Corneal Subbasal Nerve Density: A Comparison of Two Confocal Microscopes

Purpose. To compare subbasal nerve densities estimated from images recorded by the tandem scanning and the ConfoScan 4 confocal microscopes. Methods. Confocal microscopy was used to estimate subbasal nerve density in 62 corneas of 40 subjects (18 corneas of 18 normal subjects and 44 corneas of 22 patients between 1 and 12 months after LASIK). At each examination, corneas were scanned first by using tandem scanning and then by using a ConfoScan 4 confocal microscope. Subbasal nerves from 2 to 4 scans per cornea were traced by using a semi-automated nerve analysis program. Nerve density was expressed as total nerve length divided by the sample area (mu m/mm(2)). Differences in nerve density between instruments were examined by using paired tests. Results. In normal corneas, subbasal nerve density was 10,658 +/- 5,581 mu m/mm(2) (mean +/- SD) with the ConfoScan 4 and 5,534 +/- 1,850 mu m/mm(2) with the tandem scanning microscope (P<0.0001). One to 12 months after LASIK, mean subbasal nerve density was 2,477 +/- 3,514 mu m/mm(2) estimated with the ConfoScan 4 and 844 +/- 983 mu m/mm(2) estimated with the tandem scanning (P = 0.0003). Estimates of nerve density were correlated between instruments (r = 0.71, P<0.0001), although the mean difference between instruments was 2,308 +/- 3,885 mu m/mm(2) (P<0.0001). Conclusions. Mean subbasal nerve density estimated with the ConfoScan 4 was 2 to 3 times higher than density estimated with the tandem scanning confocal microscope. These differences must be considered when comparing subbasal nerve densities between studies that use different confocal microscopes.