Orthostatic Intolerance and Low Blood Pressure

blurry scene as if dizzy

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Orthostatic intolerance (OI) is an abnormal response to being upright that can cause dizziness, fainting, and other symptoms that go away when you're seated or lying down. Typically, symptoms arise when you first stand up. OI is often part of dysautonomia, which is a disorder of the autonomic nervous system.

Orthostatic intolerance may also strike after strenuous exercise, and it's a common symptom of both fibromyalgia and chronic fatigue syndrome.

The term "orthostatic" comes from "orthostasis," which literally means standing up.

What Happens With OI

In a healthy person, blood pressure increases when you stand up in order to combat the effects of gravity and keep blood going to your brain. In someone with OI, it's believed that the blood pressure drops and the heart rate increases. Abnormally low blood pressure briefly deprives the brain of blood and oxygen, causing lightheadedness.

OI Symptoms

OI may lead to other symptoms, including:

In fibromyalgia and chronic fatigue syndrome, this symptom is thought to be the result of postural orthostatic tachycardia syndrome, or POTS, which is believed to stem from dysautonomia.

Related to chronic fatigue syndrome, a 2011 study suggests that OI 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.

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