In Vitro Study
The inner surfaces of five glass capillaries were used as a model of the tear meniscus. The inner diameters and the circularity of the inner surface of the glass capillaries (Hilgenberg GmbH, Malsfeld, Germany) were confirmed by use of a hole-gauge before cutting them lengthwise in half. On the basis of preliminary studies, the medians of three consecutive measurements on the five glass capillaries (radii, 0.100 to 0.505 mm) were compared between the existing VM (Fig. 5) and the new PDM at two different sessions at the same time of day (day 1 and day 2) and after re–set up of the PDM.
In Vivo Study
Twenty subjects (10 males and 10 females; mean age, 32.3 years; range, 23 to 56 years) were randomly selected from the students and staff of the School of Optometry and Vision Sciences at Cardiff University, UK. All procedures obtained the approval of the Cardiff School of Optometry and Vision Sciences Human Ethics Committee and were conducted in accordance with the requirements of the Declaration of Helsinki. All subjects gave written informed consent before participating in the study.
Subjects were excluded if they were pregnant or breastfeeding; had a current or previous condition known to affect the ocular surface or tear film; had a history of previous ocular surgery, including refractive surgery, eyelid tattooing, eyelid surgery, or corneal surgery; had any previous ocular trauma; were diabetic; were taking medication known to affect the ocular surface and/or tear film; and/or had worn contact lenses less than 2 weeks before the study. Subjects with a history of dry eye, defined by either an item-weighted McMonnies questionnaire score higher than 14.5 or a fluorescein tear breakup time less than 10 seconds, were excluded.
The lower TMR was measured by one observer using both techniques (VM and PDM) in a randomized order. Care was taken to align both instruments consistently across data collection. The median of three consecutive measurements was recorded for both techniques. On the basis of preexperiments, median instead of mean was chosen. For both techniques, the measurement time was about 2 minutes, with a break of 1 minute between the two instruments. All assessments were of the inferior tear meniscus of the right eye directly below the pupil center with the subject looking straight ahead at a fixed target. The room temperature was 18 to 22°C and the relative humidity was 30 to 40%. To minimize diurnal and interblink variation, measurements were taken in the morning between 10 AM and noon and at 3 to 4 seconds after a blink.
Normal distribution of data was analyzed by Shapiro-Wilk test. Differences between sessions (day 1 and day 2) and instruments were analyzed using Bland-Altman plots, coefficient of repeatability (CR), and paired t-tests. The relationship between PDM and VM measurements was analyzed by Pearson product-moment correlation. Data were analyzed using SigmaPlot 12 (Systat Software Inc., Chicago, IL) and BiAS 10 (epsilon-Verlag, Darmstadt, Germany).
In Vitro Study
The measured radii of the five glass capillaries were 0.105, 0.186, 0.349, 0.394, and 0.503 mm for the PDM and 0.088, 0.169, 0.342, 0.403, and 0.534 mm for the VM. The mean difference between the measurements of the two devices was 0.0002 mm (95% confidence interval [CI], −0.0252 to +0.0256 mm; p = 0.984) (Fig. 6).
Repeated measurements from day 1 and day 2 were not significantly different for the PDM and VM (paired t-test: p = 0.468 and p = 0.775, respectively). The 95% CIs around differences indicate acceptable repeatability (95% CI: PDM, −0.0134 to +0.0074 mm; VM, −0.0282 to +0.0226 mm) and reproducibility between sessions (95% CR: 0.019 and 0.018 mm for PDM and VM, respectively) (Figs. 7 and 8).
In Vivo Study
The mean (SD) TMR of the subjects measured with the PDM was 0.34 (0.10) and 0.36 (0.11) mm of the VM. The PDM measurements were significantly correlated to measures of the VM (Pearson product-moment correlation: r = 0.940, p < 0.001). There was a nonsignificant difference between the measurements taken by the PDM and the VM (mean difference, −0.0151 mm; 95% CI, −0.0285 to −0.0018 mm; paired t-test, p = 0.124) in this cohort (Fig. 9).
Examples of a steep (r = 0.19 mm) and a flat tear meniscus radius (r = 0.37 mm) measured with the PDM are shown in Fig. 10.
With our newly developed iPod touch–based PDM, we found a good accuracy and reproducibility across the whole range of typical TMR values (Fig. 7). In contrast, the VM seemed to have the tendency to underestimate the TMR for small radii and to overestimate TMR for larger radii (Fig. 8).
This effect was also evident in the comparison between the two methods when the radii measured by the PDM seemed to be more consistent than those measured by the VM (Fig. 6). Since the experimenter was trained in maintaining the alignment of both devices, these apparent differences might be caused by differences in the design and presentation of the targets. While the VM uses metal bars, mounted coaxial with the observation system, the target of the PDM consists of digitally generated bands, which are separated from the observation system. As a result, the PDM target does not interfere with the observation system of the slit-lamp, since the VM target effectively functions as an aperture within the observation system thus influencing the depth of field. A second source of error arises from the working distance of the instrument. While the VM has a working distance of 24 mm, a longer distance of 50 mm is used by the PDM. By looking at the concave mirror formula (Fig. 3), it becomes obvious that the smaller the working distance (a), the greater the error, if the system is not exactly aligned.
In vivo, there was a good agreement between the TMR values of the two instruments. With the PDM, we found a TMR of 0.34 (0.10) mm in a group of patients with normal non–dry eyes. This was not significantly different from the TMR measured with the VM (0.36 ± 0.11 mm) and is in accordance with previously reported measurements using reflective meniscometry in subjects with normal eyes.9,17 The correlation between the two methods indicates that the PDM provides a valid measurement of TMR. For patients with dry eyes, the reported TMR, measured by reflective meniscometry, has varied between 0.22 (0.09) and 0.25 (0.09) mm,7,9,26 although some of these reports related to patients with evaporative dry eye.
While meniscometry uses specular reflection to analyze TMR, in OCT, a vertical line scan produces a cross-sectional image of the tear meniscus. On the images taken with an OCT, the 3-point method is used to fit a circle to the anterior border of tear meniscus. The TMR of the lower tear meniscus reported with this method varies from 0.25 (0.05) to 0.46 (0.40) mm for patients with normal eyes and between 0.15 (0.03) to 0.20 (0.08) mm in patients with dry eyes.18,20,21,23,32
As in this study, calibration of the original meniscometer system was carried out using glass capillaries.17 Also using glass capillaries, Kato et al.33 found no significant differences between TMR measured with the VM and an anterior segment optical coherence tomographer.
For the purpose of calculating meniscus volume, the anterior shape of the meniscus is treated as a part of a circle although it is likely to have a more complex shape.34 To understand differences in TMR measurements between reflective meniscometry and OCT, it would be helpful to describe the shape of the meniscus more precisely and to analyze the location on the meniscus where the PDM is measuring the meniscus. While OCT and the existing VM have a fixed orthogonal orientation of the target, the PDM allows rotation of the target and therefore a measurement of the meniscus under different angles in the coronal plane. This could be of value in following differences in TMR along the nasal and temporal slopes of the lid. Furthermore, the bandwidth of the target can be easily varied via the touch screen. This enables a finer grating to be projected onto the meniscus, with the possibility of obtaining a more detailed description of the tear meniscus profile.
In the literature, the measurement of tear meniscus parameters is mostly performed at the center of the lower eyelid, directly under the pupil. Some authors report TMH to be greater at the center of the lid,35 but others find no thinning of the inferior tear meniscus,36 or even that the TMH that is smaller at the center.13 These differences might be explained by the different techniques used and the different locations at which TMH was measured. At the same time, when calculating tear meniscus volume, the cross section of the meniscus is assumed to be equal along the lower lid,4,37 or a correction factor of ¾ is used to account for an unequal distribution.35,38,39 Since the PDM is mounted on a standard slit-lamp, it can be used for measurement of TMH, as well as the TMR at different locations, which will facilitate analysis of tear film distribution along the lid.
Measuring TMR is a useful noninvasive test for dry eye diagnosis.9,17–19,23 but existing techniques are either not available commercially or are too expensive for general clinical use. We have developed a PDM that permits accurate and reliable measurements of human tear meniscus radius, can be made generally available, and is suitable for use in both research and clinical practice.
Höhere Fachschule für Augenoptik Köln
(Cologne School of Optometry)
Bayenthalgürtel 6-8 D-50968
The authors thank Dr. Norihiko Yokoi for providing the videomeniscometer.
Received: February 19, 2013; accepted June 9, 2013.
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Keywords:© 2014 American Academy of Optometry
portable digital meniscometer; reflective meniscometry; tear meniscus radius; tear film; dry eye diagnosis