What Causes Flashing Lights at the Side of Your Eyes?

The most common cause of flashing lights at the side of your eyes is the vitreous gel inside the eye pulling on the retina as it shrinks and separates with age, a process called posterior vitreous detachment (PVD). This mechanical tugging stimulates retinal cells in a way the brain interprets as brief arcs or streaks of light, even though no actual light is entering the eye. But PVD is only the leading explanation among several, and the distinction matters because some causes are harmless while others signal a genuine emergency.

How the Vitreous Gel Creates Flashes

The vitreous is a clear, jelly-like substance that fills the space between the lens and the retina. In younger eyes it is firmly attached to the retinal surface, but over decades it gradually liquefies and contracts. At some point, usually after your fifties or sixties, it peels away from the retina. That separation is PVD, and it is extremely common. Most people will experience it in at least one eye during their lifetime.

The flashes happen because the vitreous does not always separate cleanly. Where it still clings to the retina, the shrinking gel exerts a tug. Retinal photoreceptor cells respond to any stimulus, mechanical or optical, so that tug registers as a flash of light. People typically describe it as a quick arc or streak in their peripheral vision, often more noticeable in dim lighting or at night. The flashes tend to recur over days to weeks as the vitreous continues to pull away, and they usually stop once the separation is complete.

PVD on its own is not dangerous. The concern is what it can drag along with it. In a prospective study of over a thousand patients referred with acute-onset flashes and floaters, roughly nine in ten had PVD confirmed at their first assessment. Among those PVD eyes, about one in ten had a retinal tear, and a smaller number had progressed to retinal detachment.¹ That ratio is worth keeping in mind: PVD is overwhelmingly benign, but its symptoms overlap almost perfectly with the symptoms of something serious.

When Flashes Signal a Retinal Tear or Detachment

A retinal tear forms when the vitreous pulls hard enough to rip the thin retinal tissue rather than simply peeling away. Fluid can then seep through the tear and lift the retina off the wall of the eye, leading to retinal detachment. Detachment is a medical emergency because the detached retina loses its blood supply and the photoreceptors begin to die. Untreated, it causes permanent vision loss in the affected area.

The tricky part is that the flashes from a tear feel identical to the flashes from a harmless PVD. Both are peripheral, both are brief, and both are caused by the same mechanical process. A few accompanying signs raise the alarm:

A sudden shower of new floaters: a burst of tiny dark spots or cobweb-like strands, especially if they appear abruptly alongside the flashes.
A shadow or curtain effect: a dark area creeping in from the side or top of your visual field, which suggests the retina is already detaching.
A rapid increase in flash frequency: flashes that were occasional and suddenly become constant or much more frequent.

The prospective study mentioned earlier also found that about three percent of patients whose initial PVD exam was clean went on to develop a retinal tear within the following two months.¹ That delayed-onset risk is why eye doctors often schedule a follow-up visit even when the first exam looks reassuring. If you notice a change in the pattern of your flashes or floaters after an initial evaluation, go back sooner rather than waiting for the scheduled appointment.

Migraine Aura and the Brain’s Own Light Show

Not all peripheral flashes originate in the eye. Migraine with aura produces visual disturbances that can look like shimmering zigzag lines, flickering lights, or expanding bright spots at the edges of vision. These visual phenomena come from the brain, not the retina, and they behave differently from vitreous-related flashes in ways that help tell the two apart.

The underlying mechanism is a wave of electrical and chemical activity that spreads slowly across the visual cortex, a phenomenon known as cortical spreading depolarization. As this wave moves across cortical tissue, it first excites neurons (producing the bright visual effects) and then suppresses them (producing a temporary blind spot that often follows the bright phase).² The whole episode typically builds over five to thirty minutes and then fades, often followed by a headache. That gradual buildup and resolution is a key distinguishing feature. Vitreous flashes are instantaneous, like a camera flash or a bolt of lightning; migraine aura tends to shimmer and evolve.

Migraine aura also affects both eyes simultaneously, even though it might seem to favor one side. Because the visual cortex processes input from both eyes, a wave traveling across one hemisphere creates a disturbance in the same side of the visual field for both eyes. If you close one eye and the visual disturbance persists, that strongly suggests a brain origin rather than an eye problem. Vitreous traction flashes are strictly monocular: they affect only the eye where the pulling is happening.

Some people experience visual aura without any headache following it. This is sometimes called “silent migraine” or “acephalgic migraine,” and it becomes more common with age. If you are over fifty and suddenly start getting shimmering zigzag patterns in your peripheral vision that resolve within half an hour and leave no lasting symptoms, silent migraine is a likely explanation, but it is still worth having it checked to rule out other causes.

Retinal Migraine and Monocular Flashing

Retinal migraine is a much rarer condition that produces visual disturbances in only one eye. Unlike typical migraine aura, the problem here is thought to involve vasospasm or reduced blood flow within the retinal vasculature itself. Patients may experience flickering lights (photopsia), blurred vision, or temporary partial blindness in one eye, sometimes followed by headache.³

This is a diagnosis of exclusion, meaning doctors need to rule out retinal tears, detachment, and vascular events like a transient ischemic attack before settling on retinal migraine. The monocular nature of the flashing can create genuine confusion: it mimics PVD symptoms in being limited to one eye, but the episodes tend to be shorter (minutes rather than weeks), and the flashes often come with a temporary dimming of vision that PVD does not cause. If you have recurrent episodes of one-sided visual flickering accompanied by brief vision loss, the evaluation will likely include both a dilated eye exam and neurological workup.

Optic Neuritis and Movement-Triggered Phosphenes

People with inflammation of the optic nerve, a condition called optic neuritis, sometimes report seeing flashes of light specifically triggered by eye movement. These phosphenes appear when you look to the side or quickly shift your gaze, and they are distinct from vitreous flashes in that they are linked to movement rather than occurring spontaneously. The phenomenon shares characteristics with the Lhermitte sign, which is a brief electric shock sensation running down the spine when the neck is flexed, seen in conditions that affect the myelin sheath of nerves.⁴

Optic neuritis is most commonly associated with multiple sclerosis, though it can also occur in isolation or alongside other autoimmune conditions. The flashes in optic neuritis are not caused by anything happening at the retina or within the vitreous. Instead, the damaged or demyelinated optic nerve fibers become hypersensitive to mechanical stimulation, and the slight stretching that occurs during eye movements is enough to trigger an electrical signal that the brain reads as light. These movement-dependent phosphenes are a useful clinical clue because they point toward the optic nerve rather than the retina, which changes the diagnostic pathway considerably.

If your flashes consistently happen when you move your eyes rather than at random, and especially if you also have pain behind the eye or a gradual decline in color vision or overall sharpness, optic neuritis is worth discussing with a doctor.

Flashes After Cataract Surgery

Peripheral flashing lights are a surprisingly common complaint after cataract surgery, even when the procedure itself goes perfectly. These unwanted visual phenomena are called dysphotopsias, and they come in two types. Positive dysphotopsias are the ones that produce visible light artifacts: patients describe glare, light streaks, starbursts, arcs, rings, halos, or frank flashes of light.⁵ Negative dysphotopsias, by contrast, produce a dark shadow or crescent, often in the far periphery of vision.

The cause is optical rather than neurological or mechanical. The artificial intraocular lens (IOL) implanted during cataract surgery has different optical properties than the natural lens it replaced. Light entering the eye at steep angles can interact with the edge of the IOL in ways that create internal reflections or shadows. One study using optical modeling showed that a distinct shadow forms between the image produced by the IOL and a second, larger image from light that misses the IOL entirely; this shadow softens rapidly when the pupil dilates even slightly.⁶ That is why post-surgical dysphotopsias are often most bothersome in bright conditions when the pupil is constricted.

The good news is that most people find these symptoms fade over weeks to months as the brain adapts to the new optics. A minority continue to be bothered long-term, and in those cases options include IOL exchange or secondary lens implantation, though these carry their own risks. If you are experiencing flashing or arcs of light after cataract surgery and your surgeon has confirmed the retina looks healthy, dysphotopsia is the most likely explanation.

Less Common Causes Worth Knowing About

Several rarer conditions can produce flashing or flickering in peripheral vision. They are unlikely to be the first explanation, but they are worth awareness, particularly if the more common causes have been ruled out.

Intraocular tumors, including choroidal melanoma, can occasionally cause visual symptoms including flashes, though they more typically present with visual field loss or distortion. One case report described a patient whose choroidal melanoma was detected during evaluation of progressive visual field loss, highlighting that tumors inside the eye can sometimes masquerade as more routine problems.⁷ The takeaway is not to worry about cancer every time you see a flash, but to make sure persistent or unusual visual symptoms get a thorough dilated eye exam.

Visual snow syndrome is a neurological condition in which people see constant tiny flickering dots across their entire visual field, somewhat like static on an old television. Although the primary symptom is the “snow” itself, many people with visual snow also report flashes, afterimages, and enhanced sensitivity to light. Standard eye exams typically come back normal because the problem originates in the brain’s visual processing rather than in the eye.⁸ If your visual disturbances are continuous rather than episodic, and if they are present in both eyes and across your entire field of vision, visual snow syndrome is a possibility worth raising with a neurologist.

And then there is an exotic one: space radiation. Astronauts have reported seeing flashes of light, particularly when trying to sleep in dark environments, since the earliest crewed space missions. These phosphenes are believed to result from high-energy cosmic ray particles striking the retina directly, causing individual photoreceptor cells to fire as if hit by light.⁹ This is obviously not a concern on the ground, where the atmosphere shields us from most cosmic radiation, but it offers a vivid illustration of the principle that the retina responds to any energy input, not just visible light.

How Doctors Tell the Causes Apart

When you show up reporting flashes of light, the first question your eye doctor will try to answer is whether something is happening inside the eye or whether the signal is coming from somewhere else in the visual pathway. A dilated fundus exam lets the doctor look directly at the retina and vitreous to check for PVD, tears, detachment, or other structural problems. This remains the standard first step.

Imaging technology adds another layer. Optical coherence tomography (OCT) is a non-invasive scan that produces cross-sectional images of the retina and can detect vitreous separation that is invisible to the naked eye during a clinical exam. One study found that clinical examination identified PVD in roughly half of confirmed cases, while OCT caught it in closer to four-fifths, with low agreement between the two methods.¹⁰ This does not mean the clinical exam is useless, but it does mean OCT has become an important complementary tool, especially in ambiguous cases.

If the eye exam is clean and the flashes persist, the evaluation shifts toward neurological causes. The pattern of the flashes guides the next steps: bilateral shimmering with a gradual onset and resolution suggests migraine aura; monocular flickering with transient vision loss points toward retinal migraine or vascular issues; movement-triggered flashes raise the possibility of optic neuritis. Brain imaging may be ordered if there is concern about demyelinating disease or other central nervous system pathology.

Practical Decisions You Can Make Right Now

Knowing the landscape of causes helps, but what most people really want to know is whether their particular flashes need urgent attention. A few practical guidelines can help you triage.

See someone within a day or two if you experience a sudden onset of flashes you have never had before, especially if they are accompanied by new floaters. This is the classic PVD presentation, and while PVD itself is harmless, the small but real risk of a retinal tear makes a prompt dilated exam worthwhile. If you also notice a shadow creeping into your peripheral vision, that suggests possible detachment and warrants same-day evaluation.

If you have had migraine with aura before and your current visual disturbance follows the familiar pattern, it is less urgent, but any change in the pattern (longer duration, new symptoms like weakness or speech difficulty, or aura without subsequent headache when you always used to get one) deserves a fresh look from a doctor. New-onset aura in someone over fifty who has never had migraines is worth evaluating to rule out other vascular or neurological causes.

Post-surgical flashes after cataract or other intraocular procedures should be mentioned at your follow-up visit. They are common and usually self-limiting, but your surgeon needs to confirm that the retina is intact and that the symptoms are optical artifacts rather than signs of a complication.

For flashes that are chronic, present in both eyes, and accompanied by persistent visual “static,” consider asking about visual snow syndrome. It is underdiagnosed, partly because standard eye exams return normal results and many patients are told there is nothing wrong. Knowing the name of the condition can help you find a specialist who takes it seriously.

Risk Factors That Increase Your Chances

Certain groups are more prone to flashing lights, and the reasons map back to the causes. Nearsightedness (myopia) is one of the strongest risk factors for both PVD and retinal tears, because myopic eyes are longer than average and the vitreous is stretched thinner. Prior eye surgery, including cataract removal and LASIK, can also accelerate vitreous changes. A history of eye trauma, even from years earlier, increases the risk of retinal tears developing when PVD eventually occurs.

For migraine-related flashes, the relevant risk factors are those for migraine itself: family history, hormonal changes, stress, and sleep disruption. Women experience migraine with aura more frequently than men, though the gap narrows after menopause.

Age affects nearly every cause. PVD becomes increasingly common after fifty. The vitreous in most people over seventy has undergone significant liquefaction. Migraine aura can appear at any age, but acephalgic migraine (visual aura without headache) becomes more common in middle age and beyond. Optic neuritis peaks in younger adults, typically between twenty and forty. The age profile of your symptoms, combined with their character and duration, gives doctors a useful starting framework even before any tests are done.