Orthostatic Intolerance: Symptoms, Types and What Causes It

"Orthostatic" means standing up, and "intolerance" means your body doesn't handle it well. Every version of it comes back to the same problem: once you're upright, your brain has a harder time getting the blood it needs. Where the system breaks down decides which label you get and, more importantly, what's likely to help. A diagnosis of POTS often gets used as a placeholder for all of it, so it's worth stepping back to the bigger category first.

Orthostatic Intolerance Symptoms

The symptoms build while you're upright, especially standing still, and improve when you sit or lie down:

• Lightheadedness or dizziness, sometimes with a sense that you're about to faint.
• A racing or pounding heart and a shaky, tremulous feeling.
• Blurred or dimming vision and muffled hearing.
• Brain fog, slowed thinking and trouble finding words.
• Weakness and fatigue, often worse after standing in line, showering or a hot day.
• Headache, nausea or shortness of breath in some people.

The POTS consensus lists lightheadedness, palpitations, tremulousness, generalized weakness, blurred vision and fatigue as the typical symptoms (Vernino, 2021).

What Causes Orthostatic Intolerance?

When you stand, gravity pulls blood toward your belly and legs, and your brain ends up above your heart. Your body answers within seconds: blood vessels tighten, the heart speeds up a little, and the brain adjusts its own vessels to keep its blood flow steady. Orthostatic intolerance happens when one or more of those responses comes up short. Common causes include:

• Low blood volume from dehydration, blood loss or a body that holds too little fluid (why some people keep dumping fluid).
• Blood pooling in the legs and belly when the arteries there don't tighten enough.
• Nerve damage, such as small fiber neuropathy, which leaves vessels unable to tighten on cue.
• Pressure sensors that misread, so the brain responds too slowly or too weakly to a change in position.
• The brain's own blood flow controls failing, including low CO2 narrowing the brain's arteries (hypocapnic cerebral hypoperfusion).
• Triggers that set it off, like a viral illness, a concussion, prolonged bed rest, heat or medications that lower blood pressure.

Types of Orthostatic Intolerance

Our team sorts orthostatic intolerance into buckets by what blood pressure, heart rate and brain blood flow do when you stand:

• Orthostatic hypotension (OH): blood pressure falls by 20 mmHg systolic or 10 mmHg diastolic or more and stays down. Classic OH shows up within 3 minutes of standing; delayed OH takes longer (Gibbons, 2006).
• Vasovagal (reflex) syncope: a trigger, like standing for a long time, heat, pain or the sight of blood, sets off a reflex that drops blood pressure, heart rate or both, and you faint.
• POTS: heart rate rises by 30 beats per minute or more within 10 minutes of standing (40 or more for ages 12 to 19), without orthostatic hypotension, with symptoms that ease lying down (Vernino, 2021). There are several types of POTS under that one label.
• Orthostatic cerebral hypoperfusion syndrome (OCHOS): blood pressure and heart rate look normal, but blood flow to the brain falls when you're upright (Novak, 2016). When low CO2 drives it, it's called hypocapnic cerebral hypoperfusion (HYCH).

OH POTS OCHOS

POTS: heart rate climbs 30 or more beats per minute and holds blood pressure up. Brain blood flow can still fall more than normal: about twice as much as in healthy controls in one tilt study (Ocon, 2009).

An illustration of the 3 patterns, not patient data.

The buckets overlap more than the labels suggest. In a study of 125 people with POTS and 127 with HYCH, the 2 groups had the same symptoms, the same drop in brain blood flow and CO2 upright and similar rates of small fiber neuropathy, and the main difference was whether the heart sped up (Novak, 2024).

Orthostatic Intolerance vs. Orthostatic Hypotension

Orthostatic hypotension is one type of orthostatic intolerance, defined by a measured fall in blood pressure. You can have orthostatic intolerance with a perfectly normal blood pressure, as in POTS and OCHOS. When blood pressure does fall, the heart rate usually explains why the body couldn't catch it, and the video above walks through 3 versions:

• The heart rate doesn't respond at all, which points to the pressure sensors not registering the drop, or the signal not getting through, a pattern called orthostatic hypotension without tachycardia.
• The heart rate falls with the pressure, as in a vasovagal faint, when the vagus nerve slows the heart so much that it briefly stops pumping enough blood to the head.
• The heart rate rises but not enough to pull blood pressure back up, the pattern our team sees most often in these cases.

Timing matters too: in a study of 230 people tilted for 45 minutes, only 46% of those whose pressure eventually fell did so in the first 3 minutes; 15% fell between 3 and 10 minutes and 39% only after 10 (Gibbons, 2006). A quick stand test can miss it.

Is Orthostatic Intolerance the Same as POTS?

POTS is one specific diagnosis inside orthostatic intolerance. Its criteria require the heart rate rise and the symptoms, and they also require that blood pressure doesn't fall (Vernino, 2021). So technically you can't be diagnosed with POTS and orthostatic hypotension at the same moment. In POTS, the faster heart is doing its job: blood pressure starts to dip, and the heart speeds up enough to pull it back to normal. Plenty of people with orthostatic intolerance don't meet that definition, either because their pressure falls or because their heart rate rises less than 30, and Canada's position statement notes that many of them get labeled POTS anyway to validate the illness (Raj, 2020).

Can You Faint From POTS?

In the strictest sense, no, and that one point clarifies the whole category. Fainting means blood flow to the brain dropped far enough to switch it off, almost always because blood pressure fell. POTS is defined by blood pressure holding, so when someone with POTS faints, a different mechanism is at work in that moment, usually a vasovagal reflex or a fall in pressure. That's worth knowing, because it's a different problem to treat. Holding blood pressure still doesn't guarantee your brain is getting enough: the brain sits above the heart, so blood has to be pushed uphill against resistance, and in POTS brain blood flow speed fell about twice as much on tilt as in healthy controls (Ocon, 2009). There's more on that in cerebral hypoperfusion: causes, symptoms and tests.

How Orthostatic Intolerance Is Tested

The standard tests track blood pressure and heart rate while you go from lying to upright, either on a tilt table or in a 10-minute standing test (Vernino, 2021). Those 2 numbers sort people into orthostatic hypotension, POTS or neither, and they say nothing about the brain. Our team adds transcranial Doppler, which measures blood flow speed in the brain's main arteries beat by beat, and capnography, which reads the CO2 in each breath, so OCHOS and HYCH don't get missed and we can see which system fails first.

At home, a squat can give you a clue. Squatting pushes blood from your legs and belly back toward the heart, and standing back up pulls it down again, so the reflexes have to respond quickly. If squatting and standing reliably sets off symptoms, the system that corrects for position changes may be slow, which our team explains in a short video on the squat test. A clue isn't a diagnosis, and testing tells you which bucket you're in.

Why the Type Changes the Treatment

If every form of orthostatic intolerance gets the same salt, fluids, compression and medication, some people will improve and others won't, because the mechanisms differ. 2 examples our team sees often:

• Blood pressure that rises on standing along with the heart rate, while brain blood flow drops. Treating the higher pressure with a blood pressure medicine can pull brain blood flow down further, and people feel worse.
• A beta blocker in POTS slows the heart and can ease palpitations and that wired feeling, which is a real benefit. In some people, though, the faster heart was what kept blood pressure and brain blood flow up, and slowing it makes lightheadedness and brain fog worse.

Medications have a place, and changes to them belong with your prescriber. What testing adds is a picture of which system is failing, so treatment can go after that mechanism directly: rebuilding upright tolerance in steps, retraining the brain's blood flow reflexes, restoring sensation where small nerves have gone quiet, or working on breathing and CO2.

Does Orthostatic Intolerance Go Away?

It often improves, and sometimes it resolves, but outcomes vary a lot with the cause. In one year-long study of POTS, 37% no longer met the criteria at follow-up, while a 10-year survey found most people still had some symptoms; we cover the research in is POTS curable? Finding the mechanism is usually what moves it.

Orthostatic Intolerance: FAQ

Keep Reading

• Types of POTS: neuropathic, hyperadrenergic, hypovolemic
• POTS vs OCHOS: why the 30 BPM line misleads you
• Cerebral hypoperfusion: causes, symptoms and tests in POTS
• What a tilt test really reveals about POTS
• How we evaluate and treat POTS

Dr. Keiser is a board-certified chiropractic neurologist (DC, DACNB, FABBIR), not a medical doctor (MD/DO). This content is for educational purposes and does not constitute medical advice. It is not a substitute for professional medical evaluation, diagnosis, or treatment. Always consult a qualified healthcare provider about your specific situation.

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