Calcium-Oxalate stone formation – USMLE
The mechanism behind calcium-oxalate stone formation is quite simple, yet I find many students don’t completely understand how it all happens… If you understand the underlying physiology then there’s no way you can be tricked by a USMLE test-writer’s attempt to lead you in the wrong direction.
Here’s how it all happens:
1st: it is necessary that you know calcium LOVES binding to fat…
2nd: it is necessary that you know 90% of calcium comes from GI reabsorption (the other 10% binds oxalate from protein breakdown in the stomach and gets excreted in the stool)
Now that you have those two facts… here’s the process of calcium-oxalate stone formation:
Normally calcium gets reabsorbed in the GI tract and performs its natural functions… 90% is reabsorbed in the GI and the other 10% is used in the stomach to bind oxalate formation as a result of protein digestion. Now, normally calcium binds oxalate in the stomach and it travels through the GI tract for excretion in the feces (this is what should happen). But, when anything that happens in the body to cause fat to move through the GI tract unabsorbed (bile duct obstruction, stress that blunts microvilli, celiac disease – or anything that will alter the reabsorption of fats), will pull all of the calcium along with it, meaning there is inadequate calcium to bind and excrete the oxalate formed.
If oxalate is formed and there is inadequate amounts of calcium to bind to it, it will misbehave and become reabsorbed in the GI tract and enter into circulation… it will search out the small amount of calcium circulating through the blood and bind to it. As this newly formed hematogenous compound travels through the vascular system, it will enter into the kidneys where it will get stuck and thus form the calcium-oxalate stone.
Causes: We know that the major causes our review books talk about include steatorrhea as a major potential cause, but in order to cover all causes the goal is to understand what can cause malabsorption… which can be any of these (not an exhaustive list):
Gall stones – don’t release bile and we don’t get fat emulsification
Stress – stressful situations stimulate release of epinephrine and NE, which constrict blood vessels and decrease the microvilli’s ability to absorb fat in the GI
Diseases – anything that lowers GI absorption (celiac disease)
Dietary – high-protein diets, ingestion of ethylene glycol
The key is to realize that absolutely anything that decreases GI absorption can lead to formation of these stones in the kidneys.
That should cover all of the important information you need to know about calcium-0xalate stone formation, and give you the tools you need to be able to derive the cause.