Many people whose eyes feel constantly dry have an adequate supply of tears. Others with a stingier tear supply don’t know what dry eye means. How can this be? Experts believe that when non-dry eyes feel dry the problem is that the tears evaporate too quickly and they blame poorly working Meibomian glands for this. The job of these special glands is to secrete an oily material delivered through openings in the lid margins that, like oil on water, slows tear evaporation. Their diagnosis is evaporative dry eyes. The problem is that the correlation between the productivity of these glands and presence of chronic dry eye symptoms not impressive. For example, many people who do have badly diseased Meibomian glands don’t complain of dry eyes.
Even more basic is the question: What are dry eye symptoms? The obvious answer is that they are the feelings we experience when our eyes are dry. In other words, they are messages from our eyes that they need more tears. Think of it as an alarm system designed to monitor the thickness of our tear films in real time and activate the release of more tears when it senses that they are about to break up. Although this typically occurs at an unconscious level, if the tearing response is inadequate, the volume of the alarm increases to conscious dry eye sensations. These sensations, a type of pain, are triggered by highly specialized nerve endings just under the surface of our corneas which are designed to sense drops in temperature, such as occur when the cooling effects of tear evaporation penetrate the tear layer. This occurs faster as it becomes thinner and its thermal insulation properties diminish. When it reaches the point of immanent break up these nerves become activated.
Obviously, the sensitivity of the tear evaporation alarm system is key to its efficient operation. If it is too low, the tear layer will break up and our vision will blur before it is rebuilt. If it is set too high, our eyes will feel dry even when they have adequate (sometimes, even overflowing) tears. I argue that hypersensitivity to tear evaporation is the underlying cause of so-called “evaporative dry eye”. What causes the chronic malfunctioning of these evaporation-sensitive nerve endings? Corneal neuropathy, which means that its nerves are damaged/diseased, is similar to that which occurs in the feet of some diabetic people. In the case of the cornea, the most common reason for persistent tear evaporation hypersensitivity is aging nerves. Nevertheless, it can also be triggered by persistent nerve damage following LASIK. These dry eye symptoms are further worsened when tear production is reduced as a consequence of dysfunctional corneal nerves.
Unfortunately, we are currently unable to reset the sensitivity of the tear evaporation alarm system to normal levels when it breaks down. Treatments focus on protecting sensitized corneal surfaces from evaporation cooling such as by improving the thermal insulating properties of tear films through the use of artificial tears and by reducing the rate of tear evaporation such as through improving the function of diseased Meibomian glands. If traditional remedies are insufficient and wearing well-sealed goggles is significantly helpful, the next step should be to test the effectiveness of wearing scleral lenses. By submerging the surface of these corneas in a pool of artificial tears covered by a vapor barrier, these devices are unique in their ability to completely block evaporation at the corneal surface (see illustration).
It is important to point out the existence of a far more devastating type of chronic corneal pain that, although commonly (and incorrectly) included in this group is described as centralized corneal pain. Although this pain syndrome can sometimes include a component simulating symptoms of dry eye, it is more commonly associated with (and sometimes dominated by) disabling photosensitivity—especially to lights of computer screens, metal halide and fluorescent fixtures. Although it feels as though it originates in the eyes and orbits, centralized pain is actually produced in the pain circuitry of the brain and projected to the region of the eyes, head (headaches), ears, face and/or jaws. The disparity between the intensity of symptoms and the appearance of these eyes (which typically look normal) is so striking that doctors are easily persuaded that they are imagined or exaggerated. Causes of this devastating pain include LASIK/PRK surgery, autoimmune diseases, fibromyalgia and generalized nerve disease. Its cause is often unknown.
Unfortunately, there are no effective treatments at this time for this disease except for a dry eye-like components (when present). Some relief can be provided by oral medicines prescribed by pain specialists. Narcotics should not be used. The limited treatment options are, for the most part, due to the fact that this condition had not been previously recognized. Fortunately, this is changing.