Computing the Minimum Rim Width: Should the Border Tissue Be Considered?

Abstract

 

Purpose

 

The Bruch's Membrane Opening - Minimum Rim Width (BMO-MRW), defined as the minimum distance between the BMO and the internal limiting membrane (ILM), is promising as a new structural parameter for glaucoma. However, in the presence of externally oblique border tissue (EOBT), the distance from the ILM to the closest point along the anterior surface of EOBT may be smaller than the BMO-MRW. In this work, we characterize the rate of EOBT, and, in cases of EOBT, characterize the rate at which any point along the anterior surface of the EOBT has a smaller distance to the ILM than the BMO-MRW.

 

Methods

 

Forty-four optic nerve head SD-OCT volumes (200×200×1024 voxels; voxel size 30×30×2 μm) from 44 human patients that presented with varying stages of glaucoma were acquired using Cirrus HD-OCT (Carl Zeiss Meditec, Inc.). After reformatting the volumes into a radial format (with 360 BMO locations), the ILM and BM surfaces were automatically segmented. The BMO was manually marked on 20 evenly spaced radial locations and interpolation was used to define the remaining BMO points. The EOBT end point was manually marked where EOBT was present. The anterior surface of the EOBT was defined by extending the BM surface to pass through the BMO and EOBT end point. The BMO-MRW was computed for each of the BMO locations. The minimum distance from the anterior surface of the EOBT (as present) to the ILM was also computed. The closest overall point to the ILM was considered to fall on the EOBT if the distance from this point to the BMO was greater than 30 μm.

 

Results

 

EOBT was present in at least one radial scan in 27/44 (61%) patients. As shown in Fig. 1a, EOBT was more common on the temporal side than on the nasal side. Of the radial scans with EOBT, the closest point to the ILM fell on the EOBT rather than the BMO in 45% of radial scans (Fig. 1b). In cases where the closest point fell on the EOBT, the average distance of this point from the BMO was 100 ± 161 μm.

 

Conclusions

 

With EOBT, the minimum distance of any point along its anterior surface appears to be smaller than the BMO-MRW approximately half of the time (Fig. 2). It may be important to consider the impact of the presence of EOBT on MRW parameters in future work.

 

 

 

Fig. 2. In the presence of EOBT, the closest point to the ILM can be the BMO (example a) or on the EOBT (example b).

 

Keywords: 627 optic disc • 438 Bruch's membrane • 629 optic nerve