Maybe I missed that pathology lecture, or maybe I was sleeping when the terms were discussed. For whatever reason, I have always had a hard time with dystrophic and metastatic calcification. I could remember that one type of calcification was simply the result of having a high calcium level, but I couldn’t really define the terms and keep straight which one was which. If you have had that problem, maybe this will help clear it up once and for all.
This type of calcification is seen in areas of necrosis. Some examples of lesions with dystrophic calcification include: atherosclerotic plaques, aging or damaged heart valves, and tuberculous lymph nodes.
The process is thought to start in membrane-bound vesicles within cells. When the membrane of a vesicle gets damaged, calcium binds to the phospholipid in the membrane. Phosphatases within the membrane add phosphate groups to the calcium, forming calcium phosphate (this is similar to the hydroxyapatite you see in regular bone). If you get enough calcium phosphate, it clumps together as a visible deposit, which rearranges itself (cool!) to generate a microcrystal (which propagates and leads to more calcium deposition).
Macroscopically, dystrophic calcification looks like tiny, white, gritty granules. Microscopically, you see basophilic granules (or sometimes amorphous deposits) either inside or outside of cells. Sometimes there is so much calcium that heterotopic bone is formed (whoa). Sometimes, single necrotic cells act like little grains of sand around which a “pearl” of calcium is deposited. This is called a psammoma body (which is appropriate since psammos is Greek for sand). Check out the very pretty psammoma body above. Psammoma bodies are particularly common in papillary carcinomas (like papillary thyroid carcinoma).
So what does it mean if you see dystrophic calcification? Well, it can just mean that the tissue has been previously injured. However, it can also indicate that there is ongoing organ dysfunction (for example, you see dystrophic calcification in calcific valvular disease). It can also be helpful if you’re trying to diagnose a carcinoma (if you have a thyroid carcinoma and you don’t know whether it’s follicular or papillary, seeing psammoma bodies is a great big clue that it’s a papillary carcinoma).
This type of calcification happens when there is hypercalcemia (this is the one thing I could remember!). It happens in normal tissues (not necrotic tissues, like dystrophic calcification). It doesn’t happen in all cases of hypercalcemia – only some. It may occur widely throughout the body, hence the term “metastatic.” I wish they would have picked a different term, because metastatic makes you think “malignant tumor” and that could make you think that this is the type of calcification you see in papillary carcinomas, which is not the case.
The process seems to affect mostly interstitial tissues of the gastric mucosa, kidneys, lungs, systemic arteries and pulmonary veins. It turns out that all of these tissues excrete acid, and therefore have an alkaline internal component, which is susceptible to calcification. You can see either amorphous deposits or discrete crystals, just like in dystrophic calcification. The deposits are formed of calcium phosphate crystals too, just like in dystrophic calcification.
So what does it mean if you see metastatic calcification? Mostly, it just means the patient is hypercalcemic, and you have to figure out why. Is it an elevated parathyroid hormone, or destruction of bone, or what? Usually the deposits in and of themselves don’t do much damage (though if you get a ton of metastatic calcification in the lung or kidney it can impair function).
Both types of calcification consist of calcium phosphate crystals. The big difference is that dystrophic calcification occurs in damaged tissue, and metastatic calcification occurs in normal tissue in the setting of hypercalcemia. Hey, I just thought of something: dystrophic = dead/damaged! Wish I would have thought of that in med school.