Measurement of central corneal thickness (CCT) is useful for diagnostic and therapeutic purposes some of which include assessment prior to refractive surgery, corneal changes after extended contact lens wear, and reliability of intraocular pressure measurements.1 Corneal thickness is determined by Orbscan, confocal microscopy, optical pachymetry and ultrasound pachymetry (gold standard). The aim of the study was to determine the accuracy and reliability of noncontact CCT measurement using a commercially available optical coherence tomogram (Stratus OCT III) (originally designed to measure retinal thickness) versus standard ultrasound pachymetry (CCT usg) and to analyze the correlation between the CCT measurements using the two devices.
Materials and Methods
CCT measurements of 60 eyes from 30 white volunteers were taken after obtaining an informed consent. Exclusion criteria included previous ocular medication use, ocular abnormality, contact lens wear, prior refractive surgery and refractive errors more than 6.0 diopters.
The Stratus OCT III has an integrated high power-condensing lens so that the retina can be imaged onto a plane within the instrument. To image the cornea [Fig. 1], the focusing system had to be defocused near the maximum to relay the image of the OCT beam onto the cornea. The subjects were asked to look at the external fixation light with the other eye, which was positioned so that the subject was looking straight ahead. Care was taken to ensure that the probe beam passing through the cornea bisected the pupil horizontally [Fig. 2]. Three consecutive measurements of the CCT using OCT (CCToct) were then performed in the center of the horizontal section and the readings were averaged. The line algorithm on the Stratus OCT III was chosen to determine the peak reading of the line representing the anterior and the posterior surface of the cornea. This was represented by the most outer spike and inner spike corresponding to a change in interface and the difference was calculated to give the corneal thickness [Fig. 3]. If a clearly delineated peak reading was not obtained then the measurements were repeated to minimize measurement error.
CCT usg was determined using an A scan ultrasonography (USG) pachymeter with a hand-held solid probe. After topical anesthesia, the probe tip of the pachymeter was held perpendicular on the central cornea. Three lowest readings were averaged to avoid measurement error due to the increase in corneal thickness peripherally. As OCT is a noncontact technique, CCToct was obtained first to avoid potential artifact caused by corneal contact involved in a scan USG pachymeter.
Data were analyzed using SPSS statistical package (version 11).
A total of 30 white subjects (12 male, 18 female) underwent CCT measurements. Subject age ranged from 30 to 75 years with a mean age of 49.5± 18.0 years (mean ±SD). The mean CCToct was 522.33±34.44 microns and mean CCTusg was 547.37± 33.08 microns. The mean difference between CCTusg and CCToct was 25.04±11.67 microns (Mean ± SD). Paired t-test showed that CCT usg was highly correlated with CCToct(r 0.93, P<0.001). Since most studies use USG pachymetry as standard instrument, the results were treated as true values and the relationship between the instruments was evaluated using linear regression analysis. The slopes of regression lines of OCT was found to be 0.980. The results of the analysis can be represented by CCToct = 0.98(CCTusg) - 13.9 (r2 = 0.886 andP < 0.001).
The mean CCT for men was 537.18 and for women was 533.29 microns.
Mean CCToct measurements were 522.48 and 522.19 microns in the right and left eye respectively. Average CCTusg was 548.83 and 548.91 microns in the right and left eye respectively.
USG pachymetry is the gold standard in measurement of CCT. However, recent literature supports the use of OCT as an accurate alternative for the same. OCT enables noninvasive noncontact, cross-sectional imaging of the eye. Its use in anterior segment imaging has been demonstrated by Izatt et al.2 Previous studies have established the correlation between corneal thickness measurement using USG pachymetry and OCT123 but the commercially available Stratus OCT III has been used for the first time in this study. Statistically, only a small sample size (7) is needed to validate this tool, as a difference of 25 microns between two instruments is acceptable in most clinical settings.
The mean difference in CCT measurement using the OCT and the USG device in our group was 25.04 microns (±11.67), compared with a mean difference of 31.9 in the study by Wong et al. and 49.4±5.9 by Bechmann et al. This difference can be explained by a possible lateral displacement of the ultrasound probe during the scan thus measuring the corneal thickness of the paracentral region of the cornea. Also, gaze monitoring is possible with the external fixation lights in Stratus OCT III, allowing precise probe beam positioning on the center of the cornea assisted by magnification.
The OCT provides qualitative assessment of different corneal layers. It can document corneal scars, corneal ulcers and intracorneal foreign bodies.1 It is useful in the assessment of corneal edema (such as Hydrops) and does not depend on a totally clear optical media.2 It has been used to assess corneal cap and stromal bed after laser in situ keratomileusis.4567 There was no significant correlation between CCToct and gender, age and right and left eyes.
This pilot study has its drawbacks. Inter-observer variations have not been reported. To our knowledge, this novel method of CCT measurement has not been tested on animal eyes. In our setting of a district general hospital an optical pachymeter was not available. Hence comparisons using this method could not be made.
Corneal thickness measurement can be performed using the commercially available Stratus OCT III which is designed mainly for posterior segment imaging and future generations of OCT may incorporate means to measure CCT with minimal or no modifications to the system. Further studies are recommended to popularize the use of OCT as an alternative in corneal thickness measurement.
I would like to thank all the volunteers who participated in this study.
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