Toxicology – Toxicant-Induced Seizures

Definition

A seizure is a transient episode of abnormal, excessive neuronal activity that may produce:

• Involuntary motor activity
• Altered or loss of consciousness
• Sensory or behavioral abnormalities
• Autonomic manifestations

In toxicology, seizures are an important manifestation of drug or chemical toxicity and are commonly generalized tonic-clonic.

Prolonged or recurrent seizures can cause severe complications including hypoxia, hyperthermia, metabolic acidosis, rhabdomyolysis, aspiration, brain injury, and death.

Pathophysiology

Toxicants can provoke seizures through several mechanisms, including:

• Reduced inhibitory GABA activity
• Excess excitatory neurotransmission
• Sodium-channel effects
• Excess catecholaminergic activity
• Metabolic abnormalities
• Hypoglycemia
• Hypoxia
• Electrolyte disturbances

Some poisonings produce seizures through a highly specific mechanism.

For example:

Isoniazid → pyridoxine depletion → impaired GABA synthesis → severe seizures

Risk Factors

The likelihood of seizures may be increased by:

• Pre-existing epilepsy
• Previous CNS injury
• Large toxic exposure
• Multiple proconvulsant drugs
• Hypoglycemia
• Hypoxia
• Electrolyte abnormalities
• Withdrawal from alcohol or sedative-hypnotic drugs

A seizure in pregnancy requires urgent evaluation for eclampsia and other obstetric causes, even when toxic exposure is also possible.

Important Toxicologic Causes

Tricyclic Antidepressants

TCA poisoning can produce:

• Seizures
• Altered mental status
• Anticholinergic findings
• Tachycardia
• Hypotension
• QRS widening
• Ventricular dysrhythmias

TCA-associated seizures plus QRS widening strongly suggest clinically important sodium-channel blockade.

Sodium bicarbonate is indicated when significant sodium-channel cardiotoxicity is present.

Bupropion

Bupropion is an important cause of drug-induced seizures.

Features may include:

• Agitation
• Tremor
• Tachycardia
• Seizures
• Altered mental status

Severe poisoning may cause:

• Recurrent seizures
• QRS or QT abnormalities
• Ventricular dysrhythmias
• Cardiogenic shock

Extended-release preparations can produce substantially delayed seizures, so an initially asymptomatic patient may still require prolonged observation after a significant exposure.

Isoniazid

Isoniazid is particularly important because seizures may be:

• Severe
• Recurrent
• Resistant to conventional anticonvulsant therapy

Mechanism:

Isoniazid → functional pyridoxine deficiency → decreased GABA synthesis → seizures

Associated findings may include:

• High-anion-gap metabolic acidosis
• Altered mental status
• Coma

Pyridoxine (vitamin B6) is the specific antidotal therapy.

Theophylline

Theophylline toxicity may produce:

• Nausea/vomiting
• Tremor
• Marked tachycardia
• Hypokalemia
• Hyperglycemia
• Dysrhythmias
• Seizures

Seizures may be severe and difficult to control.

Unlike many poisonings, significant theophylline toxicity may cause seizures without a prolonged warning period.

Stimulants

Examples include:

• Cocaine
• Amphetamines
• Methamphetamine
• Other sympathomimetics

Typical findings include:

• Agitation
• Mydriasis
• Diaphoresis
• Tachycardia
• Hypertension
• Hyperthermia

Severe poisoning may progress to:

• Seizures
• Dysrhythmias
• Rhabdomyolysis
• Metabolic acidosis
• Cardiovascular collapse

Antihistamines and Anticholinergic Drugs

First-generation antihistamines and other antimuscarinic agents can produce:

• Agitated delirium
• Mydriasis
• Dry mucous membranes
• Tachycardia
• Urinary retention
• Reduced bowel sounds
• Hyperthermia
• Seizures

Some antihistamines, particularly in severe overdose, can also produce sodium-channel blockade with QRS widening.

Camphor

Camphor exposure is an important cause of rapid-onset seizures, particularly in young children.

Manifestations may include:

• Nausea/vomiting
• Agitation
• Confusion
• Seizures

Neurologic toxicity can develop rapidly.

Lithium

Severe lithium toxicity may produce:

• Coarse tremor
• Hyperreflexia
• Myoclonus
• Ataxia
• Confusion
• Seizures
• Coma

Neurologic toxicity is particularly important in chronic or acute-on-chronic poisoning.

Severe cases may require hemodialysis.

Chloroquine and Hydroxychloroquine

Severe poisoning may cause rapid:

• Seizures
• Hypotension
• Hypokalemia
• QRS widening
• Ventricular dysrhythmias
• Cardiovascular collapse

Neurologic and cardiovascular deterioration can occur quickly.

Local Anesthetic Systemic Toxicity

Systemic local anesthetic toxicity may initially cause neurologic symptoms such as:

• Perioral numbness
• Metallic taste
• Tinnitus
• Agitation
• Tremor

This can progress to:

• Seizures
• CNS depression
• Dysrhythmias
• Cardiovascular collapse

Intravenous lipid emulsion is an important therapy for severe local anesthetic systemic toxicity, particularly when cardiovascular toxicity develops.

Carbon Monoxide

Severe carbon monoxide poisoning can cause:

• Headache
• Nausea
• Dizziness
• Confusion
• Syncope
• Seizures
• Coma

Multiple people developing similar symptoms in the same environment is an important clue.

A normal conventional pulse oximetry reading does not exclude carbon monoxide poisoning.

Hypoglycemic Agents

Insulin and insulin-secretagogue medications can cause neuroglycopenia.

Clinical findings include:

• Sweating
• Confusion
• Behavioral abnormalities
• Weakness
• Seizures
• Coma

Blood glucose should be checked immediately in any patient with an unexplained seizure.

Organophosphate and Carbamate Poisoning

Severe cholinergic poisoning may cause seizures.

Associated findings include:

• Miosis
• Salivation
• Lacrimation
• Bronchorrhea
• Bronchospasm
• Vomiting
• Diarrhea
• Sweating
• Bradycardia
• Fasciculations
• Weakness

Seizures may accompany severe CNS toxicity.

Treatment of significant organophosphate poisoning includes:

• Airway and ventilatory support
• Atropine
• Pralidoxime
• Benzodiazepines for seizures

Salicylates

Seizures are a late and concerning manifestation of severe salicylate poisoning.

Other findings include:

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

Seizures suggest severe toxicity and may accompany worsening CNS dysfunction.

Withdrawal Syndromes

Withdrawal from CNS depressants can provoke seizures.

Important causes include:

• Ethanol withdrawal
• Benzodiazepine withdrawal
• Barbiturate withdrawal

Associated findings may include:

• Tremor
• Agitation
• Tachycardia
• Hypertension
• Diaphoresis
• Hallucinations
• Hyperthermia

Benzodiazepines are central to treatment of severe alcohol or sedative-hypnotic withdrawal.

Strychnine: An Important Seizure Mimic

Strychnine causes severe painful muscular spasms rather than true epileptic seizures.

Typical findings include:

• Stimulus-induced muscle spasms
• Generalized rigidity
• Opisthotonus
• Preserved consciousness between or during early spasms

This distinction can help differentiate strychnine poisoning from generalized tonic-clonic seizures.

Nontoxicologic Differential Diagnosis

Not every seizure in a poisoned or potentially poisoned patient is caused by a toxicant.

Important alternative causes include:

Metabolic

• Hypoglycemia
• Hyponatremia
• Hypocalcemia
• Hypomagnesemia
• Uremia
• Hypoxia

Structural CNS Disease

• Intracranial hemorrhage
• Ischemic stroke
• Traumatic brain injury
• Brain tumor
• Cerebral edema

Infection

• Meningitis
• Encephalitis
• Brain abscess

Other

• Epilepsy
• Eclampsia
• Alcohol/sedative withdrawal

Psychogenic nonepileptic seizures can also mimic epilepsy, but should not be diagnosed merely from one historical or examination feature; video-EEG confirmation is the diagnostic standard when uncertainty persists.

Clinical Clues to the Toxicant

Tachycardia + Hypertension + Hyperthermia

Consider:

• Cocaine
• Amphetamines
• Other sympathomimetics
• MAOI toxicity
• Serotonin syndrome
• Severe withdrawal

Tachycardia + Hypotension

Consider:

• TCA poisoning
• Theophylline
• Chloroquine/hydroxychloroquine
• Other severe cardiotoxic poisoning

Bradycardia + Hypotension

Consider:

• Beta-blockers
• Organophosphates
• Carbamates
• Other cardiodepressant agents

Dry Skin + Mydriasis + Tachycardia

Suggests an anticholinergic toxidrome.

Potential causes include:

• First-generation antihistamines
• Antimuscarinic medications
• TCAs

Miosis + Secretions

Miosis accompanied by:

• Salivation
• Bronchorrhea
• Diarrhea
• Sweating
• Fasciculations

strongly suggests a cholinergic syndrome.

Nystagmus

May occur with:

• PCP
• Carbamazepine
• Certain anticonvulsants
• Sedative/intoxicating agents

Rigidity + Hyperthermia

Consider:

• Serotonin syndrome
• Neuroleptic malignant syndrome
• MAOI-related toxicity
• Malignant hyperthermia in the appropriate peri-anesthetic setting

Clonus and hyperreflexia favor serotonin syndrome.

Severe generalized rigidity with a slower onset favors NMS.

Complications of Prolonged Seizures

Prolonged or recurrent seizures can produce:

• Hypoxemia
• Hypercapnia
• Lactic acidosis
• Hyperthermia
• Rhabdomyolysis
• Hyperkalemia
• Acute kidney injury
• Aspiration
• Brain injury
• Cardiovascular instability

A transient lactate elevation and high-anion-gap metabolic acidosis can occur after a generalized tonic-clonic seizure and often improve as the seizure-related lactate clears.

Persistent severe acidosis should prompt investigation for another cause.

Initial Evaluation

Immediate priorities are:

Airway → breathing → circulation → stop seizure → identify reversible causes

Assess:

• Airway protection
• Ventilation
• Oxygenation
• Heart rate and rhythm
• Blood pressure
• Temperature
• Neurologic status

Obtain a rapid bedside glucose immediately.

Laboratory Evaluation

Important investigations may include:

• Glucose
• Electrolytes
• Calcium
• Magnesium
• Bicarbonate
• BUN and creatinine
• Blood gas
• Lactate
• CK

Depending on the exposure, obtain targeted testing such as:

• Salicylate concentration
• Acetaminophen concentration
• Lithium concentration
• Theophylline concentration
• Carbon monoxide testing
• Anticonvulsant concentrations

Broad urine drug screens have important limitations and should not replace toxidrome-based assessment.

ECG

An ECG is essential when toxicant-induced seizure is suspected.

Look for:

• QRS widening
• QT prolongation
• Bradycardia
• AV block
• Ventricular dysrhythmias

Seizure + QRS widening should particularly raise concern for a sodium-channel-blocking drug such as a TCA or certain antihistamines.

Neuroimaging and Lumbar Puncture

Head CT or other neuroimaging should be considered when there is concern for:

• Trauma
• Intracranial hemorrhage
• Focal neurologic deficit
• Structural CNS disease
• Unexplained persistent altered mental status

Lumbar puncture may be required when CNS infection or another appropriate neurologic diagnosis is suspected.

Management of Toxicant-Induced Seizures

First-Line: Benzodiazepines

Benzodiazepines are the preferred initial treatment for most toxicant-induced seizures.

They enhance GABA-mediated inhibition and are particularly useful because many toxic seizures result from excessive CNS excitation.

Examples include:

• Lorazepam
• Diazepam
• Midazolam

Repeated treatment may be necessary for recurrent seizures.

Persistent or Refractory Seizures

If seizures continue despite adequate benzodiazepine therapy, escalation may include:

• Additional GABAergic antiseizure therapy
• Airway control and mechanical ventilation when necessary
• Continuous EEG monitoring
• Specialist toxicology and critical-care management

Phenobarbital is an important second-line option for many toxin-induced refractory seizures.

Severe refractory status epilepticus may require anesthetic therapy in an ICU.

Role of Phenytoin

Phenytoin is generally less useful for many toxicant-induced seizures because it does not correct the mechanisms responsible for many poison-induced convulsions.

In particular, it should not be considered the preferred treatment for seizures caused by:

• Isoniazid
• Theophylline
• Withdrawal
• Many stimulant poisonings

Treatment should instead emphasize benzodiazepines and toxin-specific therapy.

Isoniazid-Induced Seizures

Isoniazid poisoning is a major exception requiring specific antidotal therapy.

Pyridoxine (vitamin B6) should be administered when isoniazid toxicity is strongly suspected, particularly with refractory seizures.

Benzodiazepines are used concurrently.

Failure to recognize isoniazid toxicity can result in persistent seizures despite conventional anticonvulsant therapy.

Neuromuscular Paralysis

Neuromuscular blocking agents may stop visible muscular activity but do not stop cerebral seizure activity.

Therefore, if paralysis is required for airway management or severe refractory convulsive activity:

Continuous EEG monitoring is important when ongoing seizure activity remains possible.

Paralysis must never be mistaken for successful treatment of the underlying seizure.

Airway Management

Endotracheal intubation may be necessary when there is:

• Persistent status epilepticus
• Severe respiratory depression
• Recurrent seizures with inadequate ventilation
• Severe aspiration risk
• Inability to protect the airway
• Need for deep sedation or anesthetic therapy

Airway management should occur alongside treatment of the seizure rather than delaying anticonvulsant therapy.

Decontamination

Do not induce vomiting.

Routine gastric lavage is generally not recommended.

Activated charcoal may be considered only in selected recent, serious, adsorbable ingestions when the airway is adequately protected.

A patient who is actively seizing or has markedly impaired consciousness has a major aspiration risk, so gastrointestinal decontamination should never take priority over stabilization.

Monitoring

Patients with significant toxicant-induced seizures should be monitored for:

• Recurrent seizures
• Respiratory depression
• Oxygenation and ventilation
• Dysrhythmias
• Hypotension
• Hyperthermia
• Rhabdomyolysis
• Electrolyte abnormalities
• Acute kidney injury

Continuous cardiac monitoring is appropriate in significant poisoning.

Continuous EEG may be required for refractory seizures, persistent unexplained coma, or patients receiving neuromuscular blockade.

Key Points

• Toxicant-induced seizures are commonly generalized tonic-clonic seizures.
• Important mechanisms include reduced GABA activity, excessive excitatory neurotransmission, metabolic abnormalities, hypoglycemia, and hypoxia.
• Common toxicologic causes include bupropion, TCAs, antihistamines, stimulants, isoniazid, theophylline, lithium, camphor, salicylates, and withdrawal syndromes.
• Always check bedside glucose immediately in an unexplained seizure.
• Obtain an ECG, because seizures may accompany life-threatening cardiotoxic poisoning.
• Seizure + QRS widening suggests possible sodium-channel-blocker toxicity.
• Isoniazid should be considered when seizures are unusually refractory, especially with compatible exposure history and metabolic acidosis.
• Pyridoxine is the specific antidotal therapy for isoniazid-induced seizures.
• Benzodiazepines are first-line treatment for most toxicant-induced seizures.
• Phenobarbital or other GABAergic therapy may be required for refractory seizures.
• Phenytoin is generally less effective for many toxin-induced seizures and is not the preferred universal second-line therapy.
• Neuromuscular paralysis eliminates visible convulsions but does not terminate cerebral seizure activity.
• Prolonged seizures can cause hyperthermia, lactic acidosis, rhabdomyolysis, hypoxia, aspiration, and brain injury.
• Always evaluate for nontoxicologic causes, including metabolic abnormalities, CNS infection, structural brain disease, epilepsy, and eclampsia.