Comparison between graticule and image capture assessment of lower tear film meniscus height

To compare graticule and image capture assessment of the lower tear film meniscus height (TMH).

Lower tear film meniscus height measures were taken in the right eyes of 55 healthy subjects at two study visits separated by 6 months. Two images of the TMH were captured in each subject with a digital camera attached to a slit-lamp biomicroscope and stored in a computer for future analysis. Using the best of two images, the TMH was quantified by manually drawing a line across the tear meniscus profile, following which the TMH was measured in pixels and converted into millimetres, where one pixel corresponded to 0.0018 mm. Additionally, graticule measures were carried out by direct observation using a calibrated graticule inserted into the same slit-lamp eyepiece. The graticule was calibrated so that actual readings, in 0.03 mm increments, could be made with a 40x ocular.

Smaller values of TMH were found in this study compared to previous studies. TMH, as measured with the image capture technique (0.13+/-0.04 mm), was significantly greater (by approximately 0.01+/-0.05 mm, p=0.03) than that measured with the graticule technique (0.12+/-0.05 mm). No bias was found across the range sampled. Repeatability of the TMH measurements taken at two study visits showed that graticule measures were significantly different (0.02+/-0.05 mm, p=0.01) and highly correlated (r=0.52, p<0.0001), whereas image capture measures were similar (0.01+/-0.03 mm, p=0.16), and also highly correlated (r=0.56, p<0.0001).

Although graticule and image analysis techniques showed similar mean values for TMH, the image capture technique was more repeatable than the graticule technique and this can be attributed to the higher measurement resolution of the image capture (i.e. 0.0018 mm) compared to the graticule technique (i.e. 0.03 mm).