Genetic Haemophgocytic Lymphohistiocytosis
Genetic HLH is also called primary HLH and may be inherited in one of two ways:
Autosomal recessive - this is where the child has 2 copies of the abnormal gene, one from each parent
X-linked - this is passed to boys from one of the mother's X chromosomes which is abnormal.
There are two types of genetic HLH. Those called Familial HLH and those that are related to a very rare syndrome associated with immune deficiencies such as Chediak-Higashi Syndrome 1 (CHS-1), Griscelli Syndrome 2 (gs-2) and X-linked proliferative syndrome (XLP).
Familial HLH
About one in every 200,000 children is diagnosed with Familial HLH. Most, around 70-80%, develop symptoms before the age of 1 and a few, approximately 10%, experience symptoms within the first 4 weeks of life. In the same family, children with familial HLH usually develop symptoms around the same age.
Acquired Haemophagocytic Lymphohistiocytosis
Acquired HLH is also called secondary HLH and can occur at any age. The frequency is unknown but it is thought to be more common than the genetic type. Like the genetic types of HLH, acquired HLH is usually triggered by an infection, often a virus. HLH can also occur in children with some cancers.
Macrophage Activation Syndrome
Macrophage Activation Syndrome (MAS) is an extremely rare condition that occurs in both children and adults with auto-immune diseases, such as rheumatoid arthritis. It has the same features as HLH but some of the initial blood changes may be less severe and problems with clotting and the function of the heart may be worse.
Like other forms of HLH, viruses have been shown to trigger MAS, but also some medications. Those suffering from MAS have a better outcome than HLH, with a survival rate of 80-90%. Treatment is similar to HLH, but less intensive.