Tucked neatly into the DEWS II definition of dry eye is perhaps the single most important word in the paragraph: homeostasis. Understanding how the normal eye works is key to understanding how to get it back on track when things go awry. The more I immerse myself in dry eye, the more obvious it becomes that I will never fully understand the complexity of the ocular surface environment. One thing I do understand is that, given an opportunity, the eye’s surface will always try to fix itself. As with the rest of our bodies, the means to accomplish repair are baked in.

You don’t need to have the intelligence of a Sheldon Cooper to understand that this exquisitely complex system is under neurological control. That’s necessary for function within an incredible variety of conditions, a key to our survival and the dominance of our species. Likewise, you don’t need to be a chemist to understand that an eye deficient in tears, either because of decreased production or excessive evaporation, will have elevated osmolarity. Whether elevated osmolarity causes dry eye or not, is beside the point to me; it defines dry eye.

Maintenance of tear film stability is another self-evident truth of a functional tear layer. Tears exist to create a smooth optical surface. They allowed our ancestors to quickly identify predators from a distance and successfully forage for food. No engineer in their right mind would have trivially created a wet environment for the ocular surface. Whether a supreme being or evolution was responsible, the ocular surface environment’s myriad components and wonderfully complicated functions strongly suggest that it was the best solution to an otherwise unsolvable problem—giving us the ability to see well. And, as with much of the rest of the body, inflammation plays a role in any attempted repair, but sometimes causes more harm than good.

As the elegance of the tears and complexity of the ocular surface environment become increasingly apparent to me as a clinician, I realize that our patients’ signs and symptoms are not just caused by any single factor, but rather, by a systematic failure of function and repair. Describing this as a loss of homeostasis may well be the biggest step in the right direction toward understanding “dry eye” that we’ve taken in quite some time.

Arthur B. Epstein, OD, FAAO Chief Medical Editor artepstein@optometricphysician.com