“A curtain came down over half the vision in one eye — but it doesn’t hurt, so it can probably wait?”

Sudden PAINLESS visual loss is an ophthalmic EMERGENCY, and the dangerous trap is the patient’s assumption that "painless means it can wait" — the absence of pain is irrelevant to urgency. Triage the differential by the story. RETINAL DETACHMENT classically gives FLASHES (photopsia) and new FLOATERS followed by a CURTAIN/shadow progressing across the field, especially in a myope or after trauma/cataract surgery, and is time-critical because the chance of saving central vision falls once the macula detaches — so it needs SAME-DAY emergency eye-service assessment. RETINAL ARTERY occlusion causes sudden, profound, painless loss (a "stroke of the eye") and in those ≥50 mandates an active GIANT CELL ARTERITIS screen (headache, jaw claudication, scalp tenderness, raised ESR/CRP) because it threatens the other eye — suspected GCA with visual loss needs immediate glucocorticoid and same-day ophthalmology review (BSR, Mackie et al 2020); RETINAL VEIN occlusion and VITREOUS HAEMORRHAGE (especially in diabetic retinopathy or on anticoagulants) are other painless causes. TRANSIENT monocular loss that fully recovers is AMAUROSIS FUGAX — a TIA of the eye — needing urgent TIA-pathway assessment even after recovery (NICE NG128: aspirin 300 mg immediately unless contraindicated, specialist assessment within 24 hours). On the phone, take a focused history (onset/speed, monocular vs binocular, curtain vs central, flashes/floaters, transient vs persistent, GCA symptoms, vascular risk) and recognise that the fundus cannot be examined remotely, so sudden persistent visual loss with a detachment story warrants same-day emergency ophthalmology, never wait-and-see. Correct the painless=benign misconception firmly but kindly, arrange same-day assessment (College of Optometrists guidance), advise against self-driving (DVLA Group 1 visual standard applies after diagnosis), and safety-net (including GCA red flags in the older patient).