Floaters are a common complaint, as are flashes to a lesser extent. The two may occur separately or together, depending on the underlying pathology. Halos are less common. It is very difficult, without the help of a slit-lamp, to determine whether they arise out of the more common, benign causes or whether they are due to one of the less common, sinister causes. There are some suggestive pointers which will be outlined here which will guide the urgency of the referral; ultimately most patients will need an ophthalmological review.

Flashes

Otherwise known as 'photopsia', flashes describe the subjective sensation of seeing a light in the absence of an objective stimulus. This may present as scintillating lights, an arc of light or a feeling like a light-bulb has just flashed on and off in the periphery of vision. Flashes are caused by improper stimulation of the eye's retina, or the optic nerve, which the brain interprets as light. This could be traction of the retina for example.

Causes of flashes¹

Retinal traction

The vitreous is the viscous filling occupying most of the globe. It is loosely attached, at its periphery, to the retina. In some parts, it is more firmly adherent. Any condition which causes it to contract (e.g. fibrosis as a primary problem or following haemorrhage, the natural progressive liquefaction that occurs with age) has the potential to result in flashes, as the vitreous pulls on the retina in the process. These conditions include:

• Vitreous detachment (may also occur with floaters)
• Retinal detachment
• Proliferative diabetic retinopathy
• Sickle cell retinopathy
• Retinopathy of prematurity

Central nervous system pathology

Conditions include:

• Papilloedema - symptoms tend to be transient, associated with straining or change in posture.
• Migraine - classically: gradually enlarging, frequently bilateral, zig-zag fortification spectra usually followed by a headache. Occasionally, migrainous visual phenomena can occur in the absence of headaches (migraine sine migraine - better known as ocular migraine) but this is a diagnosis of exclusion that can be made only after a full ophthalmological examination.
• Occipital lobe lesions (e.g. tumours or arteriovenous malformations) - tend to cause coloured shapes and blobs.
• Charles Bonnet's syndrome - complex visual hallucinations brought about by bilateral, severe visual loss.

Pseudoflashes

Although not strictly speaking flashes, the following may be described as such by the patient:

• Photophobia - usually associated with anterior segment inflammation or retinal hypersensitivity.
• Glare - a dazzle usually associated with media opacities.
• Halos - the ring effect associated with media clouding (e.g. the corneal oedema of acute angle-closure glaucoma) and occasionally, media opacities.

Floaters

Floaters are opacities floating in the field of vision which may be seen as spots, thread-like strands or squiggly lines. They are variously described by patients as 'specks', 'flies', 'spiders' and, if extensive, 'like a cobweb'. They move with eye movements and seem to dart away when you try to look at them directly. They do not follow eye movements closely, and usually drift when the eyes stop moving. There are a number of causes of floaters:¹

• Vitreous condensations - these are normal degenerative changes associated with age, resulting in translucent opacities.
• Posterior vitreous detachment (may also occur with flashes).
• Vitreous haemorrhage - (e.g. proliferative diabetic retinopathy or retinal vein occlusion) minor bleeds are seen as 'spots' in the vision (the fundus is easily visualised), major bleeds result in a severe decrease in the visual acuity (fundus is poorly visualised).
• Vitritis - inflammatory cells in the vitreous due to intermediate or posterior uveitis.
• Asteroid hyalosis - a (usually benign) condition characterised by small yellow/white particles scattered throughout the vitreous.
• Amyloidosis - sheet-like opacities.
• Tumours - (e.g. lymphoma) floaters occur as a result of vitritis.

Syneresis

When no other cause is found (approximately 25% of cases), floaters may be due to degeneration of the vitreous, where collagen fibres become thickened with vitreous liquefaction, and hence visible. These are not associated with flashes, reduced vision or defects in the visual field. They are usually of gradual onset. Patients should be reassured that these are a normal feature and require no treatment unless the situation deteriorates. They tend to "settle" at the bottom of the eye, below the line of sight, where they are less distracting. Most people learn to ignore them. Floaters can become apparent when looking at something bright, such as white paper or a blue sky. They are more likely to develop with age and are more common in people who:

• Are very short-sighted - myopia
• Are diabetic
• Have had a cataract operation

Flashes and floaters

These tend to occur with:

• Posterior vitreous detachment
• Retinal tear
• Retinal detachment
• Tumours

Halos

These are caused by light passing through water in or on the surface of the eye being broken down into its spectral colours. This results in rainbow-like coloured rings around lights or bright objects. Acute angle-closure glaucoma is the most common and the most clinically significant cause. It is a sight-threatening condition. However, there are a number of other causes:²

• Excessive formation of tears
• Oedema of the corneal epithelium from any cause (e.g. contact lens overwear)
• Corneal dystrophies in their later stages
• Chronic open angle glaucoma
• Early cataracts (glare of headlights making night-time driving impossible)
• Pigment dispersion syndrome
• Vitreous opacities
• Drugs (e.g. digitalis and chloroquine)

Document references

1. Denniston AKO, Murray PI. Oxford Handbook of Ophthalmology, OUP (2008).
2. Kunimoto DY, Kanitkar KD, Makar MS; The Wills Eye Manual (4th Ed), Lippincott, Williams and Wilkins (2004).

Internet and further reading

• Kanski J. Clinical Ophthalmology: A Systematic Approach (5th Ed) Butterworth Heinemann (2003).
• Larkin GL; Retinal Detachment. eMedicine, April 2008.
• Blanda M, Wright JT; Headache, Migraine. eMedicine, September 2008.

Acknowledgements