A 42-year-old woman attends the Emergency Department with a displaced distal radius fracture. A regional anaesthetic block is performed using ultrasound guidance. The procedure is uneventful, and she initially appears comfortable.

Approximately 15 minutes later, while awaiting manipulation, she becomes increasingly talkative and restless. She complains of feeling “strange” and intermittently anxious. Observations show a mild rise in heart rate with otherwise stable haemodynamics. There is no rash, wheeze, or hypotension.

Shortly after, she becomes confused and develops brief episodes of jaw twitching.

What is the most likely underlying cause of her deterioration?