The large area in the middle of your eyeball is filled with a clear, jelly-like substance called vitreous humour. As you get older, the vitreous humour can become less firm and strands of a protein called collagen may become visible within it. The collagen strands may appear to swirl as your eye moves.
Normally, light travels through the clear layer of vitreous humour in order to reach the retina. The retina is the light sensitive layer of cells and tissue at the back of your eye which transmits images to your brain via the optic nerve. Any objects, such as floaters, that are in the vitreous humour will cast shadows on to the retina.
Posterior vitreous detachment (PVD)
Posterior vitreous detachment (PVD) can be the result of changes that occur to the vitreous humour of the eye, as the eye gets older. PVD is a common condition that occurs in about 75% of people over 65.
With age, the central part of the vitreous humour becomes more liquid, and the outer part, known as the cortex (which contains more collagen) starts to shrink away from the retina. Floaters develop as a result of the collagen thickening and clumping together.
As well as floaters, flashing can be another symptom of PVD. Flashing may occur when the outer part of the vitreous humour pulls on the light sensitive tissue of the retina. The pulling stimulates the retina, causing your brain to interpret it as a light signal. This creates the sensation of flashing lights.
In a few cases of PVD, when the vitreous humour pulls on the retina, it can cause the tiny blood vessels in the retina to burst and bleed into the vitreous. The red blood cells may appear as tiny black dots, or they may look like smoke. However, as the blood is re-absorbed back into the retina, floaters that are caused by tears tend to disappear over the course of a few months.
In approximately half of all people, the vitreous humour has separated from the retina by the time they are 50. This doesn’t usually cause any problems, and most people aren’t even aware that it has happened.
In some cases, the vitreous humour remains attached to parts of the retina and it tears the retina as it pulls away. If the retina tears, blood that escapes into the vitreous humour can cause a ‘shower’ of lots of floaters at once. You may also see flashes of bright, white light in your vision that look a little like lightening streaks.
It is important to be aware that flashes in your vision are not necessarily a sign of retinal tears, or retinal detachment (see below). They may have another cause, such as a migraine with aura (a headache with a ‘zig-zag’ pattern across your field of vision).
Floaters and flashes do not usually cause long-term visual impairment, but if you experience them it is important that you visit an eye care specialist, such as an optometrist, in order to have an eye examination.
If you have retinal tears, you will need to be treated as soon as possible because tears can lead to retinal detachment. Retinal detachment occurs when the retina separates from the wall at the back of the eye. If this happens, it can damage your sight.
After the light has passed through the eye and reaches the retina, the retina changes the light into meaningful electric signals. The signals are sent through the optic nerve to the brain, where they are translated into the images that you see.
If the retina is damaged, the images that are received by the brain become patchy, or may be lost completely.
If you have had eye surgery, such as a cataract operation, you are more likely to experience floaters, PVD, retinal tears, and retinal detachment. In some cases, floaters may also be the result of a number of other causes including:
- inflammation (uveitis),
- eye disease, or
- eye injury.