Extreme heat: heatstroke, dehydration and risky medicines

It is the third day of a heat wave. An eighty-two-year-old woman, who lives alone on the top floor of a building with no cross ventilation, is brought in by her neighbor because she "has been confused since yesterday". She takes a diuretic, an angiotensin-converting enzyme inhibitor and a medication for overactive bladder. At triage, someone comments that she "must already have some dementia" and the case joins the regular queue. That same afternoon, a young man who works on a construction site is carried in by his colleagues after fainting at the end of his shift, with hot skin and slurred speech. Both scenes are fictional and were written for teaching. This text uses these pictures as a setting for clinical reasoning: how heat changes the probability of the hypotheses, why the order of actions matters and which biases delay the decision.

NEXO is a daily medical diagnosis game: one clinical case a day, up to six progressive clues, and ICD-10 proximity feedback across five levels.

The text avoids being a protocol. Cutoff temperatures, cooling methods and tests follow each service's protocols, under supervision. The focus here is the way of thinking: what the context adds to the history, which diagnoses compete with heat illness and why medication review is part of the examination. If you play the case of the day in NEXO, you will recognize the move: a context clue, such as the week of heat, changes the weight of all the others.

Key points

• Heat illness is a spectrum that runs from cramp and exhaustion to heat stroke, which combines a very high core temperature with central nervous system dysfunction.
• During a heat wave, acute confusion in an older adult calls for a core temperature and a neurological examination before any label of dementia or of "age".
• Rapid cooling is the only specific therapy for heat stroke, so the decision to cool comes before the full investigation.
• Hyperthermia is not a synonym for environmental heat: infection, toxidromes and medication-related syndromes must enter the differential.
• Diuretics, anticholinergics and psychotropics raise the risk, and medication review is part of the reasoning, not an administrative detail.

Why a heat wave changes the pretest probability

The same clinical picture means something different depending on the context. Acute confusion in an older person, on a mild day, opens a broad list: infection, metabolic disorder, medication, vascular event. On the third night of extreme heat, the same confusion gains a hypothesis that moves to the top of the list, without removing the others. This shift is what is called pretest probability, and the post on baseline rate in diagnosis shows how the frequency of a condition in the context changes the weight of each finding.

Heat waves raise mortality consistently on several continents. The review by Ebi and colleagues in the Lancet series on heat and health organizes the mechanisms, the most vulnerable groups and the clinical and public health responses (Ebi and colleagues, 2021). The annual Lancet Countdown report tracks heat exposure and health outcomes over the years and shows that the problem is growing (Romanello and colleagues, 2025). The World Health Organization keeps a reference page on heat and health, with guidance for services and for the public (WHO, Heat and health).

For reasoning at the bedside, the practical point is simple. When the week is hot, the question "how long has the person been exposed, where and how" enters the history of any acute picture, just as the question about travel enters the history of fever.

Who is most vulnerable

Studies of historical heat waves help build the list of who needs attention. In the Chicago heat wave, a case-control study found a higher risk of death among people who were bedridden, who did not leave home every day, who lived alone or on the top floor, and social contact worked as protection (Semenza et al., 1996). In the August 2003 heat wave in France, among older adults who lived at home, lack of mobility, prior illnesses, lack of thermal insulation and sleeping just below the roof appeared as risk factors (Vandentorren et al., 2006).

• Older adults, especially those who live alone, have reduced mobility or depend on others for daily care.
• People with cardiovascular, kidney or respiratory disease, diabetes or severe mental disorder.
• Outdoor workers, military personnel and athletes, who build up heat through physical exertion.
• Young children and pregnant people, who depend on adults for hydration and shelter.
• People taking medications that alter thermoregulation, thirst or circulating volume.

Kenny et al. reviewed how aging and common chronic diseases reduce the ability to dissipate heat, through lower sweat production and a weaker skin blood flow response (Kenny et al., 2010). This list is not meant to label anyone. It is meant to remind you to measure temperature and ask about exposure in those who arrive with a vague complaint.

The spectrum of heat illness

Heat illness is not a single entity. The Wilderness Medical Society guidelines organize the clinical forms, prevention measures and management in the field and in hospital, with recommendations graded by quality of evidence (Eifling et al., 2024). The previous 2019 version already described the same spectrum and remains useful for understanding how the recommendations evolved (Lipman et al., 2019).

At the severe end is heat stroke. Bouchama et al. define it as a rapid rise in core temperature above 40 °C with central nervous system dysfunction, and distinguish the classic form, from passive exposure to environmental heat, from the exertional form, which occurs during intense exercise (Bouchama et al., 2022). The review by Epstein and Yanovich in the New England Journal of Medicine describes the pathophysiology and management of heat stroke and is available only through the original link (Epstein and Yanovich, 2019).

Classic and exertional: two different patients

The distinction changes the reasoning. The classic form appears in waves, affects mainly older adults and people with chronic disease, sets in over hours or days and usually arrives accompanied by dehydration and other clinical problems. The exertional form affects young, healthy people, such as athletes, military personnel and workers, and can appear in minutes, sometimes on days that do not even seem extreme. The National Athletic Trainers' Association position statement describes how to recognize and treat the exertional form on site, with an emergency protocol and a gradual return to activity (Casa et al., 2015).

Form	Who usually has it	History clues	Reasoning trap
Heat exhaustion	Anyone exposed	Weakness, nausea, headache, sweating, preserved consciousness	Assuming it is just tiredness and discharging without reassessment
Classic heat stroke	Older adults, people with chronic disease or isolated	Hot days, hot home, progressive confusion	Attributing the confusion to dementia or urinary infection
Exertional heat stroke	Athletes, military personnel, outdoor workers	Intense exertion, collapse, behavior change	Attributing it to simple dehydration or hypoglycemia
Hyperthermia from another cause	Any age	New medication, drug, infectious focus, rigidity	Closing as "heat" because the week is hot

Signs that change the urgency

Some findings change the urgency right away. The first is any change in mental status, from mild confusion to seizure or coma. The second is a very high core temperature, measured by a reliable route and not just on the skin. Hypotension, persistent tachycardia and signs of poor perfusion also weigh in, along with low urine output, dark urine or intense muscle pain, which suggest kidney or muscle injury.

The presence of sweating often confuses. The idea that heat stroke always comes with dry skin leads to underestimating the exertional form, in which the person is often still sweating.

Cool first, investigate later

In heat stroke, the order of actions is part of the reasoning. Bouchama and colleagues remind us that there is no specific therapy beyond rapid cooling (Bouchama and colleagues, 2022). This reverses the usual emergency department flow, in which you investigate and then treat. Here, when the suspicion is reasonable, cooling begins while the rest of the assessment happens in parallel.

Gaudio and Grissom reviewed cooling methods and showed that large trials are lacking, but that case series favor immersion in ice water for the exertional form in young people, while studies with older adults in the classic form have used evaporative cooling with ventilation more often (Gaudio and Grissom, 2016). The choice of method depends on the patient, the place and what the team can do safely. The decision rests with the local protocol and with whoever is on shift.

What to do while cooling happens

The reasoning does not stop when cooling begins. While the temperature falls, the team measures blood glucose, assesses the airway and circulation, collects tests of kidney function, electrolytes, muscle enzymes and coagulation according to protocol, and reassesses mental status often. A rapid neurological improvement with cooling reinforces the hypothesis. The absence of improvement, or the presence of focal signs, reopens the differential.

The diagnostic pause fits well here. The post on diagnostic pause describes how to stop for a few seconds and ask what has been left unexplained, without delaying treatment that is already under way.

The hyperthermia differential: when it is not just the heat

A hot week raises the probability of heat illness, but it does not rule out the other causes of hyperthermia. The classic mistake is letting the context clue decide on its own. Four groups of hypotheses must be remembered in anyone with a high temperature and a neurological change.

The first is severe infection. Meningitis, encephalitis, sepsis and malaria in someone who has traveled can present with high fever and confusion, and environmental heat protects against none of them. The second is toxidromes. Stimulants such as cocaine and amphetamines cause hyperthermia with agitation and tachycardia. The post on stimulants in the emergency department covers this differential and what to avoid, and it is not worth repeating here. Anticholinergic poisoning produces dry, hot skin, dilated pupils, urinary retention and confusion, a picture easily mistaken for classic heat stroke in an older adult taking an anticholinergic for the bladder.

The third group is drug-related syndromes. Serotonin syndrome usually has clonus and hyperreflexia, and neuroleptic malignant syndrome usually has intense muscle rigidity and a slower onset, after starting or increasing an antipsychotic. The fourth group is endocrine and neurological causes, such as thyrotoxicosis and lesions that affect central temperature control.

Questions that separate the hypotheses

• Was there prolonged heat exposure or intense exertion in the previous hours?
• Was any medication started, increased or combined in the last few days?
• Is there substance use, even occasional, reported by the person or the family?
• Is there rigidity, clonus, tremor or focal signs on the neurological exam?
• Is there an infectious focus, neck stiffness, recent travel or contact with a sick person?

All these questions need no sophisticated test, fit in the first minutes and change the list of hypotheses. The post on differential diagnosis with clues shows how to weigh each clue without letting the first one decide everything.

Medications that make it worse: reviewing the drug list as part of the exam

In a heat wave, the medication list is part of the history just as much as the symptoms. Westaway and colleagues described the mechanisms by which common medications raise the risk of dehydration and heat illness, especially in older adults taking many medicines: diuresis and electrolyte disturbance, sedation and cognitive impairment, altered thermoregulation, reduced thirst perception, reduced sweat production, hypotension and reduced cardiac output (Westaway and colleagues, 2015). The authors highlight diuretics, especially when combined with angiotensin-converting enzyme inhibitors or angiotensin receptor blockers, anticholinergics and psychotropics.

An Australian study using dispensing data found an increased risk of hospitalization for dehydration or heat illness after the start of some of these medications in older adults (Kalisch Ellett and colleagues, 2016). For clinical reasoning, this has two consequences. The first is diagnostic: a new medication on the list is a clue that raises the probability of heat illness and also of direct toxicity. The second is preventive: medication review before summer, with individual adjustment, appears among the measures proposed by the authors themselves.

Class	Risk mechanism in heat	What to ask
Diuretics	Volume and sodium loss	Recent dose, fluid intake, urine
Angiotensin-converting enzyme inhibitors and angiotensin blockers	Hypotension and worsening kidney function with dehydration	Dizziness on standing, reduced urine
Anticholinergics	Less sweating, urinary retention, confusion	Bladder, allergy, sleep or nausea medicine
Antipsychotics and other psychotropics	Altered thermoregulation, sedation, less thirst	Recent start or dose increase
Beta-blockers	Lower cardiac response to heat and exertion (Layton and colleagues, 2020)	Dizziness, fatigue, exercise tolerance

The table is meant to organize the questions, not to guide discontinuation. Any dose adjustment is an individual decision, made with the person's care team and according to the service's protocols.

The biases that delay the decision

The extreme heat scenario brings together haste, overcrowding and patients who cannot tell their own story. It is fertile ground for shortcuts. Pat Croskerry describes the most frequent cognitive errors in diagnosis and proposes metacognition, the habit of stopping and examining one's own reasoning, as the main strategy to reduce them (Croskerry, 2003).

Attributing confusion to age

The most dangerous bias in the opening scene is accepting an older person's confusion as part of aging or of a presumed dementia. This shortcut combines age bias and premature closure. The post on implicit bias in diagnosis shows how age changes the attention a person receives. The practical way out is to ask those who live with the person how they were a week ago. New confusion is delirium until proven otherwise, and delirium has a cause.

Premature closure in both directions

Premature closure works in both directions. In one case, the team does not think of heat because the complaint is vague. In the other, the team thinks only of heat because the week is hot and lets meningitis, anticholinergic toxicity or neuroleptic malignant syndrome slip by. Both failures have the same correction: write down the main hypothesis and, next to it, the serious hypothesis that must be ruled out.

Availability bias on a day with many cases

On a shift with several patients with heat illness, the next case tends to be read the same way. Availability, the ease of recalling a diagnosis seen recently, pushes reasoning toward the most recent hypothesis. On a day with many similar cases, it is worth asking more carefully what this patient has that is different from the previous ones.

Discharge: reasoning after the emergency

The management does not end when the temperature returns to normal. Discharge, or the decision to admit, depends on where the person is going back to. A woman who lives alone in a hot apartment, with no ventilation and no one to visit her, goes back to the same environment that caused the problem. The Chicago and France studies show that social isolation and hot housing weigh as much as the underlying disease (Semenza et al., 1996; Vandentorren et al., 2006).

At discharge, it is worth recording how the temperature was measured and how it changed with cooling, which alternative diagnoses were considered and how they were ruled out, and which medications may have contributed, along with who will reassess each one. It also includes where the person will stay in the coming days, who will check how they are doing and which signs call for immediate return, explained to them and to their caregivers.

In the exertional form, the conversation is about a gradual return to activity and about the factors that contributed, such as acclimatization, hydration and work organization. The National Athletic Trainers' Association position statement offers guidance on return to exertion that serves as a reference for this conversation (Casa et al., 2015).

How NEXO helps train this reasoning

In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues. You propose a diagnosis by searching for the ICD-10 code and receive proximity feedback at five levels, which shows whether the guess landed on the exact code, the same category, the same group, the same chapter or a different chapter. This format speaks to the theme of this text: a context clue, such as the day's temperature, can pull the guess toward one chapter and hide the hypothesis that was in another.

To use the case of the day as training against premature closure, try three habits. Before each guess, write down the serious hypothesis you need to rule out. When a clue seems to explain everything, ask what it leaves unexplained. After the result, note which clue you overvalued.

Three exercises with cases

Change the context. Take a fictional case of acute confusion and reread it imagining it happened on a mild day and then during a heat wave. Note how the order of hypotheses changes.

Read the medication list. In each case involving an older adult, before looking at the tests, write down which medications may have contributed and by which mechanism.

Write the hypothesis that cannot be missed. For each picture of hyperthermia, write next to the heat illness an infection and a toxidrome that must be ruled out.

The post about case-based learning in clinical practice shows why training with cases helps consolidate this type of decision.

The limits on using the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information under curation and under creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

How do you tell heat exhaustion from heat stroke?

The central point is mental status. In heat exhaustion, the person is weak, nauseated and has a headache, but remains conscious and behaves as usual. In heat stroke, there is central nervous system dysfunction, from confusion to coma, with a very high core temperature. Any neurological change shifts management to immediate cooling and full evaluation.

Is dry skin necessary to think of heat stroke?

Sweating may be present, especially in the exertional form. Dry skin is more common in the classic form, but the presence of sweat does not rule out the diagnosis. The decision rests on core temperature, mental status and the context of exposure.

Why cool before finishing the investigation?

Because rapid cooling is the only specific therapy for heat stroke and time at high temperature increases organ damage. The investigation continues in parallel. Neurological improvement with cooling supports the hypothesis, and the absence of improvement reopens the differential.

Which medications deserve attention during a heat wave?

Diuretics, especially when combined with angiotensin-converting enzyme inhibitors or angiotensin receptor blockers, anticholinergics, antipsychotics and other psychotropics, as well as medicines that reduce the heart rate response. The review is individual and done with the person's caregiver.

How can you remember other causes of hyperthermia on a hot day?

By writing, next to the heat illness, a severe infection and a toxidrome that must be ruled out, and asking about new medicines, substance use, rigidity and clonus. This short list takes a few minutes and protects against premature closure.

What does discharge need to consider?

Where the person will return to, who will check how they are doing, which medicines will be reassessed and which signs call for coming back. In the exertional form, discharge includes the gradual return to activity program.

Final checklist

• I asked about heat exposure, exertion and housing in every acute presentation during the heat wave.
• I measured temperature by a reliable route and examined mental status carefully.
• I started cooling when heat stroke was a reasonable suspicion, without waiting for all the tests.
• I listed infection, toxidromes and medicine-related syndromes in the differential of hyperthermia.
• I reviewed the medicine list and recorded which ones may have contributed.
• I planned discharge considering the environment, isolation and follow-up.

Want to practice the reasoning with progressive clues? Play the case of the day.

References

1. Bouchama A, Abuyassin B, Lehe C, et al. Classic and exertional heatstroke. Nat Rev Dis Primers. 2022. PMID 35115565.
2. Casa DJ, DeMartini JK, Bergeron MF, et al. National Athletic Trainers' Association Position Statement: Exertional Heat Illnesses. J Athl Train. 2015. PMID 26381473.
3. Croskerry P. The importance of cognitive errors in diagnosis and strategies to minimize them. Acad Med. 2003. PMID 12915363.
4. Ebi KL, Capon A, Berry P, et al. Hot weather and heat extremes: health risks. Lancet. 2021. PMID 34419205.
5. Eifling KP, Gaudio FG, Dumke C, et al. Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness: 2024 Update. Wilderness Environ Med. 2024. PMID 38425235.
6. Epstein Y, Yanovich R. Heatstroke. N Engl J Med. 2019. PMID 31216400.
7. Gaudio FG, Grissom CK. Cooling Methods in Heat Stroke. J Emerg Med. 2016. PMID 26525947.
8. Kalisch Ellett LM, Pratt NL, Le Blanc VT, et al. Increased risk of hospital admission for dehydration or heat-related illness after initiation of medicines: a sequence symmetry analysis. J Clin Pharm Ther. 2016. PMID 27378245.
9. Kenny GP, Yardley J, Brown C, et al. Heat stress in older individuals and patients with common chronic diseases. CMAJ. 2010. PMID 19703915.
10. Layton JB, Li W, Yuan J, et al. Heatwaves, medications, and heat-related hospitalization in older Medicare beneficiaries with chronic conditions. PLoS One. 2020. PMID 33301532.
11. Lipman GS, Gaudio FG, Eifling KP, et al. Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness: 2019 Update. Wilderness Environ Med. 2019. PMID 31221601.
12. Romanello M, Walawender M, Hsu SC, et al. The 2025 report of the Lancet Countdown on health and climate change. Lancet. 2025. PMID 41175887.
13. Semenza JC, Rubin CH, Falter KH, et al. Heat-related deaths during the July 1995 heat wave in Chicago. N Engl J Med. 1996. PMID 8649494.
14. Vandentorren S, Bretin P, Zeghnoun A, et al. August 2003 heat wave in France: risk factors for death of elderly people living at home. Eur J Public Health. 2006. PMID 17028103.
15. Westaway K, Frank O, Husband A, et al. Medicines can affect thermoregulation and accentuate the risk of dehydration and heat-related illness during hot weather. J Clin Pharm Ther. 2015. PMID 26073686.
16. World Health Organization. Heat and health. Fact sheet. who.int.