Comprehensive Veterinary Toxicology: Acetaminophen, Muscle Relaxants, and Illicit Substances Study Guide to Illicit Drug Exposure

Acetaminophen Toxicity (Tylenol) General Information

  • Classification: Acetaminophen is not a Non-Steroidal Anti-Inflammatory Drug (NSAID).

  • Toxicity Mechanism: The drug itself possesses low toxicity; jedoch, toxicity arises from the formation of toxic metabolites.

    • Metabolic Pathways: Acetaminophen (APAP) is metabolized into various substances. The specific toxic metabolite of concern is NAPQI (N-acetyl-p-benzoquinone imine).

    • Pathway: NAPQI is formed via the cytochrome P450 pathway.

    • Action: This oxidative metabolite is produced in the liver and kidneys, where it acts as a free radical.

  • Target Organs and Systems:

    • Hepatotoxic: Primarily affects the liver.

    • Renal Toxicity: Occurs, though it is less common than liver involvement.

    • Red Blood Cell (RBC) Injury: Leads to oxidative damage to erythrocytes.

  • Species Sensitivity (Cats):

    • Cats are significantly more susceptible to acetaminophen toxicity than dogs.

    • Biochemical Deficiency: Cats have a glucuronyl transferase deficiency.

    • Consequence: Because they cannot conjugate the drug efficiently, their metabolic pathways become saturated much more quickly, leading to the rapid production of NAPQI.

Toxic Doses and Clinical Timeline

  • Therapeutic Index: Acetaminophen has a very narrow therapeutic index in veterinary species.

  • Hepatotoxicity Thresholds:

    • Acute ingestion: >75 - 100\,mg/kg

    • Chronic dosing: 46 mg/kg46\,mg/kg

  • General Toxicity Threshold: >150\,mg/kg

  • Methemoglobinemia Threshold: Seen at doses >200\,mg/kg

  • Lethal Doses: >250\,mg/kg

  • Timeline of Effects:

    • Onset of Action: 1−4 hours1 - 4\,\text{hours}

    • Duration of Clinical Effects: 5−7 days5 - 7\,\text{days}, though it may last longer.

  • Initial Action Item: The first step in a suspected poisoning case should be to call the Pet Poison Hotline (PPH).

Clinical Manifestations and Diagnostics

  • Clinical Signs:

    • Lethargy and depression.

    • General weakness.

    • Respiratory Distress: Tachypnea and dyspnea.

    • Cyanosis: Bluish discoloration of mucous membranes.

    • Icterus: Yellowing of tissues due to liver damage.

    • Vomiting and Hypothermia.

    • Edema: Facial or paw edema (relatively uncommon).

    • Chocolate Colored Blood: Resulting from methemoglobinemia (oxidative damage to hemoglobin).

  • Diagnostic Plan:

    • Bloodwork: Full CBC (Complete Blood Count), Chemistry panel, Electrolytes (Lytes), and a coagulation profile.

    • Urinalysis (UA): To assess renal involvement.

    • Electrolyte Monitoring: Recheck electrolytes every 24 hours24\,\text{hours} for 3 days3\,\text{days}. Recheck again at 5−7 days5 - 7\,\text{days} post-exposure (PE).

    • PCV/TS Monitoring: Recheck Packed Cell Volume (PCV) and Total Solids (TS) every 2 hours2\,\text{hours} for the first 12 hours12\,\text{hours} PE.

      • Evaluate serum color for pink (hemolysis) vs. icteric (liver damage) appearance.

      • Monitor for large drops in PCV.

    • Blood Smear: Perform every 4 hours4\,\text{hours} to assess for Heinz body anemia and fragmentary injury to Red Blood Cells.

    • Schirmer Tear Test: Perform at day 3 PE3\,PE.

      • Note on Keratoconjunctivitis Sicca (KCS): The exact mechanism leading to KCS is unknown, but the delayed onset and transient nature suggest an immune-mediated response.

Acetaminophen Treatment Plan

  • Decontamination:

    • Induce Emesis:

      • Dogs: Apomorphine (IV, SC, or IM).

      • Cats: Dexmedetomidine at a dose of 10 μg/kg IM10\,\mu g/kg\,IM.

    • Anti-emetic: Maropitant (Cerenia) following emesis.

    • Activated Charcoal: 1−3 g/kg PO1 - 3\,g/kg\,PO.

  • Specific Antidote and Support:

    • n-Acetylcysteine (NAC):

      • Loading Dose: 140 mg/kg PO140\,mg/kg\,PO or IVIV.

      • Maintenance: 70 mg/kg PO70\,mg/kg\,PO or IV q6hIV\,q6h for 5−7 doses5 - 7\,\text{doses}.

    • Hepatoprotectants: Denamarin (S-Adenosylmethionine and Silybin) administered orally on an empty stomach.

    • Supportive Care: IV fluid therapy.

    • Sedation (if needed): Dexmedetomidine or Butorphanol as needed (PRN).

Baclofen Toxicity

  • Description: A human muscle relaxant that is toxic at extremely low doses.

  • Toxic Dose Threshold: Symptoms can be observed at ingestions as low as 0.7 mg/kg0.7\,mg/kg.

  • Symptom Progression:

    • Early Signs: Inappropriate vocalization, hypersalivation, and tachycardia.

    • Advanced Signs: Ataxia, sedation, lethargy, bradycardia, hypotension, hypoventilation, coma, and Cardiopulmonary Arrest (CPA).

  • Management:

    • Decontamination: Induce vomiting only if no Central Nervous System (CNS) depression is present.

    • Treatment: Symptomatic care, Intralipid (IV Lipid) therapy, and potential mechanical ventilation.

    • Monitoring: Frequent neurological exams, blood gas analysis, blood pressure (BP), and Electrocardiogram (ECG).

Opioids and Illicit Substances

  • Opioids (Heroin, Fentanyl, Oxycodone, Morphine):

    • Clinical Signs: Initial excitement followed by CNS depression (lateral recumbency, minimal responsiveness), and hypotension.

    • Reversal Agent: Naloxone.

  • Cocaine:

    • Clinical Signs: Hyperexcitability, tremors, ataxia, seizures, mydriasis (dilated pupils), hypersalivation, tachycardia, hypertension, and hyperthermia.

  • Methamphetamine and Adderall:

    • Clinical Signs: Serotonin syndrome characterized by hyperactivity, hypersalivation, ataxia, tremors, seizures, tachycardia, arrhythmias, and hypertension.

  • Benzodiazepines (Valium, Xanax):

    • Clinical Signs: Sedation, ataxia, GI upset, and hypothermia.

    • Paradoxical Signs: Some cases show excitement, agitation, and hyperthermia.

    • Reversal Agent: Flumazenil.

  • Screening Kits:

    • OTC Human Drug Kits: Reliable for amphetamines, barbiturates, opioids, and benzodiazepines.

    • Unreliable for: THC (Marijuana), Cocaine, and PCP.

Treatment and Management of Illicit Toxicities

  • Decontamination Protocols:

    • Emesis, Activated Charcoal, or Gastric Lavage.

    • Contraindication: These are all contraindicated if the patient already displays signs of CNS depression.

  • Pharmacological Support:

    • Anxiolytics: Diazepam, Midazolam, or Acepromazine.

    • Anticonvulsants: Diazepam or Midazolam (short-term); Keppra (Levetiracetam) or Phenobarbital (long-term).

    • Tremor Management: Methocarbamol.

    • Antiarrhythmics: Lidocaine or Propranolol.

    • Anti-emetics: Cerenia (Maropitant) or Ondansetron.

  • General Care: Keep the patient in a calm environment.

Marijuana Toxicity (THC)

  • Active Agent: Tetrahydrocannabinol (THC).

  • Routes of Exposure: Inhalation, ingestion of plants, joints, baked goods, medicinal prescriptions, or vape pens.

  • Concentrated Products: Baked products are highly dangerous. Concentrates include butter products, dabs, and vaporizing oils. These often present a dual toxicity (e.g., marijuana and chocolate).

  • Potency: Newer strains of C. sativa and C. indica have higher THC levels.

  • Fatality: While coma and death are possible, they are very rare.

  • Case Study: Bailey’s Magical Trip

    • Patient: 3-year-old male neutered (M/N) Lab mix.

    • Presenting Complaint: Hind end weakness, falling over (owner suspected a stroke).

    • Clinical Findings: Mild bradycardia, urinary incontinence, ataxia/stumbling, alternating mental stupor, and an exaggerated menace response.

    • Context: Owners self-treat for anxiety or had friends over recently.

  • Marijuana Treatment:

    • Minor Cases: SQ fluids (30 ml/kg30\,ml/kg) and Cerenia injection (1 mg/kg IV1\,mg/kg\,IV or SQSQ). Recovery typically within 12 hours12\,\text{hours}.

    • Significant Mentation Alteration: Hospitalization, IV Lipid therapy, and IV fluid therapy. Call Pet Poison Helpline for severe cases.

    • Decontamination: Emesis and charcoal may help but are usually not recommended due to altered mental status (risk of aspiration).

  • Testing Reliability: Human OTC urine tests are not reliable for dogs because dogs break down THC into different metabolites than humans.

General Toxicology Management Principles

  • Step-by-Step Approach:

    1. Recognize abnormalities to generate a differential diagnosis list.

    2. Obtain an exhaustive history: Pre-existing conditions, household medications/drugs, and recent activities.

    3. Consult Resources: Use Pet Poison Helpline (PPH) or ASPCA for known toxins.

    4. Symptomatic Stabilization: If the toxin is unknown, treat the symptoms generally and provide supportive care to guide the patient through the toxicity.