Many people such as those with Sjogren’s syndrome suffer from truly dry eyes. Their eyes feel dry because they are dry and their symptoms match the visible consequences of the damage it is causing. No mystery here. On the other hand, most eyes that feel chronically dry are not dry enough to cause visible damage. The explanation is that tears evaporate too fast because certain glands in the lids (Meibomian glands) are not working well. That’s important because these glands that secrete the oil that covers the tear layer and slows its evaporation are not working well. Eyes that feel dry but are not dry (enough) are diagnosed as evaporative dry eye. But is that the core problem? I argue no. The reason: many studies have shown that the relationship between Meibomian gland dysfunction and dry eye symptoms is poor. Some people diagnosed as having dry eyes have plenty of tears (although most do not) and Meibomian glands that in other people do not cause any problems. How has this been explained? It hasn’t. But, I have a theory.
Dry eyes is the most common complaint of adults seeking eye care in the U.S. And their numbers are growing. Despite huge investments in research and major efforts on the parts of some of the most brilliant minds in medicine, treatments for these symptoms remain disappointing. Why? I believe that the long-standing belief that eyes that feel dry are dry has been the major obstacle to progress. Recent research that concludes that dry eye-like symptoms are a form of pain has, in my opinion, changed the landscape of this disease. This is not surprising. Our corneas, the transparent dome-shaped front part of our eyes through which we see our iris and pupil (see illustration), has the highest concentration of pain nerve endings in the human body—approximately 200 times that of skin. In addition to its powerful pain-generating power, corneal nerve endings are highly exposed to the potentially damaging environment from which they are protected by only a thin layer of tears and mucin. As a result, they are extremely vulnerable to being damaged. It is no wonder that a tiniest bit of dust landing on the surface of our corneas feels like a jagged rock. Nor is it surprising that chronic symptoms of dry eye-like pain are common. The open question is what, other than problem with tears, can cause these symptoms. I suggest that malfunctioning corneal nerves that make them hypersensitive to tear evaporation are the culprit.
There is another, smaller group of people who suffer from far more devastating eye pain and like those described above, the appearance of their eyes does not explain the symptoms. Some of their symptoms may mimic dryness but their eye symptoms are typically described as burning, being sliced with razor blades, pierced with needles, compressed with a red-hot poker, aching and sometimes painful sensitivity to light. Doctors sometimes include it in the wastebasket diagnosis of dry eye because they don’t know what else to call it. How can eyes that are the source of unrelenting, agonizing eye pain have no apparent cause? That is a dilemma for doctors. One explanation is that the pain isn’t “real” (tell that to the patients who have written their stories for this website). Although it is a mystery to the eye professions, it is well known to pain specialists as centralized pain. The reason for that terminology is that the pain is caused and sustained by diseased connections of the pain circuits in the brain that conduct the signals to our cerebral cortex where they are decoded into conscious feelings of pain. Although pain seems to originate in the eyes and sometimes facial and head structures, it is generated by these “misfiring” connections in the brain. What causes centralized eye pain? It can be triggered by virtually any corneal injury such as abrasion, exposure to irritating fumes, etc. Nevertheless, the classical trigger is corneal surgery; in particular laser surgery performed to eliminate the need for glasses. Considering the large numbers of people undergoing these procedures, the mystery is why some people are susceptible to developing centralized corneal pain while the great majority is not.
On the other hand, there are other types of centralized corneal pain not connected with assaults on the corneas. For example, it can be associated with various types of autoimmune diseases such as Sjogren’s syndrome, fibromyalgia, and those affecting the thyroid. Nevertheless, its cause is often unknown.
My message to patients with unexplained, chronic eye pain is that, although its causes are hidden, it is real and can be caused by the pain system itself. Centralized corneal pain is an orphan disease and, like others, requires the mobilization of patients, family and friends to make your suffering heard. Our mission is to provide that focus.