One eye suddenly goes dark, as if a curtain has been pulled shut; even if it clears up within minutes, seek emergency care for stroke on the same day

When one eye suddenly goes dark, as if a curtain has been pulled shut, it means a blockage has occurred in the blood vessels at the back of the eye. The longer the blockage persists, the less of the retina can be saved. This condition is also a form of cerebral ischemia, and the following days represent a high‑risk window for stroke. For patients in this situation, receiving evaluation and treatment on the same day reduces the likelihood of another stroke within 90 days from 10.3% to 2.1%. Seek emergency care at a hospital with a stroke center on the same day; do not wait until the next day to see an ophthalmologist.

No direct cost. However, emergency evaluation and vascular testing will take up half a day to a full day.

The 2021 scientific statement from the American Heart Association on central retinal artery occlusion — blockage of the main vessel at the back of the eye — explicitly states that acute central retinal artery occlusion is a medical emergency. Healthcare systems should recognize it as such and prioritize treatment accordingly. The longer blood flow is cut off, the less retina can be salvaged. The Oxford EXPRESS study examined two groups of patients: those with cerebral ischemia whose symptoms resolved on their own, and those with minor strokes. The former are medically termed transient ischemic attacks. After these patients received same‑day evaluation and treatment, the rate of subsequent strokes within 90 days fell from 10.3% (32 out of 310) to 2.1% (6 out of 281).

The grade B rating is due to the fact that EXPRESS was not a randomized trial; it compared outcomes before and after implementing the new protocol within the same patient population, making it difficult to isolate the effect of the intervention from other potential changes. A brief episode of darkness in one eye that resolves on its own is often dismissed as fatigue or low blood sugar. In reality, it is a type of transient ischemic attack, and the ensuing days are a critical window for stroke risk. Sudden, painless, non‑red, non‑teary loss of vision in one eye must be taken seriously. Do not wait until the next day for an ophthalmology appointment; go directly to the emergency department of a hospital equipped to handle strokes.

Mac Grory B, Schrag M, Biousse V, et al. (2021). Management of Central Retinal Artery Occlusion: A Scientific Statement From the American Heart Association. Stroke, 52(6), e282-e294：「Central retinal artery occlusion (CRAO) is a form of acute ischemic stroke that causes severe visual loss and is a harbinger of further cerebrovascular and cardiovascular events」. https://doi.org/10.1161/STR.0000000000000366 ；NHS. Symptoms of a stroke：「blurred vision or loss of sight in 1 or both eyes」「Symptoms of a stroke can sometimes stop after a short time, so you may think you're OK. Even if this happens, get medical help straight away」「you've had signs of a stroke within the last 24 hours even if they've now stopped」. https://www.nhs.uk/conditions/stroke/symptoms/ ; Rothwell PM, Giles MF, Chandratheva A, et al. (2007). Effect of urgent treatment of transient ischaemic attack and minor stroke on early recurrent stroke (EXPRESS study): a prospective population-based sequential comparison. Lancet, 370(9596), 1432-1442. https://doi.org/10.1016/S0140-6736(07)61448-2