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Orthostatic Intolerance (OI) is a general term that refers to the body not responding correctly to being upright, and symptoms are relieved by lying down (called recumbence). The body naturally compensates for being upright by constricting blood vessels, producing a mild increase in heart rate (usually under 10-15 beats per minute), and sometimes causing a slight increase in blood pressure.  This keeps blood flowing to the brain, lungs, and other organs, and circulating oxygen normally.  In people with Orthostatic Intolerance and Orthostatic Hypotension, these compensations for gravity when standing do not happen properly, sometimes causing the body to work harder to maintain blood pressure, and get enough oxygen and blood to the brain.   

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Many healthy people experience being dizzy when they stand up too fast at some point in their life, as it actually is very common, especially in the elderly. Things like dehydration, the flu, and other temporary illnesses can cause OI, and are termed "acute".  But when it happens frequently over a longer period of time, even day to day, it may be a chronic condition, and can either be primary or secondary to another disease. Finding out why it is happening is important.

 

 

The most common chronic Orthostatic Intolerance conditions that involve dysautonomia currently include Postural Tachycardia Syndrome (POTS) and Neurally Mediated Syncope (NMS), formerly known as Neurocardiogenic Syncope (NCS)​. Some physicians will use Orthostatic Intolerance as a diagnosis itself if a patient doesn't meet the diagnostic requirements for the other OI conditions, but still suffers from symptoms when upright. The definitions change frequently among physicians and researchers, so it is a very confusing topic.​ OI is also often mistaken for anxiety, as it sometimes mimics many symptoms of a panic attack, and occurs mainly in younger women. OI is also very closely associated with Chronic Fatigue Syndrome (CFS) and Fibromyalgia, and there currently is research taking place as to the exact relationship and the mechanisms behind that relationship.

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Orthostatic Hypotension is a type of Orthostatic Intolerance as well, and is defined by the American Autonomic Society as being a persistent fall in systolic/diastolic BP of more than 20/10 mm Hg within 3 minutes of assuming the upright position or during a Tilt Table Test. It is a "clinical sign" and not a disease unto itself. It can be found in people with Multiple Systems Atrophy, Pure Autonomic Failure, and secondary conditions that cause dysautonomia, such as Guillain-Barré syndrome, Diabetes, Parkinson's, and autonomic neuropathy (to name a few). Symptoms are typically worse in the early morning, with prolonged standing, after meals, with a rise in core temperature, and with activity.

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According to the 2011 Autonomic Society Consensus Statement, Orthostatic Hypotension is caused by an excessive fall of cardiac output or by defective or inadequate vasoconstrictor mechanisms. That means that blood vessels are not constricting (squeezing) as they should, and blood is pooling in the extremities and abdomen, causing a drop in blood pressure. Neurogenic Orthostatic Hypotension (NOH) is Orthostatic Hypotension due to inadequate release of norepinephrine from sympathetic vasomotor neurons leading to vasoconstrictor failure. This means that the blood vessels are not squeezing the blood vessels to keep it circulating properly because of a lack of the release of norepinepherine, which is a type of adrenalin.   

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Symptoms Include (some are relieved by lying down):

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• light-headedness​
• ​ syncope or near syncope (fainting or near fainting)​
•  blurring of vision
•  headaches and migraines​
•  problems with short-term memory and concentration
•  fatigue
•  intolerance of low impact exercise
•  palpitations
•  chest pain
•  diaphoresis (excessive sweating)​
•  tremulousness (trembling or shaking)​
•  dyspnea (air hunger)
•  nausea and/or vomiting
• hypertension when lying down (supine)

 

Find information on the most common forms of chronic Orthostatic Intolerance below, including an excellent video by Dr. Peter Rowe from Johns Hopkins on managing Orthostatic Intolerance:

Orthostatic Intolerance and Orthostatic Hypotension