Eye Floaters: When They Are Harmless, and When to Worry

You notice one against a white wall, or in a blue sky, or on a blank page. A translucent thread, or a cluster of dots, drifting slowly across your view. You try to look straight at it and it slides away, because it moves with your eye.

Most of the time this is unremarkable. But because it is unremarkable most of the time, people learn to ignore floaters generally, and that habit is the actual risk. So the important part goes first.

With that stated, the rest of this is reassurance, because the common case is genuinely benign.

What you are actually seeing

The bulk of your eye is filled with vitreous humour, a clear gel that is roughly 99% water held in a scaffold of fine collagen fibres. At birth it is uniform and firmly attached to the retina.

Over decades the gel does two things. It liquefies in pockets, and the collagen scaffold clumps together into strands and knots. Those clumps sit in the light path, and they cast a shadow on the retina behind them. What you perceive as a floater is not the clump. It is the shadow of the clump, which is why they look soft-edged and why they sharpen when your pupil constricts in bright light.

They drift because the gel moves when your eye moves, then settles a moment later. That lag is characteristic, and it is why chasing one with your gaze never works.

The event that makes them suddenly worse

As the vitreous liquefies it also shrinks, and eventually it peels away from the retinal surface. This is a posterior vitreous detachment, and despite the alarming name it is a normal ageing event that most people go through, typically from the fifties onward and earlier in short-sighted eyes.

When it happens you often notice it. The peeling releases a crop of new floaters at once, frequently including a large ring-shaped one where the gel was attached around the optic nerve. And as the gel tugs on the retina during separation, it stimulates the photoreceptors mechanically. Your retina has no way to report touch. It reports everything as light, which is why traction is perceived as a flash.

For most people the separation completes cleanly and the symptoms settle over weeks. The problem is the minority in whom the gel is stuck somewhere firmly enough that, instead of peeling away, it tears the retina.

How often that goes wrong

This is the number worth carrying. Among people who present with new floaters or flashes from a symptomatic posterior vitreous detachment, roughly 10 to 15% are found to have a retinal tear.

That is high enough to take seriously and low enough not to panic about. It also means the sensible response is not to assume the worst, but to get looked at promptly and let someone check, because the examination settles it quickly and a tear caught early is usually treated the same day with laser or freezing treatment to seal it.

Timing matters too. The risk of a tear is concentrated in the first four to six weeks after symptoms begin, which is why clinicians often want to see you again inside that window even if the first examination was clear. If you are offered a follow-up appointment for symptoms that have settled, that is why. Keep it.

What the examination involves

Worth knowing, because it sets expectations and because it explains why a quick look is not sufficient.

Retinal tears usually happen in the far periphery, which cannot be seen through an undilated pupil or with a slit lamp alone. A proper assessment means dilating drops, then indirect ophthalmoscopy examining the full 360 degrees of the retinal periphery, often with gentle indentation to bring the extreme edge into view. It is thorough rather than painful, and the dilation will blur your near vision and leave you light-sensitive for several hours.

Two practical consequences: arrange not to drive yourself home, and bring sunglasses.

If they turn out to be harmless

The honest answer is that there is not much to do, and that is usually fine.

Floaters do not dissolve, but two things happen. Some settle out of the direct visual axis over months. More importantly, your visual system learns to suppress them, in the same way it suppresses your blind spot and the blood vessels that sit in front of your own retina. Most people who were tormented by a new floater find that a year later they only notice it when they go looking.

Surgical options exist for the small number whose floaters genuinely impair function. Vitrectomy removes the gel and works, but it is intraocular surgery with real risks including cataract formation and, in a small percentage, retinal detachment. Laser vitreolysis is less invasive and the evidence for it is weaker. Neither is a reasonable trade for mild annoyance, and any ophthalmologist will say so.

Things that are not floaters

A few phenomena get filed under floaters and are worth separating.

• Migraine aura. A shimmering, often zigzag arc that expands across the field over 20 to 30 minutes and then resolves, sometimes with no headache at all. It usually affects both eyes, because it originates in the visual cortex rather than the eye. First-time aura is worth discussing with a doctor.
• Blue-field entoptic phenomenon. Tiny bright dots darting along curved paths, most visible against a bright blue sky. Those are your own white blood cells passing through retinal capillaries. Entirely normal.
• Afterimages. Persist for seconds after a bright light and fade. Floaters do not fade.

The useful discriminator is behaviour over time. Floaters drift with eye movement and stay for months or years. Aura marches across the field and disappears within the hour. Anything that arrives suddenly with flashes belongs in the first section of this article.

Floaters are also one of the few areas of eye health where the correct advice really is to see someone rather than read more, which is the same conclusion we keep reaching about protecting vision as you age: the exam is the part that cannot be substituted.