Ophthalmic surgical procedures such as vitrectomy,
1 retinal membrane cutting,
2 subretinal fluid aspiration,
3 lens refilling,
4–5 and others
6,7 require visualization of fine intraocular structures that are peeled, cut, and removed. We observed that a viscoelastic solution applied to the cornea permitted visualization of these structures for a longer duration than the standard saline drops commonly in use today, but this was merely a qualitative observation. This observation spurred us to develop a simple method to quantify the optical quality of the ocular surface, and to compare the effectiveness of a viscoelastic solution with that of saline.
8
The central tenants of Millar's
9 complaints stem from his misinterpretation of the purpose of Figure 1. The intent of this figure is to illustrate that the distortion of the specular reflection is due to topologic irregularities of the air–fluid interface on the ocular surface, and that this technique can be used to monitor tear-film stability. We elected to show how the specular reflection becomes distorted as the integrity of the lipid layer is lost and the tear film begins to evaporate. To this end, we disagree with Millar's assertion that, “it does invalidate the diagram,” as we make no claims that this diagram illustrates the precise nature of the experiment.
Many of Millar's other critiques seem to originate from his confusion about terminology. Particularly, he cites two terms: dehydration rate and tear film evaporation rate, and he is obviously uncomfortable with the unit (s−1). It seems that he overlooked the paragraph stating that, “because the ellipse axes ratio was unitless, the slope of this ratio as a function of time was given by units of inverse seconds.” Thus, the rate measured was effectively the rate of distortion of the reflection, which is presumably degraded as the tear film or saline solution evaporates. Earlier versions of the manuscript included language to this effect, but it was eliminated in the peer review process. An examination of the relationship between the optical quality of the ocular surface and the quantity of saline solution or viscoelastic are topics of future interest.
Otherwise, Millar has rightly pointed out a few minor errors and made a few suggestions that, had he been a reviewer, could have further improved the manuscript. However, our central thesis is sound: we developed a safe, noninvasive, noncontact, continuous in vivo method to quantify the optical quality of the ocular surface, and we compared a saline wetting agent used worldwide in ophthalmic surgery with a viscoelastic solution. We hope our simple technique will be used to determine the ideal viscoelastic agent that could improve patient care by maintaining the clarity of the cornea during long-duration ophthalmic procedures.