Toxicology – Antidotes for Toxic Alcohols: Fomepizole, Ethanol & Hemodialysis

Metabolic Pathways

Methanol and ethylene glycol become dangerous mainly after they are metabolized by alcohol dehydrogenase (ADH) into toxic metabolites.

• Methanol → formaldehyde → formic acid
• Ethylene glycol → glycolaldehyde → glycolic acid → glyoxylic acid → oxalic acid
• Ethanol → acetaldehyde → acetic acid

Ethanol

Ethanol has a higher affinity for ADH than methanol or ethylene glycol. By occupying the enzyme, it slows formation of their toxic metabolites and allows more of the parent alcohol to be eliminated unchanged.

Disadvantages include:

• CNS depression
• Nausea and vomiting
• Hypoglycemia
• Need for frequent serum concentration monitoring

Because of these limitations, ethanol is generally less convenient than fomepizole.

Fomepizole

Fomepizole directly inhibits alcohol dehydrogenase, preventing methanol and ethylene glycol from being converted into their harmful metabolites.

Advantages include:

• Predictable dosing
• Less CNS depression than ethanol
• No need to maintain an intoxicating serum alcohol concentration
• Can be used together with hemodialysis when necessary

Possible adverse effects include:

• Headache
• Nausea
• Dizziness
• Drowsiness
• Altered taste
• IV-site irritation

Hemodialysis

Hemodialysis can rapidly remove both the parent toxic alcohol and important toxic metabolites.

It may be needed in severe poisoning associated with:

• Significant metabolic acidosis
• Kidney failure
• Severe clinical deterioration
• High toxic-alcohol concentrations
• Visual symptoms in methanol poisoning

In ethylene glycol poisoning, dialysis is particularly useful when kidney injury or severe acidosis develops.

In methanol poisoning, dialysis removes both methanol and formate, the metabolite responsible for much of its ocular and systemic toxicity.

Additional Supportive Therapy

For ethylene glycol toxicity:

• Thiamine and pyridoxine may help direct metabolism toward less toxic products

For methanol toxicity:

• Folate or folinic acid may enhance metabolism of formate into less harmful compounds

Key Points

• Fomepizole is the preferred ADH inhibitor in most toxic alcohol poisonings.
• Ethanol works by competing for alcohol dehydrogenase but requires close monitoring and causes more adverse effects.
• Hemodialysis is used for severe methanol or ethylene glycol poisoning, especially with major acidosis or organ dysfunction.
• The major goal of antidotal therapy is to prevent formation of toxic metabolites, not simply to remove the parent alcohol.

Toxicology – Antidotes for Cyanide Poisoning

Indications

Cyanide poisoning should be suspected in patients with severe smoke inhalation or occupational exposure who develop findings such as:

• Altered mental status
• Dyspnea
• Seizures
• Coma
• Cardiovascular collapse
• Severe tissue hypoxia and lactic acidosis

Because confirmatory testing is often too slow to guide emergency care, treatment may need to begin based on the clinical picture.

Hydroxocobalamin

Hydroxocobalamin is generally the preferred antidote, especially in smoke-inhalation cases.

Mechanism

• Contains a cobalt atom that binds cyanide
• Forms cyanocobalamin (vitamin B12)
• The resulting compound is then eliminated by the kidneys

Advantages

• Does not induce methemoglobinemia
• Can be used when concurrent carbon monoxide poisoning is suspected
• Does not typically cause significant hypotension

Adverse Effects

• Red discoloration of the skin
• Red urine (chromaturia)
• Headache
• Nausea or vomiting
• Itching

Sodium Thiosulfate

Sodium thiosulfate provides sulfur that the enzyme rhodanese uses to convert cyanide into the much less toxic compound thiocyanate, which is eliminated through the kidneys.

It can be used in smoke-inhalation patients because it does not create methemoglobinemia.

Possible adverse effects include:

• Nausea
• Vomiting
• Hypotension

Nitrites – Amyl Nitrite and Sodium Nitrite

Nitrites convert normal hemoglobin into methemoglobin. Cyanide preferentially binds to methemoglobin rather than mitochondrial cytochrome oxidase, helping remove cyanide from its cellular target.

However, methemoglobin cannot effectively carry oxygen. For this reason, nitrites are generally avoided or used very cautiously when carbon monoxide poisoning or significant smoke inhalation is also present.

Rapid IV administration of sodium nitrite may also cause:

• Marked vasodilation
• Hypotension
• Tachycardia

Mechanism Summary

Cyanide antidotes work mainly by:

• Binding cyanide directly: hydroxocobalamin
• Enhancing detoxification to thiocyanate: sodium thiosulfate
• Creating an alternative cyanide-binding target: nitrites

Key Points

• Hydroxocobalamin is particularly useful in smoke-inhalation cyanide poisoning.
• Sodium thiosulfate assists the body’s natural cyanide-detoxifying pathway.
• Nitrites induce methemoglobinemia and may worsen oxygen delivery when carbon monoxide poisoning is also present.
• Cyanide treatment should not be delayed in a critically ill patient when the clinical suspicion is high.

Toxicology – Antidote: N-Acetylcysteine (NAC)

Indications

N-acetylcysteine is the primary antidote for acetaminophen (APAP) poisoning and can be given orally or intravenously. It may also be considered in some other toxicologic conditions involving oxidative stress or glutathione depletion.

For a known single acute acetaminophen ingestion, obtain a serum level at least 4 hours after ingestion and interpret it using the Rumack–Matthew nomogram. NAC is indicated when the level falls above the treatment threshold.

Treatment should also be strongly considered when:

• The time of ingestion is unknown
• The acetaminophen level is detectable with uncertain timing
• Liver enzymes, particularly AST/ALT, are elevated in a patient with possible acetaminophen toxicity

If the acetaminophen concentration is undetectable and liver enzymes are normal, clinically important toxicity is less likely.

Mechanism of Action

NAC helps protect the liver through several mechanisms:

• Replenishes glutathione, which detoxifies the reactive acetaminophen metabolite NAPQI
• Can act as a glutathione substitute and antioxidant
• Enhances sulfation pathways involved in acetaminophen metabolism
• After liver injury has already occurred, it may improve tissue oxygen delivery and reduce oxidative and inflammatory damage

Administration

Both oral and IV NAC are effective. IV therapy is often preferred when oral treatment cannot be tolerated or in patients with severe hepatic injury.

Common treatment protocols include:

• Oral NAC: loading dose followed by repeated maintenance doses over an extended course
• IV NAC: typically administered as a multi-stage infusion over approximately 21 hours

Treatment may need to continue beyond the standard protocol if acetaminophen remains detectable or liver injury is still progressing.

Monitoring

During treatment, follow:

• Acetaminophen concentration
• AST and ALT
• INR/coagulation studies
• Renal function
• Clinical signs of hepatic failure

NAC is generally continued until acetaminophen is no longer detectable and hepatic injury is clearly improving.

Adverse Effects

IV NAC can cause anaphylactoid reactions, including flushing, rash, wheezing, or hypotension. These are usually managed by temporarily slowing or stopping the infusion and providing symptomatic treatment before restarting when appropriate.

Key Points

• NAC is most effective when started early, but it can still provide benefit even after liver injury has developed.
• Do not delay treatment when significant acetaminophen poisoning is strongly suspected.
• The Rumack–Matthew nomogram applies only to a known, single, acute ingestion with a known time.
• Continued NAC may be necessary when liver enzymes remain markedly abnormal or acetaminophen is still detectable.

Toxicology – Hydrogen Sulfide Poisoning

Source

Hydrogen sulfide is a toxic gas produced during the breakdown of organic material. It may accumulate in sewers, manure pits, petroleum facilities, paper mills, and other poorly ventilated confined spaces. Because it is heavier than air, it can collect in low-lying areas.

Typical Presentation

Workers in a confined space may suddenly collapse after exposure to hydrogen sulfide. Rescuers who enter without proper respiratory protection can also become rapidly incapacitated.

Clinical Features

Lower-level exposure mainly causes mucous membrane and respiratory irritation, including:

• Runny nose
• Tearing
• Red or irritated eyes
• Headache
• Nausea and vomiting
• Dizziness
• Confusion

More severe exposure can produce:

• Sudden loss of consciousness
• Respiratory failure
• Cardiac dysrhythmias
• Coma
• Death
• Delayed pulmonary edema

Mechanism of Action

Hydrogen sulfide inhibits cytochrome oxidase, disrupting cellular oxygen utilization in a manner similar to cyanide. It also directly irritates the eyes and respiratory tract. At high concentrations, it can cause extremely rapid CNS depression and collapse.

Although hydrogen sulfide has a characteristic rotten-egg odor, prolonged or intense exposure can impair the sense of smell, making odor an unreliable warning sign.

Management

Treatment is mainly supportive:

• Immediate removal from the contaminated environment by appropriately protected rescuers
• Airway and respiratory support
• High-concentration oxygen
• Continuous cardiac monitoring
• Treatment of dysrhythmias and other complications as needed

Specialized therapies such as hyperbaric oxygen or hydroxocobalamin have been considered in severe cases, but supportive resuscitation remains the mainstay of treatment.

Key Points

• Hydrogen sulfide is especially dangerous in confined spaces.
• High concentrations can cause sudden collapse with little warning.
• The rotten-egg smell cannot be relied upon because olfactory fatigue may occur.
• Delayed pulmonary complications can develop even after the initial exposure.

Toxicology – Carbon Monoxide (CO) Poisoning

Source

Carbon monoxide is a colorless, odorless, nonirritating gas produced by incomplete combustion. Common sources include house fires, furnaces, heaters, grills, gasoline-powered generators, and motor vehicle exhaust. Methylene chloride exposure can also produce carbon monoxide after metabolism in the body.

Typical Presentation

Several people from the same household may develop headache, dizziness, nausea, and vomiting after using a fuel-burning appliance or generator in an enclosed area. Because symptoms are nonspecific, carbon monoxide poisoning can easily be mistaken for a viral illness.

Clinical Features

Common manifestations include:

• Headache
• Nausea and vomiting
• Dizziness
• Fatigue
• Confusion
• Shortness of breath
• Chest pain
• Syncope or loss of consciousness

Severe poisoning can cause neurological injury, myocardial ischemia, arrhythmias, coma, or death.

A carboxyhemoglobin (COHb) level can help confirm exposure. Pulse oximetry may appear falsely normal because standard devices cannot reliably distinguish oxyhemoglobin from carboxyhemoglobin.

Mechanism of Action

Carbon monoxide binds to hemoglobin with much greater affinity than oxygen, forming carboxyhemoglobin. This reduces oxygen-carrying capacity and also impairs release of oxygen to tissues, resulting in cellular hypoxia.

Management

• Immediately remove the patient from the source of exposure.
• Administer high-concentration oxygen.
• Monitor neurological status, ECG, and cardiac biomarkers when appropriate.
• Hyperbaric oxygen therapy may be considered in severe poisoning, especially with loss of consciousness, persistent neurological abnormalities, significant cardiac involvement, severe acidosis, or pregnancy.
• Evaluate fire victims for possible simultaneous cyanide toxicity when severe lactic acidosis or cardiovascular collapse is present.

Key Points

• Headache is the most common symptom of CO poisoning.
• Multiple people with similar symptoms in the same enclosed environment should strongly raise suspicion.
• Standard pulse oximetry can be misleadingly normal.
• Cherry-red skin is uncommon and should not be relied upon for diagnosis.
• Patients with severe smoke exposure may have both carbon monoxide and cyanide poisoning.

Toxicology – Cyanide Poisoning

Source

Cyanide exposure can occur during smoke inhalation from structural fires, especially when materials such as wool, silk, plastics, insulation, or rubber burn. It may also be encountered in certain industrial settings, some foods or plant products, and rarely from prolonged nitroprusside therapy.

Typical Presentation

A patient exposed to heavy smoke or an occupational cyanide source may rapidly develop confusion, shortness of breath, chest discomfort, seizures, or cardiovascular collapse. Cyanide toxicity should be considered in severe smoke inhalation with unexplained lactic acidosis or sudden hemodynamic deterioration.

Clinical Features

Possible findings include:

• Headache
• Nausea and vomiting
• Altered mental status
• Dilated pupils
• Rapid breathing
• Tachycardia
• Early hypertension
• Later hypotension and respiratory depression
• Chest pain
• Seizures
• Cardiovascular collapse

Laboratory abnormalities may show:

• Severe lactic acidosis
• High anion gap metabolic acidosis
• Unusually high venous oxygen content because tissues cannot effectively extract oxygen

Mechanism of Action

Cyanide inhibits cytochrome c oxidase in the mitochondrial electron transport chain. This blocks aerobic cellular respiration, preventing tissues from using oxygen despite adequate oxygen delivery and causing rapid anaerobic metabolism and lactate accumulation.

Management

Treatment requires immediate supportive care:

• High-flow oxygen and airway support
• Cardiac and hemodynamic monitoring
• Aggressive management of seizures and shock
• Hydroxocobalamin is a preferred antidotal therapy because it binds cyanide to form a less toxic compound that can be eliminated
• Sodium thiosulfate may also be used to enhance conversion of cyanide to thiocyanate

Nitrite-based antidotes induce methemoglobinemia and are generally less desirable when carbon monoxide exposure is also suspected, as commonly occurs in fire victims.

Key Points

• Think of cyanide toxicity in patients with severe smoke inhalation plus cardiovascular collapse or marked lactic acidosis.
• Cyanide prevents cells from using oxygen rather than preventing oxygen from reaching the blood.
• Hydroxocobalamin is a major antidote used in suspected cyanide poisoning.
• A markedly elevated lactate level can serve as an important indirect clue.

Toxicology – Antipsychotic Toxicity

Source

Antipsychotic medications are used to treat conditions such as schizophrenia, psychosis, severe agitation, and bipolar disorder. They include first-generation agents such as haloperidol and second-generation agents such as risperidone, olanzapine, quetiapine, clozapine, and aripiprazole.

Typical Presentation

A patient with excessive antipsychotic exposure may present with drowsiness, confusion, tachycardia, and hypotension. Severe toxicity can progress to respiratory depression, seizures, coma, or cardiac conduction abnormalities.

Clinical Features

Possible findings include:

• Sedation or agitation
• Ataxia
• Confusion
• Seizures
• Coma
• Respiratory depression
• Orthostatic hypotension

Some agents may also produce anticholinergic findings, including:

• Dilated pupils
• Dry skin and mucous membranes
• Tachycardia
• Urinary retention
• Reduced bowel sounds
• Agitation

ECG abnormalities may include:

• QRS widening
• QT prolongation
• Cardiac dysrhythmias in severe cases

Mechanism of Action

Most antipsychotics block dopamine D2 receptors to varying degrees. First-generation drugs generally produce stronger D2 blockade and therefore more extrapyramidal adverse effects. Many second-generation agents also affect serotonin receptors. Additional blockade of α1-adrenergic, histamine, and muscarinic receptors contributes to hypotension, sedation, and anticholinergic effects.

Management

Treatment is primarily supportive:

• Airway and respiratory support when necessary
• Continuous ECG and cardiac monitoring
• IV fluids for hypotension
• Vasopressors may be required for persistent hypotension
• Treat significant QRS widening or QT-related dysrhythmias according to toxicology and resuscitation guidance
• Benzodiazepines may be used for seizures or severe agitation

Key Points

• Antipsychotics can cause extrapyramidal symptoms, dystonia, neuroleptic malignant syndrome, anticholinergic effects, and orthostatic hypotension.
• Clozapine is associated with agranulocytosis.
• Seizures are uncommon overall but are more strongly associated with clozapine.
• Cardiac monitoring is important because some agents can prolong the QT interval or impair conduction.

Toxicology – Phenytoin & Fosphenytoin Toxicity

Source

Phenytoin is an anticonvulsant used for seizure control. Fosphenytoin is a water-soluble prodrug of phenytoin that is commonly used intravenously because it is better tolerated.

Typical Presentation

A patient taking phenytoin may present with unsteady gait, slurred speech, lethargy, and altered mental status. Neurological findings usually become more pronounced as the serum concentration rises.

Clinical Features

Acute toxicity is dominated by neurological symptoms, including:

• Nystagmus
• Nausea and vomiting
• Ataxia and poor coordination
• Slurred speech
• Lethargy
• Extrapyramidal movements
• Altered mental status
• Coma in severe cases

IV phenytoin can also cause:

• Hypotension
• Ventricular dysrhythmias
• Cardiovascular collapse

Chronic therapy may be associated with:

• Gingival hyperplasia
• Coarsening of facial features
• Chronic ataxia
• Liver injury

Mechanism of Action

Phenytoin inhibits voltage-gated sodium channels in neurons, reducing repetitive neuronal firing. Excessive concentrations produce predominantly cerebellar and CNS dysfunction.

Management

Treatment is mainly supportive:

• Airway and respiratory support when necessary
• Neurological monitoring
• Cardiac monitoring after IV phenytoin toxicity
• Activated charcoal may be considered in appropriate recent oral exposures
• Manage hypotension, dysrhythmias, or other complications supportively

There is no specific antidote for phenytoin toxicity.

Key Points

• Nystagmus, ataxia, and slurred speech are classic findings of phenytoin toxicity.
• Neurological toxicity generally worsens as drug levels rise.
• IV phenytoin formulations can cause significant cardiovascular toxicity.
• Fosphenytoin is generally safer for IV administration because it does not contain the propylene glycol vehicle used in traditional IV phenytoin.
• IV phenytoin may cause severe local tissue injury, sometimes referred to as purple glove syndrome.

Toxicology – Carbamazepine Toxicity

Source

Carbamazepine is an anticonvulsant used for seizure disorders, trigeminal neuralgia, neuropathic pain, and bipolar disorder.

Typical Presentation

A patient, often after accidental or intentional ingestion, may develop marked drowsiness, unsteady gait, slurred speech, and altered mental status. Severe poisoning can progress to seizures, respiratory depression, coma, and cardiac conduction abnormalities.

Clinical Features

Possible findings include:

• Sedation and confusion
• Ataxia
• Dysarthria
• Nystagmus
• Anticholinergic features
• Respiratory depression
• Seizures
• Coma

ECG abnormalities may include:

• QRS widening
• QT prolongation
• Cardiac dysrhythmias in severe toxicity

Mechanism of Action

Carbamazepine blocks voltage-gated sodium channels, accounting for both its therapeutic anticonvulsant activity and many of its toxic effects. It is structurally related to tricyclic antidepressants and also has anticholinergic and sedating properties.

Management

Treatment is primarily supportive:

• Airway and respiratory support when needed
• Continuous cardiac monitoring and serial ECGs
• Benzodiazepines for seizures
• Sodium bicarbonate for clinically significant sodium-channel blockade with QRS widening
• Serial carbamazepine concentrations may be useful because absorption can be delayed and levels may continue to rise
• Multiple-dose activated charcoal may enhance elimination in selected significant poisonings
• Hemodialysis or hemoperfusion may be considered in severe or refractory toxicity with specialist guidance

Key Points

• Although carbamazepine treats seizures, overdose can itself cause seizures.
• Ataxia, nystagmus, sedation, and anticholinergic findings are common neurological clues.
• Sodium-channel blockade can produce dangerous cardiac conduction abnormalities.
• Chronic carbamazepine therapy may be associated with hyponatremia.