Flashes of light in your vision are most commonly caused by physical tugging on the retina, the light-sensitive tissue lining the back of your eye. The single most frequent culprit is an age-related change inside the eye called posterior vitreous detachment, though migraines, blood pressure drops, and more serious conditions like retinal tears can also be responsible. Whether flashes appear in one eye or both, and what pattern they take, reveals a lot about where the problem originates.
How the Eye Creates “False” Light
Your retina is designed to convert light into electrical signals. But it can’t tell the difference between actual light hitting it and a physical force pressing or pulling on it. When something mechanically stimulates retinal cells, they fire the same electrical signals they would if light were present, and your brain interprets that signal as a flash. This happens through pressure-sensitive channels on retinal neurons that open in response to mechanical force, triggering a burst of activity identical to a light-evoked response.
This is why you might see a flash when you rub your eyes hard or get hit near the eye socket. It’s also why internal changes inside the eye, like the vitreous gel pulling away from the retina, produce flashes that look completely real even though no light is involved.
Posterior Vitreous Detachment: The Most Common Cause
The vitreous is a clear, gel-like substance that fills the space between your lens and your retina. As you age, this gel gradually liquefies. By age 50, roughly 25% of the vitreous has shifted from gel to liquid. By 80, that number reaches 62%. As the gel shrinks and collapses, it can pull away from the retina in a process called posterior vitreous detachment, or PVD.
PVD is extremely common. It affects up to 24% of people in their 50s and 87% of people in their 80s. When the vitreous tugs on the retina during this separation, you see brief flashes of light, often described as lightning streaks, typically in your peripheral vision. These flashes tend to be more noticeable in the dark or when you move your eyes quickly. Most people also notice new floaters at the same time: small dark spots, threads, or cloud-like shapes drifting across their field of view.
For most people, PVD is harmless. The vitreous eventually separates completely, the tugging stops, and the flashes fade over weeks to months. But in about 14% of people who show up with new flashes and floaters, an eye exam reveals a retinal tear, which is why new-onset flashes always warrant a professional evaluation.
Retinal Tears and Detachment
Sometimes the vitreous doesn’t separate cleanly. If it’s stuck firmly to the retina at certain points, the pulling force can tear the retinal tissue. A retinal tear on its own can often be repaired quickly, but if fluid seeps through the tear and lifts the retina off the wall of the eye, it becomes a retinal detachment, which threatens your vision.
The flashes from a retinal tear feel similar to those from a simple PVD, which is precisely why they’re so important to get checked. Warning signs that suggest something more serious include:
- A shadow or curtain creeping across part of your vision
- A sudden increase in the number of floaters, especially if they look like a shower of tiny dots
- Blurred or reduced vision that comes on quickly
If a tear is found early, your eye doctor can seal it with a laser or a freezing probe applied to the outside of the eye. Both techniques create a small scar around the tear that bonds the retina back in place and prevents fluid from getting underneath. These are outpatient procedures, and when done before the retina detaches, they’re highly effective.
Migraine Aura
Flashes caused by migraines look and behave very differently from retinal flashes. Migraine aura is a neurological event originating in the brain, not the eye, so the visual disturbance typically appears in both eyes at the same time and occupies the same region of your visual field in each eye.
The classic migraine aura starts as a small bright spot or hole of light, then expands into a crescent or C-shaped pattern with shimmering zigzag lines along its leading edge. This pattern is sometimes called a fortification spectrum because it resembles the jagged walls of a medieval fort. The whole episode usually lasts 10 to 30 minutes and may be followed by a headache, though some people get the visual aura without any head pain at all. You might also experience partial vision loss in the area behind the shimmering pattern.
The key distinction: migraine flashes are geometric, they expand and evolve over minutes, and they affect both eyes. Retinal flashes are brief, spark-like, and confined to one eye.
Neurological and Systemic Causes
When flashes appear simultaneously in both eyes but don’t follow the typical migraine pattern, they may point to a neurological or whole-body cause. A useful clinical rule: flashes in one eye suggest a problem inside that eye, while flashes in both eyes suggest something affecting the brain or circulatory system.
Potential causes in this category include transient ischemic attacks (brief episodes of reduced blood flow to the brain), significant blood pressure changes, blood sugar abnormalities, and elevated pressure inside the skull. Neurological flashes tend to appear in the same spot in both eyes (the left half or right half of your visual field), while systemic causes more often produce central flickering or diffuse shimmering across your vision.
Certain medications can also trigger flashes as a side effect. If flashes start shortly after beginning a new prescription, that connection is worth raising with your doctor. Additional red flags that point toward a neurological cause include weakness or numbness in your face or limbs, difficulty swallowing, or trouble with speech or balance.
How Quickly To Get Checked
The urgency depends on what else is happening alongside the flashes. Current referral guidelines break it down into three tiers. If you have new flashes or floaters along with a shadow or curtain across your vision, same-day evaluation is recommended, as this combination suggests the retina may already be detaching. If you have new flashes and floaters without any vision loss, you should be seen within one to two days. And if flashes are isolated, without new floaters or vision changes, an appointment within a week is generally appropriate.
The eye exam itself is straightforward. Your doctor will dilate your pupils with drops and examine the retina using a bright light and a magnifying lens, checking for tears, holes, or areas where the retina has lifted. In some cases, particularly if the view is obscured by blood or dense floaters, an ultrasound of the eye can provide a clear picture of the retina’s condition.
One Eye vs. Both Eyes: A Quick Guide
Paying attention to whether flashes appear in one eye or both is the single most useful piece of information you can bring to your doctor. You can test this by closing one eye at a time when a flash occurs. If the flash disappears when you cover one eye, it’s coming from the other eye and likely has an ocular cause like PVD or a retinal tear. If it persists regardless of which eye is open, the signal is coming from the brain, pointing toward migraine, a vascular event, or another neurological source.
Occasional, brief flashes that have been happening for years and haven’t changed are far less concerning than a sudden new onset, especially when accompanied by new floaters or any loss of vision. It’s the change in pattern that matters most.