Corneal subbasal nerve density: a comparison of two confocal microscopes

To compare subbasal nerve densities estimated from images recorded by the tandem scanning and the ConfoScan 4 confocal microscopes.

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 (microm/mm2). Differences in nerve density between instruments were examined by using paired tests.

In normal corneas, subbasal nerve density was 10,658 +/- 5,581 microm/mm2 (mean +/- SD) with the ConfoScan 4 and 5,534 +/- 1,850 microm/mm2 with the tandem scanning microscope (P<0.0001). One to 12 months after LASIK, mean subbasal nerve density was 2,477 +/- 3,514 microm/mm2 estimated with the ConfoScan 4 and 844 +/- 983 microm/mm2 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 microm/mm2 (P<0.0001).

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.