Seeing flashes of light, especially in the peripheral vision, is a visual phenomenon known as photopsia. These perceived lights are not caused by external sources but originate from mechanical or electrical stimulation within the eye or the brain. While often startling, these flashes can stem from a variety of causes, ranging from a normal, age-related change in the eye to a serious condition requiring immediate medical intervention.
Common Causes Related to Aging Vitreous
The most frequent cause of seeing flashes relates to the natural aging process of the eye’s internal structure. The main chamber of the eye is filled with the vitreous humor, a clear, gel-like substance attached to the retina at the back of the eye. With age, the vitreous undergoes syneresis, where its gel consistency breaks down and becomes more liquid-like.
This liquefaction causes the vitreous to shrink and pull away from the retina, an event known as Posterior Vitreous Detachment (PVD). As the vitreous detaches, the fine fibers connecting it to the retina tug on the light-sensitive nerve tissue. This mechanical stimulation causes the retina to send an electrical impulse to the brain.
The brain interprets this signal as a flash of light, often described as a camera flash, a streak of lightning, or a flickering arc, typically seen in the far side of the vision. These flashes usually last for only a second and are often more noticeable in dim lighting or when the eye moves. Once the vitreous fully separates from the retina, the mechanical traction stops, and the flashes generally diminish.
Warning Signs of Retinal Damage
While Posterior Vitreous Detachment carries a small risk of creating a more serious problem, the traction on the retina can be strong enough to cause a tear in the retinal tissue if the vitreous gel is abnormally adherent. A retinal tear is serious because it allows liquid from the vitreous cavity to pass through the opening and accumulate underneath the retina.
This accumulation of fluid can peel the retina away from the underlying tissue that provides its nourishment and oxygen, leading to retinal detachment. The key symptoms that signal this progression are a sudden and dramatic increase in the number or intensity of flashes. A sudden shower of new floaters, often described as a cloud of black dots or a cobweb, is another warning sign.
The most concerning symptom is the appearance of a shadow or a dark curtain that progresses across the field of vision. This indicates that the retina has detached from the back of the eye, causing a loss of vision. Because the retina lacks pain receptors, a tear or detachment is painless, making these visual changes the only indication of the emergency.
Light Flashes Caused by Migraines and Neurological Events
Flashes of light that are not related to the internal structure of the eye often stem from neurological activity in the brain’s visual processing center. These events are most commonly associated with migraines, even in people who do not experience a headache afterward (migraine aura without headache). Unlike flashes caused by vitreous traction, migraine-related visual disturbances typically appear as shimmering, zigzag lines, or geometric patterns.
These visual auras often start as a small, flickering spot near the center of vision and then gradually expand, moving toward the periphery over a period of 20 to 30 minutes. The light patterns are usually seen in both eyes simultaneously, which helps differentiate them from retinal issues that are typically restricted to one eye. The underlying mechanism is a slow wave of electrochemical activity, known as Cortical Spreading Depression, that moves across the visual cortex.
Another, less common form is the retinal migraine, which involves temporary visual disturbances such as flashes, blind spots, or even vision loss, but only in one eye. Distinguishing between flashes originating in the eye and those from a neurological source is important, and the characteristics of the visual disturbance—such as its shape, duration, and whether it affects one or both eyes—provide the necessary clues.
What Happens During an Eye Examination
Because the symptoms of a benign aging process can mimic those of an emergency, a professional examination is necessary whenever new flashes or floaters appear. The evaluation begins with a detailed review of the symptoms, including the onset, duration, and specific appearance of the flashes. The eye care professional will then perform a dilated eye examination, or fundoscopy, which involves using drops to widen the pupil.
Dilation allows the doctor to use specialized lenses to gain a clear, magnified view of the entire retina, the vitreous, and the optic nerve. The doctor specifically looks for signs of a Posterior Vitreous Detachment, as well as any small retinal tears, holes, or areas of bleeding. In some cases, a technique called scleral indentation may be used, where gentle pressure is applied to the outside of the eye to bring the far peripheral retina into view. Only after a complete examination can the cause be accurately determined and the appropriate course of action, whether simple observation or immediate treatment, be recommended.