Orthostatic intolerance (OI) is a condition where symptoms like lightheadedness, fatigue, and mental fog develop when you stand upright and improve when you lie down. It happens because your body struggles to manage the shift in blood flow that gravity causes every time you go from sitting or lying to standing. OI is an umbrella term that covers several specific conditions, including postural tachycardia syndrome (POTS) and orthostatic hypotension.
What Happens in Your Body When You Stand
When you’re lying flat, blood distributes relatively evenly throughout your body, though about 25 to 30 percent sits in the blood vessels around your abdomen and gut. The moment you stand up, gravity pulls more than 500 milliliters of blood downward into your legs and abdominal area. That’s roughly a pint of blood suddenly leaving your chest and brain.
Your body has a rapid-response system to handle this. Pressure sensors in your blood vessels detect the drop in blood flow and trigger your nervous system to speed up your heart rate, tighten your blood vessels, and push blood back up toward your brain. Your leg muscles also act as pumps, squeezing the veins in your calves and thighs to help push blood upward. Even normal breathing creates pressure changes that assist venous return.
In the first 10 to 20 seconds after standing, blood pressure can temporarily drop by 30 percent or more, even in healthy people. This brief dip is called initial orthostatic hypotension and is the reason anyone can feel momentarily woozy if they stand too fast. The difference in orthostatic intolerance is that the compensating systems don’t recover properly, so symptoms persist or worsen the longer you stay upright.
Common Symptoms
The hallmark of OI is that symptoms appear or get worse while upright and improve quickly when you sit or lie down. The most frequently reported symptoms include:
- Lightheadedness or dizziness
- Brain fog, including difficulty concentrating, reasoning, or remembering
- Fatigue and weakness
- Visual disturbances like blurred vision, tunnel vision, or seeing spots
- Nausea or abdominal pain
- Sweating or tremulousness
- Headache
- Exercise intolerance
- Fainting or near-fainting
Some of these symptoms, like nausea and sweating, come from the nervous system overreacting or underreacting to the postural change. Others, like lightheadedness and cognitive problems, come from reduced blood flow to the brain. In more severe cases, the brain simply doesn’t get enough circulation to stay conscious, leading to fainting.
Main Types of Orthostatic Intolerance
Postural Tachycardia Syndrome (POTS)
POTS is diagnosed when your heart rate increases by at least 30 beats per minute (or 40 beats per minute in adolescents) within 10 minutes of standing, without a significant drop in blood pressure. The heart speeds up as it tries to compensate for blood pooling in the lower body, but this compensation is excessive and doesn’t fully solve the problem. Cognitive difficulties and exercise intolerance are especially prominent in POTS. It disproportionately affects young women. In studies of people with hypermobile Ehlers-Danlos syndrome and related joint hypermobility conditions, POTS prevalence runs as high as 41 percent, compared to about 11 percent in the general population of those studies.
Orthostatic Hypotension
Orthostatic hypotension is defined by blood pressure dropping more than 20 points systolic (the top number) or more than 10 points diastolic (the bottom number) within three minutes of standing. Where POTS is primarily a heart rate problem, orthostatic hypotension is a blood pressure problem. The two are considered mutually exclusive for diagnostic purposes: POTS is only diagnosed when orthostatic hypotension has been ruled out.
Vasovagal Syncope
This is the classic fainting episode. It typically follows a recognizable warning phase of lightheadedness, nausea, sweating, weakness, and visual changes. The nervous system essentially overreacts, causing blood pressure and heart rate to suddenly plummet, cutting off blood flow to the brain. It can be triggered by prolonged standing, heat, emotional stress, or pain.
Who Gets Orthostatic Intolerance
OI can affect anyone, but it shows up most often in adolescents and young adults, particularly women. People with joint hypermobility disorders are at notably higher risk. In one study, 100 percent of participants with hypermobile Ehlers-Danlos syndrome reported orthostatic symptoms during tilt table testing, compared to 33 percent of controls. The connection between loose connective tissue and blood vessel regulation isn’t fully mapped out, but stretchy blood vessels may pool blood more easily.
There’s also a strong overlap with chronic fatigue syndrome (ME/CFS). In a study of 429 adults with ME/CFS, 86 percent reported orthostatic intolerance symptoms during daily life. This overlap is significant because the fatigue and cognitive symptoms of OI closely mirror those of ME/CFS, and treating the orthostatic component can sometimes improve overall function.
How It’s Diagnosed
Diagnosis starts with reproducing symptoms in a controlled setting while monitoring heart rate and blood pressure. Two common methods are used.
A tilt table test involves lying on a table that’s slowly tilted to an upright angle (usually 60 to 70 degrees) while a clinician monitors your vitals over 10 to 45 minutes. This eliminates the effect of leg muscle pumping, making it easier to see how your cardiovascular system handles the postural change on its own.
The lean test is a simpler alternative that can be done in a regular office. You lie quietly for a few minutes while a baseline heart rate and blood pressure are recorded. Then you stand slowly with your shoulders leaning against a wall, feet about 15 centimeters (roughly 6 inches) from the wall. Heart rate and blood pressure are measured at one-minute intervals for up to 10 minutes. This test is accessible, requires no special equipment, and can reliably detect both POTS and orthostatic hypotension.
For a POTS diagnosis specifically, the heart rate increase of 30 beats per minute or more must show up in at least two measurements taken at least one minute apart, there must be no significant blood pressure drop, and the patient must have frequent symptoms that worsen with standing and improve lying down. Acute dehydration and blood loss need to be ruled out first, since either can temporarily cause the same pattern.
Lifestyle Strategies That Help
For most people with OI, the first line of treatment isn’t medication. It’s a set of daily habits that support blood volume and reduce pooling.
Fluid and salt intake are the foundation. The typical recommendation is about two liters of fluid and three to five grams of salt per day. This is significantly more salt than what general health guidelines suggest, so it’s specific to people with OI (and not recommended for the hyperadrenergic subtype of POTS, where the nervous system is already in overdrive). The extra salt helps your body retain fluid, which increases blood volume and makes it easier to maintain blood pressure when standing.
Compression garments reduce the amount of blood that pools in your legs and abdomen. To be effective, they need to apply at least 30 to 40 mmHg of pressure. Both waist-high compression stockings and abdominal binders work within this range. Knee-high stockings alone are generally considered less effective because a large portion of blood pools in the abdomen, not just the legs.
Physical counter-maneuvers can help in the moment. Crossing your legs, tensing your thigh and abdominal muscles, or shifting your weight can temporarily squeeze blood back toward your heart. Avoiding prolonged standing, rising slowly from sitting or lying positions, and staying out of excessive heat are practical ways to reduce symptom triggers.
Exercise, particularly recumbent exercise like rowing, swimming, or recumbent cycling, gradually improves cardiovascular fitness and blood volume over time. Starting slowly is key, since exercise intolerance is one of the defining features of the condition. Many people with OI find that an overly aggressive exercise plan backfires, while a gradual program over several months produces real improvement.
When Medication Is Used
When lifestyle changes aren’t enough, medication may be added. The two most commonly prescribed options work through different mechanisms. One is a low-dose steroid that helps your kidneys retain salt and water, expanding blood volume. The other is a drug that directly tightens blood vessels, raising blood pressure by reducing the space available for blood to pool. Both are typically used alongside, not instead of, the lifestyle strategies described above.
Clinicians may also review existing medications that could be worsening OI. Certain blood pressure drugs, antidepressants, and diuretics can contribute to orthostatic symptoms, and adjusting or stopping them sometimes makes a noticeable difference.
Conditions Linked to Orthostatic Intolerance
OI rarely exists in isolation. Beyond the strong associations with joint hypermobility disorders and ME/CFS, it frequently overlaps with mast cell activation syndrome, autoimmune conditions, diabetes-related nerve damage, and deconditioning from prolonged illness or bed rest. In many cases, OI develops after a viral illness, surgery, or pregnancy, suggesting that a combination of immune activation, blood volume changes, and nervous system disruption can tip the balance. Identifying and managing these connected conditions often leads to better outcomes than treating the orthostatic symptoms alone.