Toxicology – Hyperthermia

Definition

Hyperthermia is an uncontrolled elevation in core body temperature caused by excess heat production, impaired heat dissipation, excessive environmental heat exposure, or a combination of these mechanisms.

It differs from ordinary fever:

• Fever: hypothalamic temperature set point is raised.
• Hyperthermia: temperature rises despite an essentially unchanged hypothalamic set point because heat production exceeds heat loss.

Severe hyperthermia, particularly around ≥40°C with CNS dysfunction, is a medical emergency and can rapidly cause multiorgan injury.

Pathophysiology

Body temperature reflects the balance between heat production and heat dissipation.

Toxicologic hyperthermia may result from:

• Severe agitation or repetitive muscle activity
• Seizures
• Muscle rigidity
• Increased metabolic activity
• Sympathetic stimulation
• Impaired sweating
• Peripheral vasoconstriction
• Environmental heat exposure
• Impaired behavioral responses to heat
• Abnormal thermoregulation

As temperature rises, cellular injury accelerates.

Severe hyperthermia can cause:

Cellular injury → rhabdomyolysis → electrolyte abnormalities → coagulopathy → hepatic/renal injury → cardiovascular collapse → multiorgan failure

Major Toxicologic Causes

Important causes include:

• Sympathomimetic toxicity
• Anticholinergic syndrome
• Serotonin syndrome
• Neuroleptic malignant syndrome (NMS)
• Malignant hyperthermia
• MAOI toxicity
• Salicylate poisoning
• Severe withdrawal syndromes
• Thyroid hormone poisoning
• Drugs causing prolonged seizures

Sympathomimetic Toxicity

Examples include:

• Cocaine
• Amphetamines/methamphetamine
• Methylphenidate
• Other stimulants

Typical findings:

• Hyperthermia
• Agitation
• Tachycardia
• Hypertension
• Mydriasis
• Diaphoresis
• Tremor
• Seizures

Hyperthermia results from increased metabolic activity, agitation, muscle activity, and sometimes impaired heat loss.

Severe cases can produce rhabdomyolysis, hyperkalemia, AKI, DIC, and multiorgan failure.

Anticholinergic Syndrome

Antimuscarinic drugs impair sweating and therefore reduce evaporative cooling.

Typical findings:

• Hyperthermia
• Mydriasis
• Tachycardia
• Dry, flushed skin
• Dry mouth
• Reduced bowel sounds
• Urinary retention
• Delirium/hallucinations

A useful distinction:

Sympathomimetic → hot and sweaty

Anticholinergic → hot and dry

Serotonin Syndrome

Serotonin toxicity commonly causes:

• Hyperthermia
• Agitation
• Diaphoresis
• Tachycardia
• Hypertension
• Hyperreflexia
• Tremor
• Clonus

Severe hyperthermia results largely from excessive muscular activity.

Clonus and hyperreflexia, particularly in the lower extremities, are important clues.

Onset is usually rapid, often within hours of serotonergic drug exposure or interaction.

Neuroleptic Malignant Syndrome

NMS is associated primarily with dopamine-receptor blockade or abrupt withdrawal of dopaminergic therapy.

Features include:

• Hyperthermia
• Altered mental status
• Autonomic instability
• Severe generalized rigidity
• Elevated creatine kinase
• Rhabdomyolysis

Compared with serotonin syndrome:

NMS → slower onset + prominent rigidity/bradykinesia

Serotonin syndrome → rapid onset + hyperreflexia/clonus

Malignant Hyperthermia

Malignant hyperthermia is a genetically susceptible skeletal-muscle hypermetabolic crisis usually triggered by certain volatile anesthetic agents or succinylcholine.

Features can include:

• Rapidly increasing carbon dioxide production
• Tachycardia
• Muscle rigidity
• Acidosis
• Hyperkalemia
• Rhabdomyolysis
• Rapidly increasing temperature

Importantly, hyperthermia may be a relatively late manifestation.

Dantrolene is the specific treatment, together with immediate discontinuation of triggering agents and aggressive supportive management.

MAOI Toxicity

Monoamine oxidase inhibitor poisoning or interactions can produce:

• Agitation
• Hyperthermia
• Hypertension or hypotension
• Altered mental status
• Neuromuscular abnormalities
• Seizures

Severe toxicity may resemble serotonin syndrome or severe sympathomimetic poisoning.

Salicylate Toxicity

Important findings include:

• Hyperthermia
• Tachypnea
• Tinnitus
• Nausea/vomiting
• Altered mental status
• Respiratory alkalosis
• High-anion-gap metabolic acidosis

Hyperthermia is particularly concerning in severe salicylate poisoning.

Withdrawal

Withdrawal from:

• Alcohol
• Benzodiazepines
• Barbiturates and other sedative-hypnotics

can produce a hyperadrenergic state with:

• Agitation
• Tremor
• Diaphoresis
• Tachycardia
• Hypertension
• Hyperthermia
• Seizures

Muscular activity and seizures can further increase heat production.

Seizures and Agitation

Any cause of prolonged:

• Seizures
• Severe agitation
• Rigidity
• Repetitive muscular activity

can generate enough heat to produce dangerous hyperthermia.

Therefore, rapid control of muscular activity is an important component of temperature management.

Nontoxicologic Differential Diagnosis

Consider:

• Environmental heat stroke
• Severe infection/sepsis
• Thyroid storm
• CNS disorders
• Status epilepticus
• Endocrine disease

Heat stroke is especially important.

Classic heat stroke involves:

Core hyperthermia + CNS dysfunction after heat exposure

Sweating may be present or absent, so dry skin is not required for diagnosis.

Clinical Features

Mild cases may cause:

• Thirst
• Weakness
• Tachycardia
• Headache
• Dizziness

Severe hyperthermia can produce:

• Agitation
• Delirium
• Seizures
• Coma
• Hypotension
• Dysrhythmias
• Rhabdomyolysis
• Acute kidney injury
• Hepatic injury
• DIC
• Multiorgan failure

Temperature Measurement

Accurate core temperature measurement is important in severe suspected hyperthermia.

Peripheral measurements such as oral, axillary, or skin temperatures may underestimate the true temperature.

A rectal temperature is commonly used in emergency heat illness because it closely reflects core temperature.

Diagnostic Clues

Hot + sweaty + agitated + mydriatic

→ sympathomimetic toxicity

Hot + dry + delirious + mydriatic

→ anticholinergic syndrome

Hyperthermia + clonus + hyperreflexia

→ serotonin syndrome

Hyperthermia + severe rigidity + dopamine antagonist exposure

→ NMS

Hyperthermia + rigidity + rapidly rising CO₂ during anesthesia

→ malignant hyperthermia

Hyperthermia + tinnitus + tachypnea + acid-base disturbance

→ salicylate toxicity

Laboratory Evaluation

Significant hyperthermia warrants evaluation for systemic complications.

Important tests may include:

• Electrolytes
• Glucose
• Renal function
• Creatine kinase
• Liver enzymes
• Blood gas
• Lactate
• Urinalysis
• Coagulation studies
• ECG

Depending on circumstances:

• Salicylate concentration
• Acetaminophen concentration
• Other targeted toxicology testing

Routine urine drug screening has limited ability to determine the cause and should not delay treatment.

Rhabdomyolysis

Hyperthermia, agitation, seizures, and rigidity can cause extensive skeletal-muscle breakdown.

Findings may include:

• Elevated CK
• Myoglobinuria
• Hyperkalemia
• Hypocalcemia early in the course
• Acute kidney injury

Renal function and electrolytes therefore require close monitoring.

Immediate Management

Severe hyperthermia requires immediate cooling and simultaneous treatment of the underlying cause.

Initial priorities include:

• Airway and ventilation assessment
• Removal from the heat source
• Removal of unnecessary clothing
• IV access
• Cardiac monitoring
• Core-temperature monitoring
• Appropriate IV crystalloid for hypovolemia
• Rapid control of seizures and severe agitation

Cooling should not be delayed while waiting for laboratory results.

External Cooling

Effective approaches depend on the cause and clinical setting.

Methods include:

• Evaporative cooling with water mist and fans
• Ice-water immersion in appropriate heat-stroke settings
• Ice packs as adjuncts
• Other active external cooling techniques

The objective is rapid reduction of dangerous core temperature while avoiding overshoot hypothermia.

Benzodiazepines

Benzodiazepines are particularly useful when hyperthermia is driven by:

• Sympathomimetic toxicity
• Severe agitation
• Seizures
• Alcohol or sedative withdrawal

They reduce muscular activity and sympathetic stimulation, thereby reducing ongoing heat production.

Severe Muscle-Driven Hyperthermia

When extreme hyperthermia persists because of severe agitation, rigidity, or muscular activity despite adequate sedation, advanced airway management and neuromuscular paralysis may be necessary.

This stops skeletal-muscle heat production.

In such circumstances, a nondepolarizing neuromuscular blocker is generally preferred.

Antipyretics

Drugs such as acetaminophen are generally not effective for toxicologic hyperthermia because the problem is not an elevated hypothalamic temperature set point.

Treatment instead requires:

• Physical cooling
• Reduction of muscle activity
• Correction of the underlying toxic syndrome

Syndrome-Specific Treatment

Serotonin syndrome

• Stop serotonergic agents
• Benzodiazepines
• Active cooling
• Cyproheptadine in selected significant cases

NMS

• Stop the causative drug
• Aggressive supportive care and cooling
• Benzodiazepines
• Dantrolene or dopamine agonists in selected severe cases

Malignant hyperthermia

• Stop triggering anesthetic
• Dantrolene
• Aggressive cooling and metabolic management

Salicylate poisoning

• Treat the salicylate toxicity
• Serum/urine alkalinization when indicated
• Hemodialysis for severe poisoning

Fluids

Hyperthermic patients can have major fluid losses from:

• Sweating
• Tachypnea
• Environmental exposure

Isotonic crystalloid is appropriate when hypovolemia is present.

However, fluids should be individualized because excessive administration can worsen pulmonary edema or other complications.

A rigid urine-output target should not replace assessment of overall perfusion, renal function, and volume status.

Decontamination

Do not induce vomiting.

Routine gastric lavage is not recommended for most poisoned patients.

Activated charcoal may be considered after selected recent ingestions when the substance is adsorbed by charcoal and the airway is adequately protected.

Decontamination must never delay rapid cooling and resuscitation in a severely hyperthermic patient.

Prognosis

Mild hyperthermia generally resolves without permanent injury.

Severe hyperthermia can cause:

• Cerebral injury
• Rhabdomyolysis
• Acute kidney injury
• Acute liver failure
• DIC
• ARDS
• Cardiovascular collapse
• Multiorgan failure
• Death

Complications may continue evolving after the temperature has normalized, so patients with severe hyperthermia require continued monitoring.

Key Points

• Hyperthermia is uncontrolled heat accumulation, not simply fever.
• Major toxicologic causes include sympathomimetics, anticholinergics, serotonin syndrome, NMS, malignant hyperthermia, MAOI toxicity, salicylates, and withdrawal syndromes.
• Sympathomimetic = sweaty; anticholinergic = dry.
• Clonus/hyperreflexia → serotonin syndrome.
• Severe rigidity → consider NMS.
• Rapid CO₂ rise and rigidity during anesthesia → malignant hyperthermia.
• Severe hyperthermia can rapidly cause rhabdomyolysis, DIC, hepatic injury, AKI, and multiorgan failure.
• Treatment requires immediate active cooling plus control of agitation, seizures, and muscle activity.
• Antipyretics generally do not treat toxicologic hyperthermia.
• Dantrolene is the specific therapy for malignant hyperthermia.
• Normalization of temperature does not mean the danger has passed; severe cases require continued monitoring for delayed organ injury.