Rare forms of congenital methaemoglobinaemia exist, but most cases of methaemoglobinaemia in adults are secondary to drugs or toxic agents. When haemoglobin is oxidised by certain drugs, it forms methaemoglobin, which imparts an intense bluish tinge to the skin, presenting clinically as cyanosis.1
This is unresponsive to oxygen therapy.2
Symptoms depend upon the concentration3
of methaemoglobin in the blood. Skin discolouration can occur in patients who are not anaemic when as little as 1.5 g/dl or approximately 10%4
of haemoglobin is in the methaemoglobin form. At a fraction of 15–20%, the patient may be relatively asymptomatic but cyanosed. At 25–50% headache, confusion and chest pain begin. Treatment is by removing the offending agent, providing supplemental oxygen and administering methylene blue intravenously.5
Methylene blue is indicated in the asymptomatic patient with a methaemoglobin blood concentration greater than 25–30%.
The drug ‘snow’ is a white powder that has recently become available in some head shops, where it is sold legally as a bath salt. Advertised as a ‘legal high’, the salt's active ingredient is mephedrone. According to TOXBASE, mephedrone (4-methylmethcathinone) can be synthesised from pseudoephedrine, or is a derivative of methcathinone, which in turn may be derived from cathinone, one of the active ingredients in khat (Cathula edulis
Forsk). Mephedrone is a psychoactive drug with effects similar to those of ‘ecstasy’ (MDMA, 3,4-methylenedioxymethamphetamine), however its stimulant properties predominate and are much more potent.6
The chemical also has a marked similarity with 4-methylamphetamine, a potent releaser of dopamine, norepinephrine and serotonin,7
thus making it neurotoxic.8
Since the patient denied using any other recreational drugs, it is believed that in this case mephedrone in the product ‘snow’ was responsible for the patient's methaemoglobinaemia. We did not use methylene blue in our case because the patient's oxygen saturation on pulse oximeter started to improve.
- Cyanosis due to de-oxygenated haemoglobin is a common presentation in the emergency department.
- Unexplained cyanosis, particularly in association with normal arterial oxygen tension suggests abnormal haemoglobin, such as methaemoglobin.
- Cyanosis due to methaemoglobin is treated with methylene blue.
- Acquired methaemoglobin is a lethal complication of many oxidising drugs in adults.