Toxicology – Neuroleptic Malignant Syndrome (NMS)

Definition

Neuroleptic malignant syndrome is a life-threatening reaction associated with severe dopamine blockade. It is classically characterized by:

• Hyperthermia
• Altered mental status
• Autonomic instability
• Severe muscle rigidity

It can occur even at therapeutic antipsychotic doses and often evolves over 1–3 days.

Common Causes

NMS is most often associated with:

• Antipsychotic medications, especially potent dopamine antagonists
• Rapid dose escalation
• Parenteral antipsychotic use
• Abrupt withdrawal of dopaminergic therapy in patients with Parkinson disease

Mechanism of Action

The syndrome is thought to result from marked reduction in dopamine activity, particularly in the hypothalamus and basal ganglia.

This leads to:

• Impaired temperature regulation
• Severe muscle rigidity
• Autonomic dysfunction
• Altered mental status

Clinical Features

Typical findings include:

• High fever
• Tachycardia
• Hypertension or labile blood pressure
• Tachypnea
• Diaphoresis
• Confusion or decreased consciousness
• “Lead-pipe” muscular rigidity
• Bradykinesia
• Leukocytosis
• Elevated creatine kinase (CK)
• Rhabdomyolysis

Complications can include acute kidney injury, dysrhythmias, respiratory failure, and shock.

Differential Diagnosis

Important mimics include:

• Serotonin syndrome
• Malignant hyperthermia
• Anticholinergic toxicity
• Sympathomimetic toxicity
• Severe infection or encephalopathy

A useful distinction is:

• NMS: lead-pipe rigidity, bradykinesia, elevated CK, slower onset
• Serotonin syndrome: clonus, hyperreflexia, GI symptoms, faster onset
• Anticholinergic toxicity: dry skin and mucous membranes rather than diaphoresis

Management

Treatment is mainly supportive:

• Immediately stop the offending dopamine-blocking drug
• Airway and cardiovascular support as needed
• IV fluids
• Active cooling for hyperthermia
• Benzodiazepines for agitation

Additional therapies sometimes used in severe cases include:

• Dantrolene to reduce muscle rigidity and heat production
• Bromocriptine or another dopamine agonist to restore dopaminergic activity

Close monitoring is needed for rhabdomyolysis, renal injury, electrolyte disturbances, and cardiac complications.

Key Points

• Think of fever + rigidity + autonomic instability + altered mental status.
• Lead-pipe rigidity strongly favors NMS over serotonin syndrome.
• CK elevation and rhabdomyolysis are common.
• Early recognition and withdrawal of the causative drug are critical.
• Symptoms often develop over days rather than within a few hours.

Toxicology – Serotonin Syndrome

Definition

Serotonin syndrome is a potentially serious condition caused by excessive serotonergic activity in the central nervous system. It can occur after increasing the dose of a serotonergic medication, taking an overdose, or combining multiple drugs that increase serotonin.

Common Causes

Drugs associated with serotonin syndrome include:

• SSRIs
• SNRIs
• MAO inhibitors
• Tricyclic antidepressants
• Triptans
• Lithium
• Certain opioids such as meperidine or tramadol
• Cocaine and other stimulants
• Cyclobenzaprine
• Dextromethorphan
• Linezolid
• St. John’s wort

Combinations involving MAO inhibitors plus other serotonergic agents are particularly dangerous.

Clinical Features

The syndrome is classically recognized by a combination of:

• Mental-status changes: agitation, confusion, anxiety, lethargy, or coma
• Autonomic hyperactivity: hyperthermia, sweating, tachycardia, hypertension, nausea, vomiting, and dilated pupils
• Neuromuscular hyperactivity: tremor, hyperreflexia, clonus, myoclonus, and increased muscle tone

Clonus and hyperreflexia, especially in the lower extremities, are particularly helpful diagnostic clues.

Symptoms usually develop rapidly, often within hours of a medication change or interacting drug exposure.

Mechanism of Action

Excess serotonin overstimulates central and peripheral serotonin receptors, particularly 5-HT₁A and 5-HT₂A receptors.

This produces the characteristic combination of altered mental status, autonomic instability, and neuromuscular excitation.

Management

Treatment includes:

• Immediately stopping all serotonergic medications
• Airway and cardiovascular support when required
• IV fluids
• Benzodiazepines for agitation, tremor, and excessive sympathetic activity
• Active external cooling for significant hyperthermia

Severe cases with extreme hyperthermia may require sedation, paralysis, and mechanical ventilation.

Cyproheptadine, a serotonin receptor antagonist, may be considered when significant symptoms persist despite supportive treatment.

Differential Diagnosis

Important conditions that can resemble serotonin syndrome include:

• Neuroleptic malignant syndrome
• Anticholinergic toxicity
• Malignant hyperthermia
• Sympathomimetic poisoning

Compared with neuroleptic malignant syndrome, serotonin syndrome generally develops more rapidly and is more strongly associated with clonus and hyperreflexia.

Key Points

• Think of the triad: mental-status changes + autonomic instability + neuromuscular hyperactivity.
• Clonus is one of the most useful bedside findings.
• Symptoms often appear within hours rather than days.
• Stop serotonergic drugs promptly and provide supportive care.
• Severe hyperthermia is caused largely by excessive muscle activity and requires aggressive supportive management.

Toxicology – Methemoglobinemia

Definition

Methemoglobinemia occurs when an excessive amount of hemoglobin is oxidized into methemoglobin, a form that cannot effectively carry and release oxygen to tissues. This produces functional tissue hypoxia despite adequate oxygen being present in the lungs.

Common Causes

Oxidizing drugs and chemicals associated with methemoglobinemia include:

• Benzocaine and prilocaine
• Phenazopyridine
• Dapsone
• Nitrates and nitrites
• Nitroglycerin
• Nitroprusside
• Nitric oxide
• Aniline compounds
• Sulfonamides
• Naphthalene
• Certain other industrial or pharmaceutical oxidizing agents

Mechanism of Action

Oxidizing substances convert the iron in hemoglobin from the normal ferrous state (Fe²⁺) to the ferric state (Fe³⁺).

Ferric hemoglobin cannot bind oxygen normally. In addition, the remaining normal hemoglobin holds onto oxygen more tightly, further reducing oxygen delivery to tissues.

Clinical Features

Early manifestations may include:

• Cyanosis
• Headache
• Dizziness
• Fatigue
• Nausea
• Mild shortness of breath

More severe poisoning can cause:

• Increasing dyspnea
• Confusion
• Altered mental status
• Tachycardia
• Seizures
• Coma
• Cardiovascular collapse

Blood may have a characteristic dark or “chocolate-brown” appearance.

Diagnosis

A characteristic finding is a saturation gap, in which oxygen measurements do not fit the patient’s clinical appearance.

Standard pulse oximetry becomes unreliable and often trends toward readings around the mid-80% range despite supplemental oxygen.

The diagnosis is confirmed by co-oximetry, which directly measures the percentage of methemoglobin. A routine arterial blood gas may show a normal or relatively preserved PaO₂ because it measures dissolved oxygen rather than hemoglobin’s ability to carry it.

Management

Treatment includes:

• Immediately stopping the responsible oxidizing agent
• Supplemental oxygen
• Supportive care and cardiovascular monitoring

Mild, asymptomatic cases may improve spontaneously as normal red-cell reducing systems convert methemoglobin back to functional hemoglobin.

Methylene blue is the principal antidote for clinically significant methemoglobinemia because it accelerates reduction of methemoglobin back to normal hemoglobin.

Special Consideration – G6PD Deficiency

Methylene blue requires NADPH for its action. In patients with G6PD deficiency, it may be less effective and can provoke hemolysis, so treatment requires particular caution and specialist guidance.

Key Points

• Methemoglobinemia results from oxidation of hemoglobin iron from Fe²⁺ to Fe³⁺.
• Cyanosis that does not improve as expected with oxygen should raise suspicion.
• Co-oximetry, rather than a routine pulse oximeter or PaO₂ alone, confirms the diagnosis.
• Methylene blue is the standard antidote for significant symptomatic toxicity.
• G6PD deficiency is important because it can complicate methylene blue treatment.

Toxicology – Opioid Withdrawal

Definition

Opioid withdrawal is a hyperadrenergic syndrome that occurs after abrupt reduction or cessation of opioids in someone who has developed physical dependence.

Symptoms usually begin sooner with short-acting opioids and later with long-acting agents such as methadone.

Typical Presentation

Common findings include:

• Yawning
• Tearing
• Runny nose
• Goose bumps
• Dilated pupils
• Sweating
• Restlessness
• Muscle aches or cramps
• Nausea and vomiting
• Diarrhea
• Tachycardia
• Hypertension

Patients often feel extremely uncomfortable and may be unable to sit still.

Severity

Unlike alcohol or benzodiazepine withdrawal, opioid withdrawal is usually not directly life-threatening, but it can still cause significant dehydration, electrolyte abnormalities, and distress.

The duration depends on the opioid involved:

• Short-acting opioids generally produce a shorter withdrawal syndrome
• Long-acting opioids can cause more prolonged symptoms

Mechanism of Action

Chronic opioid exposure causes neuroadaptation within the central nervous system.

When opioids are suddenly removed, inhibitory opioid signaling disappears and there is a rebound increase in sympathetic and noradrenergic activity, producing the characteristic withdrawal symptoms.

Management

Treatment may include:

• Buprenorphine or methadone to control withdrawal and support ongoing treatment
• Clonidine or lofexidine to reduce autonomic symptoms
• Fluids and electrolyte replacement when needed
• Antiemetics for nausea and vomiting
• Antidiarrheal therapy
• Non-opioid analgesics for muscle aches

Long-term treatment for opioid use disorder commonly includes buprenorphine or methadone, with naltrexone used in selected patients after opioid abstinence has been established.

Key Points

• Opioid withdrawal causes mydriasis, sweating, piloerection, GI upset, muscle aches, and autonomic hyperactivity.
• It is generally very uncomfortable but less medically dangerous than alcohol or benzodiazepine withdrawal.
• Buprenorphine and methadone can both treat withdrawal and support longer-term recovery.
• Symptomatic treatment can reduce autonomic and gastrointestinal symptoms.

Toxicology – Alcohol Withdrawal

Definition

Alcohol withdrawal is a hyperadrenergic syndrome that develops after abrupt reduction or cessation of alcohol in someone with chronic heavy use. Symptoms can begin within several hours and may progress over the next few days.

Typical Presentation

Early withdrawal commonly causes:

• Anxiety
• Tremor
• Sweating
• Tachycardia
• Hypertension
• Agitation

Some patients may develop symptoms overnight and notice tremor or autonomic symptoms on waking.

Neurologic Features

As withdrawal progresses, patients may develop:

• Hallucinations – visual, auditory, or tactile
• Seizures
• Delirium tremens (DTs)

Alcohol-withdrawal hallucinations can occur while cognition is still relatively preserved.

Delirium Tremens

DTs represent severe withdrawal and are characterized by:

• Marked autonomic hyperactivity
• Confusion or delirium
• Agitation
• Hallucinations
• Severe tremor
• Tachycardia and hypertension

This is a medical emergency and requires close monitoring.

Mechanism of Action

Chronic alcohol exposure increases inhibitory GABAergic tone and suppresses excitatory NMDA/glutamate signaling.

Over time, the brain compensates by:

• Downregulating GABA activity
• Upregulating NMDA activity

When alcohol is suddenly removed, the balance shifts toward excessive CNS excitation, producing autonomic overactivity, hallucinations, and seizures.

Management

Treatment is aimed at controlling CNS hyperactivity and preventing complications.

Main therapies include:

• Benzodiazepines as first-line treatment
• Fluids and correction of electrolyte abnormalities when needed
• Thiamine supplementation in patients at risk of deficiency
• Close monitoring for seizures or delirium

Severe or refractory withdrawal may require:

• Phenobarbital
• ICU-level care
• Airway support if needed
• Additional sedative agents under specialist supervision

Key Points

• Alcohol withdrawal reflects excessive CNS excitation after removal of chronic alcohol exposure.
• Early symptoms are mainly autonomic: tremor, sweating, tachycardia, and hypertension.
• Hallucinations can occur before delirium.
• Seizures and DTs indicate more severe withdrawal.
• Benzodiazepines are the mainstay of treatment.

Toxicology – When Activated Charcoal Is Helpful

Overview

Activated charcoal can reduce gastrointestinal absorption of many ingested toxins when given early, particularly when the substance is known to bind well to charcoal.

Its usefulness depends on:

• The toxin involved
• Time since ingestion
• Amount ingested
• Airway protection
• Gastrointestinal function

Substances Commonly Adsorbed by Activated Charcoal

Examples include:

• Acetaminophen
• Aspirin and other salicylates
• Amphetamines
• Atropine
• Barbiturates
• Carbamazepine
• Colchicine
• Digoxin
• Phenytoin
• Quinine
• Theophylline
• Tricyclic antidepressants
• Valproic acid
• Many other medications and organic toxins

Single-Dose Activated Charcoal

A single dose may be considered when:

• A potentially toxic substance has been ingested
• The substance is known to bind charcoal
• Presentation is early enough that meaningful drug remains in the GI tract
• The patient has a protected airway and no major contraindication

Multidose Activated Charcoal (MDAC)

MDAC involves repeated doses of activated charcoal and can enhance toxin elimination in selected poisonings.

It works by:

• Reducing continued gastrointestinal absorption
• Interrupting enterohepatic or enteroenteric recirculation
• Creating a concentration gradient that promotes movement of some drugs from blood back into the gut

Classic Drugs for MDAC

A common group to remember includes:

• Carbamazepine
• Dapsone
• Phenobarbital
• Quinine
• Theophylline

Aminophylline is also relevant because it is metabolized to theophylline.

Key Points

• Activated charcoal is most useful for toxins that are well adsorbed and still present in the GI tract.
• MDAC is reserved for a relatively small group of drugs, especially carbamazepine, dapsone, phenobarbital, quinine, and theophylline.
• Airway safety remains essential before charcoal administration.
• Charcoal should not be used automatically for every overdose; toxin-specific benefit and contraindications should always be considered.

Toxicology – Mnemonic: When Activated Charcoal Is Not Helpful

Overview

Activated charcoal can reduce gastrointestinal absorption of many toxins when given early after ingestion, but it is not effective for every substance and may be unsafe in certain situations.

Charcoal generally provides little or no benefit for substances that do not bind well to it, including:

• Caustic acids or alkalis
• Heavy metals
• Alcohols
• Hydrocarbons
• Iron
• Lithium
• Some rapidly absorbed or poorly adsorbed toxins

It should also be avoided when there is a major aspiration risk or intestinal obstruction.

Mnemonic – “CHARCOAL”

• C – Caustics / Corrosives

    Charcoal does not meaningfully adsorb strong acids or alkalis and may complicate evaluation of caustic injury.

• H – Heavy Metals

    Metals such as iron and lead are poorly bound by activated charcoal.

• A – Alcohols

    Toxic alcohols and ethanol are not effectively adsorbed.

• R – Rapidly Absorbed Toxins

    Charcoal is less useful once a toxin has already been absorbed.

• C – Cyanide

    Charcoal has limited practical usefulness in severe cyanide poisoning, where rapid antidotal and supportive treatment is the priority.

• O – Other Insoluble / Poorly Adsorbed Substances

    Some compounds simply do not bind charcoal effectively.

• A – Aliphatic Hydrocarbons

    Examples include fuels and similar hydrocarbons; aspiration risk is often more concerning than gastrointestinal absorption.

• L – Laxatives

    Additional cathartic therapy is generally not beneficial and can increase complications.

Contraindications / Situations to Avoid Charcoal

Activated charcoal should generally not be given when:

• The airway is unprotected and aspiration risk is high
• There is bowel obstruction or severe ileus
• Caustic ingestion is suspected
• Hydrocarbon aspiration risk outweighs potential benefit

Key Points

• Activated charcoal is selective, not universal.
• The mnemonic CHARCOAL helps recall substances for which it is usually ineffective or inappropriate.
• Timing matters because charcoal works best before significant absorption has occurred.
• Airway safety is more important than giving charcoal.

Toxicology – Toxins and Conditions Treatable With Dialysis

Overview

Hemodialysis can enhance elimination of certain toxins, especially substances that are relatively small, water-soluble, have a low volume of distribution, and are not highly protein bound. It is also useful when poisoning causes severe acid-base abnormalities, kidney failure, or life-threatening clinical deterioration.

A classic memory aid for major dialysis indications is “AEIOU”, while several toxicologic agents are especially important to recognize.

Isopropanol

Hemodialysis can remove isopropanol, but it is rarely required.

It may be considered in exceptionally severe poisoning with:

• Profound hypotension
• Severe CNS depression
• Markedly elevated concentrations with clinical instability

Salicylates

Hemodialysis is an important treatment for severe salicylate poisoning.

It should be considered when there is:

• Severe neurologic toxicity
• Pulmonary edema
• Significant acid-base disturbance
• Kidney dysfunction
• Progressive clinical deterioration
• Very high serum salicylate concentrations

Clinical status is generally more important than relying on a single concentration cutoff.

Theophylline / Methylxanthines

Extracorporeal removal can be useful in severe theophylline toxicity, particularly when there is:

• Refractory hypotension
• Serious ventricular dysrhythmias
• Persistent or recurrent seizures
• Severe toxicity with markedly elevated serum levels

Uremia

Uremia results from accumulation of nitrogenous waste products in advanced kidney dysfunction.

Possible manifestations include:

• Nausea and vomiting
• Weakness
• Confusion or altered mental status
• Neuropathy
• Muscle cramps
• Seizures
• Pericarditis or pericardial effusion

Dialysis removes accumulated toxins and helps correct associated fluid, electrolyte, and acid-base abnormalities.

Methanol

Hemodialysis removes both methanol and formate, its major toxic metabolite.

It is especially important when methanol poisoning causes:

• Significant metabolic acidosis
• Visual abnormalities
• Kidney dysfunction
• Severe clinical deterioration
• High methanol concentrations

Dialysis is usually combined with inhibition of alcohol dehydrogenase using fomepizole.

Barbiturates

Dialysis may be considered in selected cases of severe barbiturate poisoning, particularly with long-acting agents such as phenobarbital and severe persistent toxicity.

Hemodialysis is generally more effective than peritoneal dialysis for toxin removal.

Lithium

Lithium is highly dialyzable and hemodialysis may be required in severe poisoning.

Important indications include:

• Significant neurologic toxicity
• Seizures or coma
• Kidney failure
• Serious cardiovascular instability
• Persistently high or rising lithium concentrations

Repeat dialysis may sometimes be necessary because lithium can redistribute from tissues back into the bloodstream after treatment.

Ethylene Glycol

Hemodialysis removes ethylene glycol and its toxic metabolites.

It may be needed when there is:

• Severe metabolic acidosis
• Acute kidney injury
• Major electrolyte abnormalities
• Severe clinical toxicity
• High ethylene glycol concentrations

Treatment is usually combined with fomepizole to prevent continued formation of toxic metabolites.

Key Points

• Dialysis decisions should be based on the patient’s clinical condition, laboratory abnormalities, toxin characteristics, and serum concentration, rather than a single number alone.
• Important dialyzable toxins include salicylates, lithium, methanol, ethylene glycol, and theophylline.
• Hemodialysis is usually preferred over peritoneal dialysis when rapid toxin removal is required.
• Severe neurologic toxicity, refractory shock, major acidosis, or kidney failure are common reasons to consider extracorporeal treatment.

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125. Toxicology – Antidote: Hyperbaric Oxygen Therapy (HBOT)

Indications

Hyperbaric oxygen therapy may be considered for selected severe poisonings involving asphyxiant toxins, particularly carbon monoxide poisoning. It has also been discussed as an adjunct in severe hydrogen sulfide or cyanide toxicity, although evidence is much less established for these exposures.

Asphyxiant Toxins

Asphyxiants interfere with normal aerobic metabolism and oxygen utilization.

They can be divided broadly into:

• Simple asphyxiants: Reduce the amount of available oxygen in the surrounding environment.
• Systemic asphyxiants: Interfere with oxygen transport or prevent cells from using oxygen normally.

Examples include:

• Carbon monoxide: Forms carboxyhemoglobin and interferes with oxygen delivery
• Cyanide: Inhibits mitochondrial cytochrome oxidase
• Methemoglobin-forming agents: Oxidize hemoglobin so it cannot effectively carry oxygen
• Hydrogen sulfide: Can inhibit mitochondrial respiration in a manner similar to cyanide

Mechanism of Action

HBOT exposes the patient to 100% oxygen at pressures greater than normal atmospheric pressure, usually in a specialized hyperbaric chamber.

This greatly increases the amount of oxygen dissolved directly in the plasma, allowing oxygen delivery to tissues even when hemoglobin-dependent transport is impaired.

In carbon monoxide poisoning, HBOT also:

• Accelerates removal of carbon monoxide from hemoglobin
• Improves tissue oxygenation
• May reduce ongoing neurologic injury in selected severe cases

Clinical Use

For carbon monoxide poisoning, HBOT may be considered when significant features are present, such as:

• Loss of consciousness
• Persistent neurologic abnormalities
• Myocardial ischemia or serious cardiac involvement
• Severe metabolic acidosis
• Pregnancy with significant exposure
• Other evidence of severe poisoning

Exact indications vary, so consultation with a hyperbaric medicine specialist, poison center, or medical toxicologist is recommended.

Risks and Contraindications

Potential complications include:

• Ear or sinus barotrauma
• Oxygen toxicity
• Pulmonary barotrauma
• Claustrophobia

An untreated pneumothorax is the major absolute contraindication because increased chamber pressure can worsen trapped pleural air.

Key Points

• HBOT increases the amount of dissolved oxygen in plasma.
• Its best-established toxicologic role is in selected cases of severe carbon monoxide poisoning.
• Its role in cyanide and hydrogen sulfide poisoning is much less certain and should not delay established antidotal and supportive treatment.
• Hyperbaric treatment requires specialist consultation and access to an appropriate facility.

Toxicology – Antidotes: Atropine & Pralidoxime (2-PAM)

Indications

Atropine and pralidoxime are used for cholinergic poisoning, especially from:

• Organophosphate insecticides
• Nerve agents
• Selected carbamate insecticides

These poisonings result from excessive acetylcholine activity caused by inhibition of acetylcholinesterase.

Mechanism of Action – Atropine

Atropine is a competitive muscarinic acetylcholine receptor antagonist.

It blocks the effects of excess acetylcholine at muscarinic receptors and is especially important for treating:

• Excess bronchial secretions
• Bronchospasm
• Bradycardia
• Other muscarinic manifestations

Atropine does not reverse the underlying acetylcholinesterase inhibition and has limited effect on skeletal muscle weakness.

Mechanism of Action – Pralidoxime (2-PAM)

Organophosphates bind to and inhibit acetylcholinesterase, causing acetylcholine to accumulate.

Pralidoxime can reactivate the inhibited enzyme by removing the organophosphate from acetylcholinesterase, provided this is given before the enzyme-toxin complex undergoes “aging.”

Aging refers to a chemical change that makes the organophosphate–acetylcholinesterase bond effectively irreversible.

Pralidoxime is particularly helpful for nicotinic and neuromuscular effects, including:

• Muscle weakness
• Fasciculations
• Respiratory muscle paralysis

Management

Treatment priorities include:

• Airway and respiratory support
• Rapid atropine administration for significant secretions and respiratory compromise
• Pralidoxime for suspected organophosphate toxicity
• Benzodiazepines for seizures, marked agitation, or severe muscle activity
• Appropriate decontamination while protecting healthcare personnel from secondary exposure

Atropine may need to be repeatedly administered and titrated to improvement in airway secretions and ventilation, rather than to a fixed total dose.

Carbamate Poisoning

Carbamates generally inhibit acetylcholinesterase reversibly and for a shorter duration than organophosphates.

• Atropine remains important
• The benefit of pralidoxime is less certain, but it may be considered in severe or unclear cholinergic poisoning when organophosphate exposure cannot be excluded

Key Points

• Atropine treats muscarinic symptoms, especially excessive secretions and bronchospasm.
• Pralidoxime reactivates acetylcholinesterase before aging occurs.
• Early pralidoxime is most important in significant organophosphate poisoning.
• Severe cases may require unusually large cumulative amounts of atropine.
• Benzodiazepines are used when seizures or severe CNS manifestations occur.