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Methaemoglobinaemia
In methaemoglobinaemia the iron in haemoglobin is oxidised to Fe3+ and cannot carry oxygen; the patient is cyanosed despite oxygen, the ordinary oximeter saturation is "stuck" around 85% while the PaO2 is normal, and treatment for symptoms or a high level is methylene blue.
Why it matters in the exam
The typical question describes a patient who received a local anaesthetic throat spray (benzocaine) before a procedure, or takes dapsone, or an infant given nitrate-rich well water, who develops cyanosis that does not improve with oxygen. The decisive clue is a "saturation gap": a normal PaO2 on the blood gas, a low oximeter saturation, and chocolate-brown blood.
Treatment thresholds and contraindications are also tested: methylene blue 1–2 mg/kg IV over 5 minutes when the level is above 20% with symptoms, or above 30% even without symptoms, repeatable after one hour. In G6PD deficiency methylene blue is ineffective and may cause haemolysis — then ascorbic acid is given, and in severe cases exchange transfusion or hyperbaric treatment. Methylene blue inhibits MAO and may cause serotonin syndrome in patients on SSRIs.
Remember the vulnerable patients: young infants, especially with diarrhoea and dehydration, because of low activity of the enzyme that reduces methaemoglobin, and patients with anaemia or cardiopulmonary disease who have symptoms at lower levels. Diagnosis is by co-oximetry, which measures the methaemoglobin percentage directly, and not by a standard blood gas that calculates saturation from the PaO2. The most important step is stopping the causative agent.
Key points
- Fe3+ does not carry oxygen — cyanosis that does not respond to oxygen.
- Saturation gap: normal PaO2, saturation around 85%, brown blood.
- Causes: benzocaine, prilocaine, dapsone, nitrites, primaquine.
- Methylene blue 1–2 mg/kg IV above 20% with symptoms or above 30%.
- In G6PD deficiency — no methylene blue; ascorbic acid or exchange transfusion.
Summary table
| MetHb level | Symptoms | Treatment | Note |
|---|---|---|---|
| Below 15% | Cyanosis only | Stop the agent, oxygen | Oximeter saturation slightly low |
| 20–30% | Headache, breathlessness, anxiety | Methylene blue if symptomatic | Marked saturation gap |
| 30–50% | Confusion, tachycardia, severe fatigue | Methylene blue 1–2 mg/kg IV | May repeat after one hour |
| Above 50% | Seizures, arrhythmias, acidosis, coma | Methylene blue, full support | Life-threatening |
| G6PD deficiency | Any level | Ascorbic acid, exchange transfusion, hyperbaric treatment | Methylene blue is forbidden |
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Sources
- Goldfrank's Toxicologic Emergencies, 11th edition (2019) — Methemoglobin inducers
- Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 9th edition (2020) — Dyshemoglobinemias
- Iolascon A et al., Recommendations for diagnosis and treatment of methemoglobinemia (American Journal of Hematology), 2021 — Acquired methemoglobinemia
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.