Hyporeflective Core Drusen Cause an Artifactual Bending of Bruch's Membrane on OCT Images: Optical Mechanism and Proposed Quantitative Metrics.

Purpose: To investigate the optical mechanism and impact of the artifactual bending of Bruch's membrane (BM) beneath hyporeflective core drusen (hypoRCD) on optical coherence tomography (OCT). Methods: This observational and experimental laboratory study analyzed volumetric OCT scans with hypoRCD from an age-related macular degeneration (AMD) cohort. The optical mechanism underlying BM bending was examined using tissue phantoms with different oil droplets to model different refractive indices (RIs) of hyporeflective cores (hypoRCs). Three metrics were proposed to quantify hypoRCD: (1) hypocore bending index (CoreBI), (2) drusen bending index (DruBI), and (3) maximum bending index (Bmax). The impact of BM bending on choriocapillaris flow deficit (CCFD) analysis was assessed by comparing CCFD measurements before and after correcting BM segmentation under hypoRCD. Results: BM bending was consistently observed beneath the core region of hypoRCD in both structural OCT and optical attenuation coefficient (OAC) B-scans. Phantom experiments demonstrated that the bending was associated with differences in the RIs of the hypoRCs. The degree of bending, quantified by the CoreBI, increased with the RI of the core material (P < 0.001). The CCFD percentage beneath hypoRCD was significantly reduced by 25.9% ± 7.7% (P = 0.005) after correcting the BM segmentation. Morphological changes associated with the bending could be quantified using the three proposed metrics. Conclusions: BM bending induced by hypoRCD is an artifactual OCT feature that reflects the optical properties of the hypoRCs. This morphological characteristic may improve our understanding of hypoRCD, raise cautions in quantitation of CCFDs under hypoRCD, and provide additional insights into their role in AMD.