Flashes and Floaters

Floaters are caused by small pieces of debris that float in the vitreous humour of the eye.

They occur behind the lens and in front of the retina. The lens is a small clear structure that focuses light as it enters the eye. The retina is the light-sensitive tissue that lines the inside surface of the back of the eye.

Vitreous humour is a clear, jelly-like substance that fills the space in the middle of the eyeball. It is mostly (99%) made up of water. The other 1% is made up of substances that help maintain the shape of the vitreous.

Floaters occur as part of the natural ageing process. Some people may have posterior vitreous detachment (PVD), which is where the vitreous jelly comes away from the retina (see below). This may cause a sudden increase in the number of floaters.

Ageing process

As you get older, your risk of developing floaters increases. They tend to occur in people over 40 years of age, and are most common in those in their 60s and 70s. However, some younger people may also develop floaters.

As you get older, the vitreous humour in the middle of your eyeball can become softer, and strands of a protein called collagen may become visible within it. The collagen strands may appear to swirl as your eyes move.

Normally, light travels through the clear layer of vitreous humour to reach the retina. The light-sensitive layer of cells at the back of your eye that form the retina transmit images to your brain via the optic nerve. Any objects that are in the vitreous humour, such as floaters, will cast shadows onto the retina.

Posterior vitreous detachment (PVD)

Floaters can be a symptom of posterior vitreous detachment (PVD). This is a common condition that occurs in around three-quarters of people over 65 years of age. PVD can occur as the result of changes to the vitreous humour as the eye gets older.

As your eye ages, 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 lights can be another symptom of PVD. This can occur if the outer part of the vitreous humour pulls on the light-sensitive tissue of the retina. The pulling stimulates the retina, which causes your brain to interpret it as a light signal and creates the sensation of flashing lights.

Retinal tears

In about half of all people, the vitreous humour has separated from the retina by the time they are 50 years of age. This does not usually cause any problems and most people are not even aware it has happened.

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 humour. The red blood cells may appear as tiny black dots or they may look like smoke. However, as the blood is absorbed back into the retina, these floaters tend to disappear over the course of a few months.

In some cases, the vitreous humour remains attached to parts of the retina and tears the retina as it pulls away. If the retina tears, blood that escapes into the vitreous humour can cause a shower of floaters all at once. You may also see flashes of bright, white light in your field of vision that look similar to lightning 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 zigzag pattern across your field of vision).

Retinal detachment

If you have a retinal tear, you will need to be treated as soon as possible because tears can lead to retinal detachment. This is where the retina separates from the wall at the back of the eye which can damage your sight.

Retinal detachment is rare, affecting about 1 in every 10,000 people each year.

After the light has passed through the eye and reaches the retina, the retina changes the light into meaningful electrical signals. The signals are sent through the optic nerve to the brain, where they are translated into the images you see.

If the retina is damaged, the images received by your brain will become patchy or may be lost completely.

In rare cases, floaters may be a sign of retinal detachment. If your retina is detached, you will usually experience:

• a large number of floaters

• bright, flashing lights

• some loss of vision

Other causes

If you have had eye surgery, such as cataract surgery, you are more likely to experience floaters, PVD and, more rarely, retinal tears and retinal detachment. In some cases, floaters may also be caused by: 

• infection

• uveitis

• eye disease

• eye injury

Floaters are also more common in people who have:

• short-sightedness (myopia) – where distant objects appear blurred, while close objects can still be seen clearly

• diabetes – a lifelong condition that causes a person's blood sugar level to become too high