The MastAttack 107: The Layperson’s Guide to Understanding Mast Cell Diseases, Part 20

I answered the 107 questions I have been asked most in the last four years. No jargon. No terminology. Just answers.

29. Why do I swell up when I have a reaction? Where does the fluid come from and where does it go?
• Your body feeds its cells by keeping blood circulating. The blood passes by cells. The cells pull nutrients, oxygen, and other things they want out of the blood. In return, the cells release their wastes into the bloodstream to carry them away to a place where they can be broken down.
• Mast cells make and release many chemicals. These chemicals are commonly called mediators because they mediate many reactions in the body that affect the body in many different ways.
• Mast cell mediators do many things. They can make blood vessels get looser or tighter to help control heart rate and blood pressure. Their ability to make blood vessels relax is the important point when considering swelling. When mast cells release certain mediators, the vessels relax and get a little wider.
• Vessels are made of a complicated network of cells and fibers. It’s like an afghan blanket: some parts of mostly solid and in other places, there are holes you can put your fingers through. When the vessels relax, those holes get larger so it’s easier for things to fall through the holes. In this case, what falls through the hole is fluid from the blood.
• Keep in mind that blood is a mixture of many things. For right now, let’s divide it into cells and everything else. Everything else is a liquid with some stuff dissolved in it.
• When the blood vessels relax, that liquid from the blood trickles out of the blood vessel and goes into the tissue. In some cases, if the blood vessels relax enough, cells actually fall out of the bloodstream and end up in tissue, too.
• The problem is that once you fall out of the bloodstream, you can’t just turn around and go right back in. That’s why swelling takes longer to subside than other symptoms, even with appropriate treatment.
• Everyone is familiar with the bloodstream. Less familiar is the lymphatic circulatory system. Lymphatic circulation is how your body moves things that fall out of the blood back to the bloodstream. This process is slower than processes that can release other symptoms and can sometimes take days.
• When you swell up, fluid falls out of your bloodstream and gets stuck in your tissues. The cells nearby will absorb some of the fluid and take up molecules they can use. However, if there is a lot of extra fluid there, the cells nearby cannot take up all of the fluid. Gradually, your lymphatic system sucks up that fluid and brings it back toward the heart so it can get back into the bloodstream.
• When you get hives (urticaria), it happens because fluid falls out of the bloodstream in a layer of tissue in the skin called the upper dermis.
• When you get angioedema, it happens because fluid falls out of the bloodstream in lower portions of tissue in the skin called the dermis, subcutaneous tissue, mucosa, and submucosa.
• The fact that the process for hives and angioedema is so similar and really distinguished only by which tissue layer they affect is the reason urticaria and angioedema so commonly occur together and are discussed together.

For more detailed reading, please visit the following posts:
Chronic urticaria and angioedema: Part 1
Chronic urticaria and angioedema: Part 2
Chronic urticaria and angioedema: Part 3
Chronic urticaria and angioedema: Part 4
Chronic urticaria and angioedema: Part 5
The Provider Primer Series: Management of mast cell mediator symptoms and release

Just a horse

This past spring, I started developing a rash on my back.  It was macular and itchy and swelled and turned red when I touched it. 
“It looks like urticaria pigmentosa except it’s not dark,” one doctor told me.  It’s important to note here that my rash pigmentation has been unusual for my entire life.  I have, on several occasions, had rashes misdiagnosed because they were “too pink” or “too faint.”  I don’t know the reason for this, but it happens.  “I’m sure it’s some kind of mast cell issue, your skin is very reactive,” he followed up.
“That is definitely urticaria pigmentosa,” another doctor told me.  He touched the spots and they puffed up and got itchy.  “See, it has a positive Darier’s sign.”  My skin will urticate will very little provocation so I was not convinced.  But I figured I was probably bound to have UP eventually, so I wasn’t very concerned.
“It is probably a mast cell rash, but you should get it biopsied just in case,” a third doctor told me.  By this point, the rash was all over my back and shoulders.  It was itchy, but not all the time.  I scheduled an appointment with a dermatologist. 
I saw the dermatologist on Thursday.  She took one look at it and said, “Oh, that’s not cutaneous mastocytosis.  That’s a harmless fungal rash.  It’s more common in people who are immunosuppressed.  I’ll give you a cream.”
We had a good laugh over the fact that when you have a rare disease, everyone assumes it is the cause of all your symptoms.  She told me a funny story about a patient with several rare diseases who had a “mysterious rash” that the residents couldn’t identify.  It was tinea versicolor, a very common fungal rash.  The residents had assumed it was something exotic and had not considered more mundane options.
Then there was a small fire in the building while I was dressed only in a gown, educating the visiting PCP about systemic mast cell symptoms from skin reactions.  I threw my clothes on and ran outside as the fire department arrived.  Always lively. 
Mast cell disease is hard to manage in part because it can cause so many problems.  But just because it can cause all of them doesn’t mean it does. 
Mast cell patients are zebras, often many times over.  But even zebras mingle with horses once in a while.