Jul 20, 2026 · By Anna B. Albright · Co-founder & CEO · 14 min read

Heat and dysautonomia: why temperature breaks autonomic regulation

Heat stress cuts tolerance to a simulated orthostatic challenge by roughly 80 percent in healthy young people. For someone whose autonomic nervous system is already working without reserve, summer sets a hard physiological ceiling. Here is what the mechanism actually is, who it affects, and an honest accounting of how thin the intervention evidence still is.

What heat actually does to autonomic control

Thermoregulation and blood pressure regulation compete for the same resource. When core temperature rises, the hypothalamus responds by dumping heat through the skin, and the only way to move heat to the skin is to move blood there. Cutaneous blood flow at thermoneutral rest sits around 0.3 litres per minute. Under substantial passive heat stress it can climb to 5 to 7 litres per minute, which in a person with a resting cardiac output of 6 or 7 litres per minute represents a wholesale redistribution of the circulation toward the periphery.

The cutaneous vascular bed is highly compliant, so it pools that blood rather than simply passing it along. Combine that with fluid losses through sweating and the result is a measurable reduction in central blood volume. Central venous pressure falls, and ventricular filling and stroke volume fall with it. Heart rate rises to defend cardiac output, and much of the cardiovascular reserve that a person would normally deploy on standing up has already been spent on staying cool.

Then add gravity. Standing shifts a further 500 to 800 millilitres of blood into the lower body and splanchnic circulation. In a normothermic healthy adult, this is handled without conscious effort. Under heat stress it becomes a genuine challenge, and the experimental data on that point are unambiguous.

That result is the single most useful piece of physiology in this subject, because of what the third bar shows. If heat intolerance were caused by heat damaging autonomic signalling, restoring blood volume would not fix it. It does fix it. The deficit is haemodynamic.

The tilt table data tell the same story from a different angle. In a study of nine healthy subjects undergoing ten minutes of 60 degree head-up tilt, no participant became presyncopal in the normothermic condition. Under heat stress, several did. Cooling the skin surface during the heated tilt prevented the fall in cerebral blood flow velocity by raising mean arterial pressure, and no participant became presyncopal in that condition either.

Where the failure actually happens

It is common to read that heat impairs the baroreflex. That is not what the literature shows. Baroreflex sensitivity is broadly preserved under heat stress, and the problem sits downstream. Cutaneous arterioles have been maximally dilated in service of heat dissipation, so when the baroreflex correctly identifies a falling pressure and signals for vasoconstriction, the vessels have very little left to give. The signal arrives and the vessels cannot answer it.

This matters clinically, because it changes what an intervention has to accomplish. There is no reflex arc to repair. The task is to give the reflex something to work with, either by reducing the thermoregulatory demand on the cutaneous circulation or by increasing the circulating volume the reflex is managing.

Which populations carry this risk

Heat intolerance in dysautonomia is usually discussed in the context of POTS, but the affected population is considerably broader, and much of it sits inside the working-age workforce.

POTS

The prevalence figure most often cited, 1 to 3 million Americans, comes from the 2019 NIH expert consensus meeting, and that document is candid about its own basis. It states that prevalence "has not been properly studied" and that the estimate is based largely on clinical experience. No epidemiological survey underlies it. Anyone quoting a precise POTS population number is quoting a considered guess made by clinicians who see the condition often.

What is measured is the diagnostic trend. An interrupted time-series analysis of 65.1 million adult patient records across 64 US healthcare organisations found new POTS diagnoses rising from 4.21 to 22.66 cases per month across the cohort after March 2020, with incidence going from 1.42 to 20.3 per million person-years. The honest reading is that this curve measures diagnosis rates rather than disease onset. A pre-pandemic incidence of 1.42 per million person-years is implausibly low for a condition thought to affect one to three million people, which tells you the baseline was capturing a small fraction of real cases. Increased awareness and the introduction of a dedicated ICD-10 code are unadjusted confounders in that trend.

The demographics are more solid. POTS is roughly 80 to 94 percent female depending on the sample, with tertiary referral series reporting around 87 percent and large community surveys reporting 94 percent. Age at onset is younger than most summaries suggest: a survey of 4,835 patients found a mean onset age of 21 with a mode of 14, while the frequently quoted figure of around 30 is age at diagnosis. That gap is itself the finding. Peer-reviewed data from a Canadian cohort put diagnostic delay at 5.0 years for women and 3.0 years for men.

The functional consequence is substantial. In a study of employment and economic loss, 50.2 percent of POTS patients were not employed in the previous three months, 70.5 percent had lost income, and 36 percent had lost more than $10,000 over twelve months. That sample was community-recruited and skews toward more severely affected patients, so treat it as an upper bound rather than a population average.

Sweating abnormalities specifically

Impaired sudomotor function would make heat intolerance in POTS mechanically worse, because it removes evaporative cooling as an escape valve and forces even more of the thermoregulatory burden onto skin blood flow. The prevalence data are messy. A study of 30 women with POTS found abnormal quantitative sudomotor axon reflex testing in 56 percent, in a patchy lower-extremity distribution, while the Mayo Clinic series of 152 patients found sudomotor abnormality in roughly half. Larger and less selected series report substantially lower rates, down toward 20 percent. The pattern that emerges is that small tertiary-centre samples produce the alarming numbers, and the NIH consensus explicitly cautions that these estimates come from small or selected populations.

Multiple sclerosis

Roughly 914,000 US adults have MS, based on a claims algorithm applied to 2010 census data. Heat sensitivity is the defining feature of Uhthoff's phenomenon, and the primary survey on the question, covering 256 respondents, found 58 percent reporting heat sensitivity. Higher figures of 60 to 80 percent circulate widely in review articles but are not what that survey measured.

Spinal cord injury and diabetic autonomic neuropathy

Approximately 308,600 people in the US live with traumatic spinal cord injury, though that point estimate carries a stated range of 259,000 to 394,000 and derives from a statistical model published in 1995 and extrapolated forward to current population figures. Injury above the sympathetic outflow disrupts both vasomotor and sudomotor control below the lesion, and the higher the lesion, the larger the thermoregulatory deficit.

Cardiovascular autonomic neuropathy is the largest population by raw count and the least discussed. Of the 40.1 million Americans with diabetes, including approximately 11 million undiagnosed, prevalence of cardiovascular autonomic neuropathy runs 16.8 percent in type 1 and 22.1 percent in type 2 under strict criteria requiring at least three of six abnormal autonomic function tests. Those figures come from a clinic-attending population studied in 1993, so treat them as directional rather than current.

Age, the largest unlabelled group

Thermoregulatory capacity declines with age independent of any diagnosis. Adults aged 50 and over store roughly 1.3 to 1.8 times more body heat than adults aged 19 to 30 at the same heat load, across exercise and passive exposure and in both humid and dry conditions. Sweat rate falls and the skin blood flow response blunts. Workers aged 55 and over now make up 23.1 percent of the US civilian labour force, up from 15.6 percent in 2004 and projected to reach 23.6 percent by 2034. The heat-vulnerable share of the workforce is growing for demographic reasons alone.

What the intervention evidence actually shows

This is where the subject requires more honesty than it usually gets. The mechanistic case for cooling heat-intolerant patients is strong. The clinical trial case is uneven, and in POTS it barely exists.

Multiple sclerosis: real but modest

A 2023 systematic review and meta-analysis pooled 13 studies and 384 participants across liquid-perfused garments, phase-change vests, a thigh cuff and a palm-cooling device. It found improvements in walking capacity and functional mobility, some improvement in strength and balance, and improvements in core temperature, skin temperature, thermal sensation and subjective fatigue. Manual dexterity did not improve. The authors declined to identify a superior garment type and concluded that patients should experiment to find their own preference, which is a fair reflection of how heterogeneous the underlying studies are.

The methodologically strongest single trial remains a 2003 multicentre sham-controlled double-blind crossover study of 84 MS patients using liquid-perfused garments. Effects were modest, and the timed 25-foot walk did not change with acute cooling. Blinding a cooling garment is close to impossible, which is exactly why that sham-controlled design carries more weight than the volume of open-label work around it.

Spinal cord injury: mixed to negative

A systematic review covering ten cooling studies in spinal cord injury found three outright null on core temperature and two more null on performance despite achieving thermal effects. Sample sizes ran from 3 to 19 participants. Benefit clustered in tetraplegia, the group with the largest sudomotor deficit, and largely disappeared in paraplegia and in able-bodied controls. One trial that did improve time to exhaustion also produced significantly lower peak power output when cooling was applied during exercise, which is a trade-off. Nearly all of this work was conducted in trained wheelchair athletes during exercise, which is not the exposure profile of a heat-intolerant person doing an ordinary shift.

POTS: one uncontrolled pilot

As of July 2026 the entire prospective cooling literature in POTS is a single study: an open-label, uncontrolled pilot of a wrist-worn thermal comfort device in 22 patients over four weeks, with a median age of 36.5 and a 96 percent female sample. It reported statistically significant improvements on self-reported temperature disturbance domains, with p values of 0.047, 0.039 and 0.015 across multiple domains in a sample of 22 and no apparent multiplicity correction. There was no control arm and no blinding, and the significant findings were all patient-reported quality of life items rather than objective measures. The device manufacturer subsequently promoted the study as clinical validation of its product.

A search of ClinicalTrials.gov conducted in July 2026 returned no registered cooling garment or cooling device trial in POTS or dysautonomia. The paper's own framing concedes the gap, noting that cooling ties, vests and portable fans have been evaluated in healthy athletes but not in patients with POTS.

What does have evidence in POTS

Compression is the strongest non-pharmacological finding. A randomised crossover trial of 30 patients tested four conditions during ten minutes of head-up tilt: no compression, lower leg only, abdominal and thigh, and full abdominal plus leg. Upright heart rate fell in a dose-dependent fashion from 109 to 103 to 97 to 92 beats per minute. The clinically useful detail is that abdominal compression captured most of the benefit while legs-only was the weakest condition, which points at splanchnic capacitance rather than calf pooling as the primary target.

Volume expansion has a reproducible acute effect. Bolus water drinking of around 500 millilitres blunts the orthostatic heart rate rise and improves tilt tolerance. Intravenous saline acutely improves cardiac output and orthostatic tolerance, but that evidence is acute physiology and case series only, with no randomised trials against placebo or against oral strategies, and the guideline direction has moved away from routine indwelling access because of line infection and thrombosis risk.

Exercise training has the best longitudinal data. Three months of progressive training beginning in recumbent and rowing positions and advancing to upright increased peak oxygen uptake by about 8 percent, cardiac mass by about 8 percent and blood volume by about 6 percent, and lowered upright heart rate by roughly 9 beats per minute. Ten of nineteen patients no longer met POTS criteria at the end. That study had no randomised non-exercise control, had marked selection and attrition, and was never designed as a heat intervention.

Heat acclimatisation deserves a specific warning. No acclimatisation data exist in POTS or dysautonomia. The only clinical-population data are two small spinal cord injury studies. Standard acclimatisation protocols work by deliberately imposing repeated hyperthermic and orthostatically stressful exposures, which is a difficult thing to recommend to a population defined by orthostatic intolerance.

Where this meets employment law and workplace standards

For working-age patients, the practical question is usually not what a trial showed but what an employer is obliged to provide. Two frameworks apply.

The Americans with Disabilities Act, as amended in 2008, defines disability functionally, and circulatory function is expressly covered as a major bodily function. POTS, MS, spinal cord injury and autonomic neuropathy are all strong candidates for coverage, and the EEOC's enforcement guidance on reasonable accommodation uses an air conditioning breakdown affecting an employee with multiple sclerosis as one of its worked examples. The Job Accommodation Network maintains temperature sensitivity as a discrete limitation category, with documented accommodations including cooling garments, portable cooling equipment, modified break schedules, flexible scheduling and telework. There appears to be no reported EEOC enforcement action turning specifically on heat accommodation, which suggests the legal hook exists and is under-used.

The occupational side is moving more slowly than most coverage implies. The proposed federal OSHA heat standard, which would cover roughly 36 million workers with an 80 degree Fahrenheit initial heat index trigger and a 90 degree high-heat trigger requiring paid rest breaks, is still in the proposed rule stage. A supplemental notice of proposed rulemaking is scheduled for December 2026, with final action targeted for October 2027, which pushes real compliance obligations to 2028 at the earliest. Enforcement in the interim runs through the General Duty Clause and a revised National Emphasis Program effective April 2026 that retargets inspections to 55 high-risk industries. Eight states have their own enforceable heat rules, with California, Washington, Oregon and Maryland the most protective, and Virginia enacted legislation in April 2026 directing adoption of a standard by 2028.

What this means in practice

The physiology is settled enough to act on. Heat competes with blood pressure regulation for the same circulating volume, and people whose autonomic reserve is already spent lose the competition first. The interventions with the clearest support are the ones that address volume and capacitance directly, which is why abdominal compression outperforms leg compression and why acute volume expansion reverses heat-induced orthostatic intolerance almost entirely under laboratory conditions.

The cooling question is more open than the marketing around it suggests. Reducing skin temperature reduces the thermoregulatory demand on the cutaneous circulation, and in healthy heat-stressed volunteers that translated into preserved cerebral perfusion and no episodes of presyncope. Whether it translates into meaningful function in a POTS patient over a working day is an empirical question that nobody has answered, because nobody has run the trial. The MS literature would predict a real and modest effect. That expectation is reasonable, and it remains an expectation.

If you work in occupational safety and you have employees with any of these diagnoses, the operational implication is specific. Standard heat protocols are calibrated to the physiology of a healthy adult, and they assume a thermoregulatory reserve that a meaningful share of your workforce does not have. The acclimatisation ramp that protects a healthy new hire is the same exposure pattern that destabilises someone with orthostatic intolerance, which means the protocol that reduces risk for most of a crew can raise it for a few. Individual accommodation is the only approach the physiology supports, and under the ADA it is also the one an employer is likely obliged to consider.

Frequently asked questions

Why does heat make POTS symptoms worse?

Heat forces blood to the skin for heat dissipation, with cutaneous flow rising from roughly 0.3 to 5 to 7 litres per minute. That volume comes out of central circulation, reducing cardiac filling and stroke volume. In someone whose autonomic system is already using its full reserve to defend blood pressure when upright, heat removes the compensation that was holding things together. In healthy volunteers, heat stress alone cut tolerance to a simulated orthostatic challenge by roughly 80 percent.

Does heat impair the baroreflex?

No. Baroreflex sensitivity is largely preserved. The failure is at the effector: cutaneous vessels are already maximally dilated for thermoregulation, so there is little constrictive reserve left when the reflex calls for vasoconstriction. The signal arrives and the vessels cannot answer it.

Do cooling vests work for POTS?

Nobody knows, because the trial has not been run. As of July 2026 the entire prospective cooling literature in POTS is one open-label uncontrolled pilot of a wrist device in 22 patients with self-reported outcomes. The cooling garment trial evidence comes from multiple sclerosis, where effects are real and modest, and the mechanistic evidence comes from healthy heat-stressed volunteers.

What has the best evidence for heat intolerance in POTS?

Compression, and specifically abdominal compression. A randomised crossover trial in 30 patients cut upright heart rate from about 109 to 92 beats per minute with full compression, with abdominal compression alone capturing most of the effect.

How many people in the US have heat-sensitive autonomic conditions?

POTS is estimated at 1 to 3 million Americans, though that figure is expert opinion rather than a measured survey. Roughly 914,000 adults have multiple sclerosis, of whom 58 percent reported heat sensitivity in the primary survey on the question. About 308,600 people live with traumatic spinal cord injury. Among 40.1 million Americans with diabetes, cardiovascular autonomic neuropathy affects roughly 17 to 22 percent under strict diagnostic criteria.

Is heat acclimatisation safe for people with dysautonomia?

There is no data either way in dysautonomia. Acclimatisation protocols work by imposing repeated hyperthermic and orthostatically stressful exposures, which is difficult to justify in a population defined by orthostatic intolerance without evidence.

Is POTS covered by the Americans with Disabilities Act?

The ADA does not maintain a list of qualifying conditions, but the 2008 amendments define disability functionally and expressly include circulatory function as a major bodily function. POTS is a strong candidate for coverage, and the Job Accommodation Network documents specific accommodations for both POTS and temperature sensitivity.

Selected references

1. Rowell LB. Cardiovascular aspects of human thermoregulation. Circ Res. 1983;52(4):367-379.
2. Smit AA, Halliwill JR, Low PA, Wieling W. Pathophysiological basis of orthostatic hypotension in autonomic failure. J Physiol. 1999;519(Pt 1):1-10.
3. Lucas RAI, Ganio MS, Pearson J, Crandall CG. Acute volume expansion preserves orthostatic tolerance during whole-body heat stress in humans. J Physiol. 2009;587(Pt 13):3323-3333.
4. Wilson TE, Cui J, Zhang R, Witkowski S, Crandall CG. Skin cooling maintains cerebral blood flow velocity and orthostatic tolerance during tilting in heated humans. J Appl Physiol. 2002;93(1):85-91.
5. Cui J, Durand S, Levine BD, Crandall CG. Effect of skin surface cooling on central venous pressure during orthostatic challenge. Am J Physiol Heart Circ Physiol. 2005;289(6):H2429-33. PMID 16024573.
6. Crandall CG, Wilson TE. Human cardiovascular responses to passive heat stress. Compr Physiol. 2015;5(1):17-43.
7. Vernino S, Bourne KM, Stiles LE, et al. Postural orthostatic tachycardia syndrome: state of the science and clinical care from a 2019 NIH Expert Consensus Meeting, Part 1. Auton Neurosci. 2021;235:102828.
8. Dulal D, Maraey A, Elsharnoby H, Chacko P, Grubb B. Impact of COVID-19 pandemic on the incidence and prevalence of postural orthostatic tachycardia syndrome. Eur Heart J Qual Care Clin Outcomes. 2025;11(5):698-704. PMID 39775762.
9. Shaw BH, Stiles LE, Bourne K, et al. The face of postural tachycardia syndrome: insights from a large cross-sectional online community-based survey. J Intern Med. 2019;286(4):438-448.
10. Thieben MJ, Sandroni P, Sletten DM, et al. Postural orthostatic tachycardia syndrome: the Mayo Clinic experience. Mayo Clin Proc. 2007;82(3):308-313. PMID 17352367.
11. Peltier AC, Garland E, Raj SR, et al. Distal sudomotor findings in postural tachycardia syndrome. Clin Auton Res. 2010;20(2):93-99. PMID 20035362.
12. Bourne KM, Sheldon RS, Hall J, et al. Compression garment reduces orthostatic tachycardia and symptoms in patients with postural orthostatic tachycardia syndrome. J Am Coll Cardiol. 2021;77(3):285-296. PMID 33478652.
13. Bourne KM, Nerenberg KA, Stiles LE, Shaw BH, Raj SR, et al. Postural orthostatic tachycardia syndrome is associated with significant employment and economic loss. J Intern Med. 2021;290(1):203-212.
14. Bourne KM, Sheldon RS, Raj SR, et al. Sex differences in the diagnostic experience of patients with postural orthostatic tachycardia syndrome. CJC Open. 2021;3(12 Suppl):S44-S54.
15. Fu Q, VanGundy TB, Galbreath MM, et al. Cardiac origins of the postural orthostatic tachycardia syndrome. J Am Coll Cardiol. 2010;55(25):2858-2868.
16. Raj SR, et al. Oral and intravenous hydration in the treatment of orthostatic hypotension and postural tachycardia syndrome. Auton Neurosci. 2022;241:102911.
17. Stevens CJ, Singh G, Peterson B, Vargas NT, Périard JD. The effect of cooling garments to improve physical function in people with multiple sclerosis: a systematic review and meta-analysis. Mult Scler Relat Disord. 2023;78:104912.
18. Schwid SR, Petrie MD, Murray R, et al. A randomized controlled study of the acute and chronic effects of cooling therapy for MS. Neurology. 2003;60(12):1955-1960. PMID 12821739.
19. Flensner G, Ek AC, Söderhamn O, Landtblom AM. Sensitivity to heat in MS patients: a factor strongly influencing symptomology, an explorative survey. BMC Neurol. 2011;11:27.
20. Wallin MT, Culpepper WJ, Campbell JD, et al. The prevalence of MS in the United States: a population-based estimate using health claims data. Neurology. 2019;92(10):e1029-e1040.
21. Rasminsky M. The effects of temperature on conduction in demyelinated single nerve fibers. Arch Neurol. 1973;28(5):287-292.
22. Grossmann F, Flueck JL, Perret C, Meeusen R, Roelands B. The thermoregulatory and thermal responses of individuals with a spinal cord injury during exercise, acclimation and by using cooling strategies: a systematic review. Front Physiol. 2021;12:636997.
23. Miglis MG, Seliger J, Machnik JV, Shaik R, Larsen NW, Sinn D. Use of a thermal comfort wearable improves temperature intolerance in patients with postural tachycardia syndrome. Auton Neurosci. 2026.
24. National Spinal Cord Injury Statistical Center. Traumatic spinal cord injury facts and figures at a glance. University of Alabama at Birmingham, 2025.
25. Lasfargues JE, Custis D, Morrone F, Carswell J, Nguyen T. A model for estimating spinal cord injury prevalence in the United States. Paraplegia. 1995;33(2):62-68.
26. Ziegler D, Gries FA, Spüler M, Lessmann F. Prevalence and clinical correlates of cardiovascular autonomic and peripheral diabetic neuropathy in patients attending diabetes centers. Diabete Metab. 1993;19(1 Pt 2):143-151. PMID 8314418.
27. Centers for Disease Control and Prevention. National Diabetes Statistics Report. Atlanta, GA: US Department of Health and Human Services, 2024.
28. Millyard A, Layden JD, Pyne DB, Edwards AM, Bloxham SR. Impairments to thermoregulation in the elderly during heat exposure events. Gerontol Geriatr Med. 2020;6:2333721420932432.
29. Larose J, Boulay P, Sigal RJ, Wright HE, Kenny GP. Age-related decrements in heat dissipation during physical activity occur as early as the age of 40. PLoS One. 2013;8(12):e83148.
30. US Bureau of Labor Statistics. Civilian labor force by age, sex, race, and ethnicity. Employment Projections program, 2025.
31. Occupational Safety and Health Administration. Heat Injury and Illness Prevention in Outdoor and Indoor Work Settings, Notice of Proposed Rulemaking. 89 FR 70698, August 30, 2024. RIN 1218-AD39.
32. Occupational Safety and Health Administration. National Emphasis Program, Outdoor and Indoor Heat-Related Hazards. Directive CPL 03-00-024, effective April 10, 2026.
33. US Equal Employment Opportunity Commission. Enforcement Guidance on Reasonable Accommodation and Undue Hardship under the ADA. Notice 915.002, revised 2002.
34. Job Accommodation Network. Temperature Sensitivity, and Postural Orthostatic Tachycardia Syndrome (POTS). askjan.org, accessed July 2026.

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