Discrepancy between central and midperipheral corneal thickness measurements obtained with slit-scanning pachymetry...

To compare corneal thickness measurements obtained by a slit-scanning method with those obtained by noncontact specular microscopy, with a focus on the midperipheral (3.0 mm) region of the cornea.

Department of Vision Sciences, Glasgow Caledonian University, Glasgow, United Kingdom.

In adults with up to 8.50 diopters of myopia, single measurements of corneal thickness were performed with specular microscopy (Topcon SP-2000P) at central, 3.0 mm nasal, and temporal midperiphery locations. Slit-scanning pachymetry (Orbscan II) measurements were then taken and data extracted for 2.0 mm diameter sample zones at the central, 3.0 mm nasal, and temporal locations.

The mean corneal thickness by specular microscopy was 0.524 mm in the central zone, 0.586 mm in the temporal location, and 0.547 mm in the temporal location. The mean uncorrected slit-scanning pachymetry readings were 0.584 mm, 0.668 mm, and 0.658 mm, respectively, and the mean 0.92 acoustic factor-corrected readings, 0.538 mm, 0.614 mm, and 0.603 mm, respectively. At central locations, the corrected slit-scanning measurements agreed with specular microscopy measurements to within +/-0.014 mm (95% confidence interval, -0.019 to +0.045 mm). However, even with correction, discrepancies at the midperipheral locations were as high as 4 times greater (temporal).

Results show that the slit-scanning pachymeter can substantially overestimate more peripheral thickness measurements of the cornea than specular microscopy or ultrasound pachymetry and that the differences cannot be corrected using the same acoustic factor (0.92). Off-center slit-scanning thickness measurements, therefore, cannot be used interchangeably with those obtained with other pachymetry methods, especially before refractive surgery.