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

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

The American Heart Association's 2021 scientific statement on central retinal artery occlusion (where the main blood vessel in the fundus is blocked) states: "Acute central retinal artery occlusion is an emergency." The medical system should identify it as an emergency and prioritize it accordingly. The longer the period of blood flow interruption, the less retina can be salvaged. The Oxford EXPRESS study looked at two types of patients: minor stroke and cerebral ischemia that improved on its own (medically known as transient ischemic attack). This group of patients was later changed to same-day assessment and same-day treatment. After the change, the proportion of recurrent strokes within 90 days decreased from 10.3% (32/310) to 2.1% (6/281). The abstract of this scientific statement also states that central retinal artery occlusion is a type of acute ischemic stroke and is also a precursor to subsequent cerebrovascular and cardiovascular events. The UK National Health Service (NHS) lists "blurred vision or loss of vision in one or both eyes" as a stroke symptom and specifies that even if symptoms stop on their own after a while, one must seek medical attention immediately. Those who experienced stroke symptoms within the past 24 hours, even if they have since stopped, should also call emergency services.

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

Grade B is assigned because EXPRESS is not a randomized trial. It compares two different time periods—before and after the change in practice—within the same population, and other conditions between the two periods may have also changed, which cannot be separated. A momentary blackout that recovers after a few minutes is easily dismissed as being due to "fatigue" or "low blood sugar." The NHS specifies that even if symptoms stop on their own, one must seek medical attention immediately; for the EXPRESS patient group, if not treated on the same day, 10.3% suffered another stroke within 90 days. Sudden loss of vision in one eye, even if there is no pain or redness, must be taken seriously. Do not wait for an ophthalmology outpatient appointment the next day; go directly to the emergency department of a hospital with a stroke center.