Straight Lines versus Conical Curves – The Corneal Subbasal Nerve Plexus in Keratoconus

Abstract

Purpose : Keratoconus is a corneal ectasia of unknown etiology, usually affecting both eyes with progressive and non-inflammatory thinning of the central cornea resulting in irregular astigmatism and visual impairment. To end up with reliable and resilient conclusions in regard of the configuration of the corneal subbasal nerve plexus (SNP), more than nine confocal in vivo images from one eye are required. Thus, this study analyzed quantitatively a) the density and b) the winding course of the SNP in keratoconus based on ten confocal in vivo microscopy (CIVM) images per affected eye.

Methods : The SNPs of patients with keratoconus (n=10) recruited from the Homburg Keratoconus Center (HKC) and of healthy patients without keratoconus (n=10) were examined by 10 CIVM images of one eye per patient. The diagnosis was ensured by clinical examination and corneal topography. Exclusion criteria were neurodegenerative diseases, ocular trauma, previous corneal surgery, coexisting corneal pathology and keratitis. Analyses of the images were performed using the open-source software ImageJ. The SNP density was calculated as the total length of nerve fibres per frame area and the tortuosity of the SNP was quantified by measuring a) the amplitude of the conical bulges and b) the area under the curve (AUC) formed by the SNP (Fig.1).

Results : Statistical analysis of the SNP density in 200 images using a Mann-Whitney-U-Test revealed the SNP density to be significantly lower in keratoconus (17.4 ± 2.1 mm/mm²) than in healthy corneae (24.9 ± 3.8 mm/mm², p<0.05; mean ± SD). Values (median with 25th/75th percentile) of amplitudes and AUCs were significantly higher in keratoconus (amplitude 35/25/47µm and AUC 2709/1573/5202µm²) than in healthy corneae (amplitude 23/19/28µm and AUC 1510/1262/2085µm², p<0.05).

Conclusions : CIVM allows visualizing even slightest alterations within the corneal SNP in keratoconus including a) a significantly lower density and b) an enhanced winding course of the SNP. The significantly lower SNP density observed in keratoconus appears to confirm the hypothesis of a neurodegenerative aspect of the disease, to be correlated with the severity and further course of the disease in the future.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Fig.1: CIVM images of the SNP (keratoconus, 400x 400µm); A, SNP tracing (red); B, SNP tracing (red), AUC (hatched), baseline (yellow), amplitude of the conical bulge formed by the SNP (black).