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What is HLH?

Hemophagocytic Lymphohistiocytosis

HLH stands for Hemophagocytic Lymphohistiocytosis and is a life-threatening immunodeficiency.  It affects people of all ages and ethnic groups.  Common symptoms are fevers, enlarged spleen, low blood counts and liver abnormalities.  At least half of the patients with HLH will also experience inflammation of the brain which can lead to a wide variety of neurologic problems.

HLH has traditionally been divided into genetic forms (Familial HLH or FHLH) or acquired forms which may be associated with infections, autoimmune conditions or cancer.  HLH patients have overactive histiocytes and lymphocytes, which are white blood cells that can cause swelling, redness, heat, pain and malfunction/damage of organs when they attack and accumulate on healthy lymph tissue (ed: Liver, Spleen, Lymph Nodes).  To date, 9 genetic causes of FHLH have been discovered, accounting for about 50% of childhood HLH cases in the U.S.A.  FHLH can be inherited in either an autosomal recessive manner (where both parents are carriers) or in an x-linked manner affecting only males.  HLH qualifies as an orphan or rare disease with a reported prevelance ranging from 1/150,000, although this may be an underestimate.

The symptoms of HLH result from an overactive and poorly controlled immune response involving T lymphocytes and macrophages.  This abnormally intense immune response results in prolonged and highly elevated levels of cytokines (molecules which, in minute quantities, normally regulate immune functions).  This so-called “hypercytokinemia” is potentially damaging to all the major vital organs.  The hallmark of hypercytokinemia is Hemophagocytosis, the process for which the disorder is named.  Hemophagocytosis is an abnormal consumption of blood cells by macrophages which are turned on by the high cytokine levels, resulting in the low blood counts.

While some patients experience HLH symptoms which can wax and wane or respond well to steroids initially, many patients suffer from progressive illness which can lead to death if appropriate immune suppressive treatment is not begun in time.  Ultimately, children and young adults who have HLH on the basis of known genetic defects will require a hematopoietic cells transplant (BMT) to cure them.  Patients without a known genetic cause who experience progressive HLH or recurrent episodes are also considered for BMT.  At present, with the more widespread use of Reduced Intensity Conditioning (RIC) protocols accompanied by careful selection of donor and stem cell product to be used – long term survival rates after HCT have improved from 50-70% up to 90% in experienced HCT centers.

HLH is not a cancer.  Over the years cancer treatments have been used in patients with HLH.  Consequently, Hematologists and Oncologists who treat cancer also treat children with HLH.

Because the disease is relatively new from a research perspective (1985) and causes as well as symptoms are still being discovered, may cases go undiscovered (or too late) and lead to fatality for the patient. It is important to catch the illness early.

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