First, people who are about to have a colonoscopy are told that they cannot eat or drink things before the procedure. That means that they stop taking in fluids and keeping themselves hydrated. Any person who stops eating and drinking has the chance to become dehydrated. This happens a lot with elderly people who do not have good access to water or other food and fluids.
The second dehydrating reason is the colon cleaning preparation. Normally a person will have to drink a large amount of a solution called Go-lytely or something similar. The solution contains a large amount of a glycol based compound that causes diarrhea. This eliminates all of the feces that exists in the stool so that the doctor can move the scope around in the colon and see what is going in.
A colonoscopy is not possible without the preparation as it will be too packed with stool. Even the CT colonoscopy procedure requires a bowel preparation in order to get the patient ready to have the doctor look around. This solution will dehydrate anyone who drinks it and it the usual culprit for dehydration problems.
The symptoms of dehydration are many. Dehydration causes people to feel lightheaded. This is because of the reduction of blood volume that circulates around the body. With a reduced blood volume, it is hard for the blood to be pumped up to the brain and perfuse the cortical tissue properly. This lack of perfusion makes people feel faint and causes other symptoms of dehydration. Someone who is experiencing this problem will also have something called orthostatic hypotension. When they stand up quickly they will feel the head rush of blood leaving their cranium.
This can be measured by having a person have their pulse and blood pressure taken when they are lying down and seated and then comparing to the blood pressure when they are standing. A difference in pulse of at least 20 beats can indicate that the person is experiencing dehydration and that they may be having problems providing blood to their brain. There are many things that can cause similar symptoms and this is a good test to differentiate the various problems.
These problems are easily resolved. Sometimes that doctors will give the patient a large amount of saline solution via an IV in the office if the patient has lost too much blood volume and is not feeling good. This effectively solves the problem by giving a large amount of salt and water quickly and does not ruin the preparation. After the procedure is done, the person can fix their own dehydration symptoms by eating an drinking normally to restore their blood volume.