ADC Practice Question — Local anaesthetic toxicity

What is the most likely diagnosis based on the slate-grey skin discolouration and reduced SpO2 after multiple articaine injections?

Pharmacology questions in the ADC written exam test applied prescribing decisions, not rote recall of drug names. This exam-level vignette tests your ability to work through clinical details and arrive at the correct management.

Mr. Sam Reeves, a 37-year-old man, is having four teeth restored with composite fillings under bilateral inferior alveolar nerve blocks plus infiltrations. The total local anaesthetic administered is 7 cartridges of 4% articaine with adrenaline 1:100,000. Twenty minutes into the second quadrant, the dental nurse notices the patient's skin has developed a slate-grey discolouration. His SpO2 reading on the pulse oximeter shows 82% but he is conscious and reports feeling 'a bit short of breath.' He has no allergies and no medical history. What is the most likely diagnosis based on the slate-grey skin discolouration and reduced SpO2 after multiple articaine injections?

A. Vasovagal syncope causing peripheral cyanosis
B. Methaemoglobinaemia — a rare but serious complication of articaine and prilocaine, presenting as slate-grey cyanosis unresponsive to supplemental oxygen
C. Anaphylaxis to the local anaesthetic causing cyanosis
D. Acute myocardial infarction causing circulatory failure
E. Acute asthma attack triggered by the dental procedure

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Sources cited

Therapeutic Guidelines v4 Odell's 3rd & 4th Ed IADT 2020 Code of Conduct CMS (Evans) Infection Control + more

Show the answer and cited explanation

Correct answer: B

Methaemoglobinaemia presents as slate-grey cyanosis (distinct from the blue cyanosis of hypoxia) that characteristically does NOT respond to supplemental oxygen. It occurs when haemoglobin iron is oxidised from ferrous (Fe²⁺) to ferric (Fe³⁺) state, preventing oxygen binding. Articaine and prilocaine carry the highest risk among dental local anaesthetics. The SpO2 reading of 82% reflects the methaemoglobin's effect on pulse oximetry.

Source: Ch 24/8: Methaemoglobinaemia — slate-grey cyanosis, articaine/prilocaine risk, unresponsive to O2

Remy's study tip — For pharmacology vignettes, read the patient’s medical history and allergies BEFORE looking at the drug options.

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How do I approach an unfamiliar Local anaesthetic toxicity vignette?

Extract key clinical data from the stem before reading options. There are usually 3–4 critical variables that determine the correct answer.

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What is the most likely diagnosis based on the slate-grey skin discolouration and reduced SpO2 after multiple articaine injections?

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