Critical care fellow here. We definitely see BUNs over 100 in the ICU. The record high I've personally see is 236.
As mentioned above, there is an encephalopathy that results from azotemia. You will also see delirium in patients immediately after dialysis (if BUN drops too much too soon) because of the osmotic effect = temporary cerebral edema.
There are also negative effects on platelet aggregation. I've had to give DDAVP to uremic pts in the OR before who had normal coags and patelet counts but had a bleeding diathesis which was presumably related to their azotemia.
Im going to second this, then take in the direction of Step 1.
BUN has osmotic effect. So does Na. So does glucose. In fact, you can estimate the serum osmoles with the equation
2*Na + Glc/18 + Bun/2.8
So at BUNs of 10 (normal) the osmotic effect is (divided by 2.8) meh, negligible. But at levels in the triple digits (triple digits are required to call it "uremic encephalopathy") they start to have effect. Do the math if you don't believe me:
BUN 10 = Osmolals of 10/2.8 = 3
BUN 150 = Osmolals of 150/2.8 = 50
Your serum osmoles are about 280 normally, so I hope you can feel that "3" is not the big of a deal while "50" is a substantial portion of your serum osmoles. If you haven't kept up to this point, you probably won't be able to continue...
You've got alot of BUN around. It has osmotic effect. It gets into the brain. Lots of osmoles in the blood, same amount as always in the brain. What does water do? It travels to the area of highest concentration to low concentration (flows from more water, low concentration, to less water, high concentration). The brain, seeing a high BUN in the blood, shifts water from the cells to the blood. Plump happy cells get dehydrated, going to sad crumpled floppy cells. Sad cells
🙁
Now lets take PMPMD's dialysis example. Body has a lot of BUN, sad cells have already occurred. But they aren't dead. So, the body has adjusted. Now, the happy renal fellow (in his first year) sees a BUN of 236 (osmoles of what? 236/2.8 = 80ish) and says "ILL SAVE THE DAY!" and takes the BUN from 236 to 10. You just lost 80 osmoles from the blood. Now in the blood is "less stuff" and in the cells is relatively "more stuff." What does water do? It travels from the blood and into the brain. At first you might think "rehydrating the cells, thats good." But oh, as with everything in medicine, it is about the rate. You take crumpled sad cells (who, in their constant depression have accepted their fate, and like a dog in an electric cage, have adjusted), and FLOOD them with water. Much like a brittle water balloon they swell. Brain swelling? Bad. WORSENING the encephalopathy by causing a rebound edema, or, worse, popping the cells altogether.
In short: BUN causes problems because of water shifts in the brain. Because every BUN is divided by 3 (while Na is multiplied by 2) it takes a lot of BUN to make an osmolar impact. It all boils down to general chemistry, water flowing from an area of more water to an area of less water.