Dry eye is a disease of the tears and surface of the eyes that is characterized by discomfort, visual disturbance, and tear instability with damage to the front parts of the eye. This condition is very common with between 10–20% of adults affected, although severe disease is less frequent.
The literal meaning of dry eye is a simplification. Whilst it can result from tear deficiency, owing to insufficient production or excessive loss, common usage of the term encompasses compositional anomalies of tears and recognizes the role of inflammation.
In most cases of dry eye the cause is not tear deficiency, rather it is due to evaporative loss as a result of abnormal secretions from the meibomian glands within the eyelids or blinking anomalies.
Meibomian gland dysfunction alters the composition of their oily secretions that usually cap the tear film and retard evaporation. These secretions may be excessive and froth along the lid margin, or have a raised melting point and so become solid and block the gland. This dysfunction may occur as a result of the skin disorders seborrhoeic dermatitis or acne rosacea, or reflect hormone levels.
Poor delivery of water to the surface to the eye is usually caused by autoimmune damage to the lacrimal gland, which is often associated with hormone anomalies or generalised autoimmune disease such as rheumatoid arthritis. Less commonly tear deficiency results from damage to the sensory nerves of the cornea, most commonly after laser eye surgery, or as a result of scarring to the secretory ducts of the lacrimal gland.
Dry eye causes the eyes to appear red and feel irritable. The different symptoms of patients may in part reflect differences in word choice, but includes dryness, soreness, burning and itch.
A smooth tear film is essential to see clearly, and so dry eye is also associated with intermittent blurring of vision.
There are a number of tests used to detect dry eye and to determine its cause. At its most basic this typically would involve examining the eyes using a slitlamp biomicroscope and the instillation of a coloured dye. More often there is more to be done to investigate dry eye that can realistically be performed as part of a routine eye examination. For our more advanced dry eye patients we offer a dry eye clinic where further investigations can be performed and alternative solutions offered.
Tear substitutes increase the volume of fluid at the ocular surface. These treatments frequently have a positive effect in mild and moderate dry eye, although improvements are generally transient and incomplete.
The length of time that an eyedrop remains at the ocular surface is primarily dependent on their viscosity. It does not follow that more viscous preparations are always better because they cause blurred vision and in the case of petroleum derivatives reduce the wettability of the ocular surface and thus the stability of resident tears. Best results with these eyedrops are achieved by instilling small amounts frequently.
Multidose products often contain preservatives that can cause irritation when used more than 4-6 times a day or in severe dry eye. When this happens it is preferable to use products that avoid the preservative benzalkonium chloride or use preservative-free formulations.
A wide product range is available on the market suitable for various aspects of dry eye. Upon a full dry eye workup an appropriate product can be advised.
Reducing tear evaporation
Evaporative tear loss can be minimised by increasing atmospheric humidity, reducing ocular surface exposure, and addressing meibomian gland disease.
Humidity can be raised during the day with a vaporiser or humidifier, and wrap-around spectacles increase humidity around the eye.
Heat and forced air currents increase evaporation rate, thus air conditioning and car heaters should be avoided. In shared offices where control of air conditioning is not possible, a screen to prevent air blowing directly on the eyes is of assistance. Computer users benefit from positioning their terminals below eye level, which decreases the area of exposed ocular surface.
Meibomian gland dysfunction can be improved with the regular use of warm compresses or lid scrubs as described in the section on blepharitis.
Tears drain from the eye through tiny holes in the eyelids close to the nose called puncta. These holes can be blocked with small plugs that may either be temporary or permanent, and thus increase the length of time that the tears are present in the eye. This is often more effective than frequent dosing with ocular lubricants in cases of tear deficient dry eye. This is a very specialized service and is only offered locally at the hospital eye service.
It is well established that malnutrition, normally associated with poverty or lack of food in developing countries, but also occurring in developed nations as a result of inappropriate food selection, eating disorders, and overzealous fad diets, can contribute to dry eye. However it does not necessarily follow that dry eye can be successfully treated by dietary means in patients with good diets, although limited beneficial effects are possible with this approach, and have been reported.
Most interest on this issue has focused on omega-3 and omega-6 fatty acids. In the UK there is a tendency for a large bias towards omega-6s in our diets, which has been linked to cardiovascular disease, autoimmune disorders, depression and cancer. Given the data from other areas of healthcare it seems prudent to not recommend supplements that only boost omega-6 levels. Instead, efforts to increase intake of omega-3s should be encouraged. Regardless of any effect on ocular dryness this behaviour is likely to have considerable general health benefits. This can be done most effectively and enjoyably through increasing the consumption of fish (government guidelines recommend at least twice a week).
Supplements are available for those unwilling to change their diets, but whole foods are preferable because they contain other substances that are important for health – capsules are also associated with “fishy burps”. Fish oil based supplements should be avoided in those with fish or seafood allergies, for these individuals flaxseed oil is more appropriate. Omega-3 supplements are cautioned against in those taking the anticoagulants warfarin or clopidogrel because it also thins the blood. Anecdotally, the majority of clinicians report that almost no patient is cured by taking supplements in isolation, although many feel that there is some benefit, particularly when used as part of a management package.
Regarding hydration, intuitively, general body dehydration can only aggravate dry eye. This can be avoided by regular fluid intake during the day by an amount guided by urine colour.