Spatial Relationship between Bruch’s Membrane Opening and Lamina Cribrosa Determines Optic Disc Tilting

Abstract

 

Purpose

 

To investigate the association between tilted disc geometry and location of the lamina cribrosa (LC) relative to Bruch’s membrane opening (BMO).

 

Methods

 

Normal subjects with axial length less than 26.5 mm were recruited. Serial horizontal and vertical enhanced depth imaging optical coherence tomography (EDI OCT) B-scans of the optic nerve head (interval between scans, ~30 μm) were obtained for one eye of normal subjects. After reconstructing a 3-dimensional image of BMO and the anterior LC insertion points, the distance between the centroids of BMO and the LC was measured (Fig A). The direction of lateral LC displacement from BMO was measured as an angle (θ₁) between the line connecting the two centroids and temporal horizontal line (Fig B). Disc ovality index (longest disc diameter/shortest disc diameter) was measured using infrared disc photographs. The direction of disc tilting was measured as an angle (θ₂) between the axis of shortest disc diameter and the temporal horizontal line (Fig C).

 

Results

 

23 normal eyes (23 subjects) were included (mean age, 42±15 years). Mean distance between the centroids of BMO and the LC was 222±93 (range, 70 to 368) µm. Mean disc ovality index was 1.14±0.08 (range: 1.00 to 1.28) mm. The distance between the two centroids was significantly correlated with the disc ovality index (p<0.001, R=0.739; Fig D). The direction of lateral LC displacement from BMO (θ₁) was also significantly correlated with the direction of disc tilting (θ₂) (p<0.001, R=0.896; Fig E).

 

Conclusions

 

Lateral displacement of the LC from BMO underlies optic disc tilting. The severity and direction of lateral LC displacement from BMO determines the severity and direction of optic disc tilting.

 

 

(A and B) Green and blue dotted circles = anterior lamina cribrosa (LC) insertion points and Bruch’ membrane opening (BMO), respectively. Green and blue dots = centroids of the LC and BMO, respectively. (A) Red double arrow = distance between the two centroids. (B) Angle θ₁ = direction of lateral LC displacement from BMO. (C) Angle θ₂ = direction of disc tilting; yellow dotted lines = longest and shortest disc diameters.

 

Keywords: 577 lamina cribrosa • 438 Bruch's membrane • 627 optic disc