DEWMD: A lean definition of dry eye
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When I write for myself, in my personal blog and elsewhere, I routinely begin by defining the terms I will be addressing. In order to have a useful discussion on any topic, it is important to agree on what it is that you are talking about.
I have long been uncomfortable with the various definitions of dry eye disease (DED). In some ways, definitions can be like mission statements, or at least they should be. Both should be simple and succinct. A mission statement can be partisan, espousing and promoting an agenda. A definition should not have a point of view. As much as possible, one should be bound by consensus, free of both controversy and conflict.
A definition is simply a place to start, not a road map for the journey.
I recently came across a new definition of DED from Tsubota and colleagues, a group including Stephen Pflugfelder, prompting me to think about how we have defined the entity over time. Over the years, the definition of DED has become more and more complex. Rather than seek the most narrow and elegant option, each new iteration has attempted to include the entirety of dry eye. Etiology, pathophysiology and treatment guidance have all made their way into the newer definitions. Like a mission statement that seeks to convey all of the nuances of how one will seek to address some cause or another, each subsequent effort has suffered to some degree from mission bloat.
Did you know that the first definitions of DED were first published in 1995? No? Me either. Every review I have ever seen gives “founder’s” credit to the Delphi panel of 2006. It turns out that the Delphi panel never actually defined DED (more in a moment). As a North American-based eye doctor, did you know that there has been a parallel arc of development in the U.S. and Japan? I admit to a certain degree of “homerism” when it comes to DED. Canada and the EU have been notably slow to adopt many (most?) of the diagnostic and therapeutic breakthroughs that have entered our daily use here in the U.S. In contrast, Japan has a long and storied history when it comes to DED. Indeed, as we will see, they have done a very good job at the definition thing over the years. See if you agree.
National Eye Institute (U.S.), 1995: “Dry eye is a disorder of the tear film due to tear deficiency or excessive evaporation, which causes damage to the interpalpebral ocular surface and is associated with symptoms of ocular discomfort.”
Japan Dry Eye Society (JDES), 1995: “Ocular surface epithelial damage caused by qualitative or quantitative abnormalities of tears.”
Boy, these two are just money. You could make a case that everyone should have just combined the best of these definitions and gone about the business of finding ways to diagnose and treat DED. Sure, you could ask for a nod to visual symptoms, and I would happily agree, but these two are short, sweet and elegant. Note also that the JDES predates the Tear Film & Ocular Surface Society (TFOS) DEWS I by a decade.
Delphi panel (mostly U.S.), 2006: “The dysfunctional tear syndrome (DTS) is diagnosed primarily by symptoms and clinical signs and is triggered or maintained by inflammation.” This is my minimalist distillation of the stream of comments in the original publication. Members of the panel would find this too spartan and add: “Level of severity should determine treatment; clinical tests are not useful in diagnosis, staging or evaluating treatment response,” a harbinger of the excesses to follow.
JDES, 2006: “Dry eye is the chronic disease in tears and corneal/conjunctival epithelia caused by various factors. It may accompany irritating symptoms and/or visual disturbances.”
TFOS DEWS I (international panel), 2007: “Dry eye is a multifactorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability with potential damage to the ocular surface. It is accompanied by increased osmolarity of the tear film and inflammation of the ocular surface.”
The JDES offering is still pretty tight and tidy. There is a nod to DED symptoms, a plus. The specific mention of surface “epithelia” is curious. DEWS I is a little wordy, eh? In truth, this would have been awfully good if they quit after the first sentence. Osmolarity, which can only be measured by a “clinical test,” is included in direct conflict with Delphi. That is curious, too. Does osmolarity define DED? Why not lactoferrin levels, a contemporary diagnostic test equally associated with DED?
JDES, 2016: “Dry eye is a multifactorial disease characterized by [an] unstable tear film causing a variety of symptoms and/or visual impairment, potentially accompanied by ocular surface damage.”
TFOS DEWS II, 2017: “Dry eye is a multifactorial disease of the ocular surface characterized by a loss of homeostasis of the tear film, and accompanied by ocular symptoms, in which tear film instability and hyperosmolarity, ocular surface inflammation and damage, and neurosensory abnormalities play etiological roles.”
Tsubota, 2020: “Dry eye is a multifactorial disease characterized by a persistently unstable and/or deficient tear film (TF) causing discomfort and/or visual impairment, accompanied by variable degrees of ocular surface epitheliopathy, inflammation and neurosensory abnormalities.”
Phew, DEWS II is a mouthful. I challenge anyone to get through it on a single breath. The definition from the Tsubota group is similar, no? Once again, JDES is a model of brevity. All three recent definitions reflect the now universally recognized functional aspect of DED by including tear instability. Including hyperosmolarity in the TFOS definition after implicitly acknowledging DTS is puzzling; osmolarity is low as often as it is high. Are osmolarity (TFOS), damage (all three) and neurosensory abnormalities (TFOS, Tsubota) defining characteristics? JDES tempers the inclusion of “damage” with the modifier “potentially.”
If we return to my original premise, the best, most helpful definitions portray a deep consensus in a concise package. Both TFOS and JDES have altered their definitions of DED as our understanding of DED has evolved. JDES was first to the table, proposing an elegant definition of DED 5 years before the founding of TFOS in 2000 and more than 10 years before the Delphi panel. You could simply slap a statement about symptoms on that 1995 definition and call it a day, but what fun would that be?
Here, then, is DEWMD (Darrell E. White, MD: “Doomed”), a modern, minimalist definition of DED: “Reduced quantity or quality of the tear film resulting in ocular discomfort, visual disturbance or both.” Short. Sweet. Everything you need and nothing you do not. There is plenty more to be said about DED (etiology, pathophysiology, diagnostic and treatment pathways), but nothing else that belongs in a definition.
- References:
- Behrens A, et al. Cornea. 2006;doi:10.1097/01.ico.0000214802.40313.fa.
- Craig JP, et al. Ocul Surf. 2017;doi:10.1016/j.jtos.2017.05.008.
- Ocul Surf. 2007;doi:10.1016/s1542-0124(12)70081-2.
- Shimazaki J. Invest Ophthalmol Vis Sci. 2018;doi:10.1167/iovs.17-23475.
- Tsubota K, et al. Int J Mol Sci. 2020;doi:10.3390/ijms21239271.
- For more information:
- Darrell E. White, MD, can be reached at SkyVision Centers, 2237 Crocker Road, Suite 100, Westlake, OH 44145; email: dwhite@healio.com.
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