Orthostatic intolerance

Orthostatic intolerance is a form of low blood pressure that can cause dizziness.

Prevalence
Orthostatic intolerance is a common dysfunction in ME/CFS Estimates of the rate of orthostatic intolerance in Chronic Fatigue Syndrome and myalgic encephalomyelitis patients vary widely, with estimates as low as 50% to as high as 97% of patients.

Orthostatic intolerance is also commonly associated with fibromyalgia

Orthostatic intolerance is diagnosed by a tilt table test. It is part of the Institute of Medicine report's proposed diagnostic criteria for Systemic Exertion Intolerance Disease.

Postural orthostatic tachycardia
Postural orthostatic tachycardia syndrome (POTS) is one of a group of disorders that have orthostatic intolerance (OI) as their primary symptom. The primary symptom of OI is lightheadedness or fainting. In POTS, the lightheadedness or fainting is also accompanied by a rapid increase in heartbeat of more than 30 beats per minute, or a heart rate that exceeds 120 beats per minute, within 10 minutes of rising.