Orthostatic intolerance (OI) is a condition defined by the development of symptoms that occur specifically when an individual assumes an upright posture, such as sitting or standing. These symptoms are typically relieved or significantly lessened when the person sits or lies down again (recumbence). OI represents a failure of the body’s mechanisms to adequately regulate blood flow and pressure against the force of gravity upon standing. This inability to compensate for the postural change means OI is an umbrella term for syndromes that share this common characteristic of difficulty maintaining an upright posture.
The Body’s Response to Standing
When a person moves from a lying or sitting position to standing upright, gravity immediately pulls a significant volume of blood downward, causing a redistribution of approximately 500 to 750 milliliters into the lower extremities and splanchnic (abdominal) circulation. This gravitational shift causes the central blood volume returning to the heart to decrease, leading to a temporary reduction in the heart’s stroke volume and cardiac output. In a healthy individual, this drop is sensed instantly by specialized pressure receptors, called baroreceptors, located in the carotid arteries and aortic arch.
The baroreceptors quickly activate the sympathetic branch of the autonomic nervous system. This activation triggers widespread vasoconstriction, involving the tightening of blood vessels, particularly in the lower body and abdomen, to force blood back toward the heart and brain. Simultaneously, the heart rate increases to maintain cardiac output, ensuring that blood pressure and cerebral blood flow remain stable. This coordinated process normally restores blood pressure within 30 to 60 seconds of standing, ensuring continuous perfusion of the brain.
In individuals with orthostatic intolerance, this compensatory mechanism fails, often leading to excessive “venous pooling” in the lower body. The failure to constrict blood vessels adequately means that the necessary volume of blood does not return to the heart, resulting in a sustained drop in the amount of blood reaching the brain. The autonomic nervous system’s response may be insufficient, delayed, or inappropriate, preventing the body from overcoming the gravitational challenge of upright posture.
Common Signs of Orthostatic Intolerance
The symptoms of orthostatic intolerance arise directly from insufficient blood flow to the brain and the body’s overcompensatory efforts. The most frequent complaint is lightheadedness or dizziness, often described as presyncope (the feeling that one is about to faint). This can be accompanied by visual disturbances, such as blurred vision, tunnel vision, or temporary “gray-outs” or “black-outs” as blood flow to the eyes is compromised.
A rapid or pounding heart sensation (palpitations or tachycardia) is common as the heart attempts to pump faster to compensate for reduced blood return. Patients may also experience generalized weakness, profound fatigue, and heaviness in the legs. Other symptoms include nausea, headache, difficulty concentrating, and increased sweating or tremulousness. These uncomfortable signs typically improve rapidly once the person lies down, removing the gravitational stress on the circulation.
Factors Contributing to Orthostatic Intolerance
Orthostatic intolerance can stem from a wide variety of underlying issues that compromise the cardiovascular or nervous systems. Transient or secondary causes are often the most common, including simple dehydration, which reduces overall blood volume and makes compensation difficult. Prolonged periods of bed rest, such as during post-surgical recovery or due to illness, can also temporarily decondition the cardiovascular system, reducing its ability to respond to standing.
Certain medications, particularly those used to treat high blood pressure, such as diuretics or vasodilators, may exacerbate or cause OI symptoms as a side effect. More chronic or primary forms of OI can be related to problems with the autonomic nervous system itself, a condition known as autonomic neuropathy. This nerve damage can be a complication of other diseases, such as diabetes, or be associated with neurodegenerative disorders like Parkinson’s disease.
Orthostatic intolerance encompasses several specific syndromes. These include Postural Orthostatic Tachycardia Syndrome (POTS), characterized by an excessive increase in heart rate upon standing without a significant drop in blood pressure. Neurally Mediated Hypotension (NMH) involves a sudden, inappropriate drop in blood pressure and heart rate, often leading to fainting (vasovagal syncope). Another form is orthostatic hypotension, defined by a sustained drop in blood pressure within three minutes of standing.
Confirming a Diagnosis and Treatment Options
The diagnostic process begins with a detailed history and physical exam, including measuring blood pressure and heart rate while the patient is lying down, sitting, and standing. This initial evaluation often involves a simple Active Stand Test, where vital signs are recorded at regular intervals for several minutes after the patient stands up. If the diagnosis remains unclear, specialized testing is employed to confirm the body’s inability to regulate circulation under gravitational stress.
The Head-Up Tilt Table Test is a common diagnostic procedure where the patient lies flat on a motorized table that is then tilted to an upright angle, typically 60 to 70 degrees. This allows doctors to continuously monitor heart rate and blood pressure without the use of leg muscles, which can interfere with the assessment of the autonomic response. The test is particularly useful for identifying conditions like POTS or delayed orthostatic hypotension, which may not be apparent during a brief standing test.
Management for orthostatic intolerance generally focuses first on non-pharmacological interventions. Lifestyle modifications are paramount, often involving increasing fluid intake to expand blood volume and increasing dietary salt to help the body retain that fluid. Wearing compression garments, such as waist-high stockings or abdominal binders, is a mechanical strategy used to reduce venous pooling in the lower body.
Patients are also taught physical counter-maneuvers, such as tensing the leg and gluteal muscles or squatting, to help push blood back to the central circulation when symptoms begin.
If non-drug measures are insufficient, pharmacological treatments may be introduced. Medications often work by increasing blood volume (e.g., the mineralocorticoid fludrocortisone) or by causing vasoconstriction to narrow the blood vessels (e.g., midodrine). Other agents like pyridostigmine may be used to enhance the signaling of the autonomic nervous system. The goal of treatment is to relieve orthostatic stress and improve standing tolerance without causing supine hypertension.