The use of Bowman's layer vertical topographic thickness map in the diagnosis of keratoconus

Abstract

Purpose: To evaluate the use of Bowman's layer (BL) vertical topographic thickness maps in diagnosing keratoconus (KC).

Design: Prospective, case control, interventional case series.

Participants: A total of 42 eyes: 22 eyes of 15 normal subjects and 20 eyes of 15 patients with KC.

Intervention: Bowman's layer 2-dimensional 9-mm vertical topographic thickness maps were created using custom-made ultra high-resolution optical coherence tomography.

Main outcome measures: Bowman's layer average and minimum thicknesses of the inferior half of the cornea, Bowman's ectasia index (BEI; defined as BL minimum thickness of the inferior half of the cornea divided by BL average thickness of the superior half of the cornea multiplied by 100), BEI-Max (defined as BL minimum thickness of the inferior half of the cornea divided by BL maximum thickness of the superior half of the cornea multiplied by 100), keratometric astigmatism (Ast-K) of patients with KC, and average keratometric (Avg-K) readings.

Results: In patients with KC, BL vertical thickness maps disclosed localized relative inferior thinning of the BL. Inferior BL average thickness (normal = 15±2, KC = 12±3 μm), inferior BL minimum thickness (normal = 13±2, KC = 7±3 μm), BEI (normal = 91±7, KC = 48±14), and BEI-Max (normal = 75±8; KC = 40±13) all showed highly significant differences in KC compared with normal subjects (P< 0.001). Receiver operating characteristic (ROC) curve analysis showed excellent predictive accuracy for BEI and BEI-Max with 100% sensitivity and specificity (area under the curve [AUC] of 1) with cutoff values of 80 and 60, respectively. The AUC of inferior BL average thickness and minimum thickness were 0.87 and 0.96 with a sensitivity of 80% and 93%, respectively, and a specificity of 93% and 93%, respectively. Inferior BL average thickness, inferior BL minimum thickness, BEI, and BEI-Max correlated highly to Ast-K (R = -0.72, -0.82, -0.84, and -0.82, respectively; P< 0.001) and to Avg-K (R = -0.62, P< 0.001; R = -0.59, P = 0.001; R = -0.60, P< 0.001; and R = -0.59, P = 0.001, respectively).

Conclusions: Bowman's layer vertical topographic thickness maps of patients with KC disclose characteristic localized relative inferior thinning. Inferior BL average thickness, inferior BL minimum thickness, BEI, and BEI-Max are qualitative and quantitative indices for the diagnosis of KC that accurately correlate with the severity of KC. In our pilot study, BEI and BEI-Max showed excellent accuracy, sensitivity, and specificity in the diagnosis of KC.

Figure 1

9 mm vertical 2-dimensional ultra-high resolution optical coherence tomography (UHR-OCT) composite image of the cornea of a keratoconus patient: Note that the composite image consists of three images, A (superior), B (central) and C (inferior) images of the respective parts of the cornea. Specular reflection is present through the center of each image denoting that UHR-OCT scanning probe is perpendicular on the area of the cornea under study. The 3 presets from each part of the UHR-OCT image show a magnified image of the anterior part of the cornea. In the presets, doted lines represent the demarcation lines for the anterior surface of the epithelium (red), interface between the epithelium and Bowman’s layer (pink) and the interface between the Bowman’s layer and the anterior stroma (green). Bars are 100 μm.

Figure 2

Topographic thickness maps of the Bowman’s layer (BL) of a control subject (blue) and a keratoconus patient (red; same patient as in figure 1). The BL thickness map of the keratoconus patient discloses an inferior localized thinning of the layer, whereas the normal subject map discloses a more uniform thickness without that inferior localized thinning. In the normal subject, BL average thickness on the inferior half of the cornea, BL minimum thickness on the inferior half of the cornea, Bowman’s ectasia index (BEI) and Bowman’s ectasia index-maximum (BEI-Max) are 15 μm, 14 μm, 90 and 81, respectively, whereas in the keratoconus patient, those are 12 μm, 9 μm, 68 and 53, respectively.

Figure 3

Combined receiver operating characteristics (ROC) graph of Bowman’s layer (BL) total average thickness, BL average thickness of the inferior half of the cornea, BL minimum thickness of the inferior half of the cornea, Bowman’s ectasia index (BEI) and Bowman’s ectasia index-Maximum (BEI-Max). Note that area under the curve for BEI and BEI-Max is 1.