What Is HLH?
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. 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 since there is little government funding for much needed research. HLH is considered to be rare, but specific incidence is unknown.
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.
The onset of FHL is usually early in life, and a permanent cure can only be achieved through a bone marrow transplant. In some ways, histiocytosis is similar to cancer, and has historically been treated by oncologists with chemotherapy and steroids to try and quiet the disease, ultimately needing a bone marrow transplant, but it is not cancer.
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.
The Doctors at Cincinnati Children's Hospital treat more cases of HLH each year than any other pediatric hospital in the world.
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