Page 7 of 14

PE14.3 | Paracetamol Poisoning — SDL Guide

Learning Objectives

• Identify the risk factors for paracetamol poisoning in children, including accidental ingestion, intentional overdose, and therapeutic misadventure
• Explain the pathophysiology of paracetamol-induced hepatotoxicity through NAPQI production and glutathione depletion
• Recognise the four clinical phases of paracetamol poisoning and the deceptive apparent improvement in Phase II
• Interpret the Rumack-Matthew nomogram to determine whether N-acetylcysteine treatment is indicated
• Apply the correct management: activated charcoal within 1 hour, N-acetylcysteine (NAC) protocol with correct paediatric dosing, and supportive care for hepatic failure
• Explain why the NAC window (most effective within 8–10 hours) is the critical therapeutic decision point

INSTRUCTIONS

Paracetamol (acetaminophen) is the most widely available over-the-counter analgesic and antipyretic in India — and in overdose, a cause of potentially fatal hepatic failure that is both preventable and treatable if identified early. The insidious danger is the Phase II apparent recovery, where the child looks well for 24–72 hours after a large ingestion while hepatocellular destruction is already occurring. By the time jaundice, coagulopathy, and encephalopathy appear in Phase III, the window for effective antidote therapy may have passed. This module teaches you to recognise the poisoning early, use the Rumack-Matthew nomogram correctly, and initiate N-acetylcysteine (NAC) within the therapeutic window.

References

• Ghai Essential Pediatrics, 9th Edition — Chapter on Poisoning (textbook)
• Nelson Textbook of Pediatrics, 21st Edition — Acetaminophen Toxicity (textbook)
• IAP Guidelines on Management of Common Childhood Poisoning (guideline)

---

Version 2.0 | NMC CBUC 2024

Clinical Presentation of Paracetamol Poisoning

The clinical presentation of paracetamol poisoning is characterised by four sequential phases that span 4–7 days following a significant overdose. Understanding this temporal pattern is essential because the most dangerous phase (Phase III, hepatic failure) is preceded by a period of apparent well-being that can falsely reassure both the family and an inexperienced clinician. The critical clinical insight is that normal examination and absence of symptoms in the first 24 hours do NOT rule out serious paracetamol toxicity — the biochemical injury has already begun. Final-year students must resist the temptation to reassure based on appearance alone and instead act on the ingested dose and the Rumack-Matthew nomogram result.

Phase I (0–24 hours): Initial symptoms. The child may experience nausea, vomiting, anorexia, malaise, and pallor. These symptoms are non-specific and mild, often dismissed as a viral illness. Some children with large overdoses remain entirely asymptomatic in Phase I. This phase does not correlate with the eventual severity of hepatic injury.

Phase II (24–72 hours): Apparent recovery. Phase I symptoms resolve. The child appears clinically well. This is the most treacherous phase — the patient looks recovered, but hepatocellular necrosis is actively occurring. Transaminases (ALT, AST) begin to rise silently. Families and clinicians who are not aware of this phase may conclude that no harm occurred. Right upper quadrant tenderness may develop.

Phase III (72–96 hours): Peak hepatotoxicity. This is the fulminant phase if the overdose was large and treatment was not given. Features of acute hepatic failure appear: jaundice, hepatomegaly with right upper quadrant pain, coagulopathy (INR rise, bleeding), hypoglycaemia, metabolic acidosis, oliguria, and hepatic encephalopathy (ranging from confusion to deep coma). ALT and AST peak — values of >1000 U/L or even >10,000 U/L in severe poisoning. Renal failure (hepatorenal syndrome or direct tubular toxicity) may accompany hepatic failure.

Phase IV (days 4–7): Resolution or death. With adequate NAC treatment given early, the vast majority of patients recover completely, including biochemical normalisation. Without treatment, severe cases progress to fulminant hepatic failure and death. Survivors without transplantation have complete hepatic regeneration.

Phase	Timing	Key Features	Danger
I	0–24 h	Nausea, vomiting, malaise (may be asymptomatic)	May reassure falsely — injury already beginning
II	24–72 h	Apparent recovery; transaminases rise silently	Most dangerous for missing treatment window
III	72–96 h	Jaundice, coagulopathy, encephalopathy, renal failure	Fulminant hepatic failure, death
IV	Day 4–7	Recovery (with treatment) or death (without)

Clinical Phases and Mechanism of Paracetamol Poisoning

Pathophysiology: Glutathione Depletion and Hepatotoxicity

The pathophysiology of paracetamol poisoning centres on a single toxic metabolite — NAPQI (N-acetyl-p-benzoquinone imine) — that is produced in small amounts during normal therapeutic use but accumulates to dangerous levels in overdose. Understanding this mechanism makes the rationale for both the dose threshold and the NAC antidote immediately logical.

Normal paracetamol metabolism: At therapeutic doses, paracetamol is metabolised primarily by hepatic Phase II reactions — approximately 90% undergoes glucuronidation and sulfation, both producing water-soluble, non-toxic conjugates that are excreted in urine. A minor fraction (approximately 5–10%) is oxidised by CYP2E1 (and CYP3A4) to produce NAPQI, a highly reactive electrophile. At therapeutic doses, NAPQI is rapidly and completely detoxified by conjugation with glutathione (a tripeptide antioxidant in hepatocytes), producing a harmless mercapturate that is excreted. Hepatocytes contain sufficient glutathione under normal circumstances to handle this small NAPQI burden without injury.

Overdose mechanism — glutathione depletion: When paracetamol is ingested in toxic amounts (>150 mg/kg in children), the glucuronidation and sulfation pathways become saturated, and proportionally more paracetamol is channelled through CYP2E1, generating a larger NAPQI load. Glutathione stores are finite and become depleted — once stores fall below approximately 30% of normal, NAPQI is no longer adequately detoxified. The unquenched NAPQI then binds covalently to hepatocyte proteins and mitochondria, disrupting cellular respiration and causing centrilobular hepatic necrosis (zone 3 of the hepatic acinus, where CYP2E1 activity is highest and oxygen tension is lowest).

Why centrilobular? Zone 3 hepatocytes (perivenular, surrounding the central vein) are most susceptible because CYP2E1 is most concentrated there and oxygen tension is lowest — making them most vulnerable to oxidative mitochondrial injury.

Factors that increase susceptibility: Conditions that reduce glutathione stores or upregulate CYP2E1 increase toxicity at lower paracetamol doses: fasting and malnutrition (glutathione synthesis requires cysteine), chronic alcohol use (CYP2E1 induction), and concurrent hepatic disease.

How NAC works: N-acetylcysteine (NAC) is a cysteine prodrug — it replenishes hepatocyte cysteine (the rate-limiting substrate for glutathione synthesis), thereby restoring glutathione stores and providing additional capacity to detoxify NAPQI. It also acts as a direct NAPQI scavenger and improves hepatic microcirculation. Its efficacy is greatest when given before or during NAPQI accumulation (within 8–10 hours), though it retains some benefit up to 24 hours.

Paracetamol Metabolism and Hepatotoxicity in Overdose

Risk Factors and Epidemiology

Paracetamol poisoning in children occurs through three distinct epidemiological patterns, each with different risk factors, clinical profiles, and prevention implications. Understanding these patterns helps the clinician anticipate which patients are at highest risk and direct counselling appropriately. In India, the universal availability of paracetamol as an OTC medication, its perceived safety ('it's just paracetamol'), and its use across all socioeconomic groups make it the most common pharmaceutical agent in paediatric poisoning.

Provided image

Pattern 1 — Accidental ingestion (children <6 years): Young children ingest tablets or syrup found at home, typically from unlocked cabinets or caregivers' handbags. Doses are often lower and less likely to reach the toxic threshold (>150 mg/kg), but accurate dose estimation can be difficult. Risk factors include storage of medications within reach, multiple caregivers administering doses independently, and liquid formulations in attractive packaging.

Pattern 2 — Intentional overdose (adolescents): The most clinically dangerous pattern — adolescents in distress may ingest large numbers of tablets impulsively. Paracetamol is chosen because of its accessibility rather than lethality intent; the adolescent may not realise that the harm is delayed and that they 'feel fine' for 24–72 hours. This group requires both medical and psychiatric management. Suicide attempt in adolescence carries significant recurrence risk and requires formal psychiatric assessment.

Pattern 3 — Therapeutic misadventure: Repeated supratherapeutic dosing — giving slightly higher-than-recommended doses multiple times over days — can accumulate toxicity, particularly in children who are febrile, fasting, or malnourished (all states that reduce glutathione). This pattern presents with chronic, progressive hepatic injury rather than an acute single-ingestion history.

Risk factors for greater toxicity at any given dose:
- Malnutrition or prolonged fasting (reduced hepatic glutathione)
- Concurrent use of CYP2E1-inducing medications
- Pre-existing hepatic disease
- Young age with proportionally higher per-kg doses from accidental ingestion