Salicylate Poisoning

Salicylate poisoning is potentially fatal. Salicylates are present in aspirin medications, peptobismol and in high concentrations of oil of wintergreen (methylsalicylate: 1 teaspoon = 7g of salicylate). The local poisons service should be contacted for advice:

• UK National Poisons Information Service - www.npis.org - Tel: 0870 600 6266
• Toxbase (see below for link: www.spib.axl.co.uk):¹ provides information about routine diagnosis, treatment and management of patients exposed to drugs, household products, and industrial and agricultural chemicals.

Children and adults who have ingested less than 120 mg/kg aspirin and have no symptoms do not require hospital admission but ingestion of more than 250 mg/kg aspirin is likely to cause moderate toxicity and ingestion of more than 500 mg/kg aspirin causes severe and possibly fatal toxicity. Risk factors for death include:¹

• Children and elderly
• Late presentation
• Pulmonary oedema, central nervous system features, hyperpyrexia
• Acidosis
• Salicylate concentration above 700 mg/L
• Chronic overdose

• Mild poisoning causes nausea, vomiting, tinnitus, lethargy or dizziness.
• More severe poisoning causes dehydration, restlessness, sweating, warm extremities with bounding pulses, increased respiratory rate, hyperventilation and deafness.
• Hyper-, normo-, or hypoglycaemia (all with intracellular glucose depletion) may occur.
• Uncommon features include haematemesis, hypokalaemia, hyponatraemia/hypernatraemia, hypocalcaemia, thrombocytopenia, increased INR, intravascular coagulation, renal failure and non-cardiac pulmonary oedema.
• Central nervous system: confusion, disorientation, coma and convulsions (less common in adults than in children).

• Plasma salicylate concentrations:¹
  • The severity of poisoning cannot be assessed from plasma salicylate concentrations alone and the clinical and biochemical features should also be considered.
  • Should be measured urgently for patients who are thought to have ingested more than 120 mg/kg of aspirin as well as those who have taken methyl salicylate or salicylamide.
  • The sample should be taken at least 2 hours (symptomatic patients) or 4 hours (asymptomatic patients) following ingestion as it may take several hours for peak plasma concentrations to occur.
  • A repeat sample should be taken after a further 2 hours in patients with suspected severe toxicity following recent ingestion because of the possibility of continuing absorption. Measurements should be repeated until concentrations are falling.
• Renal function and electrolytes, full blood count, INR/PTR, urinary pH, and blood glucose.
• Plasma potassium should be checked every 3 hours and maintained at between 4.0-4.5 mmol/L.
• Arterial blood gases: some degree of acid-base disturbance is present in most cases.
  • Adults and older children over the age of 4 years: mixed respiratory alkalosis and metabolic acidosis, with normal or high arterial pH.
  • Young children: metabolic acidosis is common.

The likelihood of toxicity can be gauged to a degree by:

• The ingested dose:¹
  • Greater than 120 mg/kg body weight: likely toxicity is mild.
  • Greater than 250 mg/kg body weight: likely toxicity is moderate.
  • Greater than 500 mg/kg body weight: likely toxicity is severe, possibly fatal.
• Salicylate concentration:
  • Severity of poisoning cannot be assessed from plasma salicylate concentrations alone but salicylate intoxication is usually associated with plasma concentrations greater than 350 mg/L (2.5 mmol/L). Most adult deaths occur in patients whose concentrations exceed 700 mg/L (5.1 mmol/L).
• Clinical grading:
  • Mild (nausea, vomiting, tinnitus)
  • Moderate (hyperventilation and confusion)
  • Serious (hallucinations, seizures, coma, cerebral/pulmonary oedema)
• Acid-base staging:
  • Stage I: Blood pH>7.4, Urine pH>6.0 - Respiratory alkalosis, increased urinary excretion of bicarbonate.
  • Stage II: Blood pH>7.4, Urine pH<6.0 - Metabolic acidosis with compensating respiratory alkalosis, urine H excretion, intracellular K depletion.
  • Stage III: Blood pH<7.4, Urine pH<6.0 - Severe metabolic acidosis and hypokalaemia.

Treatment must be in hospital where plasma salicylate, pH and electrolytes can be measured. Absorption of aspirin may be slow and the plasma-salicylate concentration may continue to rise for several hours, requiring repeated measurement of plasma-salicylate concentration.

• General measures for poisoning
• Consider oral activated charcoal (50 g for an adult, 1g/kg for a child) if ingested more than 120 mg/kg body weight salicylate within 4 hours (gastric emptying may be slowed).
• A second dose of charcoal may be warranted in patients whose blood level continues to rise, suggesting delayed gastric emptying, or who have taken enteric coated preparations where absorption may be slower.
• Gastric lavage if patient has ingested more than 500 mg/kg body weight salicylate within 1 hour.
• Aggressive rehydration
• Have a low threshold to give glucose as intracellular glucose depletion may not be reflected in the blood glucose level.
• Urinary alkalinisation:
  • 50mmol (mild toxicity) to 100mmol (mod-sev) sodium bicarbonate (1-2mmol/kg in children) in 5% Dextrose at 200ml/hr (2-4ml/kg/hr) aiming for urine output of 1-2ml/kg at pH 7.5-8.5.
  • Potassium at 20-40mmol/L should be added to keep [K ] 4.0-4.5mmol/L as hypokalaemia inhibits alkalinisation.
• Whole bowel irrigation should be considered in large enteric coated salicylate ingestions.
• Forced diuresis: should not be used as does not enhance salicylate excretion and may cause pulmonary oedema.
• Haemodialysis is the treatment of choice for severe poisoning and should be seriously considered in patients with:¹
  • Plasma concentrations greater than 700 mg/L (5.1 mmol/L)
  • Renal failure
  • Congestive cardiac failure
  • Non-cardiogenic pulmonary oedema
  • Convulsions
  • CNS effects not resolved by correction of acidosis
  • Persistently high salicylate concentrations unresponsive to urinary alkalinisation
  • Severe metabolic acidosis.
• Patients under 10 years or over 70 years have increased risk of salicylate toxicity and may require dialysis at an earlier stage.
• ICU referral is suggested if >300mg/kg ingested, serious clinical features, salicylate concentration>500mg/L or Stage II or III acid-base grading.

• Small retail pack size²
• Measures to prevent accidental overdose by children, e.g. safe storage, child-proof caps

This record has been revised by Dr C Tidy with additional existing content contributed by Dr A M Bonsall.