Section 2.4: General Toxicology: Overdose, Antidotes & Adverse Drug Reactions (ADRs)

Section 2.4: General Toxicology: Overdose, Antidotes & Adverse Drug Reactions (ADRs)

Introduction to Clinical Toxicology

Clinical toxicology focuses on the diagnosis, management, and prevention of poisoning and adverse drug reactions (ADRs). In Saudi Arabia, the SFDA maintains rigorous pharmacovigilance systems to monitor ADRs. For the SPLE, candidates must identify the clinical presentations of common drug toxicities (acetaminophen, digoxin, opioids, and tricyclic antidepressants), know their specific antidotes and molecular mechanisms, and master the standard classification of ADRs.

Common Overdoses and Specific Antidotes

1. Acetaminophen (Paracetamol) Overdose

Acetaminophen is widely available and a frequent cause of drug-induced liver injury.

• Pathophysiology: At therapeutic doses, acetaminophen is metabolized via Phase II glucuronidation and sulfation. A tiny fraction is oxidized by CYP2E1 to the highly reactive, electrophilic metabolite N-acetyl-p-benzoquinone imine (NAPQI), which is rapidly detoxified by glutathione. In an overdose, the glucuronidation and sulfation pathways saturate, and more drug is shunted through CYP2E1. Hepatic glutathione stores deplete. NAPQI then binds covalently to hepatic proteins, causing mitochondrial damage and centrilobular hepatic necrosis.
• Clinical Presentation: Initial phase (0-24 hours) may be asymptomatic or show mild GI distress. Phase 2 (24-72 hours) presents with right upper quadrant pain, elevated liver transaminases (AST/ALT), and bilirubin. Phase 3 (72-96 hours) is marked by hepatic encephalopathy, coagulopathy, and renal failure.
• Antidote: N-acetylcysteine (NAC). NAC acts as a precursor for glutathione synthesis and can also directly conjugate with and neutralize NAPQI. To prevent hepatotoxicity, NAC should ideally be administered within 8 hours of ingestion. The decision to treat is guided by the Rumack-Matthew Nomogram, which plots serum acetaminophen concentration against time since ingestion (valid starting 4 hours post-ingestion).

2. Digoxin Toxicity

Digoxin is a cardiac glycoside with a narrow therapeutic range (0.5 - 2.0 ng/mL).

• Pathophysiology: Digoxin inhibits the membrane-bound Na+/K+-ATPase pump in cardiomyocytes. This increases intracellular sodium, which shuts down the sodium-calcium exchanger ($NCX$), leading to intracellular calcium accumulation and increased contractility. In toxic amounts, this leads to delayed afterdepolarizations, causing cardiac arrhythmias.
• Predisposing Factors: Hypokalemia potentiates digoxin toxicity because digoxin competes with potassium for binding to the Na+/K+-ATPase pump. High calcium or low magnesium also increases risk.
• Clinical Presentation: Nausea, vomiting, abdominal pain, neurological signs (confusion, delirium), and classic visual changes: xanthopsia (yellow-green vision or halos around lights). Cardiac signs include bradycardia, AV blocks, and ventricular arrhythmias (such as bidirectional ventricular tachycardia).
• Antidote: Digoxin-Specific Antibody Fragments (Digibind/Digifab). These are Fab fragments from sheep antibodies that bind directly to free digoxin in the extracellular space. The resulting large complex is biologically inert and excreted in the urine. Indications include life-threatening arrhythmias, severe hyperkalemia (>5.0 mEq/L in acute overdose), or ingestion of >10 mg in adults.

3. Opioid Overdose

Opioids (e.g., morphine, codeine, tramadol, fentanyl) are agonists at mu-opioid receptors.

• Pathophysiology: Mu-opioid receptor activation in the brainstem reduces responsiveness to carbon dioxide, causing respiratory depression.
• Clinical Presentation (The Opioid Triad):
  1. Coma/altered mental status
  2. Miosis (pinpoint pupils) - exceptions include meperidine and propoxyphene, which do not cause miosis due to anticholinergic effects.
  3. Respiratory depression (respiratory rate < 12 breaths/minute, cyanosis).
• Antidote: Naloxone. Naloxone is a competitive opioid receptor antagonist with high affinity for the mu-receptor. It rapidly reverses respiratory depression. Clinical Pearl: Naloxone has a shorter half-life (30-90 minutes) than most opioids. The patient must be monitored closely for rebound respiratory depression as the naloxone wears off, which may require repeat doses or a continuous infusion.

4. Tricyclic Antidepressant (TCA) Overdose

TCAs (e.g., amitriptyline, imipramine) have complex toxicological profiles blocking multiple receptors:

• Fast Sodium Channel Blockade: Causes QRS widening (>100 ms), QT prolongation, and life-threatening ventricular arrhythmias (torsades de pointes).
• Alpha-1 Adrenergic Receptor Blockade: Causes severe peripheral vasodilation and refractory hypotension.
• Muscarinic Receptor Blockade: Causes anticholinergic toxidrome (dry mouth, blurred vision, mydriasis, urinary retention, hyperthermia, delirium).
• GABA Receptor Blockade: Induces seizures.
• Antidote: Intravenous Sodium Bicarbonate ($NaHCO_3$). Administered when the QRS interval exceeds 100 ms or in the presence of hypotension/arrhythmias. Sodium bicarbonate acts by:
  1. Increasing extracellular sodium concentration, which helps overcome the competitive blockade of cardiac sodium channels.
  2. Alkalizing the blood (target pH 7.45 - 7.55), which increases the un-ionized fraction of the TCA, decreasing its binding affinity for the sodium channel.

Classification of Adverse Drug Reactions (ADRs)

ADRs are classified into categories to guide clinical management:

• Type A (Augmented): Dose-dependent and predictable based on the known pharmacology of the drug. Account for 80% of all ADRs. Typically resolved by reducing the dose. Examples: bleeding with warfarin, hypoglycemia with insulin, orthostatic hypotension with doxazosin.
• Type B (Bizarre/Idiosyncratic): Dose-independent, unpredictable, and unrelated to the drug's primary pharmacology. Often immunologically mediated (hypersensitivity) or due to genetic variations (e.g., hemolysis in G6PD deficient patients exposed to primaquine). High mortality. Requires immediate discontinuation. Examples: anaphylaxis to penicillin, Stevens-Johnson Syndrome (SJS) with carbamazepine.
• Type C (Chronic/Continuous): Relates to long-term cumulative dose and duration of therapy. Examples: osteoporosis or hypothalamic-pituitary-adrenal (HPA) axis suppression from chronic prednisone use.
• Type D (Delayed): Occurs long after drug exposure, sometimes years later or in the offspring of patients. Examples: teratogenicity (fetal hydantoin syndrome from phenytoin), carcinogenesis.
• Type E (End of Use): Occurs upon abrupt withdrawal of a drug. Prevented by tapering the dose. Examples: rebound hypertension after stopping clonidine, withdrawal seizures after stopping benzodiazepines.