More evidence that corneal nerves may be involved in progression of keratoconus

The authors examined whole cornea button mounts from 14 patients who underwent keratoplasty for advanced keratoconus. They stained them with acetylcholinesterase using a Hamamatsu NanoZoomer digital pathology microscope system. The results were compared with normal controls. Central nerves of keratoconic corneas showed significant morphological abnormalities, supporting the idea of corneal nerve involvement in the pathophysiology and progression of keratoconus.

The acetylcholinesterase method stains the nerve and nerve-related structures densely and can be used to stain whole mounts of cornea, allowing examination of the central, peripheral, subbasal and stromal nerves for any abnormality. It also faintly stains the epithelial cells and keratocyte nuclei. This is the first time it has been used to investigate nerve changes in keratoconus.

Seventy-one percent of keratoconic corneas demonstrated central stromal nerve changes, including thickening, tortuosity, nerve spouting and overgrowth. The central stromal nerves of the keratoconic corneas were significantly thicker (18.9 ± 14.7 μm) than those of controls (8.11 ± 3.31 μm; P <0 .001) and also than those of keratoconic corneas examined in several in vivo confocal microscopy studies. They also showed abnormal tortuosity and overgrowth, which has not been shown in previous investigations. These abnormalities could be attributable to the severity of the keratoconus and the comprehensive nature of whole-mount technique.

The thickness of peripheral stromal nerves (12.6 ± 3.1 μm) was similar to that of controls (14.86 ± 5.60 μm; P = .072). Sub-basal nerves showed changes in the form of loss of radial orientation and increased tortuosity, especially at the cone apex. At the cone base, a concentric arrangement of sub-basal nerves was found in 43 percent of cases. Localized thickenings of sub-basal nerves also were observed at their origin from the bulbous terminations of sub-Bowman nerves. The terminal bulbs, too, were enlarged. The mean diameter of the sub-basal nerves in keratoconus (4.11 ± 0.60 μm) did not differ from that of the controls (4.0 ± 0.61 μm; P = .422).

No clear evidence is available to explain why these nerves are becoming thicker and showing these abnormal morphologic features. The fact that they are mainly thickened at the conal region, sparing the peripheral innervation, suggests a stimulus for thickening that is predominantly confined to the cone. It has been postulated that the increased diameter of the central stromal nerves could be the result of pulling of the nervous tissue toward the visual axis by the keratoconic protrusion and therefore facilitating imaging of the more peripheral and thicker nerve fibers that have been dragged to the center instead of the nerves that normally cover the corneal center. This study contradicts this hypothesis, because the central stromal nerves were genuinely larger than the peripheral nerves in the same cornea.

Although the limited number of cases does not enable definitive conclusions on the relationship between corneal nerve changes and the duration of disease and contact lens wear, the authors note that the most severe nerve changes were found in patient 10, who had the longest disease and contact lens wear among the patients.