Three-Dimensional Bowmans Microlayer Optical Coherence Tomography for the Diagnosis of Subclinical Keratoconus

Abstract

Purpose : To report the use of Bowmans Microlayer Optical Coherence Tomography 3-dimensional (3D) maps (B-MLT) in the diagnosis of subclinical keratoconus (KC).

Methods : Forty eyes (17 normal and 23 subclinical KC) were imaged using a high definition optical coherence tomography (Envisu R2210, Bioptigen, Buffalo Grove, IL, USA) with 6 mm radial cuts centered on corneal vertex. Subclinical KC was defined as patients with normal clinical examination and Placido topography (TMS-3; Tomey, Erlangen, Germany) but abnormal tomography (Pentacam; Oculus, Wetzlar, Germany). Custom-made code automatically segmented Bowmans layer (BL). Segmentations were edited as needed then reconstructed to produce BL color-coded 3D thickness and bullseye maps. Bullseye maps were divided into 14 different regions (Figure 1) and the mean and minimum Bowman’s layer thickness and Bowman Ectasia index (BEI) were calculated for each region and compared between groups (Figure 2). BEI was defined as minimum thickness of BL in each region of the inferior cornea divided by the mean thickness of BL in the corresponding region of the superior cornea multiplied by 100.

Results : B- MLT color coded and bullseye maps were successfully created for all studied eyes. In subclinical KC, B-MLT maps disclosed localized relative thinning of BL. In subclinical KC, BL minimum thickness was significantly less in C₁, C_2, C₅ regions (p < 0.01). BEI was significantly lower in subclinical KC as compared to normal eyes in region C₁, C_2, M_1, M_2, M_4, M_5, M_6, O₄ and O₅ (70 ± 11, 70 ± 12, 72 ± 12, 71 ± 11, 73 ± 13, 62 ± 19, 71 ± 13, 66 ± 19, 60± 20 vs. 83 ± 8, 83 ± 11, 80 ± 9, 81 ±9, 82 ±8, 80 ± 11, 80 ±12, 78 ±15, 78± 20; P< 0.05). There was no significant difference in mean BL thickness between both groups.

Conclusions :
B- MLT is a novel technique for the diagnosis of subclinical keratoconus. BL 3D thickness maps disclose a significant localized relative thinning that can be quantified using BL minimum thickness and BEI index.

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

 

Figure 1: Normal corneal layers and bullseye maps (right eye)

 

"3D Bowmans layer thickness maps and bullseye Bowmans Ectasia Index (BEI) maps in a normal subject (above) as compared to a subclinical keratoconus patient (below)."