Acute loss of vision has 3 general causes:

Opacification of normally transparent structures through which light rays pass to reach the retina (eg, cornea, vitreous)
Retinal abnormalities
Abnormalities affecting the optic nerve or visual pathways

The most common causes of acute loss of vision are

Vascular occlusions of the retina (central retinal artery occlusion Central Retinal Artery Occlusion and Branch Retinal Artery Occlusion Central retinal artery occlusion occurs when the central retinal artery becomes blocked, usually due to an embolus. It causes sudden, painless, unilateral, and usually severe vision loss. Diagnosis... read more , central retinal vein occlusion Central Retinal Vein Occlusion and Branch Retinal Vein Occlusion Central retinal vein occlusion is a blockage of the central retinal vein by a thrombus. It causes painless vision loss, ranging from mild to severe, and usually occurs suddenly. Diagnosis is... read more )
Ischemic optic neuropathy (often in patients with temporal arteritis Giant Cell Arteritis Giant cell arteritis involves predominantly the thoracic aorta, large arteries emerging from the aorta in the neck, and extracranial branches of the carotid arteries. Symptoms of polymyalgia... read more )
Vitreous hemorrhage (caused by diabetic retinopathy Diabetic Retinopathy Manifestations of diabetic retinopathy include microaneurysms, intraretinal hemorrhage, exudates, macular edema, macular ischemia, neovascularization, vitreous hemorrhage, and traction retinal... read more or trauma)
Trauma Overview of Eye Trauma Common causes of eye injury include domestic accidents (eg, during hammering or exposure to household chemicals or cleaners), assault, sporting injuries (including air- or paint pellet-gun injuries)... read more

In addition, sudden recognition of loss of vision (pseudo-sudden loss of vision) may manifest initially as sudden onset. For example, a patient with long-standing reduced vision in one eye (possibly caused by a dense cataract Cataract A cataract is a congenital or degenerative opacity of the lens. The main symptom is gradual, painless vision blurring. Diagnosis is by ophthalmoscopy and slit-lamp examination. Treatment is... read more ) suddenly is aware of the reduced vision in the affected eye when covering the unaffected eye.

Presence or absence of pain helps categorize loss of vision (see table Some Causes of Acute Vision Loss Some Causes of Acute Vision Loss ).

Most disorders that cause total loss of vision when they affect the entire eye may affect only part of the eye and cause only a visual field defect (eg, branch occlusion of the retinal artery or retinal vein, local retinal detachment Retinal Detachment Retinal detachment is separation of the neurosensory retina from the underlying retinal pigment epithelium. The most common cause is a retinal break (a tear or, less commonly, a hole—rhegmatogenous... read more ).

Less common causes of acute loss of vision include

Anterior uveitis Overview of Uveitis Uveitis is defined as inflammation of the uveal tract—the iris, ciliary body, and choroid. However, the retina and fluid within the anterior chamber and vitreous are often involved as well.... read more (a common disorder, but one that usually causes eye pain severe enough to trigger evaluation before vision is lost)
Aggressive retinitis
Certain drugs (eg, methanol, salicylates, ergot alkaloids, quinine)

History of present illness should describe loss of vision in terms of onset, duration, progression, and location (whether it is monocular or binocular and whether it involves the entire visual field or a specific part and which part). Important associated visual symptoms include floaters Floaters Floaters are opacities that move across the visual field and do not correspond to external visual objects. With aging, the vitreous humor can contract and separate from the retina. The age at... read more , flashing lights, halos around lights, distorted color vision, and jagged or mosaic patterns (scintillating scotomata). The patient should be asked about eye pain and whether it is constant or occurs only with eye movement.

Review of systems should seek extraocular symptoms of possible causes, including jaw or tongue claudication, temporal headache, proximal muscle pain, and stiffness (giant cell arteritis Giant Cell Arteritis Giant cell arteritis involves predominantly the thoracic aorta, large arteries emerging from the aorta in the neck, and extracranial branches of the carotid arteries. Symptoms of polymyalgia... read more ); and headaches (ocular migraine).

Past medical history should seek known risk factors for eye disorders (eg, contact lens use, severe myopia, recent eye surgery or injury), risk factors for vascular disease (eg, diabetes Diabetes Mellitus (DM) Diabetes mellitus is impaired insulin secretion and variable degrees of peripheral insulin resistance leading to hyperglycemia. Early symptoms are related to hyperglycemia and include polydipsia... read more , hypertension Hypertension Hypertension is sustained elevation of resting systolic blood pressure (≥ 130 mm Hg), diastolic blood pressure (≥ 80 mm Hg), or both. Hypertension with no known cause (primary; formerly, essential... read more ), and hematologic disorders (eg, sickle cell anemia Sickle Cell Disease Sickle cell disease (a hemoglobinopathy) causes a chronic hemolytic anemia occurring almost exclusively in blacks. It is caused by homozygous inheritance of genes for hemoglobin (Hb) S. Sickle-shaped... read more or disorders such as Waldenström macroglobulinemia Macroglobulinemia Macroglobulinemia is a malignant plasma cell disorder in which B cells produce excessive amounts of IgM M-proteins. Manifestations may include hyperviscosity, bleeding, recurring infections... read more or multiple myeloma Multiple Myeloma Multiple myeloma is a cancer of plasma cells that produce monoclonal immunoglobulin and invade and destroy adjacent bone tissue. Common manifestations include lytic lesions in bones causing... read more that could cause a hyperviscosity syndrome).

Family history should note any family history of migraine headaches Migraine Migraine is an episodic primary headache disorder. Symptoms typically last 4 to 72 hours and may be severe. Pain is often unilateral, throbbing, worse with exertion, and accompanied by symptoms... read more .

Vital signs, including temperature, are measured.

If the diagnosis of a transient ischemic attack is under consideration, a complete neurologic examination is done. The temples are palpated for pulses, tenderness, or nodularity over the course of the temporal artery. However, most of the examination focuses on the eye.

Eye examination Evaluation of the Ophthalmologic Patient The eye can be examined with routine equipment, including a standard ophthalmoscope; thorough examination requires special equipment and evaluation by an ophthalmologist. History includes location... read more includes the following:

Visual acuity is measured.
Peripheral visual fields are assessed by confrontation.
Central visual fields are assessed by Amsler grid.
Direct and consensual pupillary light reflexes are examined using the swinging flashlight test.
Ocular motility is assessed.
Color vision is tested with color plates.
The eyelids, sclera, and conjunctiva are examined using a slit lamp if possible.
The cornea is examined with fluorescein staining.
The anterior chamber is examined for cells and flare in patients who have eye pain or conjunctival injection.
The lens is checked for cataracts using a direct ophthalmoscope, slit lamp, or both.
Intraocular pressure is measured.
Ophthalmoscopy is done, preferably after dilating the pupil with a drop of a sympathomimetic (eg, 2.5% phenylephrine), cycloplegic (eg, 1% cyclopentolate or 1% tropicamide), or both; dilation is nearly full after about 20 minutes. The entire fundus, including the retina, macula, fovea, vessels, and optic disk and its margins, is examined.
If pupillary light responses are normal and functional loss of vision is suspected (rarely), optokinetic nystagmus is checked. If an optokinetic drum is unavailable, a mirror can be held near the patient’s eye and slowly moved. If the patient can see, the eyes usually track movement of the mirror (considered to be the presence of optokinetic nystagmus).

Acute loss of vision is itself a red flag; most causes are serious.

Diagnosis of acute vision loss can be begun systematically. Specific patterns of visual field deficit Types of Field Defects help suggest a cause. Other clinical findings also help suggest a cause for acute vision loss Some Causes of Acute Vision Loss :

Difficulty seeing the red reflex during ophthalmoscopy suggests opacification of transparent structures (eg, caused by corneal ulcer Corneal Ulcer A corneal ulcer is a corneal epithelial defect with underlying inflammation usually due to invasion by bacteria, fungi, viruses, or Acanthamoeba. It can be initiated by mechanical trauma... read more , vitreous hemorrhage, or severe endophthalmitis Endophthalmitis Endophthalmitis is an acute panuveitis resulting most often from bacterial infection. Most cases of endophthalmitis are caused by gram-positive bacteria, such as Staphylococcus epidermidis... read more ).
Retinal abnormalities that are severe enough to cause acute loss of vision are detectable during ophthalmoscopy, particularly if the pupils are dilated. Retinal detachment Retinal Detachment Retinal detachment is separation of the neurosensory retina from the underlying retinal pigment epithelium. The most common cause is a retinal break (a tear or, less commonly, a hole—rhegmatogenous... read more may show retinal folds; retinal vein occlusion may show marked retinal hemorrhages; and retinal artery occlusion may show pale retina with a cherry-red fovea.
An afferent pupillary defect (absence of a direct pupillary light response but a normal consensual response) with an otherwise normal examination (except sometimes an abnormal optic disk) suggests an abnormality of the optic nerve or retina (ie, anterior to the optic chiasm).

In addition, the following facts may help:

Monocular symptoms suggest a lesion anterior to the optic chiasm.
Bilateral, symmetric (homonymous) visual field defects suggest a lesion posterior to the chiasm.
Constant eye pain suggests a corneal lesion (ulcer or abrasion), anterior chamber inflammation, or increased intraocular pressure, whereas eye pain with movement suggests optic neuritis.
Temporal headaches suggest giant cell arteritis or migraine.

Erythrocyte sedimentation rate (ESR), C-reactive protein, and platelet count are done for all patients with symptoms (eg, temporal headaches, jaw claudication, proximal myalgias, stiffness) or signs (eg, temporal artery tenderness or induration, pale retina, papilledema) suggesting optic nerve or retinal ischemia to exclude giant cell arteritis.

Other testing is listed in the table Some Causes of Acute Vision Loss Some Causes of Acute Vision Loss . The following are of particular importance:

Ultrasonography is done to view the retina if the retina is not clearly visible with pupillary dilation and indirect ophthalmoscopy done by an ophthalmologist.
Gadolinium-enhanced MRI of the brain and orbits is done for patients who have eye pain with movement or afferent pupillary defect, particularly with optic nerve swelling on ophthalmoscopy, to diagnose multiple sclerosis Multiple Sclerosis (MS) Multiple sclerosis (MS) is characterized by disseminated patches of demyelination in the brain and spinal cord. Common symptoms include visual and oculomotor abnormalities, paresthesias, weakness... read more .

Causative disorders are treated. Treatment should usually commence immediately if the cause is treatable. In many cases (eg, vascular disorders), treatment is unlikely to salvage the affected eye but can decrease the risk of the same process occurring in the contralateral eye or of a complication caused by the same process (eg, ischemic stroke).