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Comprehensive Ophthalmology Free Papers 6.0.3) using the corneal reflectivity profile. The central cornea was identified from the peak of the reflectivity profile on the horizontal axis. The calipers were then aligned on the peak reflections at the anterior and posterior boundaries of the cornea, in the axis of the corneal apex. The two measurements were averaged for right eye only to avoid any bias. Ultrasound testing was also performed twice for right eye only. The cornea was first anesthetized with one drop of pro-paracaine hydrochloride 0.5% (Paracain, Sunways, India). It would be very difficult to assess whether the two instruments took measurements at the same exact location. However, to ensure that ultrasound CCT was measured at the center of the cornea, we placed the probe tip at the very center of the cornea and kept the pachymeter horizontal. If the patient moved and the probe decentered, then the measurements were repeated. Ultrasound pachymetry was performed in each case by one of two authors (Dr. MJ and Dr. AJ) to measure the inter individual variability. Ten sequential measurements were obtained from the center of the cornea and averaged. Values with standard deviation (SD) of 5 micron or less were considered suitable for inclusion. Statistical Analysis. Pearson correlation was used to assess the strength of correlation of the two measurements. The measurements were also compared with the paired t-test. Since AS-OCT measurements involved caliper measurements made by the observer all eyes were remeasured by a second observer to study inter observer agreement. RESULTS Mean age was 32.9 +/- 16.1 years with 41 male and 49 female patients. Mean CCT with ultrasonic pachymetry for observer 1 and 2 were 523.2 +/- 28.6 microns and 522.5 +/- 30.9 respectively. Mean CCT with AS-OCT for observer 1 and 2 were 526.6 +/- 26.7 and 525.8 +/- 26 microns respectively. The value with AS OCT is slightly higher than with ultrasonic pachymetry but the P value is not significant. The P value was 0.405 and 0.436 between ultrasonic pachymetry and AS OCT for observer 1 and 2 respectively which is not significant. Correlation coefficient between ultrasonic pachymetry and AS-OCT was 0.95 (observer 1) and 0.93 (observer 2). Correlation between the two observers, to assess the inter individual variability for ultrasonic pachymetry and AS-OCT was 0.96 and 0.97 respectively. DISCUSSION Doughty and Zaman 2 (300 patients) found that mean normal CCT was 530 microns for slit-lamp-based optical pachymetry and 544 microns for ultrasonography. Subsequent reports using retinal OCT adapted for the anterior segment 3–5 suggest a similar effect; other authors have reported no significant difference 6 or even thicker measurements of CCT by OCT. 7 473
70th AIOC Proceedings, Cochin 2012 AS-OCT has the advantages over ultrasonic pachymetry in that it is a noninvasive, non contact modality, as well as having the ability to examine other structures as mentioned above. The disadvantage is that it is slower to perform in a busy clinic than a hand-held pachymeter and requires greater expertise. Unlike ultrasonic pachymetry, AS-OCT can easily be used to assess regional differences in the cornea and the facility for the patient to fixate on a target allows more accurate identification of the central corneal surface. Zhao et. al. 8 has reported CCT using AS-OCT found a systematic difference between AS-OCT and ultrasonic pachymetry. It is unclear whether ultrasound or OCT measurements more accurately reflect the true corneal thickness. Kim et. al. 9 presented their data with another AS-OCT machine that is SL- OCT (Heidelberg Eye Explorer v1.5.9.0) and found that though the correlation was excellent between the two machine 0.82 there was a significant difference in the average standard deviation (SD) around 26.3 micron and the AS-OCT measurements were slightly lower than the pachymetry measurements. Our measurements showed that there was excellent correlation between the two methods and the AS-OCT showed a minimally higher value which has been reported earlier by Fishman et. al.. 6 and Leung et. al.. 7 We did not see much difference in the SD range also and the two methods correlated well for both the observers separately. For OCT, uncertainty of the true index of refraction of infrared radiation in the cornea may create a source of error in calculating the CCT. 8 There could also be small calibration errors in either system. In conclusion, AS-OCT is a promising non contact and reproducible diagnostic method that is comparable to ultrasonic pachymetry in the evaluation of corneal thickness and the inter observer variability is low, though the AS-OCT may been slightly more time consuming but has the advantage of being totally noninvasive. REFERENCES 1. Wolffsohn JS, Davies LN. Advances in anterior segment imaging. Curr Opin Ophthalmol 2007;18:32–8. 2. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures: a review and metaanalysis approach. Surv Ophthalmol 2000;44:367–408. 3. Wirbelauer C, Scholz C, Hoerauf H, et. al.. Noncontact corneal pachymetry with slit-lamp–adapted optical coherence tomography. Am J. Ophthalmol 2002;133:444- 50. 4. Wong AC, Wong CC, Yuen NS, Hui SP. Correlational study of central corneal thickness measurements on Hong Kong Chinese using optical coherence tomography, Orbscan, and ultrasound pachymetry. Eye 2002;16:715–21. 474
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