Orthostatic intolerance (OI) refers to the inability to be vertical — sitting in a chair, or standing — without developing symptoms.
The simple act of standing requires a complex set of interactions involving the autonomic nervous system, blood vessels, muscle and heart which work to keep the blood from draining from out of our upper body in our abdomen and/or lower body.
When a failure in this system results in inadequate amounts of blood getting to the brain symptoms like dizziness, fainting, cognitive problems and more can result. Tack on other symptoms common to OI such as fatigue, postexertional malaise, stimulus overload and you have a condition that looks very much like ME/CFS.
OI is common enough in ME/CFS that every patient ought to consider seriously whether they have it. Home tests are available for POTS and hyperventilation.
- POTS - Postural orthostatic tachycardia syndrome (POTS) is the most common type of OI found in ME/CFS. In POTS standing causes an immediate, large increase in heart rate (>30 bpm in adults; >40 bpm in adolescents) as the heart struggles to keep the blood flowing to the brain and upper body.
Exercise in ME/CFS can induce temporaryepisodes of POTS.
- Hyperventilation - A ‘Postural Orthostatic Syndrome of Hyperventilation identified by Dr Natelson is probably next most common.
- NHM (neurally mediated hypotension) - results when blood pressure drops after being upright for prolonged periods.