Eye flashes are a person's perception of bright flashes of light, flickering lights, or streaks of light that do not correspond to external sources. Eye floaters are specks or strings that appear to move through a person's field of vision but do not correspond to external objects. Flashes and floaters are common.
Causes
Eye flashes result when something besides light from the environment stimulates the retina, which is the light-sensing structure at the back of the eye. This stimulation causes the retina to send a signal to the brain. The brain may interpret the signal as a simple, sudden flash of light that can look like lightning, spots, or stars (photopsia). Photopsias can occur when the eyes are rubbed.
Eye floaters are seen when an object in the eyeball casts a shadow on the retina, which is the light-sensing structure at the back of the eye. This is often interpreted as an apparent object or objects floating in the external field of vision. A person may think they are seeing a swarm of flies, gnats, or specks but then realize they are not actually present.
An Inside Look at the Eye
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The most common cause of eye flashes and floaters is
Shrinking of the jellylike substance that fills the eyeball (vitreous humor)
In early adulthood people may notice occasional floating strands in the vision of one or both eyes relating to contraction of the vitreous humor. These types of floaters (called contraction or idiopathic vitreous floaters) rarely indicate anything harmful. Between about age 50 to 75 years, however, the vitreous humor continues to shrink and tugs on the retina from time to time. These tugs may stimulate the retina, giving the illusion of light, seen as flashes. Over time, the vitreous humor usually pulls completely away from the retina (vitreous detachment) as a normal part of aging.
Less common but more serious causes include
Tear in the retina
Bleeding in the vitreous humor
Inflammation of the vitreous humor
Sometimes migraines Migraines A migraine headache is typically a pulsating or throbbing pain that ranges from moderate to severe. It can affect one or both sides of the head. It is often worsened by physical activity, light... read more cause vision symptoms. These vision symptoms may be white, jagged flashing lines that appear first in the middle of the field of vision and then spread across the entire field of vision (not single objects like floaters). They typically then resolve over about 20 minutes, disappearing first from the peripheral field of vision and last from the center of the field of vision. People may not have a headache with them. These symptoms are called ocular or visual migraine. People can also have similar symptoms or lose vision in part of an eye for about 10 to 60 minutes, often before a migraine headache begins (called migraine aura). In these cases, symptoms are caused by a phenomenon in the brain, not in the retina.
Flashes of light can also result from a blow to the back of the head ("seeing stars"), probably because of stimulation of the part of the brain where vision is interpreted.
Tumors (for example, lymphoma Overview of Lymphoma Lymphomas are cancers of lymphocytes, which reside in the lymphatic system and in blood-forming organs. Lymphomas are cancers of a specific type of white blood cells known as lymphocytes. These... read more 
) of the eye and vitritis (inflammation of the vitreous humor) are rare causes of floaters. Foreign objects Corneal Abrasions and Corneal Foreign Bodies Foreign bodies in the cornea cause abrasions, resulting in pain and redness, and lead to infections, even after they are removed. Most of these injuries are minor. (See also Overview of Eye... read more in the eye can cause floaters but usually cause other symptoms, such as vision loss 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 , 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 , or 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 , that are more troublesome than floaters.
Evaluation
Not every instance of eye flashes or floaters requires an immediate evaluation by a doctor. 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 flashes or floaters, certain symptoms and characteristics are cause for concern. They include
Sudden increase in floaters
Repeated, often lightning-like flashes of light
Complete or partial loss of vision (often feeling like part of the vision is covered by a shade or curtain)
Recent eye surgery or eye injury
Eye pain
When to see a doctor
Although most often flashes and floaters are not serious, people who have warning signs should see an eye doctor as soon as possible. They may have a serious vitreous or retinal disorder, and waiting a few days, or sometimes even hours, may lead to a permanent loss of vision. People without warning signs who have begun to notice a few flashes and floaters should see a doctor when practical, although a delay of a few days or more is unlikely to be harmful. People who have had flashes or floaters for some time and have no other symptoms should have an eye examination at some point, but timing is not critical.
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 flashes or floaters and the tests that may need to be done (see table Some Causes and Features of Eye Flashes and Floaters Some Causes and Features of Eye Flashes and Floaters 
).
Doctors ask the person to describe the flashes and floaters and then ask
When the person noticed symptoms
What characteristics the flashes floaters have (for example, shapes, movement, or whether they recur)
Whether symptoms occur in one or both eyes
Whether the person sees flashing lights or whether any part of vision is missing or seems covered by a curtain
Whether the person has had an eye injury or eye surgery
Whether other symptoms (such as blurred vision, eye redness, eye pain, or headaches) are present
Whether the person is nearsighted
Whether the person has disorders that may affect vision, such as diabetes or immune system disorders (for example, AIDS)
The eye examination is the most important part of the physical examination. Doctors check sharpness of vision, eye movements, and the pupil's response to light. They also check the eyes for redness and the visual field for areas of vision loss.
Ophthalmoscopy is the most important part of the eye examination. Doctors first use drops to dilate the pupils. Then they use an ophthalmoscope What Is an Ophthalmoscope? 
(a light with magnifying lenses that shines into the back of the eye) to examine the inside of the person's eyes, including as much of the retina as possible. If one of the serious causes of flashes or floaters seems possible, an evaluation by an ophthalmologist is critical. An ophthalmologist is a medical doctor who specializes in the evaluation and treatment (surgical and nonsurgical) of eye disorders.
Pressure in the eye (intraocular pressure) is measured after a drop of anesthetic is put into the eye.
Doctors put a drop of fluorescein stain in the eye and then use a slit lamp What Is a Slit Lamp? 
(an instrument that enables a doctor to examine the eye under high magnification) to examine the entire eye.
Testing
Doctors can often identify less serious causes of eye flashes and floaters during an examination, but if a serious disorder seems possible, doctors refer people to an ophthalmologist to make a diagnosis. The ophthalmologist does more detailed ophthalmoscopy and may order testing. For example, for vitreous inflammation due to infection, testing to identify organisms suspected of causing infection may be needed.
Treatment
Vitreous contraction floaters are floaters due to shrinking of the jellylike substance that fills the back part of the eyeball, called the vitreous humor, and require no treatment. Many flashes are also due to shrinking of vitreous substance and do not require treatment.
Sometimes if a person has many floaters that interfere with vision, doctors may use a hollow needle to remove the vitreous humor from the eye and replace it with salt water. This surgical procedure is called vitrectomy. However, many doctors think vitrectomy should not be done for floaters because the procedure may cause detachment of the retina or cataracts and because sometimes floaters remain afterward.
Other disorders causing symptoms are treated. For example, surgery is done to repair a detached retina. Antimicrobial drugs are used for infections causing vitreous inflammation.
Key Points
People who have flashes or floaters but no warning signs rarely have a serious disorder.
People with flashes or floaters and warning signs may need to be referred to an ophthalmologist.
