Criticality of the measurement of corneal thickness in specular reflection by digital biomicroscope

Purpose: The aim was to evaluate the effects on corneal thickness (t) measurement, of asymmetry of the CCD position for a digital biomicroscope in specular reflection. Methods: t was deduced from the distance between reflexes from anterior and posterior corneal surfaces using a biomicroscope (Takagi SM70 N), with the illuminator either, (a) farther from sensor side (150 eyes) or, (b) closer to it (134 eyes). The distance between reflexes was also measured on a glass slide and a reference lens, with nominal thicknesses of 1580 and 520 μm, respectively. Corneal thickness (tPACH) was also measured by pachymeter (Canon TX-20 P). Results: When biomicroscope asymmetry was ignored, t for the glass slide was (a) 1760 and (b) 1404 μm. Correcting for the asymmetry provided corresponding values of 1588 and 1591 μm. For the lens, t was (a) 696 and (b) 543 μm, or 642 and 497 μm, when using the approximation of parallel surfaces (APS). Correcting for the asymmetry gave 565 and 552 μm (578 and 564 μm, with APS). Mean corneal t was (a) 560 and (b) 467 μm, (564 and 468 μm, with APS). Correcting for asymmetry gave (a) 506 and (b) 529 μm (508 and 530 μm, with APS). Mean tPACH was 552 μm. Conclusion: Biomicroscope asymmetry critically affects corneal thickness measurement in specular reflection. Induced errors can be accounted for and corrected, however. While the correction to the curvature radius is clinically relevant, it plays a minor role compared to asymmetry.