Haemophagocytic syndrome, or haemophagocytic lymphohistiocytosis (HLH), is a hyperinflammatory disorder characterised by uncontrolled activation of the immune system. It can result from mutations in multiple genes involved in cytotoxicity or occur secondary to a range of infections, malignancies or autoimmune rheumatic diseases. In the latter case, it is also known as macrophage activation syndrome (MAS). Characteristic features are persistent fever, hepatosplenomegaly, petechial/purpuric rash, progressive cytopenias, coagulopathy, transaminitis, raised C reactive protein, falling erythrocyte sedimentation rate, hypertriglyceridaemia, hypofibrinogenaemia and extreme hyperferritinaemia often associated with multi-organ impairment. Distinguishing HLH from systemic sepsis can present a major challenge. Criteria for diagnosis and classification of HLH and MAS are available and a serum ferritin >10 000 µg/L is strongly supportive of HLH. Without early recognition and appropriate treatment, HLH is almost universally fatal. However, with prompt referral and advancements in treatment over the past two decades, outcomes have greatly improved.
- macrophage activation syndrome
- haemophagocytic syndrome
- Infectious Diseases
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Haemophagocytic syndrome, also known as haemophagocytic lymphohistiocytosis (HLH), is a rare but potentially fatal multi-system inflammatory disorder characterised by uncontrolled hyperactivation of macrophages (histiocytes) and T lymphocytes. Primary HLH is a genetic disease resulting from recessive defects in the cytolytic pathway of natural killer (NK) cells and cytotoxic T lymphocytes (CTLs).1 It predominantly affects children, although onset in adulthood has been reported.2 Secondary, or acquired, HLH is triggered by a wide variety of infections, malignancies and autoimmune diseases. Secondary HLH in the context of rheumatic diseases, such as systemic juvenile idiopathic arthritis (sJIA), is often designated macrophage activation syndrome (MAS).3 ,4 Although termed ‘secondary’, on some occasions HLH can be the presenting manifestation of the underlying condition in a previously well child. It can be difficult to distinguish from systemic sepsis or autoimmune diseases.5 ,6 Given the risk of significant morbidity and mortality, recognition of HLH/MAS and its early management are important skills for all paediatricians.
The purpose of this article is to raise awareness of HLH/MAS and provide a guide to its correct and early diagnosis. It will also provide a summary of epidemiology, aetiology and pathogenesis. For details on specific treatments of HLH and MAS, including immunosuppression, pro-apoptotic chemotherapy and haematopoietic stem cell transplantation, readers are referred to several recent reviews.1 ,2 ,4
A previously well, 3-year-old Caucasian boy was referred to the paediatric team with a fever and rash. He had not been completely well for 2 weeks with coryza, sore throat and intermittent temperatures. He had deteriorated over the previous week with a reduced appetite and a fluctuating, red blanching rash. There was no history of diarrhoea, vomiting, cough or altered consciousness. He had no significant medical or family history, no regular medications or foreign travel. Initial assessment showed a febrile, unwell child with heart rate 130/min, blood pressure 96/55 mm Hg, respiratory rate 40/min, oxygen saturation 97% in air and temperature 39°C. He had a blanching, macular, erythematous rash on his torso and upper legs, coryza, a red throat but without exudate and several moderately enlarged cervical lymph nodes. There was poorly localised, mild abdominal tenderness and a palpable liver edge. Complete examination of the musculoskeletal system revealed no signs of arthritis. Cardiovascular, respiratory and neurological examinations were normal.
Initial investigations were performed including blood and urine cultures, bacterial and viral throat swabs, anti-streptolysin O titre and Epstein-Barr virus (EBV) serology. Baseline blood results are shown in table 1. He was clinically felt to be septic and was started on intravenous ceftriaxone. He was also given intravenous fluids due to poor oral intake.
Over the next 48 hours, he continued to have fever over 38°C and the results of blood and urine cultures yielded no growth. EBV viral capsid antigens IgM and IgG were positive suggesting a recent infection. On the third day of admission, he appeared to be clinically deteriorating with lethargy, petechial rash, cool peripheries, tachycardia and reduced urine output. Repeat blood test results are shown in table 1 with concerning changes, consistent with HLH, highlighted in bold.
Recognising the features of persisting fever, multi-organ involvement (renal, hepatic and potentially neurological), cytopenias, coagulopathy and increasing C reactive protein (CRP) despite treatment with a broad-spectrum antibiotic, the diagnosis of HLH was considered. The child was managed in a high-dependency environment and appropriate emergency resuscitation and stabilisation treatment instituted. His condition was discussed with the paediatric intensive care team at the local tertiary hospital. Additional investigations were requested including: erythrocyte sedimentation rate (ESR), ferritin, fasting triglycerides, fibrinogen, D-dimers, lactate dehydrogenase and aspartate aminotransferase. The results are shown in table 1.
Primary, or genetic, HLH is a rare disease with an estimated incidence of 0.12/100 000 children per year in a Swedish study7 and 0.34/100 000 children per year in studies from Japan.8 The prevalence of all cases of HLH under 18 years of age has been estimated as 1.07/100 000.9 In general, however, HLH is under-recognised and the true prevalence, particularly of secondary HLH, is likely to be higher.10 In the context of sJIA, MAS occurs in 7–13% of patients.11 Evidence of subclinical MAS based on bone marrow findings was present in 53% of patients with sJIA at diagnosis.12
Primary HLH is of genetic origin and results from mutations in an increasing list of genes associated with the cytolytic and apoptotic pathways (table 2, reviewed in ref. 13). Although it is the more-likely form of disease to be seen in infants and very young children, mutations and potentially pathogenic polymorphisms have been identified in older children and adults with apparently secondary (infection-triggered or rheumatic disease-triggered) HLH. Conversely, infections may be the trigger for active HLH even in young children with genetic mutations. Therefore the distinction between ‘primary’ and ‘secondary’ may be blurred.
Viruses are the most common aetiological agent for infection-triggered HLH with EBV being the precipitant in 74% of children where an infectious cause was identified.14 Of note, in male patients, an association has been identified between EBV-associated HLH and X-linked lymphoproliferative syndrome types 1 (XLP1) and 2 (XLP2) and X-linked immunodeficiency with magnesium defect, Epstein-Barr virus infection and neoplasia (XMEN) syndrome caused by mutations in SH2D1A, XIAP and MAGT1 genes, respectively.13 ,15
Herpes simplex virus (HSV) has been recognised as a relatively common trigger for severe HLH in neonates. It was identified as the cause in 30% in a case series (n=20) from Japan and was associated with a poor prognosis.16 Other viral triggers for HLH include cytomegalovirus (CMV), adenovirus, influenza A, dengue and Ebola.4 ,17 Leishmaniasis can also be a cause for HLH with reports from a wide range of countries including within Europe.18
Malignancies, particularly lymphomas and leukaemias, can be a trigger for HLH although less commonly in children than in adults. In the younger age group, acute B-lymphoblastic leukaemia is the most frequently associated malignancy.10 HLH may present before the underlying malignancy is detected and this should be considered during the diagnostic work-up.
Among rheumatic diseases, secondary HLH/MAS is most frequently associated with sJIA. It has also been reported in the context of Kawasaki disease, juvenile systemic lupus erythematosus, polyarticular JIA, juvenile dermatomyositis, anti-phospholipid syndrome and mixed connective tissue disease.4 MAS can be triggered by a flare of the underlying disease or may be the first presentation of the rheumatic diagnosis.19 Methotrexate and biological drugs used to treat the rheumatic diseases have also been suspected as causing MAS in some cases.20 ,21 Haemophagocytic syndrome is a recognised complication following haematopoietic stem cell transplantation used to treat severe JIA.22 ,23
Immunodeficiency is a characteristic feature of some of the genetic diseases associated with primary HLH, such as Chediak-Higashi syndrome and Griscelli syndrome type 2. Acquired immunosuppression, either iatrogenic or resulting from HIV infection, has also been identified as causing HLH. It is not clear if this results from decreased immune surveillance or more frequent infections.24
During normal immune responses, cells of the innate immune system such as macrophages and dendritic cells are activated, via toll-like receptors, by molecules from micro-organisms called pathogen-associated molecular patterns. These cells phagocytose pathogens, present antigens and activate the adaptive immune system. NK cells and CTLs recognise and destroy pathogen-infected cells via a cytolytic pathway by release of granules containing the lytic proteins perforin and granzymes.1 They also produce pro-inflammatory cytokines such as interferon γ and tumour necrosis factor α (TNF-α) that activate and recruit macrophages to tissues. NK cells and CTLs play a role in removal of antigen-presenting cells (APCs) by inducing apoptosis, thus providing a form of negative feedback.25
In genetic HLH, molecular defects in the cytolytic pathway result in failure of destruction of target-infected cells and APCs, which results in a ‘cytokine storm’. Elevated levels of TNF α, interleukin 6 (IL-6), IL-1 and others cause high fever and infiltration of tissues with activated macrophages and lymphocytes, leading to multi-organ inflammation and damage.26
In acquired, non-genetic, forms of HLH it is infection, malignancy or autoimmune disease, which shifts the immune system balance. In EBV-induced HLH, the infected B lymphocytes act as APCs and proliferate more rapidly than can be controlled by EBV-specific CTLs. The resulting immune system hyperactivation produces the clinical picture of HLH.27
Clinical features and diagnosis
Critical to diagnosis of HLH is an awareness of the disease and a high degree of suspicion in children with some of the clinical features. Each feature alone is non-specific and may be caused by a wide range of diseases. However, the combination of clinical and laboratory signs, their severity and the changes over time facilitate correct diagnosis. The characteristics that point to a diagnosis of HLH/MAS are shown in table 3. Many of these are also seen in severe systemic sepsis and some consider HLH and severe sepsis to be phenotypes on a spectrum of hyperinflammatory reactions.28 The cardinal features of HLH are fever, hepatosplenomegaly and cytopenias, particularly with failure to respond to initial anti-infective treatments.
HLH can affect all organ systems. Neurological manifestations have been reported at presentation in up to 30% and include cranial nerve palsies, ataxia, encephalopathy and seizures. Elevated cerebrospinal fluid protein or cell count is found in approximately half of patients.27 In the neonatal age group, presentation of HLH may be associated with isolated central nervous system (CNS) involvement or with fulminant liver failure.29 In the latter case, it may be almost indistinguishable from neonatal haemochromatosis, although this is not associated with fever, cytopenias or hypertriglyceridaemia.16
The diagnostic criteria for HLH were developed for the HLH-2004 study and are shown in table 4.30 In a child with the appropriate acute clinical presentation, a diagnosis can be made on the basis of identification of mutations in HLH-associated genes or presence of five of the eight diagnostic criteria. In children other than neonates, fever and splenomegaly are present in 90–100% at diagnosis of HLH.31
Hyperferritinaemia is a crucial marker for active HLH/MAS. It is an acute-phase reactant and may be raised in several inflammatory or infective processes at a lower level. However, ferritin >10 000 µg/L in children was found to be 90% sensitive and 96% specific for HLH.32 Figure 1 illustrates that ferritin can be a helpful test for stratifying patients with a systemic sepsis/HLH presentation.
When considering other laboratory measures, such as cytopenias, transaminases and ESR, an important feature to recognise is that it is the change in parameters over time, rather than absolute values, which may indicate progression to HLH at an earlier stage. For example, neutrophils and platelets may be within the laboratory normal range but it is the drop from previously elevated levels associated with acute infection or active autoimmune disease that may herald development of HLH.33 The paradoxically falling ESR despite increasing systemic inflammation is thought to be secondary to decreasing fibrinogen resulting from fibrinogen consumption and liver dysfunction.4 Measuring ESR and CRP together is helpful: a dropping ESR coupled with persistently elevated or rising CRP is an important sign of HLH.
A significant problem with HLH-2004 criteria is that measurement of NK cell function and soluble interleukin 2 receptor α chain (sIL-2Ra, CD25) are available only in specialised immunology or research laboratories. In addition, it may be inappropriate to undertake bone marrow biopsy under general anaesthetic in a critically unwell child with coagulopathy in order to detect haemophagocytosis. While the presence of haemophagocytosis in bone marrow can help to confirm the diagnosis of HLH, it is frequently absent, particularly in the early stages.34 ,35 A multinational study of 362 patients with MAS complicating sJIA reported that only 60.7% of patients whose bone marrow was examined showed haemophagocytosis.36 The above tests, therefore, may be unavailable or non-diagnostic leading to critical delay in initiation of appropriate treatment.
Partly because of these difficulties, one study used HLH-2004 criteria excluding those for NK cell function, sIL-2Ra level and tissue haemophagocytosis in patients with sJIA suspected of having MAS, therefore using thresholds of 3/5 or 4/5 criteria. These adapted HLH-2004 guidelines were found to have high specificity but low sensitivity for diagnosis of MAS associated with sJIA.37 A further problem in this context was the overlap in clinical features between HLH diagnostic guidelines and those seen in the rheumatic conditions underlying MAS; this limits their usefulness in the paediatric rheumatology population.
In order to address the specific difficulties of distinguishing active sJIA from MAS, consensus-derived classification criteria for MAS in sJIA have recently been published.38 These are summarised in table 4. The platelet count ≤181×109/L and fibrinogen ≤3.6 g/L do overlap with the normal range, but it is these results in the context of a child with significant systemic inflammation that raise suspicion of MAS.
Once HLH has been diagnosed, the underlying cause for the syndrome should be investigated including a search for genetic mutations. Figure 2 provides an example algorithm, which initially uses flow cytometry analysis to identify deficiencies in key proteins listed in table 2. A molecular genetic analysis, which may take several weeks, is subsequently used to confirm specific mutations.
Infections may trigger HLH in patients both with and without a genetic disease. Screening for viruses using PCR including EBV, CMV, parvovirus B19, adenovirus, HSV, human herpes virus 6 and varicella zoster virus has been recommended.27 If a bone marrow biopsy is performed, a sample should also be sent for PCR looking for leishmania infection.
In patients with a known rheumatic disease who develop MAS, it is not yet clear whether screening tests for primary HLH are also necessary. One study reviewed retrospectively 21 patients who presented with MAS and also had at least one primary HLH screening test (NK cell perforin expression; NK cell granule release assay; anti-CD3 stimulation of CD8 lymphocytes; in male patients, signalling lymphocyte activation molecule-associated protein (SAP) and X-linked inhibitor of apoptosis protein (XIAP) expression).40 In this group 3/21 (14%) ultimately had a diagnosis of primary HLH and the authors therefore recommend screening in all children whose first rheumatologic presentation is with MAS.
Details of the treatments for HLH/MAS are beyond the scope of this review. In broad terms, however, primary and secondary HLH, with the exception of MAS, are treated according to HLH-2004 guidelines with an 8-week induction period of dexamethasone, etoposide and ciclosporin and intrathecal methotrexate and steroids in selected patients with CNS involvement.30 In time, etoposide chemotherapy may be replaced by use of the anti-T-cell antibody alemtuzumab if a multicentre prospective study being conducted in France shows equal or improved efficacy (personal communication, Dr Despina Moshous). Continued chemotherapy and haematopoietic stem cell transplantation using reduced intensity conditioning therapy are recommended for genetic, familial and persistently active disease. In resolved, non-familial, non-genetic disease, treatment may be stopped after the initial 8-week period. High-dose intravenous methylprednisolone is the most common first-line treatment for MAS with ciclosporin or intravenous immunoglobulin added in patients not responding rapidly to steroids.4 In all cases, an adequate anti-microbial therapy is important to treat an identified infectious trigger and if there is ongoing diagnostic uncertainty between severe systemic sepsis and HLH. In the case of EBV-associated HLH, depletion of B lymphocytes using the monoclonal antibody rituximab in combination with steroids, etoposide and/or ciclosporin chemotherapy has been shown to be effective.41
HLH is a potentially life-threatening syndrome that may present, particularly in young children, as a result of genetic mutations or being triggered by infections, malignancies or rheumatic diseases. Early recognition and instigation of appropriate treatment are crucial to prevent a cytokine storm progressing to cause multi-organ failure. Increased awareness of the condition is the first step, together with knowledge of the key features including persistent fever, cytopenias and extreme hyperferritinaemia. Children with HLH often require high dependency/intensive care and collaborative working between specialists is essential to maximise chances of a favourable outcome.
Contributors ESS reviewed the literature and wrote the article. CGS and AVR made significant contributions to discussion of content and review/editing of the article prior to submission. All authors read and approved the final manuscript.
Funding ESS is funded by a National Institute for Health Research (NIHR) Rare Disease Translational Research Collaboration (RD-TRC) clinical research fellowship.
Competing interests None declared.
Provenance and peer review Commissioned; externally peer reviewed.
↵i This is a fictitious case for illustrative purposes, therefore no patient/parent consent is required.
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