Floaters and flashes of light: when should you see a doctor?
Tiny threads, dots or cobwebs drifting across your field of vision, sometimes with brief flashes of light: these symptoms are very common and most often harmless. Certain signs, however, call for a retinal examination within 24 to 48 hours.
Floaters that have been present for years are almost always harmless. By contrast, a shower of new black dots, repeated flashes of light or a dark veil in the field of vision call for a retinal examination within 24 to 48 hours: they are signs of a possible tear.
Where do “floaters” come from?
The inside of the eye is filled with a transparent gel, the vitreous, which occupies the space between the lens and the retina. Over time, this gel becomes liquid and small clumps of fibres form in it. They cast a moving shadow on the retina: these are floaters, or myodesopsia.
They are more visible against a bright background — a sky, a white wall, a screen — and seem to dart away when you try to look at them, because they move with the movements of the eye.
Posterior vitreous detachment, a natural process
Between the ages of 55 and 65 on average, the vitreous eventually separates from the surface of the retina: this is posterior vitreous detachment (PVD). It occurs earlier in cases of high myopia, after cataract surgery or after an eye injury. It is not a disease, but a normal stage in the ageing of the eye.
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Why do we see flashes of light?
As the vitreous detaches, it exerts traction on the retina in places. The retina, mechanically stimulated, sends a signal that the brain interprets as light: hence the sensation of a flash, typically brief, located at one side of the field of vision and more noticeable in the dark.
These flashes should not be confused with the visual aura of an ocular migraine, which tends to last 20 to 30 minutes, affects both eyes and draws shimmering zigzag lines.
Signs that should prompt you to be seen quickly
- The sudden appearance of many floaters, like a shower of black dots or soot.
- Repeated flashes of light, especially if they are recent or becoming more intense.
- A dark veil or curtain encroaching on one edge of the visual field.
- A sudden drop in vision or distortion of straight lines.
In a minority of cases, vitreous detachment is accompanied by a retinal tear. If found in time, it is treated with a laser in a few minutes, at the practice. Left untreated, it can progress to a retinal detachment, which then requires emergency surgery. This is the whole value of a prompt examination.
How the examination works
The consultation begins with detailed questions: since when, one eye or both, which associated symptoms. After measuring visual acuity, the pupil is dilated with eye drops in order to examine the whole retina, right out to its periphery, where most tears occur.
The examination may be completed with ultra-wide-field retinal photography, a macular OCT and, if the vitreous is cloudy or bloodstained, an ocular ultrasound. Dilation makes vision blurred and light-sensitive for three to four hours: it is best to plan not to drive.
And if the examination is normal?
This is the most common case, and it is good news. No treatment is then needed: over the weeks, the floaters drift out of the line of sight and the brain learns to ignore them. A check-up is often offered a few weeks later, because a tear can appear later on. Any change in symptoms means you should be seen again without delay.