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NON-CONTACT optical coherence tomography (OCT) can be used to measure corneal thickness reliably and may be more accurate than ultrasound pachymetry, reported Peter Breil MD at the annual ASCRS Symposium on Cataract, IOL and Refractive Surgery. In order to compare and assess the reliability of the two methods for measuring corneal thickness, Dr Breil and two colleagues each examined 30 eyes using both OCT (4Optics; Lübeck, Germany) and ultrasound (DGH 500; DGH Technology, Exton, PA). Each examiner measured each eye three times with both technologies in a random sequence. The results showed both techniques had acceptable intra and inter-observer reliability, with the inter-observer reliability higher for ultrasound (98%) compared with OCT (86%). However, for each examiner and for all measures combined, values obtained using OCT were significantly higher than the cornea thickness recorded using ultrasound pachymetry. Mean corneal thickness values were 590 (range: 568 to 618) microns for OCT and 559 (range: 532 to 582) microns for the ultrasound technique. "OCT is a novel approach for non-contact measurement of corneal thickness at the slit-lamp, and it can be used as well to measure other anterior segment structures up to the iris. This study indicates that it offers moderately high reliability and may provide a more accurate determination of corneal thickness compared with ultrasound pachymetry because its use does not involve applanation of the corneal surface," Dr Breil said. The eyes included in the study were all healthy. Statistical analyses of the data were performed in co-operation with the Coordination Centre for Clinical Trials at Johannes Gutenberg University, Mainz, Germany and included descriptive analyses using quartiles, ANOVA testing to assess intra and inter-observer reliability and the sign-test to identify significant global and examiner-associated deviations. The analyses of intra-observer data for OCT showed no statistically significant differences in the results of the first, second and third measurement for each examiner. There was also no statistically significant variation in the inter-observer values. Mean corneal thickness values measured with OCT were 584, 591 and 596 microns. Using ultrasound, there was closer agreement for both the intra and inter-observer measurements of corneal thickness, but again, neither the intra nor inter-observer variations were statistically significant. The mean corneal thickness values for the three examiners using ultrasound pachymetry were 563, 558 and 561 microns. "We were actually surprised to see the agreement with the repetition of the ultrasound measurements. Since the cornea may be dipped progressively more with each applanation, we would expect to see a lower thickness by the third measurement. Perhaps all three measurements were performed very carefully to avoid depression," Dr Breil said. Although the OCT measured corneal thickness was greater than that determined using ultrasound pachymetry for each examiner, there was variation among the examiners in the deviation between the two values. The difference between the mean cornea thickness measured with OCT and ultrasound was 25 microns for one examiner compared with 40 microns for the other two. There was no apparent explanation to account for that difference. Peter Breil MD |
