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 loss of vision in one or both eyes (blindness Vision Loss, Sudden 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... read more ) is considered something different.
Causes
Blurred vision has four general mechanisms:
Disorders affecting the retina, the light-sensing structure at the back of the eye
Clouding of normally transparent eye structures (cornea, lens, and vitreous humor—the jellylike substance that fills the eyeball) that light rays must pass through to reach the retina
Disorders affecting the pathways of nerves that carry visual signals from the eye to the brain (such as the optic nerve)
Imperfect focusing of light rays on the retina (refractive errors)
An Inside Look at the Eye
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Certain disorders can have more than one mechanism. For example, refraction can be impaired by early cataracts or by the reversible lens swelling caused by poorly controlled diabetes.
Some disorders that cause blurred vision are more likely to cause other symptoms that prompt people to seek medical attention, such as 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 and eye redness Eye Redness Eye redness refers to a red appearance of the normally white part of the eye. The eye looks red or bloodshot because blood vessels on the surface of the eye widen (dilate), bringing extra blood... read more (for example, acute corneal disorders such as abrasions, ulcers, herpes simplex keratitis, or herpes zoster ophthalmicus).
Common causes
The most common causes of blurred vision include
Refractive errors Refractive Disorders (such as nearsightedness, farsightedness, or astigmatism), the most common cause overall
What Is Astigmatism?
Astigmatism is an irregularity in the curvature of the cornea or lens. That is, the cornea or lens is curved differently in different directions. This difference causes light traveling in different planes to be focused differently. For example, vertical lines may be in focus when horizontal lines are not (or vice versa). The irregularity can be in any plane, however, and is often different in each eye. A person with astigmatism (each eye should be tested separately) tends to see certain lines more boldly (that is, in better focus) than the others. Astigmatism can be corrected with prescription eyeglasses or contact lenses. It often occurs together with nearsightedness or farsightedness. The diagram below is of a standard chart used to test for astigmatism in one eye at a time.  |
Less common causes
Rare disorders that can cause blurred vision include
Inherited disorders affecting the optic nerve, called hereditary optic neuropathies (for example, dominant optic atrophy and Leber hereditary optic neuropathy)
Corneal scarring due to vitamin A deficiency (rare in developed nations)
Evaluation
The following information can help people decide when a doctor's evaluation is needed and help them know what to expect during the evaluation.
Warning signs
In people with blurred vision, certain symptoms and characteristics are cause for concern. They include
Sudden change in vision
Severely reduced vision, particularly in only one eye, even if the symptoms began gradually
Loss of a specific area in the field of vision (called a visual field defect)
Human immunodeficiency virus (HIV) infection Human Immunodeficiency Virus (HIV) Infection or AIDS Human Immunodeficiency Virus (HIV) Infection or other condition affecting the immune system (for example, use of chemotherapy or other immune suppressing drugs)
When to see a doctor
People who have warning signs should usually go to an emergency department right away. People who have a bodywide disorder that sometimes causes retinal damage (for example, diabetes, high blood pressure, or sickle cell disease) should see an eye doctor as soon as is practical, usually within a few days. However, if vision has been deteriorating gradually for months or years but has not been severely impaired and there are no warning signs, waiting a week or longer is usually not harmful.
Eye examinations should be done by an ophthalmologist or optometrist. Ophthalmologists are medical doctors who specialize in the evaluation and treatment (surgical and nonsurgical) of all types of eye disorders. Optometrists are health care practitioners who specialize in the diagnosis and treatment of refractive errors (which are treated by prescribing glasses or contact lenses). However, optometrists can often diagnose certain other eye problems and then refer people to an ophthalmologist for treatment. People should usually see an ophthalmologist if they have warning signs.
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 of the blurred vision and the tests that may need to be done (see table Some Causes and Features of Blurred Vision Some Causes and Features of Blurred Vision 
).
Doctors ask many questions about the person's symptoms because it is important to understand exactly what the person means by blurred vision. For example, people who have actually lost vision in part of their visual field (visual field defect) may describe this sensation only as blurred vision. The presence of other eye symptoms, such as eye redness, sensitivity to light, floaters, a sensation of sudden flashes of light that can look like lightning, spots, or stars (photopsias), and pain at rest or with eye movement, helps doctors determine the cause. Doctors also ask about the effects of darkness (night vision) and bright lights (for example, causing blur, star bursts, or halos) and whether the person wears corrective lenses.
Doctors also ask questions about symptoms of possible causes and about the presence of disorders that are known to be risk factors for eye disorders (for example, high blood pressure, diabetes, HIV infection or AIDS, and sickle cell disease).
Examination of the eyes may be all that is necessary.
Testing visual acuity (sharpness of vision) is the first step. Ideally, acuity is measured while the person stands about 20 feet (6 meters) from a standard eye chart (Snellen chart) posted or projected on a wall. Each eye is measured separately while the other eye is covered. Visual acuity is measured with and without the person's own glasses. Sometimes the doctor has the person look through a device that has a pinhole. This device can usually correct refractive errors almost completely but does not correct vision that is blurred due to other causes.
The eye examination is also important. The doctor carefully examines the entire eye using an ophthalmoscope What Is a Slit Lamp? 
(a light with magnifying lenses that shines into the back of the eye), slit lamp Tonometry A person who has eye symptoms should be checked by a doctor. However, some eye disorders cause few or no symptoms in their early stages, so the eyes should be checked regularly (every 1 to 2... read more (an instrument that enables a doctor to examine the eye under high magnification), or both. Often the eyes are dilated for ophthalmoscopy with eye drops. Pressure inside the eye (intraocular pressure) is measured (called tonometry Tonometry A person who has eye symptoms should be checked by a doctor. However, some eye disorders cause few or no symptoms in their early stages, so the eyes should be checked regularly (every 1 to 2... read more ).
Symptoms and findings during the eye examination often help suggest a cause. For example, if visual acuity returns while corrective lenses or a pinhole device is used and the remainder of the eye exam is normal, simple refractive error is usually the cause of blurring.
Testing
Testing depends on the suspected cause of the blurred vision (see table Some Causes and Features of Blurred Vision Some Causes and Features of Blurred Vision ). People with symptoms of bodywide disorders should have appropriate testing. For example, testing might include blood tests for diabetes, systemic lupus erythematosus, or HIV infection, blood pressure monitoring, and possibly electrocardiography if high blood pressure or a kidney disorder is suspected.
Treatment
Essentials for Older People
Although some decrease in visual acuity normally occurs with aging, acuity normally is correctable to 20/20 with refraction, even in very old people.
Key Points
Refractive error is the most common cause of blurred vision.
If refractive error is the problem, corrective lenses or surgery (particularly if cataracts are the cause of the change in refractive error) can usually correct the blurriness.
Doctors check visual acuity and determine whether glasses or a pinhole device corrects the problem, and if it does not, they dilate the eye with drops and carefully examine the retina.
People with warning signs should see a doctor as soon as possible.
Because glaucoma is common, intraocular pressure is usually measured.
