Floaters and flashes in the eyes

The specks, threads and semi-transparent cobwebs that drift across the field of vision and slip away the moment you look at them are familiar to almost everyone. On their own they are usually harmless. But this subject has a second half, and it is the reason to read to the end: exactly the same visual phenomenon, when it appears suddenly, can be the first and only warning of a retinal detachment, a condition measured in days and sometimes in hours. The important question here is therefore not whether floaters are dangerous, but whether they are old ones or have just arrived.

When you need an eye specialist the same day

Do not put the examination off to a convenient day and do not wait to see whether it settles, if any of the following has happened:

• a sudden shower of new floaters all at once — a swarm of midges, a rain of soot, a cloud of specks — especially if there were none before or noticeably fewer;
• flashes of light — brief lightning streaks, sparks or arcs at the edge of vision, more obvious in the dark and when the eye moves;
• a dark curtain, veil or shadow creeping in from one side, from above or from below, as though part of the view had been drawn over;
• vision has dropped abruptly, or a fixed dark patch has appeared in it;
• it all began after a blow to the eye or the head, after recent eye surgery, or after laser treatment;
• the floaters come with eye pain, redness or discomfort in bright light.

What is needed is an examination of the back of the eye with the pupil dilated. It is the only way to see a retinal tear or detachment, and a plain sight test is no substitute. Go the same day to an emergency eye service or to a hospital; if there is no way to do that, call an ambulance on the single European number 112.

Why the hurry. The retina is the thin light-sensitive layer lining the inside of the eye. When it detaches it lifts away from the layer that feeds it and begins to die for lack of oxygen. As long as the detachment has not reached the central area responsible for sharp vision, sight recovers well; once it has, it no longer recovers fully, even after successful surgery. That is why a delay of a few days costs vision, while a small tear found in time is often sealed with laser as an outpatient.

Long-standing floaters are a different matter

If the specks and threads:

• have been with you for months or years;
• have not become noticeably more numerous;
• come without any flashes;
• do not block off part of the field of vision;
• and your sharpness of vision is unchanged —

then this is an ordinary feature of the ageing vitreous, not a disease. They show up best against a pale background: a white wall, snow, the sky, a screen. They drift with the movement of the eye and lag slightly behind when the gaze stops; catching one by looking straight at it is impossible, they always slide away.

They need no treatment, and with time they intrude less: some of the opacities settle below the line of sight, and the brain stops attending to them the way it stops noticing spectacle frames. That habituation is not self-deception but a real property of the visual system. There is a condition attached, though: you can only get used to something that stays the same. Any fresh change in the picture sends you back to the section above.

Vitreous detachment, the commonest cause

The inside of the eye is filled with the vitreous, a clear gel that in youth lies firmly against the retina. With age the gel liquefies, shrinks and at some point separates from the retina. This is called posterior vitreous detachment, and it happens to most people — usually after fifty, and earlier in short-sighted eyes.

The process itself is not a disease, but it is what produces both symptoms at once. The floaters are condensations and strands of gel now drifting freely and casting their shadow on the retina; the most striking is often a large ring or comma, the imprint of the place where the gel was anchored beside the optic nerve. The flashes are a mechanical tug: as it pulls away, the gel drags on the retina, and the retina can answer in only one way — with a signal of light.

An important caveat. Vitreous detachment usually ends well, but in roughly one person in ten the retina tears in the process, and a tear can go on to become a detachment. The two cannot be told apart by how they feel: the complaints are identical. That is precisely why floaters and flashes appearing for the first time are examined rather than waited out. And even if the first examination is clear, any new deterioration means going back.

Who should be more watchful

• Short-sighted people. A myopic eye is longer, the peripheral retina is stretched and thinner; vitreous detachment arrives earlier and tears are commoner. The stronger the short sight, the more this counts.
• People over fifty, simply because of age-related change in the gel.
• Anyone who has had eye surgery, above all lens replacement for cataract, and also laser treatment.
• After injury to the eye or head, whether straight away or weeks later.
• Anyone who has already had a detachment in the other eye, and anyone with a detachment in the family.
• In diabetes: altered retinal vessels can bleed into the vitreous, and the person then sees a sudden downpour of black specks or a hazy veil.
• In inflammation of the inner coats of the eye, where floaters come together with pain, redness and discomfort in light.

What looks like flashes but is not

The commonest thing mistaken for flashes is migraine aura, and telling them apart is not hard once you know the signs. An aura is not a brief spark but a picture that lives its own life for twenty or thirty minutes: it usually starts as a shimmering spot near the centre, grows into a jagged, trembling or iridescent arc, drifts slowly out towards the edge of vision and disappears. It is seen with both eyes and does not vanish if one eye is closed, because it arises in the brain rather than in the eye. A headache may follow, but in many people the aura passes with no headache at all. The flashes of vitreous detachment behave differently: they are instantaneous, they repeat, they belong to one eye and to its edge, they are more obvious in the dark and on sudden eye movement, and they build no picture at all.

A few other look-alikes:

• sparks and darkening on standing up quickly — that is about blood pressure and circulation rather than the eye, and it passes within seconds;
• a brief flash when you rub an eye or sneeze — mechanical stimulation of the retina, harmless in itself;
• a single fixed dark or distorted patch exactly in the centre, with straight lines looking wavy: that points to the macula, is a separate subject and also needs examining;
• sudden painless loss of vision in one eye, complete or like a blind coming down, even if it lasts only minutes: that is a vascular emergency, call an ambulance.

What the examination involves

It is straightforward and nearly always the same. Sharpness of vision and the field of vision are checked, the front of the eye is inspected, then drops are put in to widen the pupil and, twenty to thirty minutes later, the back of the eye is examined in detail, always including the far periphery of the retina, which is where most tears hide. Sometimes a special lens resting lightly on the eye is added, and an ultrasound scan if opacities or blood make the back of the eye impossible to see.

Plan the day so that afterwards you do not have to drive: for several hours close vision will be blurred and light will feel harsh, and sunglasses are worth taking. If a tear is found without detachment, it is usually walled off with laser in one short session. If the retina has already detached, surgery is needed and is done urgently. And if everything is clear, you will be told which signs to watch for and what should bring you straight back — that list is worth listening to closely, because it is the same one this page opens with.

Separately: long-standing floaters are not operated on for comfort alone without solid reasons. Procedures on the vitreous are not cosmetic, they carry their own risks, and the decision is taken only in the uncommon cases where the opacities genuinely get in the way of living.

Online consultation

This conversation is rarely a substitute for the examination; it is there to work out whether you need to go today. The doctor will ask what they would ask face to face: when it started, one eye or both, whether the specks have multiplied, whether there are flashes, whether there is a shadow at one side, whether there was an injury or an operation, how short-sighted you are. From the answers they will separate the quiet picture from the one that needs a dilated examination of the back of the eye the same day, and will explain exactly where to go with it. They will consider whether what you describe is a migraine aura. They will tell you what to expect from the examination and why you should not drive afterwards. And they will name the signs that mean stopping all waiting and leaving at once. What an online consultation cannot do is look at your retina: with sudden floaters, flashes or a shadow it is no replacement for being examined.

This material is for information only and does not replace medical advice.