Corneal Subbasal Nerve Density: A Comparison of Two Confocal Microscopes

Abstract

Purpose: : To compare corneal subbasal nerve density from images recorded by the ConfoScan 4 confocal microscope with densities recorded by the Tandem Scanning confocal microscope.

Methods: : Subbasal nerve density was determined in 98 confocal examinations selected from 42 corneas (21 patients) before and after LASIK. Eight examinations were before LASIK and the remainder were after LASIK, when nerve density is known to be reduced. At each examination, corneas were scanned first by using a Tandem Scanning confocal microscope (Tandem Scanning, Reston, VA) and then by using a ConfoScan 4 confocal microscope (Nidek, Inc., Fremont, CA). Images of nerves were selected from 1 to 4 scans of each cornea. Nerves were traced by using a semi-automated nerve analysis program (NeuronJ, NIH and E Meijering, et al. 2004, Cytometry, 58A:167-176) in images from the ConfoScan 4 and by using a custom tracing program in images from the Tandem Scanning microscope. Mean nerve density was expressed as total nerve length divided by the sample area (µm/mm²). The sample area was 0.058 mm² for the ConfoScan 4 (42% screen area) and 0.166 mm² for the Tandem Scanning microscope (full screen area). Differences in nerve density between instruments were examined by using paired t-tests.

Results: : For all eyes, subbasal nerve density was 3,919 ± 5,371 µm/mm² (mean ± SD) with the ConfoScan 4 and 1,249 ± 1,793 µm/mm² with the Tandem Scanning microscope (P<0.0001, n=98). In post-LASIK corneas, subbasal nerve density was 3,057 ± 4,106 µm/mm² with the ConfoScan 4 and 804 ±964 µm/mm² with the Tandem Scanning microscope (P<0.0001, n=90). In normal corneas, subbasal nerve density was 13,716 ± 5,826 µm/mm² with the ConfoScan 4 and 5,418 ± 1,785 µm/mm² with the Tandem Scanning microscope (P=0.005, n=8). Estimates of nerve density between instruments were correlated (r=0.59, P<0.001, n=98), although the mean difference was 2,670 ± 4,557 µm/mm² (p<0.0001, n=98).

Conclusions: : Although measurements of subbasal nerve density with the two confocal microscopes were correlated, estimates of density with the ConfoScan 4 were 2.5 to 4 times higher than those with the Tandem Scanning microscope, likely because of better nerve visibility in ConfoScan 4 images. These absolute differences should be considered when comparing results from studies that use different microscopes.

Keywords: microscopy: confocal/tunneling • cornea: clinical science • imaging/image analysis: clinical