Poisoning with Substances that cause Methaemoglobinaemia

Description

Medicine treatment

---

D74.9 + (T41.3/T41.4/T46.3 + X44.99/X64.99/Y14.99)

DESCRIPTION

• Substances causing methaemoglobinaemia include nitrites, nitroglycerine, dapsone, mothballs (naphthalene), local anaesthetics, phenazopyridine, chlorates, and anilines.
• Nitrites are used to cure meat in the formal and informal butchery sector.
• Patients present with:
  • Deep cyanosis with only mildly reduced oxygen saturation
  • CNS depression, and
  • arrhythmias.

---

Note:
Methaemoglobinaemia causes patients to appear cyanosed with falsely high conventional pulse oximetry readings and normal PaO₂. Blood gas analysis using co-oximetry is required to specifically measure methaemoglobin levels.

---

MEDICINE TREATMENT

• Oxygen via facemask.

In symptomatic cases or patients with high methaemoglobin values > 30%:

• Methylene blue (methylthionine chloride) 1% dilute solution, slow IV infusion, 1–2 mg/kg administered over 5 minutes.
  • Repeat in 1 hour and, if necessary, 4 hourly up to a total dose of 7 mg/kg.
  • Side effects include precordial pain, restlessness, and dyspnoea.
  • After administration of methylene blue, oxygen saturation may drop initially.

In life-threatening cases not responding to methylene blue, or if methylene blue is not available, exchange transfusion may be considered. Refer to the Poisons Information Helpline for advice on treatment and possible alternatives to methylene blue.

Click here to access the pdf.

---

Standard Treatment Guidelines and Essential Medicines List for South Africa, Adult Hospital Level, 2020-2024_Version 2.0_27 March 2025

Chapter 19 Poisoning_19.22 Poisoning with Substances that cause Methaemoglobinaemia