What is sinus histiocytosis?

Sinus histiocytosis is a rare human disorder of unknown cause, usually indicated by inflammation of the lymph glands, either subcutaneous or deep tissue. It predominantly affects the young, particularly during the first two decades of life. It is usually benign, though sometimes chronic, and in rarer cases it can metastasize to other parts of the body through the vessels of the lymphatic system. Since it was first identified in 1969, an effective cure has been elusive, with most doctors recommending close clinical observation without any treatment for the possibility of spontaneous self-healing.

"Histiocytosis" refers to a class of conditions characterized by the excess production and accumulation of histiocytes, which are normally a beneficial autoimmune macrophage released by littoral cells lining the inner endothelial wall of lymph nodes. Lymph nodes are small ball-shaped hollow organs that are distributed throughout the reticulated immune system of the human body. Within the lymphatic sinuses, immune cells such as histiocytes function as a filter or trap for foreign particles and pathogens. Although swelling of a lymph node is quite common, particularly when fighting an infection, a disorder of the lymph nodes is called lymphadenopathy.

Sinus histiocytosis with massive lymphadenopathy (SHML) is also called Rosai-Dorfman disease, named for the two collaborating physicians who first described the condition. Most cases occur from birth to 15 years of age and manifest as large but painless enlargements of the lymph nodes on both sides of the neck, accompanied by fever and other indicators associated with infection. A biopsy will confirm hyperactive littoral cells, distended sinus canals, and an elevated histiocyte count. If there is additional evidence of dermatopathy or an aberrant skin condition, it is likely that histiocytes have escaped the confines of the lymph node and have begun to attack surrounding skin cells.

A sinus histiocytosis that has spread to sites other than the lymph nodes is described as extranodal and occurs in about a quarter of all cases. The skin, bone, and upper respiratory tract are common destinations, but an extranodal metastasis can attack anywhere in the body, including the central nervous system, genitourinary system, and sinuses. Treatment and recovery can take years or decades, depending on the number of lymph nodes and the type of extranodal organs affected by SHML. Meanwhile, patients are at increased risk of exposure to their compromised immune systems. Therefore, some doctors prefer aggressive therapies for this disease.

The etiology, or cause, of sinus histiocytosis is unknown. DNA studies of offending histiocytes have found no evidence of change, suggesting that the disease is a reactive condition, possibly in response to an as yet undiscovered virus. The fact that it generally follows a benign clinical course, often with spontaneous resolution and few or no lasting negative effects, supports this theory. On the other hand, especially in extranodal cases, the disease behaves similarly to malignant cancers, including periods of exacerbation and remission. Corresponding therapies (surgical excision of lymph nodes, chemotherapy, and radiation) have had inconsistent degrees of efficacy.

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