Secondly, instil a single drop of pilocarpine and wait 30 minutes: if the pupil remains large the cause is pharmacogenic (or local, that is, within the eye) not neurogenic, and the patient does not need to be admitted for further investigations. Firstly, take a detailed history of possible exposure to chemicals with antimuscarinic properties that may have got on to the patient’s hands. For patients who are otherwise well with no other neurological symptoms I suggest two simple and cheap preliminary checks before the pupil sign is labelled neurogenic. The clinical approach to an unreactive pupil in emergency medicine is fraught with anxiety over missing an acute neurosurgical emergency-in particular, coning or a posterior communicating artery aneurysm. Inadvertent muscarinic blockade from contamination of his contact lens was confirmed by demonstrating no miosis after topical administration of pilocarpine drops. On closer questioning the patient explained that shortly before inserting his contact lenses that morning he had been manually crushing Vallergan tablets (an antihistamine with antimuscarinic properties) to administer to his sick daughter via her gastric feed. My examination confirmed an unreactive mid-sized left pupil with no ptosis or ophthalmoplegia and no other neurological deficits. When the results of all these investigations came back as normal, a neuro-ophthalmic opinion was sought. He was sent to the accident and emergency department and later admitted for urgent investigations, which included computed tomography, magnetic resonance imaging and angiography of the head, and a battery of blood tests. Various explanations have been suggested, the most common being that it is due to direct pressure on the third nerve. The pupil abnormality was spotted by an observant staff nurse on the paediatric ward where he was visiting his daughter. I was recently referred a 34 year old man admitted for investigation of an asymptomatic fixed dilated left pupil. Williams et al describe a patient with a fixed dilated pupil after using a prescription hand cream with antimuscarinic properties.
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