INTRODUCTION AND DEFINITIONS

Hemophagocytic lymphohistiocytosis (HLH) is also known as autosomal recessive familial hemophagocytic lymphohistiocytosis (FHL), familial erythrophagocytic lymphohistiocytosis (FEL), and viral-associated hemophagocytic syndrome (VAHS). This aggressive and potentially life-threatening disease most often affects infants from birth to 18 months of age, but cases in older children and adults have been reported [1-5].

Use of the term "primary HLH" to denote the presence of an underlying genetic disorder and "secondary HLH" to denote presence of the HLH phenomenon occurring secondary to another condition (eg, viral illness, autoimmune disease, lymphoma) has caused a great deal of confusion among clinicians. This reflects the fact that in patients with inherited mutations predisposing to HLH, clinical symptoms are often triggered by viral infections.

• In pediatric patients, the same gene mutations can be present in both situations and there is no specific test with the ability to quickly define these two categories.
• In adult patients, genetic mutations are much less common, suggesting that most represent "secondary HLH."

With few exceptions, the clinical presentation and outcome are the same for both. A review has been written to help clinicians understand the many signs and symptoms of HLH besides the eight diagnostic criteria that can help with diagnosing this illness as well as the subtleties of its treatment [6].

HLH and a similar disorder, the macrophage activation syndrome (MAS), which occurs primarily in patients with juvenile idiopathic arthritis or systemic lupus erythematosus, will be discussed here [6].