Sarcoidosis

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Causes

The cause of sarcoidosis is unknown. Because the lungs and thoracic lymph nodes (the lymph nodes located in the chest) are the most frequently involved organs, some physicians and researchers suspect that sarcoidosis may be caused by something that enters the body through the lungs, that is, something that is inhaled such as a virus or bacteria, or an unidentified environmental toxin.

In particular, there is some evidence that sarcoidosis may be associated with Mycobacterium tuberculosis, the bacterium that causes tuberculosis. Sometimes sarcoidosis and tuberculosis occur simultaneously, suggesting they may have the same cause. Also, scientists have found M. tuberculosis DNA in infected sarcoidosis tissue, suggesting the bacteria is present and may be causing the disease. On the other hand, it may be that a patient with sarcoidosis has a suppressed immune system, making them more susceptible to tuberculosis. Or, a patient with TB may be more susceptible to sarcoidosis. One may not have anything to do with the other except that if you have one, your body is defenseless against the other. Sarcoidosis also resembles berryliosis, a disease caused by inhalation of the metal berrylium.

Geographical clustering
There is some evidence of geographical clustering, meaning the disease tends to occur with greater frequency in certain geographical areas. This suggests an environmental cause (i.e., if everyone in the area is breathing the same environmental toxin, and if sarcoidosis is caused by that toxin, people living in the area where the toxicity is high are more likely to develop the disease) or transmissible agent (i.e., if one person in the area has sarcoidosis, and it is infectious, other people in the area are likely to get infected).

Immunological disorder
No matter whether the trigger is a chemical toxin or bacteria, sarcoidosis involves a malfunctioning immune system. It is not really clear what the problem is and how it begins. It may even have a partial genetic basis.

HLA
It is not clear whether there is a link between characteristics of sarcoidosis and certain HLA (human lymphocyte antigens) patterns or not. HLAs are a type of histocompatibility antigens (proteins on the surface of the cells that serve as an important part of the body's defense against foreign invaders, such as viruses and bacteria). There are many different types of HLAs and many different forms of each type of HLA.

Genetics
Familial sarcoidosis occasionally occurs, suggesting that there may be an inherited genetic component.

Incidence and Prevalence
The frequency of sarcoidosis (the percentage of a population infected at a given time) is largely unknown because many cases are asymptomatic and never reported. Often, sarcoidosis is discovered by a chest x-ray done for other purposes. Or, it is not seen until after death, during an autopsy.

According the the NIH's National Heart, Lung, and Blood Institute, the occurrence of sarcoidosis was considered rare in the United States until fairly recently. It was detected in large numbers during a mass chest x-ray screening of U.S. military personnel in the 1940s. In 1990, Congress declared a Sarcoidosis Awareness Day to increase awareness of the disease. Although usually considered fairly mild, it can be a very serious, life-threatening condition, especially in certain racial and ethnic groups. For example, in the United States, young African-American men suffer the most severe form of sarcoidosis and have the highest mortality rate.

Unlike most lung diseases, sarcoidosis actually occurs more frequently in nonsmokers than smokers. It is not clear why.

Geographical
The frequency of sarcoidosis varies from place to place. In some areas it is high, and in others it is low. In Sweden, for example, where sarcoidosis is considered very common, more than 60 people out of every 100,000 are affected; in New York, 39 out of every 100,000 are affected, and in Spain, only 1.2 per 100,000 are infected. Sarcoidosis is rarely reported in the Middle East, China, SE Asia, or among the Inuit or Native North Americans.

Racial variation
Within a geographical area, the frequency and course of the disease vary considerably among racial groups. In one study of sarcoidosis in London, for example, the incidence of sarcoidosis in W. Indian and Asian immigrants is 10 times higher than in the indigenous Caucasian population. Also, in the immigrant population, there is a greater need for corticosteroid treatment and a lower chance for full recovery.

In another study, researchers found that sarcoidosis in S. Africa occurs in 23 out of every 100,000 black persons, 12 of every 100,000 mixed race persons, and 4 of every 100,000 Caucasians.

In the United States, various studies have shown that sarcoidosis occurs in 10–80 of every 100,000 African Americans, compared to less than 8 of every 100,000 Caucasians.

Sex
African American women develop sarcoidosis twice as often as African American men. Caucasian women and men are equally likely to develop it.

Age
The highest prevalence is in 25-34 year olds.

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