Flashes and floaters are common symptoms which often affect normal eyes. They usually occur because of changes that happen in the jelly-like substance inside the eye (which is called the vitreous humour). The most common causes need no treatment, do not prevent your normal visual tasks and tend to settle by themselves. However, flashes and floaters can occasionally be warning symptoms of retinal tear or retinal detachment.
Haloes are bright circles which seem to surround a source of light. They are also referred to as glare. They are a common symptom, particularly in older people. They can be unpleasant and uncomfortable and lead to temporary dazzling, and this may particularly affect driving at night. They can sometimes be a sign of underlying eye conditions such as glaucoma.
What are flashes?
Flashes are unexplained brief lights seen in one or both eyes. They often occur on the edges of vision and they are fairly common. Each flash, which can vary from a bright light to almost a sparkle, lasts a varying length of time. Periods of flashing can go on for several months. The symptoms are often most noticeable when going from a light to a dark room.
What causes flashes?
Flashes are most commonly due to age-related changes in the vitreous humour. The vitreous humour is the jelly-like substance which fills the inside of the eye, between the lens and the retina. The vitreous humour is contained in a fine membrane and this is attached to the retina at the back and the lens at the front.
As we age, the vitreous humour shrinks and as it does so it can pull on the retina. This can cause flashes because the pulling triggers nerves in the retina and they send signals to the seeing nerve (optic nerve). Eventually the vitreous membrane tends to pull right off the retina, a condition called posterior vitreous detachment. This condition is harmless in itself, and in fact it happens to almost everyone eventually. The vitreous is detached from the retina in 75% of people aged over 65 and this is usually harmless.
The flashes of vitreous detachment may occur in one or both eyes. If they occur in both eyes this is because the same thing is happening in each eye separately (but since your eyes are usually very similar this is not unlikely to occur at the same time). Sometimes, as the vitreous pulls on the retina, it can tear it, causing a retinal tear or a retinal detachment. However, most vitreous detachment does not harm the retina.
Flashes can also relate to migraines. Some people with migraine experience flashing lights. Usually, in migraine, these occur in both eyes simultaneously. They can last up to an hour and tend to increase to a maximum before fading away and being replaced by a headache, which is typically one-sided and which may or may not be severe.
Charles Bonnet syndrome is a condition experienced by people, usually elderly people, whose vision is deteriorating. The brain, deprived of real visual information, can make things up instead, particularly in conditions of low light. Patients may sometimes see flashes, although more frequently they see complex visual images like children or animals, which can look very real.
Can flashes be serious?
Most flashes are caused by changes in the vitreous humour which are related to age and which are harmless. Occasionally flashes can be a sign that the retina is at risk of being torn or detached. Increasing, persistent or constant flashes all suggest strong pulling on the retina and may mean that you are at risk of retinal damage. Flashes accompanied by a shadow coming down over your vision is suggestive of retinal detachment.
Some people are at greater risk of retinal detachment than others, including those who have already had a retinal detachment in the other eye, those with inflammatory eye disorders such as uveitis, or degenerative conditions of the retina, those who have had significant eye trauma or surgery, and those with a family history of retinal detachment. People who are extremely short-sighted (correction more than -6.00D - your optician can tell you what your correction is) are at higher risk, as the globe of their eye tends to be longer so that the vitreous is more likely to pull away.
What are floaters?
Floaters are shapes (opacities) floating in the field of vision. They may look like spots, threads, spiders or cobwebs. They move as you move your eye and can seem to dart away when you try to look at them. They drift about inside the eye rather than staying still. They tend to be more obvious when bright objects, such as a blue sky, are being viewed.
Most floaters are also caused by changes in the vitreous humour. Most commonly this is due to normal ageing of the eye, when opacities form in the clear jelly and drift around. These kinds of floater are not associated with flashes or reduction in your vision and they tend to come on gradually. They also tend to 'settle' at the bottom of the eye, below the line of sight. After a while you will find them less noticeable. They are more common in those who are short-sighted, those who have had eye surgery and those who have diabetic eye disease.
Floaters can also occur after posterior vitreous detachment. In this case there will be a sudden obvious increase in the number of floaters. Flashes may also occur. Again, there should be no loss of vision and most cases settle without causing any problems.
Bleeding into the vitreous humour (vitreous haemorrhage) will also lead to the formation of floaters. However, in this case the floaters represent blood in the jelly. If the bleeding is major then vision may be affected. For more information on this condition see the separate leaflet called Vitreous Haemorrhage.
Floaters will result from any internal damage to the back of the eye. Retinal tears and retinal detachments also cause floaters, and these will vary with severity, depending on the severity of the damage.
Less common causes of floaters include inflammation of the eye (posterior uveitis) and, more rarely still, tumours affecting the eye.
Are floaters serious?
Floaters are usually not serious. However, you should see your doctor or optician, or visit the A&E department, if any of the following apply:
- They come on suddenly.
- There are large quantities of them.
- They are particularly disturbing.
- They are associated with other eye symptoms such as pain, severe headaches, changes in your vision, grey shadows in your vision or with new onset of flashes.
- You have previously experienced retinal detachment, have had recent eye injury or eye surgery, have other eye conditions affecting the retina, or you have very high short-sightedness (myopia).
- You already have vision in only one eye because of a prior condition, and you experience any new symptoms in your vision.
What are haloes?
Haloes are rainbow-like coloured rings around lights or bright objects. They usually occur because there is extra water in the layers of the eye. The most common and important cause of this is acute glaucoma. If you have glaucoma, you have increased pressure in your eye. This is a very painful condition which can threaten your sight if not treated promptly. However, another cause - chronic glaucoma - comes on more quietly and is not painful.
Many other conditions can cause you to experience haloes. These include watering or tearful eyes, overuse of contact lenses, cataracts and opacities in the vitreous humour. Some prescribed medicines may also cause you to see haloes, including digoxin and chloroquine.
Are haloes serious?
Because haloes can be a sign of increased pressure in your eye (glaucoma) then it is important you see your doctor or optician if you develop persistent haloes. It is also important that you do not drive in conditions where haloes may be affecting your vision - for example, when driving at night. If you have any doubts about your fitness to drive it is your responsibility to speak to the DVLA, who will advise you.
Who develops flashes, floaters and haloes?
Most people will notice occasional floaters, as there are often small opacities and crystals in the vitreous. Because more marked floaters, together with flashes and haloes, are mostly caused by conditions occurring naturally in older eyes, most people who experience them are over 60 years of age, although occasional floaters are not uncommon in people in their 40s and 50s.
Children and young adults may also experience flashes, floaters and haloes, particularly if there has been trauma or surgery to the eye or if they have other existing eye disease. These might include inflammatory conditions of the eye like uveitis, and conditions which can affect the retina like sickle cell disease and the form of retinopathy that can affect very premature babies.
When should I worry about flashes, floaters or haloes?
You should seek urgent advice about floaters and flashes if they are very marked or sudden in onset. You should also seek urgent advice if they are associated with pain, or changes in your vision, of if both floaters and flashes are occurring together. You should always seek advice if you develop persistent haloes. You should seek advice for any new symptoms, even if less severe than this, if you have previously lost the sight in one of your eyes, so that your new symptoms affect your only functioning eye.
Your first port of call, depending on the severity and timing of your symptoms, may be your optician, GP surgery or A&E department. Most opticians are able to check the pressures in your eyes in order to rule out glaucoma. Many will have equipment to allow them to fully examine the back of your eye for signs of damage to the retina. This equipment (called a slit lamp) is also available in A&E departments. Most GPs do not have slit lamps but your GP will be able to tell you if your symptoms suggest that you need to be seen by an optician or in A&E.
Further reading and references
; Royal National Institute of Blind People (RNIB)
; Flashes, floaters, and a field defect. BMJ. 2013 Nov 4347:f6496. doi: 10.1136/bmj.f6496.
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