Comparison of optical and ultrasound pachometry

(1) To determine the agreement between optical and ultrasound pachometry for central corneal thickness measurements used to "correct" applanation intraocular pressure (IOP) readings. (2) To determine the inter- and intra-observer variability of optical and ultrasound pachometry.

Central corneal thickness (CCT) was measured in a masked manner using optical and ultrasound pachometry in 50 normal eyes. To assess intra- and inter-observer variability, multiple masked measurements were obtained in 51 eyes (optical pachometry) and 34 eyes (ultrasound pachometry). Agreement was determined by a published technique that uses the mean of the differences, standard error (SE) and standard deviation (SD).

The mean difference in CCT between optical and ultrasound pachometry was 0.001 mm (SD 0.031 mm; SE 0.00439 mm). The mean inter-observer difference for the optical pachometer was 0.019 mm (SD 0.049 mm; SE 0.0069); the mean intra-observer difference was 0.003 mm (SD 0.017; SE 0.0.0024). The mean inter-observer difference for ultrasound pachometry was 0.001 mm (SD 0.009; SE 0.0015) and the mean intra-observer difference was 0.002 mm (SD 0.011; SE 0.0019).

Ultrasound pachometry is the more reliable method for the measurement of central corneal thickness used to correct applanation IOP values. Optical pachometry had good intra-observer variability. The range of error in IOP correction for corneal thickness (inter-observer) that can occur using the ultrasound pachometer is -1.2 mmHg to +1.4 mmHg as compared to -5.6 mmHg to +8.5 mmHg with the optical pachometer.