Castleman disease is a group of rare disorders characterized by lymph node enlargement, specific microscopic changes to the lymph nodes, and a broad range of symptoms and laboratory findings. The lymph nodes, and the cells that reside in them (lymphocytes), are an integral part of our immune system that help us fight invading organisms. For reasons unknown, these lymph nodes undergo some transformations that result in overproduction of lymphocytes and other inflammatory compounds.
The two main subtypes are Unicentric Castleman disease (UCD) and Multicentric Castleman disease (MCD). As the name suggests, the enlarged lymph nodes in UCD appear in only one region of the body. UCD tends to have milder symptoms and rarely affects organs. The multicentric types are characterized by lymph node enlargement in multiple regions of the body and the symptoms tend to be more severe than UCD.
The multicentric subtype can be further classified into two categories: HHV-8 positive Multicentric Castleman disease and idiopathic Multicentric Castleman disease. HHV-8 is a common virus that does not result in disease for the overwhelming majority of individuals. Unfortunately, a small proportion of individuals with this virus, especially those with suppressed immune systems, may develop a form of MCD. Although we know that the virus is involved, we still do not know why only a few patients with this virus develop the disease.
The other category is idiopathic Multicentric Castleman disease (iMCD). Idiopathic refers to the fact that we do not know the causative agent. We know that these individuals are HHV-8 negative and therefore have a different origin for the disease. iMCD also has important differences in symptoms, disease course, and treatment from HHV-8 positive disease. Furthermore, some iMCD patients can develop a specific subtype called TAFRO characterized by Thrombocytopenia (low platelet count), Anasarca (ascites, swelling), Fever or elevated C-reactive protein (inflammation marker), Reticulin fibrosis (evaluated in bone marrow biopsy), and Organomegaly (hepatomegaly/splenomegaly). Diagnosis of iMCD-TAFRO requires thrombocytopenia, anasarca, fever/elevated C-reactive protein, and either reticulin fibrosis or organomegaly.