What are the risks of using throat numbing sprays that can cause carboxyhemoglobinemia (elevated levels of carboxyhemoglobin in the blood)?

Risks of Throat Numbing Sprays Causing Carboxyhemoglobinemia

Throat numbing sprays containing benzocaine can cause potentially life-threatening methemoglobinemia, with levels exceeding 10% causing profound cyanosis unresponsive to oxygen therapy and levels above 70% potentially being lethal. 1, 2

Mechanism and Risk Factors

Throat numbing sprays, particularly those containing benzocaine (such as Hurricaine), can cause rapid oxidation of hemoglobin from the ferrous (Fe²⁺) to ferric (Fe³⁺) state, resulting in:

• Formation of methemoglobin, which cannot carry oxygen
• Leftward shift of the oxyhemoglobin dissociation curve, decreasing oxygen delivery to tissues
• Potential development of carboxyhemoglobinemia as a secondary effect

Risk factors for developing significant methemoglobinemia include:

• Young age (infants have 50-60% of adult erythrocyte CYB5R activity) 1
• Pregnancy
• Renal insufficiency
• Concurrent use of other oxidizing drugs
• G6PD deficiency (contraindication for methylene blue treatment) 1

Clinical Presentation

Symptoms correlate with methemoglobin levels:

• 10-20%: Cyanosis, chocolate-brown blood
• 20-30%: Anxiety, headache, dizziness, tachycardia
• 30-50%: Fatigue, confusion, dizziness, tachypnea
• 50-70%: Arrhythmias, seizures, coma

• 70%: Death 1, 3

A key diagnostic clue is the discrepancy between pulse oximeter readings (≤90%) and arterial oxygen partial pressure (≥70 mmHg), present in 91.8% of cases 3.

Treatment Algorithm

1. For asymptomatic patients with MetHb <20%:

   • Discontinue the offending agent
   • Provide supportive care and oxygen

2. For symptomatic patients or MetHb >20%:

   • Administer methylene blue 1-2 mg/kg IV over 3-5 minutes
   • Can repeat at 1 mg/kg if no response within 30-60 minutes
   • Maximum total dose: 7 mg/kg (higher doses risk toxicity) 1

3. For patients with G6PD deficiency or if methylene blue fails:

   • Consider ascorbic acid as alternative therapy
   • For severe cases, exchange transfusion or hyperbaric oxygen therapy 1

4. Monitor for rebound methemoglobinemia:

   • Can occur up to 18 hours after methylene blue administration, especially with benzocaine on mucous membranes
   • Reported cases with methemoglobin values as high as 59.9% 3

Prevention Recommendations

• Benzocaine sprays should be avoided entirely due to unpredictable absorption and risk 3
• If local anesthetics must be used:
  • Limit spray application to 1 second 2
  • Consider alternative agents to benzocaine
  • In adults, keep prilocaine doses below 5.0 mg/kg (reduced to 3.2 mg/kg with renal insufficiency and 1.3 mg/kg if using other oxidizing drugs) 3
  • Avoid prilocaine in children under 6 months, pregnant women, and patients taking other oxidizing drugs 3

Monitoring

• Standard pulse oximetry is unreliable for detecting methemoglobinemia
• Laboratory spectrophotometry (CO-oximeter) is required for accurate measurement of both methemoglobin and carboxyhemoglobin levels 4
• Either arterial or venous blood can be used for measurement

Important Caveats

• A single spray of benzocaine can induce methemoglobinemia 3
• Complications include hypoxic encephalopathy, myocardial infarction, and death 3
• Patients may develop severe cyanosis despite being awake and conversant 2
• "Cherry red" skin coloration is a rare diagnostic sign and should not be relied upon 4