Clinical assessment
The patient presents with….
Methaemoglobinaemia can be asymptomatic or cause a range of hypoxic symptoms.
Table 3: Symptoms related to percentage MetHb
| MetHb as % of total Hb* | Typical Symptoms+ |
| <10 | None |
| 10-20 | Cyanosis |
| 20-30 | Anxiety, headache, light headedness, tachycardia |
| 30-50 | Fatigue, confusion, dizziness, tachycardia, tachypnoea |
| 50-70 | Coma, seizures, arrhythmias, respiratory depression |
| > 70 | Death |
* for a Hb of 15 g/dL
+ symptom severity does not always reliably correlate to MetHb level but is affected by individual factors including pre-existing co-morbidities
Cyanosis is usually the first sign and patients can initially appear very well for the level of cyanosis. This is because cyanosis secondary to methaemoglobinaemia appears at 1.5 g/dL of MetHb, whereas cyanosis secondary to hypoxaemia appears at 5 g/dL of deoxygenated haemoglobin, which represents a much greater reduction in oxygen carrying capacity.
Patients often have a low SpO2 on pulse oximetry but again appear well for the degree of hypoxia. A notable feature of methaemoglobinaemia is a low SpO2 on pulse oximetry which does not improve with supplemental oxygen. This can be the first pointer to the diagnosis of methaemoglobinaemia [4].
Learning bite
In any patient who presents with cyanosis or low SpO2 out of keeping with their clinical state, and which does not improve with supplemental oxygen, think methaemoglobinaemia.