The corneal nerve density in the sub-basal plexus decreases with increasing myopia: a pilot study

Myopia can cause many changes in the health of the eye. As it becomes more prevalent worldwide, more patients seek correction in the form of glasses, contact lenses and refractive surgery. In this study we explore the impact that high myopia has on central corneal nerve density by comparing sub basal nerve plexus density measured by confocal microscopy in a variety of refractive errors.

Seventy healthy adult subjects between the ages of 21-50 years participated in this study. The study took place in two phases with no overlapping subjects (n = 30 phase 1 and n = 40 phase 2). In both phases an autorefraction, keratometry reading, corneal thickness measure and confocal corneal scan of the sub basal nerve plexus were performed for both eyes. There were 11 hyperopes (+0.50 to +3.50DS), six emmetropes (-0.25 to +0.50DS), 30 low myopes (-5.50 to -0.50DS), and 23 high myopes (-5.50DS and above). In the second phase of the study additional tests were performed including an axial length, additional corneal scans, and a questionnaire that asked about age of first refractive correction and contact lens wear. Corneal nerves were imaged over the central cornea with a Nidek CS4 confocal microscope (460 × 345 μm field). Nerves were evaluated using the NeuronJ program for density calculation. One eye was selected for inclusion based on image quality and higher refractive error (more myopic or hyperopic).

As myopia increased, nerve density decreased (t₁  = 3.86, p < 0.001). We also note a decrease in data scatter above -7 D. The relationship between axial length values and nerve density was also significant and the slope was not as robust as refractive error (t₁  = 2.4, p < 0.04). As expected there was a significant difference between the four groups in axial length (F₃  = 19.9, p < 0.001) and age of first refractive correction of the myopic groups (14.9 vs 11.5 years; t₄₆  = 2.99 p < 0.01). There was no difference in keratometry readings or corneal thickness between the groups (F₃  = 0.6, p = 0.66 and F₃  = 1.2, p = 0.33 respectively).

Corneal nerve density in the sub-basal plexus decreased with increasing myopia. This could have implications for corneal surgery and contact lens wear in this patient population.