Loss of vision is considered sudden if it develops within a few minutes to a couple of days. It may affect one or both eyes and all or part of a field of vision. Loss of only a small part of the field of vision (for example, as a result of a small retinal detachment Detachment of the Retina Detachment of the retina is separation of the retina (the transparent, light-sensitive structure at the back of the eye) from the underlying layer to which it is attached. People notice a sudden... read more ) may seem like blurred vision Vision, Blurred Blurred vision is the most common vision symptom. When doctors talk about blurred vision, they typically mean a decrease in sharpness or clarity that has developed gradually. Sudden, complete... read more 
. Other symptoms, for example eye pain Eye Pain Eye pain may be severe and seem sharp, aching, or throbbing, or people may feel only mild irritation of the eye surface or the sensation of a foreign object in the eye (foreign body sensation)... read more , may occur depending on the cause of vision loss.
An Inside Look at the Eye
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Causes
Sudden loss of vision has three general causes:
Clouding of normally transparent eye structures
Abnormalities of the retina (the light-sensing structure at the back of the eye)
Abnormalities of the nerves that carry visual signals from the eye to the brain (the optic nerve and the visual pathways)
Light must travel through several transparent structures before it can be sensed by the retina. First, light passes through the cornea (the clear layer in front of the iris and pupil), then the lens, and then the vitreous humor (the jellylike substance that fills the eyeball). Anything that blocks light from passing through these structures, for example, a corneal ulcer or bleeding into the vitreous humor, or disrupts the transmission of nerve impulses from the back of the eye to the brain can cause loss of vision.
Most of the disorders that cause total loss of vision when they affect the entire eye may cause only partial vision loss when they affect only part of the eye.
When the Visual Pathways Are Damaged
Nerve signals travel along the optic nerve from each eye. The two optic nerves meet at the optic chiasm. There, the optic nerve from each eye divides, and half of the nerve fibers from each side cross to the other side. Because of this arrangement, the brain receives information via both optic nerves for the left visual field and for the right visual field. Damage to an eye or the visual pathway causes different types of vision loss depending on where the damage occurs.  |
Common causes
The most common causes of sudden, painless loss of vision are
Blockage of a major artery of the retina (central retinal artery occlusion Blockage of Central Retinal Arteries and Branch Retinal Arteries An artery in the retina (the transparent, light-sensitive structure at the back of the eye) may become blocked, causing sudden, painless loss of vision. Doctors typically make the diagnosis... read more )
Blockage of a major vein in the retina (central retinal vein occlusion Blockage of Central Retinal Veins and Branch Retinal Veins A vein in the retina (the transparent, light-sensitive structure at the back of the eye) may become blocked, causing sudden, painless loss of vision. Doctors typically make the diagnosis by... read more )
Blood in the jellylike vitreous humor near the back of the eye (vitreous hemorrhage)
Eye injury
Sudden retinal artery blockage can result from a blood clot or small piece of atherosclerotic material that breaks off and travels into the artery. The artery to the optic nerve can be blocked in the same ways and can also be blocked by inflammation (such as may occur with giant cell [temporal] arteritis). A blood clot can form in the retinal vein and block it, particularly in older people with high blood pressure or diabetes. People with diabetes are also at risk of bleeding into the vitreous humor.
Sometimes what seems like a sudden start of symptoms may instead be sudden recognition. For example, a person with long-standing reduced vision in one eye (possibly caused by a dense cataract) may suddenly become aware of the reduced vision in the affected eye after covering the unaffected eye.
Less common causes
Less common causes of sudden loss of vision (see table Sudden Loss of Vision Some Causes and Features of Sudden Loss of Vision 
) include stroke or transient ischemic attack (TIA), acute glaucoma, retinal detachment, inflammation of the structures in the front of the eye between the cornea and the lens (anterior uveitis, sometimes called iritis), certain infections of the retina, and bleeding within the retina as a complication of age-related macular degeneration.
Evaluation
Sudden loss of vision is an emergency. Most causes are serious.
When to see a doctor
All people who experience a sudden loss of vision should see an ophthalmologist (a medical doctor who specialize in the evaluation and treatment—surgical and nonsurgical—of eye disorders) or go to the emergency department right away.
What the doctor does
Doctors first ask questions about the person's symptoms and medical history. Doctors then do a physical examination. What they find during the history and physical examination often suggests a cause and the tests that may need to be done (see table Sudden Loss of Vision Some Causes and Features of Sudden Loss of Vision ).
Doctors ask the person to describe when loss of vision occurred, how long it has been present, and whether if has progressed. People are asked whether loss affects one or both eyes and whether loss is total or affects only a specific part of the field of vision. Doctors also ask about other vision symptoms such as floaters, flashing lights, halos around lights, distorted color vision, jagged or mosaic patterns, or eye pain. Doctors ask about symptoms that are not related to the eyes and risk factors for disorders that may cause eye problems.
The physical examination concentrates primarily on the eyes, but doctors may also do a general physical examination.
For the eye examination, doctors first carefully check sharpness of vision (visual acuity), usually by having the person read letters on a chart, first while one eye is covered and then with both uncovered. Doctors check how the pupils narrow (constrict) in response to light and how well the eyes can follow a moving object. Color vision may be tested. Doctors examine the eyes and eyelids using a slit lamp What Is a Slit Lamp? 
(an instrument that enables a doctor to examine the eye under high magnification) and measure pressure in the eye. Ophthalmologists, after instilling drops that dilate the pupils, examine the retina thoroughly with a slit lamp or light that is shone from a head lamp through a hand-held instrument.
Testing
The presence or absence of pain helps narrow the list of possible causes of sudden vision loss considerably (see table Sudden Loss of Vision Some Causes and Features of Sudden Loss of Vision ). If vision returns quickly on its own, transient ischemic attack and ocular migraine are among the likely causes.
Often findings during the eye examination provide enough information for doctors to diagnose the cause of loss of vision. Sometimes, however, testing is needed depending on what disorders are suspected. The following tests are of particular importance:
Ultrasonography is done if the retina is not clearly visible during an ophthalmoscopic examination.
Gadolinium-enhanced MRI of the orbit and/or the brain is done for some people with eye pain and certain other symptoms and when optic nerve swelling is seen during the eye examination.
Erythrocyte sedimentation rate (ESR) and C-reactive protein level (blood tests that indirectly measure inflammation in the body) are done and the number of platelets in the blood (platelet count) are sometimes measured, particularly in people over age 50 who have headache.
Treatment
The disorder causing the loss of vision is treated as rapidly as possible, although treatment may not be able to save or restore vision. However, prompt treatment may decrease the risk of the same process occurring in the other eye.
Spotlight on Aging: Vision Loss in Older People
Key Points
Sudden loss of vision is an emergency, so people should go directly to a hospital.
The presence or absence of pain helps indicate which causes are most likely.
If vision returns quickly on its own, transient ischemic attack and ocular migraine are among the likely causes.
