Flashes and floaters: which ones matter

Floaters are the specks, threads and cobwebs that drift across your vision, most visible against a bright plain background like the sky or a white wall. They move when you move your eye and lag slightly behind, which is how you know they are inside the eye rather than on the surface.

They are extremely common and usually harmless. But a sudden change in them is one of the few eye symptoms where the timing of your response genuinely determines the outcome.

What they are

The cavity behind the lens is filled with vitreous, a clear gel held together by a fine collagen scaffold. With age the gel liquefies and the collagen clumps, and the clumps cast shadows on the retina. Those shadows are floaters.

Eventually the vitreous shrinks enough to peel away from the retina at the back of the eye. This is posterior vitreous detachment, it happens to most people at some point, and it is not in itself dangerous.

What makes it worth attention is the moment of separation. As the gel pulls away it tugs on the retina. The tug is perceived as a flash of light — usually a brief arc in the peripheral vision, more noticeable in the dark. And where the vitreous is unusually firmly attached, the pull can tear the retina instead of separating cleanly.

Why a tear matters

A retinal tear allows fluid to pass behind the retina and lift it away from the wall of the eye. That is a retinal detachment, and a detached retina loses its blood supply and its function. If the central macula detaches, the sharp vision it provides may not fully recover even after successful surgery.

A tear found before it detaches is sealed with laser in a clinic appointment lasting minutes. A detachment requires surgery, carries a real chance of incomplete recovery, and is more urgent the closer it comes to the centre.

The gap between those two outcomes is a matter of days, and it is the entire reason this symptom is treated as urgent.

What to act on

Seek assessment within twenty-four hours for:

• A sudden shower of new floaters — many at once, where before there were a few or none.
• Flashing lights, particularly repeated arcs in the periphery, in the dark.
• A curtain, shadow or veil coming across part of your vision from one side. This means detachment has already begun, and it is an emergency.
• A sudden loss or blurring of vision in one eye.
• A single large new floater, sometimes described as a ring or a fly, which can mark the point of separation.

The assessment requires dilating drops and a careful examination of the peripheral retina. It is not something that can be judged by how the eye looks or by how good the central vision is — a substantial tear can sit in the far periphery with acuity entirely normal.

What does not need urgent attention

Floaters you have had for years that have not changed. A slow, gradual increase over a long period. Floaters that are annoying but stable.

These are worth mentioning at a routine examination, and they are not an emergency.

Who is at higher risk

People who are significantly short-sighted — a longer eye means a more stretched, thinner retina — anyone who has had cataract surgery, anyone with a previous detachment in either eye, anyone with a family history of detachment, and anyone who has had a significant blow to the eye or head.

If you are in one of those groups, the threshold for getting new symptoms checked should be lower, not higher.

Living with ordinary floaters

They usually become less noticeable, partly because they settle out of the line of sight and partly because the brain learns to disregard them. Looking up and down briskly can shift one out of the way.

Treatments exist — laser to break up large floaters, or vitrectomy to remove the gel — but both carry risks that are disproportionate for a nuisance, and they are reserved for the small number of cases where floaters genuinely disable someone's vision.

The useful discipline is simply to know your own floaters, so that you notice when something new arrives.