‘Dry eyes’ is a very common problem; but can be complex to treat. It’s an unhelpful term as some people actually experience watery eyes, and most ophthalmologists are moving towards terms such as ‘ocular surface disease’ (OSD) or ‘dysfunctional tear syndrome’.
Typical symptoms are discomfort, grittiness, or blurry vision when trying to concentrate on reading, driving, or computer work. This blurring often improves on blinking or rubbing the eyes. More severe symptoms can include red, painful eyes, and severely reduced vision.
The normal tear film
The healthy eye is coated by a film of tears, which not only lubricates and protects the eye; but is also is the first layer to focus incoming light. Consequently, a normal tear film is necessary for normal vision, and a disturbed tear film will cause visual problems.
The tear film is comprised of three layers: a mucous layer which is a wetting agent and helps to distribute the tears evenly, an aqueous (‘water’) layer over this, and finally a lipid (‘oil’) layer which stops the aqueous layer from evaporating too quickly. Each of these layers is produced by glands on or around the eye, and dysfunction of these glands can result in dry eye symptoms.
The ‘sleep’ we often find in our eyes in the morning is simply the crystallised lipid layer that is left behind when the water layer has evaporated.
What causes ocular surface disease (OSD)?
For many years, it was assumed that OSD was simply a result of deficient production of one of the tear film layers, and that replacing this layer would resolve the symptoms.
For mild OSD this is true (see below); but recent research has shown that moderate or severe OSD is primarily an inflammatory problem. The initial trigger can be mild OSD, blepharitis, an irregular corneal or conjunctival surface, or problems with eyelid position and function.
Chronic dryness of the ocular surface results in inflammation that damages the tear glands, and damages the nerves that inform the brain about the amount of tears in the eye. This creates a viscious cycle, and although artificial tears can give symptomatic relief, the cause remains untreated.
Unstable tears or not enough tears?
Mild OSD is often due to tear film instability.
The normal tear film is stable for over ten seconds without blinking. If there’s a deficiency in the lipid layer, the underlying aqueous layer evaporates quickly, turning a smooth tear film into patches of tears, similar to rain on a window. This distorts and scatters light, resulting in blurred vision.
At the same time, the patchy tear film leaves the surface of the cornea exposed to the air. This can cause discomfort and a reflex over-production of tears: although the eye is classed as ‘dry’, there are often tears running down the cheek!
Unstable tears often cause discomfort, grittiness, or blurry vision when trying to concentrate on reading, driving, or computer work. This blurring often temporarily improves on blinking or rubbing the eyes, because these actions redistribute the tear film over the cornea.
Insufficient tear production
Each of the three layers of the tear film is made by specialised glands on or around the eye. Any disease process which damages these glands will reduce their efficiency. As described above, inflammation of the ocular surface damages tear glands, and also reduces the feedback from the ocular surface to the brain.
In very severe forms of OSD, chronic disease destroys the corneal nerves and the patient is unaware of the fact that their eyes are extremely dry. Sometimes the only complaint is of reduced visual acuity: once treatment is started, the ocular surface recovers and the vision improves.
How can OSD be treated?
There are three parts to treating OSD:
- Treat the tear deficiency
- Treat the inflammation
- Treat the trigger
Treating mild OSD
For mild OSD, the first of these may suffice. Artificial tear-drops or gels top up the natural tear production and compensate for the drying effects of central heating, air-conditioning, or aeroplane cabin air.
Just like skin moisturisers, there is no ‘one size fits all’, and it’s worth trying several drops to see which works best. There are dozens on the market – all available over-the-counter from your pharmacist.
Broadly speaking, lubricants fall into three categories:
- Drops – lightweight, short-acting, but minimal blurring on instillation
- Ointment – heavyweight, long-lasting, but blur the vision. Good for overnight use.
- Gel – a halfway-house: last longer than drops, but less blurring than ointment
Once the cause is established, treatment can be given accordingly: lid hygiene to manage blepharitis, aqueous or lipid substitutes, and anti-inflammatory drops for ocular surface inflammation.
Often dry eye treatment is management, rather than cure:
Punctal plugs are also helpful: by blocking the tear drainage ducts, they keep any tears the eye can produce – and any drops the patient instills – in the eye for longer.