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

Abstract 

Purpose

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

Methods

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 0.0018mm. 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 0.03mm increments, could be made with a 40× ocular.

Results

Smaller values of TMH were found in this study compared to previous studies. TMH, as measured with the image capture technique (0.13±0.04mm), was significantly greater (by approximately 0.01±0.05mm, p=0.03) than that measured with the graticule technique (0.12±0.05mm). 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±0.05mm, p=0.01) and highly correlated (r=0.52, p<0.0001), whereas image capture measures were similar (0.01±0.03mm, p=0.16), and also highly correlated (r=0.56, p<0.0001).

Conclusions

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. 0.0018mm) compared to the graticule technique (i.e. 0.03mm).

Keywords: Tear meniscus height, Tear prism, Tear film, Repeatability