Hot, gritty, stinging, burning or just generally tired eyes may all be symptomatic of dryness. Surprisingly so can watery eyes!
Tears are produced by a variety of tiny glands on and around the eye and lids and are composed of 3 layers. The epithelial cells which form the outer layer of the cornea and conjunctiva (which covers the sclera- the white of the eye) are smeared with mucus on every blink. The mucus acts as a wetting agent- the next layer up is the watery (Aqueous) layer, which would not stick to the epithelium, but is attracted to the mucus which does stick to the eye- the outer layer of epithelial cells normally is covered by little finger like projections, called microvilli which “grip” the mucus. Over these is a layer of oil- this floats on the watery layer and reduces water lost by evaporation.
Dry Eye may occur because of a deficiency in production of one or more layers of tears, an increased evaporation, abnormal corneal epithelium or decreased sensitivity, or an incorrectly positioned or damaged eyelid.
Some people experience dry eye symptoms occasionally- perhaps after a long day in-front of the PC, whilst others (most likely older ladies) have dry eye symptoms regularly, perhaps even daily. Anyone’s eyes can dry up, but only those who experience discomfort regularly would be diagnosed with Dry Eye Syndrome. Dry eye problems are usually “only” a comfort problem- the cornea is very quick to heal, which means that the damage rarely becomes cumulative.
After a blink, the tears soon begin to destabilise, the watery layer begins to thin until the lipid layer which is also attracted to the mucus “jumps” across the thinned layer. The time taken for the tears to destabilise is called the tear break up time. This creates a dry spot on the cornea and microscopic damage to the cells in that area. The irritation caused by this may not reach a conscious level, but in a normally sensitive cornea it will cause a blink. If this happens often enough the little dry spots will eventually cause discomfort.
When on the move, our eyes are constantly in motion- navigating in between people and objects, looking out for traffic and the like and it helps the eyes to make large movements if they are closed. In this situation an individual will probably blink 12 to 14 times per minute- on average a blink every 5 seconds.With an average tear break up time of 12 seconds, this would mean that the average person would be blinking more frequently than necessary to prevent tear break up and the dry spots it causes. Even this person may suffer dry eyes in certain circumstances- concentration decreases the blink rate, as does the decrease in eye movements when looking at a VDU for example- in this circumstance they may suffer symptoms. VDU screens are worse for this than paperwork, as we look at them straight ahead- when reading paper we look down, the upper lid drops down leaving less area for evaporation and a thickening of the tear film in front of the open part of the eye.
As we age, tear production gradually decreases and certain medications used to treat high blood pressure, anti-inflammatories and sleeping tablets may cause or exacerbate dry eye. Some medical conditions, such as Rheumatoid Arthritis can damage the aqueous producing glands.
Some cases of dry eye syndrome cannot be cured- they are caused by decreased production, but it can be effectively treated with artificial tears. These contain no drugs, but are a thick liquid which bulks up the eye’s natural moisture. Used regularly these can prevent the dryness damage and the discomfort it causes. The frequency of use needs to be worked out by the individual- in a warm dry atmosphere they will need them more often for example.
If the problem is because of a deficiency in the oily layer (which results in increased evaporation) the cause may be treatable- the Meibomian glands in the eyelid produce the oily layer and if a significant number become blocked this can cause problems, but this case is easily treatable with eyelid care.
We would highly recommend the MGDRx Eyebags to help alleviate most dry eye symptoms. Below is a video about them- you may need to install Flash Player to see the video.
Most dry eye symptoms can be significantly helped or completely resolved with lid care to increase the amount of the oily lipid layer in the tears. This helps to seal the tears in and reduce evaporation of the underlying watery layer
The tears drain through 2 tiny ducts, one in the upper, one in the lower lid, and sometimes blocking these ducts can bring relief by reducing tears drainage. This is done as a reversible trial initially, but can be made more permanent if beneficial.
Abnormal lid positions may be correctable surgically if it was causing problems- for example an eyelid which is too loose would not spread the tears properly, this is relatively common in the elderly.
And finally- when advising a patient that their eyes are dry, they often retort “But they are watery”.
A dry eye can easily become watery, because the eye’s only response to irritation is to flush- whether the cause be a piece of grit, chemical irritation such as chopping an onion or corneal dessication. It used to be thought that the Lacrimal gland (a large gland which sits in the bony orbit above the eye) which produces the emergency flush or tears required in these circumstances was unaffected in dry eyes, and that the watery part of the normal tears was produced by the tiny accessory lacrimal glands. It was thought that whilst the accessory glands may suffer from a decreased output, the large gland remained unaffected (but only came into play in an emergency flush). It is now thought that all glands are affected, which produces a decrease in baseline production of tears, but the main gland still has spare capacity which only gets used when the eye becomes irritated.