LETTER TO THE EDITOR: Reply to Letter by Dr. C. J. Radke

Dear Editor,

We thank Dr. Radke for his three clarifying remarks.¹

First, we agree that PrCTT is quite variable. As an example, using data from a recent study of 20 normal subjects² the range of PrCTT was 1.98 to 9.17 μm, measured at the center of the cornea, 2 seconds after a blink, and using the first measurement for each subject. Thus the ratio of maximum to minimum thickness, 4.6:1, is similar to that of Creech et al.,³ 5.7:1, which was based on measurement of meniscus radius. This indicates that variability of meniscus radius is probably the main cause of variability of tear film thickness. However, while these ratios are similar in the two studies, our thickness values are less than half of those reported by Creech et al.³ Two factors which could contribute to this discrepancy are, first, the constant factor to be used in the equation for tear film thickness (compare Equations 1 and 2 in our review⁴) and, second, the proposal that the (upper) meniscus radius during tear film deposition may be less than the (lower) meniscus radius which is measured between blinks, and which was used by Creech et al. in their calculated thickness.³

Second, with regard to whether the tear film breakup can be caused by the black lines near the tear menisci, McDonald and Brubaker⁵ state “It is a regular observation that when tear fracture occurs in an eye under observation, it occurs in an area of meniscus-induced thinning.” With regard to confirmatory evidence for tear breakup from staining, Korb and Herman⁶ state “Certain forms of staining … may, however, be now at least partially explained. An example is superior lid margin staining, which appears to be related to the action of the tear meniscus at the superior lid margin, a phenomenon where thinning of the tear film with resultant tear film fracture and resultant staining can occur.” Thus there is support in the literature for the idea that the black line can cause tear film breakup, which may, in turn, cause ocular surface damage and staining.

Third, with regard to Radke's comments on the role of surface asperities in tear film breakup, he is correct in the assertion that mechanical rupture of the tear film is not possible over a water-wet asperity, but this does not rule out the possibility that tear film rupture could occur from the combined effects of evaporation and mechanical forces, cf. Fig. 5D, Fig. 5E and Fig. 5F in our review.⁴ Radke¹ states that “the asperity argument demands repeated breakup at the same locations upon each blink.” Some evidence for this behavior is provided by Rengstorff,⁷ who states “Successive observations on the same eyes revealed that dry areas reoccurred at the same location in about half the eyes.” It also seems possible that breakup could be triggered by one or more raised cells which are about to be sloughed from the epithelial surface; hence breakup could occur in another place if these cells were dislodged by the next blink. Tear film breakup may who also be triggered by “mucus strands”⁸ which are normally buried in the tear film⁹; the position of breakup could change if such strands are moved by the blink action. Concerning Radke's concluding discussion of asperity size, we think that these could have a considerably smaller height than he suggests, because the tear film is often less than 5 μm thick, and our recent unpublished analysis indicates that evaporation rates in “natural” viewing conditions (i.e., without a chamber in front of the eye) may be considerably higher (perhaps up to about 10 u.m/min) than those which have been reported.¹⁰ In keratitis sicca, Brown¹¹ described the role of elevations of the corneal surface as follows: “when the irregularity protruded farther than the tear-film thickness, dryness could be observed above the elevation after blinking.”

We agree with Radke that there is little experimental evidence for breakup over asperities of the normal epithelial surface; the current state of uncertainty about mechanisms of breakup emphasizes the need for additional observations and experimental studies on which to base theory. For example, breakup has been seen to be triggered by white (thick) spots in the lipid layer which may cause tear thinning by outward flow from surface tension gradients (Marangoni flow).¹² Breakup occurred wherever these white spots were located, and this observation throws doubt on the popular assumption that breakup is usually triggered by regions of non-wettable epithelium. It also seems probable that a number of different mechanisms could be involved in tear film breakup.