Risk stratification

Low risk patients can be treated with supportive care and observation whilst high risk patients require active treatment to reverse the methaemoglobinaemia [5].

Traditionally active treatment is only indicated if:

• MetHb > 30% regardless of whether symptomatic or not

or

• MetHb < 30% and symptomatic

Some authors challenge this threshold as MetHb is not the only indicator of disease severity [6]. Significant variation exists between individuals and additional factors should be considered when stratifying risk:

1) Evidence of tissue hypoxia due to MetHb (e.g acidosis, end-organ dysfunction, altered mental state, myocardial ischaemia or arrhythmia).

2) Co-morbidities. Pre-existing cardiovascular, respiratory and haematological conditions or acute conditions (e.g. sepsis) which hinder oxygen delivery will reduce the threshold for symptoms and active treatment.

3) Hb level. Anaemic patients will be symptomatic at a lower concentration of MetHb.

Two patients with a MetHb of 1.5 g/dL will both be cyanotic. But if one has a Hb of 15 g/dL (i.e. 10% MetHb) they will be otherwise well, whereas if the other has a Hb of 7.5 g/dL (i.e. 20% MetHb) they will be symptomatic.

4) Chronic or acute methaemoglobinaemia. A low level methaemoglobinaemia can be normal for a patient with cytochrome-b5-reductase deficiency and not require emergency treatment. Furthermore patients with a chronic methaemoglobinaemia will have developed compensatory mechanisms and have a degree of tolerance lacking in naive patients [1].

Learning bite

• Active treatment is indicated for patients with MetHb > 30% and those who have evidence of tissue hypoxia due to methaemoglobinaemia.
• Risk assessment should not end at MetHb level but also consider pre-existing and active conditions.