Caloric testing is useful in determining whether there is a weakness of one or both of the vestibular nerves. While lying supine (on the back), head elevated 30 degrees, cool and warm air or water is infused into the ear canal. The air or water cools or warms the fluid in the horizontal semicircular canal, which is on the other side of the eardrum. This creates a current (cooler fluid is more dense and moves toward the ground while warmer fluid is less dense and moves away from the ground) with the end result being the creation of nystagmus. Cooling of the inner ear fluid creates nystagmus away from the tested ear and warming of the inner ear fluid creates nystagmus toward the tested ear. Many use the "COWS" mnemonic, which stands for "Cold Opposite Warm Same". The amount of nystagmus generated by cooling or warming the inner ear fluid can be measured to determine whether the response is normal in each ear or different from one ear to the other. In most labs, a difference of up to 25% is considered normal, while a difference greater than 28% is abnormal.
To learn more about the different parts of the VNG, follow the links below: