Why Do Autoimmune Conditions “Collect”? – Part 1
If you’re reading this article, it might be because you saw the title and thought “yes, this is me!” If so, you are not alone. Outside of our chronic autoimmune community, the idea of “collecting” conditions may seem bizarre, but within our community, it’s a topic that comes up frequently. Here’s an example of what I hear on a daily basis from members:
“Why is your first autoimmune disease like a magnet for others?”
“Does anyone else here also suffer from Crohn’s Disease and Sjogren’s?”
“I must have hit the jackpot, I have rheumatoid arthritis AND psoriatic arthritis!”
The research surrounding this phenomenon is fragmented and still in its infancy, but my purpose for writing this article is to try and summarize a bit of it for you. In Part 1, most importantly, I want to share some relevant medical terms that you can use to start your own digging. Here are three I think you’ll find interesting.
“What!” you say? “There’s an actual term for this?” Yes! Polyautoimmunity describes the presence of more than one autoimmune disease in an individual. It’s perhaps been most closely studied in relation to autoimmune thyroid diseases, but the scope spreads beyond that. It’s estimated that up to 20% of the population may suffer from at least one autoimmune condition, and within that population there may be up to a 35% incidence of polyautoimmunity.1 The mechanisms beyond polyautoimmunity are not well understood, but may include scientific mouthfuls like “molecular mimcry” and “epitope spreading”; we’ll explore these and other theories in Part 2.
Multiple Autoimmune Syndrome (MAS)
The term “Multiple Autoimmune Syndrome” was first fully described in 1988 by Humbert and Dupond and is a more specialized description of polyautoimmunity.2 Even though it was first proposed more than three decades ago, MAS is a term just now catching on, and describes those of us with at least three autoimmune conditions. Here’s another interesting fact: of those people with three autoimmune conditions, one of the conditions is usually dermatological.3
There are currently three “types” of MAS, and these serve to group together autoimmune conditions that tend to be concurrent.3 For example, Type II includes rheumatoid arthritis, Sjogren’s, scleroderma, cirrhosis, and autoimmune thyroid disease. How many of you could add a Type IV that includes psoriatic arthritis, Sjogren’s, psoriasis, non-alcoholic fatty liver disease (NAFLD)*, fibromyalgia, and Hashimotos? The study of MAS is, by extension, the study of what autoimmune conditions are most likely to “group” together.
If MAS and polyautoimmunity are where various autoimmune diseases are present in the same individual, familial autoimmunity is where many different autoimmune diseases (or the same autoimmune disease) are present within the same family. Familial autoimmunity suggests a genetic susceptibility that may underlie autoimmunity. The most common autoimmune disease in familial autoimmunity is autoimmune thyroid disease (Graves Disease, Hashimotos, etc.), followed by Systemic Lupus Erythematosus and rheumatoid arthritis.4
What makes it so hard to tease out the underlying genetic factors is that the responsible genes are not what scientists call “high penetrance,” and therefore do not exhibit classic inheritance characteristics that would make them easy to pin down. In addition, family members from the same household are likely to have a number of overlapping environmental factors as well, which further complicates the issue. Identical twin studies are one of the best tools for separating genetic from environmental factors, but the only PsA twin study to date found no difference in PsA rates among identical and non-identical twins, so the search for non-genetic causes continues!5
My hope is that Part 1 of this article has demonstrated that epidemiology agrees with us: once you have one autoimmune condition, you’re more likely to develop another one. In addition, there are clusters of autoimmune diseases that are more likely to group together. Your suspicions are not just in your head, there are established terms to describe this phenomenon and there is more and more research being done every day.
*Editor's Note: NAFLD is typically categorized as a metabolic syndrome and not an autoimmune disorder. An example of an autoimmune disorder specifically impacting the liver is autoimmune hepatitis.6
How do you plan to recognize PsA Awareness Month?