Abstract: Aqueous deficient vs. evaporative dry eye (maybe, maybe not)

This study illustrates quite emphatically (if I understand it correctly) how these terms aqueous deficient dry eye and evaporative dry eye are too frequently used to distinguish between two conditions that really may NOT have any nice simple clear diagnostic distinctions.

Looks like we're graduating from "The tear film has three layers... blah blah blah" and going right back to "Golly the tear film and its diseases sure are complex."

The first thing that jumped out at me when I skimmed this abstract was the absence of any difference in meibomian gland dropout between the two types. What does it mean? That people with ADDE eventually lose their MGs so we're really not looking at ADDE vs EDE but rather ADDE+EDE vs EDE standalone? Or that the criteria for the initial classification was just plain wrong? Or that meibum doesn't really play all that big a role in retarding evaporation? Or, or, or... the possibilities are endless. I found only questions, not answers here.

Tear Physiology of Aqueous Deficiency and Evaporative Dry Eye.
Optom Vis Sci. 2009 Sep 17. [Epub ahead of print]
Khanal S, Tomlinson A, Diaper CJ.
*PhD daggerPhD, DSc, FAAO double daggerFRCS, FRC(Opth) School of Natural Sciences, University of Western Sydney, New South Wales, Australia (SK), Department of Vision Sciences, Glasgow Caledonian University, Glasgow, Scotland, United Kingdom (AT), and Southern General Hospital, South Glasgow University Hospitals NHS Trust, Glasgow, Scotland, United Kingdom (CJMD).

PURPOSE.: To determine the differences in tear physiology between aqueous deficiency dry eye (ADDE) and evaporative dry eye (EDE), and evaluate their utility in diagnosis.

METHODS.: Fifty-six dry eye patients were classified into 30 ADDE and 26 EDE according to the recently published Dry Eye Workshop criteria. A range of tear physiology measures comprising of tear evaporation, turnover rate (TTR), distribution, volume and osmolarity, and meibomian gland dropout were measured in these patients. The effectiveness of the tests, singly and in combinations, in differentiating between the dry eye subtypes was evaluated by retrospective allocation into groups and by Receiver Operative Characteristics (ROC) curve analysis.

RESULTS.: Statistically significant differences were seen for TTR and tear evaporation (with lower values for ADDE) between ADDE and EDE, but no significant differences were seen for tear osmolarity, volume, distribution, and meibomian gland dropout scores. Differentiation of ADDE and EDE by a cut-off value of 11%/min for TTR was found to have a sensitivity of 86%, specificity of 75%, positive predictive value 89%, negative predictive value 69%, and overall accuracy 83%. The area under the curve on the ROC curve was 0.83. For tear evaporation, a cut-off of 60 g/mh was found to have a sensitivity of 77%, specificity of 55%, positive predictive value 38%, negative predictive value 80%, and overall accuracy 58% in subtype differentiation. The area under the curve was 0.59 on the ROC curve. The distribution curve of the evaporation rates for ADDE and EDE, showed an overlap coefficient of 0.76 indicating that tear evaporation is within a similar range in these two dry eye subtypes.

CONCLUSIONS.: Tear turnover is significantly lower in ADDE than EDE, but there is considerable overlap of tear evaporation between the two dry eye subtypes. Tear osmolarity and turn over tests can be conducted sequentially to effectively diagnose dry eye and its subtypes.