Topographic, Pachymetric And Epithelial Thickness Distribution Characteristics Of Forme Fruste Keratoconus: A Comparison With Keratoconic And Normal Eyes

To compare the topographic, pachymetric and epithelial distribution characteristics of forme Fruste Keratoconus (FFKC) eyes with keratoconic and normal eyes using elevation based Scheimpflug imaging (Pentacam AXL, Oculus, Optikgerate, GmbH, Wetzlar, Germany) and high speed spectral domain optical coherence tomography (SDOCT) [Revo Nx, OPTOPOL Technology, Poland]. 

Cornea and Refractive Services of Advanced Eye Centre, Post Graduate Institute of Medical Education and Research (PGIMER), Chandigarh, India.

This observational study included normal (control) eyes [n=67], unilateral keratoconus [n=25] and FFKC eyes [n=25]. FFKC eyes were contralateral clinically and topographically normal eyes of patients with unilateral keratoconus. Topographic parameters for all eyes were obtained from the Pentacam AXL. Average pachymetric and epithelial thickness distribution using the SDOCT was obtained over the i) central 2 mm zone ii) 8 sectors each  in the 2-5 mm  and 5-7 mm zones (superior [S], inferior[I], nasal [N], temporal [T], superonasal [SN], inferonasal [IN], superotemporal [ST] and inferotemporal[IT]). Central and sector difference analysis (SN-IT, S-I, ST-IN and T-N differences) for corneal and epithelial thickness were compared across groups.

Simulated mean K (43.6 ± 1.3 D) of the FFKC eyes  was comparable to control eyes( 43.8 ± 1.5 D; p = 0.99) but less than keratoconic eyes (47 ± 4.6 D; p =0.004). Thinnest pachymetry of FFKC eyes (447.9 ±56.2µ) was comparable to keratoconus eyes (437.2 ±59.9 µ; p=0.99) but less than control eyes (493.1 ± 29.1 µ; p <0.001). Central epithelial thickness of FFKC eyes and keratoconus eyes (52.7 ±7.4 µ and 52.4 ± 8 µ) was comparable (p=1) but thinner than that of normal eyes (60.4 ± 8.3 µ)(both p’s <0.001). S-I pachymetry difference at 5 mm was greater for FFKC eyes (42.9 ± 44.2 µ ) than control eyes (23.1 ± 23.9 µ) (p=0.008). S-I epithelial thickness difference of FFKC eyes (-1.00 ± 5.99 µ) was less than that of control eyes (-5.39 ± 10.50 µ)

FFKC eyes were similar to the normal eyes for front surface topography. The pachymetric features of FFKC eyes however resembled the keratoconic eyes for thinnest pachymetry and the regional variability across superior and inferior sectors. The epithelial profile of the FFKC eyes also mimicked the pattern of the ectatic eyes as manifested by a lower central epithelial thickness as well as the drift of the  S-I epithelial thickness difference to a less negative value indicating  an early thinning of the inferior epithelium. A closer scrutiny of  the interplay between these three parameters i.e topography , pachymetry and epithelial thickness distribution in patients seeking  keratorefractive surgery is essential  for safe screening.