Orthostatic intolerance is an abnormal response to being upright that can cause dizziness and possibly even fainting. Typically, someone with this problem will be fine when seated or lying down, then become dizzy when they stand up. Orthostatic intolerance may also strike after strenuous exercise.
The term "orthostatic" comes from "orthostasis," which literally means standing up.
In these conditions, the symptom is believed to be the result of dysregulation in the autonomic nervous system, called dysautonomia. This dysfunction is suspected of causing the blood pressure to fall and the heart rate to rise upon standing. In a healthy person, blood pressure increases when you stand up in order to combat the effects of gravity. The abnormally low blood pressure briefly deprives the brain of blood and oxygen, causing lightheadedness.
Orthostatic intolerance may lead to other symptoms, including:
- Visual disturbance (e.g., seeing spots, flashing lights)
- Problems with breathing or swallowing
- Paleness of the face
In chronic fatigue syndrome, one study suggests that orthostatic intolerance may be linked to a heart abnormality in which the left ventricle (one of the chambers of the heart) is smaller than normal.
Outside of illness, orthostatic intolerance can result from problems other than dysautonomia, such as dehydration.