Flashes and floaters: harmless or an eye emergency?

A thread drifting across the page as you read, a few specks against a blue sky: many people know these floaters, which the French call mouches volantes, ‘flying flies’. Most of the time they are harmless. There are, however, warning signs you should not wait out – because only an examination can show what lies behind them.

What is floating around in your eye

The inside of your eye is filled with the vitreous, a clear gel that makes up about two-thirds of the eye’s volume. According to a Swedish study from 2017, it is around 99 per cent water, plus fine collagen fibres and hyaluronic acid. Over the years the gel liquefies – by the age of 80, around 62 per cent of it – and fibres clump together into strands and specks.

What you see are not the specks themselves but the shadows they cast on the retina, explains the American Academy of Ophthalmology (AAO). That is why they stand out against bright, even surfaces such as the sky. According to the AAO, these floaters are generally not dangerous and usually become less noticeable over time.

Posterior vitreous detachment: common and usually harmless

At some point the shrinking vitreous comes away from the retina: a posterior vitreous detachment (PVD). It is a normal part of ageing: the Swedish study cites reported rates of up to 24 per cent in people aged 50 to 59 and up to 87 per cent in those aged 80 to 89. In short-sighted people or after an injury, it can happen earlier.

Typical signs:

• New floaters. Sometimes a distinct ring: tissue that has come away from the head of the optic nerve (a Weiss ring).
• Flashes of light. They occur when the vitreous tugs on the retina and stimulates it mechanically. According to the AAO, they are most noticeable in the dark and can come and go for weeks or even months.

The symptoms usually settle, sometimes only after several months. The detachment itself generally needs no treatment, provided the retina is not damaged.

When it becomes serious: retinal tears and detachment

Where the vitreous is still firmly attached to the retina, it can tear it as it pulls away. If fluid passes through the tear beneath the retina, the retina lifts off its base: a retinal detachment.

How often is there more to it? A review of 17 studies published in JAMA in 2009 found a retinal tear in 14 per cent of people with sudden floaters or flashes, and the Swedish study (365 patients) in 14.5 per cent; for the first examination, the AAO’s 2024 guideline gives 5.4 to 8 per cent, depending on the study. So the great majority do not have one.

In the Swedish study, whose participants all had acute symptoms, having flashes as well made no measurable difference; reduced vision, blood in the eye and symptoms that had started less than 24 hours earlier did raise the risk. New flashes are still a reason to be examined promptly.

If a small blood vessel tears as the vitreous pulls away, blood in the vitreous can appear as a swarm of fine dark dots – what German-speaking doctors call Russregen (‘soot rain’), which University Hospital Zurich lists as a warning sign. With this kind of bleeding, the AAO guideline reports at least one tear in about two-thirds of patients.

Two things make this tricky. A retinal detachment does not hurt, as London’s Moorfields Eye Hospital points out. And according to the AAO guideline, symptoms alone cannot reliably tell a harmless PVD from one with a tear. Only an examination of the back of the eye, out to the edge of the retina, can tell.

These warning signs are an emergency

See an ophthalmologist straight away, the same day at the latest – or go to an emergency department outside practice opening hours – if you notice:

• a sudden burst of new floaters or a shower of dark dots
• new flashes of light, especially together with new floaters
• a dark shadow, wall or curtain covering part of your field of vision
• newly blurred or distorted vision

Even if just one or a few new floaters suddenly appear – a ring, for example – and nothing else seems wrong, have your eyes checked promptly, ideally within 24 to 48 hours according to Moorfields Eye Hospital: a tear can be behind these, too.

Practical points:

• Don’t drive yourself. Your pupils will usually be widened with drops for the examination; according to the US National Eye Institute (NEI), your vision may be blurry and your eyes sensitive to light for a few hours afterwards.
• Don’t wait it out. The earlier a detachment is treated, the better the results, says the AAO – especially while the macula, the area of sharpest vision, is still attached.
• Stay alert afterwards. Even if no tear is found at first, one still turns up in the following weeks in around 2 to 3.4 per cent of patients. Without risk findings, the AAO guideline usually recommends a check-up after four to six weeks. If new floaters appear or your vision gets worse, get back in touch straight away.

Not every flicker comes from the retina: zigzag lines or heat-wave effects lasting up to an hour can, according to the AAO, be part of a migraine, even without a headache. If in doubt, have it checked.

Who is at higher risk

Overall, retinal detachment is rare: it affects about 1 to just under 2 in 10,000 people a year. Risk factors listed in the AAO guideline include:

• Short-sightedness. According to one study, the risk was about four times higher with just 1 to 3 dioptres and around ten times higher above 3 dioptres. The longer the eye, the higher the risk. Read more: Why myopia is on the rise.
• Previous surgery inside the eye, such as cataract surgery or lens exchange. Overall, a detachment after modern cataract surgery is uncommon: a US registry analysis of more than 3 million operations found roughly one case per 500 in the first year. The risk is higher, the AAO guideline notes, in long, short-sighted eyes, in men and in younger people. After lens exchange for high short-sightedness, one study found the cumulative risk rose from 2 to 8 per cent over seven years.
• Eye injuries, whose effects may only show years later.
• A detachment in the other eye (risk for the second eye: around 10 per cent) or in a parent, sibling or child.
• Lattice degeneration, a thinning at the edge of the retina found in 6 to 8 per cent of people.

Worth knowing: laser eye surgery only reshapes the cornea; the eye stays the same length. According to the AAO guideline, large studies have not found a higher risk afterwards than in eyes with the same degree of short-sightedness – so the warning signs still apply.

Caught early, treatable

A fresh retinal tear can usually be sealed with laser or targeted freezing (cryotherapy), generally in the clinic, says the NEI: the resulting scar holds the retina in place. According to the AAO guideline, at least half of untreated fresh horseshoe tears that cause symptoms and on which the vitreous keeps pulling lead to a detachment; prompt treatment cuts this risk to under 5 per cent. The scar can take up to a month to hold fully, so follow-up visits are part of the plan.

A retina that has already detached usually needs surgery, for example removal of the vitreous (vitrectomy) or a buckle stitched to the outside of the eye. According to the AAO, more than 95 per cent of uncomplicated detachments can be reattached, sometimes only after more than one operation. Full vision is not guaranteed afterwards, especially if the macula was already affected.

Harmless floaters usually need no treatment. Vitrectomy is an option for very troublesome floaters, but it carries its own risks and, according to the AAO, is seldom necessary; for laser treatment, the AAO guideline says there is not yet enough evidence.

So new warning signs belong in the hands of an eye doctor straight away, and even a single new floater should be looked at promptly. Familiar floaters that have not changed for a long time, on the other hand, can be examined without hurry: examining the back of the eye is part of the basic check-ups in our ophthalmology service, and you can book an appointment via our contact page.