancy or other causes of cytopenia.
- Genetic Testing: For mutations in HLH-associated genes (e.g., PRF1, UNC13D, STXBP2) to differentiate primary HLH, especially if atypical features or family history.
- Viral Studies: EBV PCR, CMV PCR, Parvovirus B19, HIV, etc., to rule out infectious triggers of secondary HLH.
Monitoring:
- Daily CBC, LFTs, coagulation panel, ferritin, CRP.
- Repeated sCD25 and sCD163 to assess disease activity and response to treatment.

Diagnostic Criteria for MAS in sJIA (2016 EULAR/ACR/PRINTO Classification Criteria): A patient with known or suspected sJIA is classified as having MAS if he or she fulfills the following criteria: 1. Fever 2. Ferritin >684 ng/mL 3. AND any 2 of the following laboratory criteria: * Platelets ≤181 x 10^9/L * AST >48 U/L * Triglycerides >156 mg/dL (1.75 mmol/L) * Fibrinogen ≤360 mg/dL (3.6 g/L)

Note: These criteria are highly sensitive and specific for MAS in sJIA but may not apply to other forms of HLH or MAS in other rheumatic diseases. The HLH-2004/2009 criteria are often used for general HLH, but their applicability to MAS in rheumatic diseases has limitations due to the high baseline inflammatory state.

HLH-2004 Diagnostic Criteria (used for general HLH, including MAS in other contexts): Diagnosis of HLH is established if one of the following is met: 1. A molecular diagnosis consistent with HLH. 2. Fulfillment of 5 out of 8 criteria: * Fever (peak temperature ≥38.5°C for >7 days) * Splenomegaly (palpable spleen >3 cm below costal margin) * Cytopenias (affecting ≥2 cell lines): Hb <9 g/dL (age <4 weeks: <10 g/dL), Platelets <100 x 10^9/L, Neutrophils <1.0 x 10^9/L * Hypertriglyceridemia (fasting triglycerides ≥3.0 mmol/L [265 mg/dL]) and/or hypofibrinogenemia (fibrinogen ≤1.5 g/L) * Ferritin ≥500 μg/L * Soluble CD25 (sIL-2Rα) ≥2400 U/mL * Low or absent NK cell activity * Hemophagocytosis in bone marrow, spleen, lymph nodes, or CSF (absence does not exclude HLH)

Imaging

Recommended:
- Abdominal Ultrasound/CT: To confirm hepatosplenomegaly and assess for lymphadenopathy. May show ascites or gallbladder wall thickening.
- Chest X-ray: To rule out pneumonia or pulmonary involvement.
Advanced:
- Brain MRI with Gadolinium: Recommended if there are any neurological symptoms (seizures, altered mental status, focal deficits) to detect CNS involvement (e.g., leukoencephalopathy, periventricular lesions, enhancement).
- PET-CT: Rarely used, mainly if malignancy is a strong differential.

Management

Management of MAS is an emergency requiring immediate and aggressive immunosuppression to halt the cytokine storm and prevent irreversible organ damage.

Acute Management

Immediate (First-hour interventions):
- Admission to an intensive care unit (ICU) for close monitoring and supportive care.
- STAT lab draws: CBC, LFTs, coagulation panel, ferritin, D-dimer, triglycerides, sCD25.
- Initiate broad-spectrum antibiotics if infection cannot be immediately ruled out.
- Fluid resuscitation and hemodynamic support.
- Secure airway if neurological compromise or respiratory distress.
Stabilization (Early management priorities):
- High-Dose Corticosteroids: The cornerstone of therapy.
  - Methylprednisolone pulse therapy: Typically 30 mg/kg/day (maximum 1 g/day) intravenously for 3-5 days, then rapidly tapered over 2-4 weeks.
- Consider Cyclosporine A (CsA): If there is no rapid improvement with corticosteroids alone, or as part of initial therapy for severe cases.
  - Dosage: 2-7 mg/kg/day orally or intravenously in divided doses. Aim for trough levels of 150-250 ng/mL.
- Intravenous Immunoglobulin (IVIG): Can be considered, especially if infection is a significant concern or as an adjunct therapy.
  - Dosage: 1 g/kg/day for 2 days or 400 mg/kg/day for 5 days.
- Supportive Care:
  - Transfusion of blood products (platelets, fresh frozen plasma) for active bleeding or severe thrombocytopenia/coagulopathy.
  - Correction of electrolyte imbalances.
  - Nutritional support.
  - Infection prophylaxis (Pneumocystis jirovecii prophylaxis once steroids are started).
Emergency (Crisis management):
- Anakinra (IL-1 receptor antagonist): A rapidly acting targeted therapy increasingly used, especially in sJIA-associated MAS. Can be considered early or if inadequate response to steroids and CsA.
  - Dosage: 2-10 mg/kg/day subcutaneously or intravenously, divided twice daily (max 100 mg/dose). Higher doses (up to 10 mg/kg/day) or continuous IV infusion are used for severe, life-threatening cases.
- Etoposide (VP-16): Potent cytotoxic agent, part of the HLH-94/2004 protocol for primary HLH. Reserved for severe, refractory MAS that fails to respond to steroids, CsA, and biologics, or if there is strong suspicion of underlying primary HLH. Etoposide has significant myelosuppression and long-term risks (e.g., secondary malignancies).
  - Dosage: 150 mg/m² IV 2 times per week for 2 weeks, then weekly, often tapered. Requires careful monitoring of blood counts.
- Tocilizumab (IL-6 receptor antagonist): Less evidence for acute MAS treatment compared to anakinra, but may be considered in sJIA patients already on tocilizumab who develop MAS, or for managing the underlying sJIA flare post-MAS.
- Ruxolitinib (JAK1/2 inhibitor): Emerging as a potential therapy for refractory HLH/MAS, by inhibiting IFN-γ signaling. Not yet standard first-line for MAS.

Chronic Management

Pharmacological:
- Tapering Immunosuppression: Once MAS is controlled (ferritin trending down, cytopenias resolving, clinical improvement), corticosteroids and other acute therapies are gradually tapered. The duration of therapy depends on the severity of MAS and the underlying condition.
- Management of Underlying Disease: Continue/reinstitute therapy for the underlying rheumatic disease (e.g., sJIA, SLE) to prevent future MAS flares. This may involve conventional DMARDs or biologics.
Non-pharmacological:
- Regular follow-up with rheumatology and hematology/immunology specialists.
- Education for patients/families on early warning signs.
- Monitoring for infections.
Monitoring:
- Regular clinical assessments and laboratory monitoring (CBC, LFTs, CRP, ferritin) until full recovery.
- Long-term monitoring for underlying rheumatic disease activity.
- Screening for genetic HLH if there were atypical features, recurrence, or very young age of onset.

Treatment Algorithms

First-Line (Initial Response):
1. Suspect MAS: In a patient with underlying rheumatic disease (especially sJIA) and new onset fever, cytopenias, hepatosplenomegaly, and high ferritin.
2. Initiate High-Dose Methylprednisolone: 30 mg/kg/day IV (max 1g/day) for 3-5 days.
3. Concurrent: Start broad-spectrum antibiotics (if infection cannot be ruled out) and aggressive supportive care.
4. Monitor response: Daily labs (CBC, LFTs, PT/aPTT, fibrinogen, D-dimer, ferritin, CRP).
Second-Line (Inadequate Response):
1. No significant improvement within 24-48 hours of high-dose steroids: Consider adding Cyclosporine A (2-7 mg/kg/day IV/PO) and/or Anakinra (2-10 mg/kg/day SC/IV).
2. For sJIA-MAS: Anakinra is often favored due to its rapid onset and targeting of IL-1 pathways relevant to sJIA pathogenesis. It can be added or used instead of CsA based on clinician judgment and local guidelines.
Refractory Cases (Life-threatening or unresponsive to conventional treatment):
1. Consider Etoposide: If multi-organ failure progresses or no response to steroids, CsA, and anakinra after several days. This therapy requires careful risk/benefit assessment due to severe side effects.
2. Ruxolitinib: Emerging option for highly refractory cases, particularly if high IFN-gamma signatures persist.
3. Tocilizumab: In sJIA-MAS, if IL-6 is deemed a dominant cytokine, but generally less potent in acute MAS.
4. Rituximab: Rarely used, for severe cases with prominent B-cell activation, or if malignancy is suspected.
5. Alemtuzumab (anti-CD52): Very potent lymphocyte-depleting antibody, used in very severe, highly refractory cases, often in primary HLH.
6. Allogeneic Hematopoietic Stem Cell Transplantation (HSCT): The only curative option for genetic forms of HLH, generally not indicated for reactive MAS unless primary HLH is strongly suspected or proven after a severe, relapsing course.

Complications

Common:
- Coagulopathy and bleeding (DIC).
- Acute liver failure.
- Acute kidney injury.
- Severe infections (due to immunosuppression and neutropenia).
- Anemia requiring transfusion.
Serious:
- Multi-organ failure (cardiac, pulmonary, renal, hepatic).
- Central Nervous System (CNS) involvement: seizures, coma, permanent neurological deficits.
- Gastrointestinal bleeding.
- Secondary opportunistic infections.
- Drug toxicities (e.g., myelosuppression from etoposide, nephrotoxicity from cyclosporine).
Long-term:
- Chronic organ damage (e.g., liver cirrhosis, renal impairment).
- Neurological sequelae (e.g., cognitive impairment, epilepsy).
- Increased risk of malignancy (secondary to etoposide).
- Recurrence of MAS, especially if underlying disease is not well-controlled.

Prognosis

Factors:
- Prompt Diagnosis and Treatment: Early initiation of aggressive immunosuppression significantly improves outcomes.
- Severity at Presentation: Patients with multi-organ failure, severe CNS involvement, or profound cytopenias have a poorer prognosis.
- Response to Initial Therapy: Rapid normalization of laboratory parameters (especially ferritin, platelets) and clinical improvement are favorable signs.
- Underlying Cause: MAS secondary to sJIA generally has a better prognosis than primary HLH or HLH secondary to malignancy, assuming proper treatment.
- Availability of Targeted Therapies: Access to drugs like anakinra has improved outcomes in sJIA-MAS.
Survival:
- Historically, MAS carried a high mortality rate (up to 50% or more).
- With modern aggressive management, particularly the early use of high-dose corticosteroids and IL-1 antagonists, mortality rates in sJIA-associated MAS have significantly decreased, though precise, consistently updated figures are challenging to provide given the rarity and variable presentations.
- Mortality remains high (around 20-30%) for severe, refractory cases or those presenting with established multi-organ failure.
Quality of Life: Survivors may experience long-term complications related to organ damage (especially CNS) or from the chronic management of their underlying rheumatic disease.

Prevention

Primary (Preventing initial occurrence):
- No specific measures can prevent MAS in patients with predisposing conditions.
- Early and effective control of the underlying systemic inflammatory disease (e.g., sJIA) with appropriate therapies (e.g., biologics like IL-1 or IL-6 inhibitors) might theoretically reduce the risk of MAS by preventing severe flares.
Secondary (Early detection and intervention):
- High Index of Suspicion: Clinicians caring for patients with systemic rheumatic diseases must be highly vigilant for signs of MAS, especially during flares, infections, or after medication changes.
- Prompt Laboratory Monitoring: Routine monitoring of CBC, LFTs, and inflammatory markers in high-risk patients can help detect early changes.
- Education: Educate patients and their families about the warning signs of MAS.
Tertiary (Preventing complications):
- Aggressive and Timely Treatment: As outlined in the management section, rapid initiation of immunosuppression is key to preventing organ damage and death.
- Appropriate Supportive Care: Management of coagulopathy, infections, and organ dysfunction.
- Long-term follow-up: To address chronic complications and manage the underlying disease to prevent recurrence.

Special Populations

Pediatric:
- MAS is most common in children with sJIA. The 2016 EULAR/ACR/PRINTO MAS criteria are specifically designed for this population.
- Dosages of medications must be carefully weight-based (e.g., methylprednisolone 30 mg/kg/day, anakinra up to 10 mg/kg/day).
- Children, especially infants, may present with atypical features or rapid deterioration, requiring immediate attention. Genetic testing for primary HLH should be considered more readily in very young children with HLH/MAS.
Geriatric:
- MAS is less common but can occur in older adults, often in the context of AOSD or malignancy.
- Diagnosis can be challenging due to multiple comorbidities and polypharmacy masking or mimicking MAS symptoms.
- Increased risk of complications from immunosuppression (e.g., infections, osteoporosis). Dosing adjustments for renal/hepatic impairment may be necessary.
Pregnancy:
- MAS in pregnancy is rare but serious, posing risks to both mother and fetus.
- Management is complex, requiring a multidisciplinary approach (rheumatologist, maternal-fetal medicine, hematologist).
- Treatment choices must balance efficacy with fetal safety. High-dose corticosteroids are generally considered safe during pregnancy. Cyclosporine and anakinra have been used, but data on fetal safety are more limited. Etoposide is generally contraindicated due to high fetal toxicity.
- Close monitoring of fetal well-being is essential.
Comorbidities:
- Renal/Hepatic Impairment: Dose adjustment of renally/hepatically cleared drugs (e.g., cyclosporine, etoposide) is critical. Risk of drug accumulation and toxicity.
- Infection: MAS can be triggered by infection and immunosuppressive therapy increases infection risk. Requires aggressive diagnostic workup and empirical broad-spectrum antibiotics.
- Malignancy: HLH/MAS can be a paraneoplastic syndrome. Consider malignancy workup if no clear rheumatic trigger or atypical features.
- Immunodeficiency: Patients with primary or secondary immunodeficiencies are at higher risk of developing HLH/MAS in response to triggers.

Clinical Pearls

Diagnostic:
- The "Ferritin Bomb": While high ferritin is a hallmark of MAS, it's not specific. However, values >10,000 ng/mL (or rapidly rising from a lower baseline) are highly suggestive.
- The ESR/CRP Paradox: A low or normal ESR despite very high CRP and clinical inflammation is a crucial early clue in MAS due to consumption of fibrinogen. Don't be reassured by a "normal" ESR.
- Early Thrombocytopenia: Often the first and most consistent cytopenia. A sudden drop in platelet count in a patient with sJIA should trigger immediate concern for MAS.
- Bone Marrow Aspiration: Hemophagocytosis may be absent in early stages or patchy. A negative biopsy does not rule out MAS. Its primary utility is to exclude other diagnoses like malignancy.
- The sJIA-MAS Criteria: Use the 2016 EULAR/ACR/PRINTO criteria for diagnosis in sJIA, as they are tailored for this population.
Treatment:
- Time is Tissue: Early, aggressive immunosuppression is paramount to prevent irreversible organ damage. Do not delay treatment awaiting all confirmatory tests.
- Steroids First: High-dose corticosteroids are the first-line therapy.
- Anakinra for sJIA-MAS: Increasingly recognized as a highly effective and rapid-acting therapy for sJIA-associated MAS, often preferred over CsA if not responding to steroids.
- Etoposide is a Last Resort: Reserved for truly refractory or life-threatening cases due to its significant toxicity.
- Treat the Underlying Disease: Once MAS is controlled, ensure the underlying rheumatic disease is adequately managed to prevent recurrence.
Pitfalls:
- Misdiagnosis as Sepsis: MAS can mimic severe sepsis, especially with high fever, cytopenias, and coagulopathy. A lack of response to antibiotics, combined with extremely high ferritin and the ESR/CRP paradox, should prompt consideration of MAS.
- Over-reliance on Bone Marrow Biopsy: A negative biopsy for hemophagocytosis can be misleading and delay diagnosis.
- Under-dosing Steroids: Insufficient steroid dosing in early stages may lead to progression.
- Ignoring Subtle Lab Changes: Small but consistent drops in platelet count or paradoxical ESR changes can be early warning signs.

Recent Updates

Guidelines:
- The 2016 EULAR/ACR/PRINTO Classification Criteria for MAS in sJIA provided standardized diagnostic criteria, improving diagnostic consistency.
- Updated consensus treatment recommendations emphasize earlier and more prominent roles for biologic agents, particularly IL-1 inhibition (anakinra), in sJIA-associated MAS.
Evidence:
- Growing evidence supports the efficacy and favorable safety profile of anakinra for sJIA-MAS, leading to its increased use as a second-line or even early first-line agent, often combined with steroids.
- Research into other targeted therapies like ruxolitinib (JAK inhibition) and emactuzumab (anti-CSF1R) is ongoing for refractory HLH/MAS, targeting different pathways of macrophage activation.
- Understanding of the genetic predisposition for MAS in rheumatic diseases is evolving, with studies identifying risk alleles and pathways that may predispose individuals to hyperinflammation.
Controversies:
- Optimal timing and sequencing of therapies: While high-dose steroids are first-line, the precise timing for adding cyclosporine, anakinra, or other agents remains an area of ongoing debate and depends on severity and response.
- Role of Etoposide: Its use is decreasing in non-malignancy associated MAS, especially sJIA-MAS, given the efficacy of targeted biologics and its significant toxicity. However, it remains vital for refractory cases or when primary HLH is highly suspected.
- Applicability of HLH-2004 criteria vs. MAS criteria: The HLH-2004/2009 criteria are broad for HLH, but their specificity for MAS in the context of an underlying rheumatic disease (where inflammatory markers are already high) is debated, leading to the development of disease-specific criteria.

Key References

Guidelines:
- Minoia F, Bovis F, Davì S, et al. Development and Validation of a Multicriteria Alert System for Macrophage Activation Syndrome in Systemic Juvenile Idiopathic Arthritis. Arthritis Rheumatol. 2017;69(11):2217-2227. (Describes the EULAR/ACR/PRINTO MAS criteria)
- Henter JI, Horne A, Aricó M, et al. HLH-2004: A Revised Protocol for the Diagnosis and Treatment of Hemophagocytic Lymphohistiocytosis. Pediatr Blood Cancer. 2007;48(2):124-131. (Standard for HLH diagnosis and treatment protocols)
Studies:
- Davì S, Minoia F, Pistorio A, et al. Performance of the 2016 EULAR/ACR/PRINTO criteria for macrophage activation syndrome in patients with systemic juvenile idiopathic arthritis. Ann Rheum Dis. 2019;78(3):358-365. (Validation of MAS criteria)
- Crayne CB, Cron RQ. Anakinra in the treatment of macrophage activation syndrome. Curr Opin Rheumatol. 2019;31(4):339-347. (Review on anakinra in MAS)
Reviews:
- Schulert GS, Cron RQ. Macrophage activation syndrome. Rheum Dis Clin North Am. 2020;46(3):479-493. (Comprehensive review)
- Henderson LA, Cron RQ. Macrophage activation syndrome and secondary hemophagocytic lymphohistiocytosis in childhood rheumatic diseases. Pediatr Rheumatol Online J. 2020;18(1):31.