Central corneal thickness: Z-ring corneal confocal microscopy versus ultrasound pachymetry

PURPOSE: To compare the repeatability and validity of corneal pachymetry by a corneal confocal microscope with a z-axis adapter (Confoscan 4.0 with z-ring adapter: z-CS4) versus ultrasound (US) pachymetry in the measurement of central corneal thickness (CCT). METHODS: CCT in 44 eyes of 44 subjects was determined with z-CS4. Z-CS4 exams were used to estimate the repeatability of thickness measurement by z-ring adapter for this confocal microscope. Intraclass Correlation Coefficient (ICC) between two different z-CS4 users was also determined. CCT in the same 44 eyes was determined with US pachymetry and measurements were compared with z-CS4 CCT. RESULTS: Z-CS4 CCT showed high intrainstrument reproducibility (ICC = 0.989; 95%CI 0.982-0.993; P <0.0001). Mean difference among three CCT consecutive measures, in the same eye, was 0.8 ± 11.1 μm. High correlation was found between two users (ICC = 0.896; 95%IC 0.830-0.937; P <0.0001). Z-CS4 CCT showed high correlation with US pachymetry (ICC = 0.921; 95%CI 0.851-0.958; P <0.0001). Mean corneal thickness determined was statistically different with the two methods (US: 512.6 ± 65.8 μm; z-CS4: 487.8 ± 60.1 μm; P <0.0001). CONCLUSION: Z-CS4 seems an accurate, noninvasive and reproducible technique for CCT evaluation and confirms that central cornea is thinner when measured with confocal microscopy compared to ultrasounds.