Prilocaine induced methaemoglobinaemia in a medically compromised patient. Was this an inevitable consequence of the dose administered?

Patient M, a 45-year old female, was admitted for extractions with local anaesthetic, sedation and monitoring. This was to be carried out on an in patient basis due to the patient's extensive medical history which included rheumatic heart disease, previous deep vein thrombosis and severe ulcerative colitis. Patient M also gave a history of allergy to penicillin and lignocaine. The procedure was completed without incident, local anaesthesia having been achieved using Citanest with Octapressin (prilocaine 30 mg/ml and felypressin 0.03 unit/ml 2 ml cartridge). However, on recovery Patient M's oxygen saturations dropped to 90% on air and although she had no symptoms, the levels could not be improved with supplemental oxygen. A diagnosis of methaemoglobinaemia (MetHb) was established, a rare complication associated with the administration of prilocaine. Patient M was transferred to the high dependency unit and was given methylthioninium chloride (methylene blue) intravenously. Her oxygen saturations quickly returned to normal and she was discharged the following day. This case highlights issues relating to the risk of developing MetHb, what is considered the maximum safe dose of prilocaine and some incongruities the authors feel exist in the literature.