The Importance of Signal Strength Index in Optical Coherence Tomography Angiography: A Study of Eyes with Pseudoexfoliation Syndrome

Zia Sultan Pradhan,1 Shruthi Sreenivasaiah,1 Thanemozhi Srinivasan,1 Sujani Shroff,1 Sathi Devi,1 Dhanaraj AS Rao,1 Carroll AB Webers,2 Narendra K Puttaiah,1 Harsha Laxmana Rao1,2 1Department of Glaucoma, Narayana Nethralaya, Bangalore, India; 2Department of Ophthalmology, University Eye Clinic Maastricht, University Medical Center, Maastricht, the NetherlandsCorrespondence: Zia Sultan Pradhan, Department of Glaucoma, Narayana Nethralaya, 121/C, Chord Road, 1st “R” Block, Rajajinagar, Bangalore, 560010, India, Tel +91 80 66121300, Email zedpradhan@gmail.comPurpose: To evaluate the effect of the signal strength index (SSI) on a comparison of the vascular and structural OCT measurements between eyes with pseudoexfoliation syndrome (PXF) and healthy controls of Asian-Indian origin.Methods: In this cross-sectional study, 33 eyes of 33 PXF patients and 40 healthy eyes of 40 controls underwent OCT and OCT angiography (OCTA). Eyes with intraocular pressure (IOP) > 21mmHg, glaucomatous disc changes, or any other ocular pathology were excluded. Peripapillary vessel density (VD) and retinal nerve fiber layer (RNFL) thickness were determined from the optic disc scans. Parafoveal VD and ganglion cell complex (GCC) thickness were measured from the macular scans. These parameters were compared between the groups using mixed effect models after adjusting for clinical confounders such as IOP, as well as SSI of the scans.Results: The 2 groups were demographically similar. Average RNFL (94μm vs 100μm, p = 0.01) and GCC (91μm vs 95μm, p = 0.03) were thinner in the PXF group compared to controls. The average peripapillary VD appeared lower in the PXF groups compared to controls (58.2% vs 58.8%, p = 0.04), but after adjusting for IOP and SSI, no difference was noted (p = 0.39). After accounting for confounders, parafoveal VD in the PXF group was significantly lower compared to controls (44.3% vs 46.8%, p = 0.008).Conclusion: Peripapillary RNFL thickness, parafoveal GCC thickness and parafoveal VD were decreased in eyes with PXF when compared to controls. VD measurements are associated with the SSI and, therefore, clinicians and researchers evaluating OCTA scans quantitatively must consider the SSI value during analysis and interpretation.Keywords: optical coherence tomography angiography, signal strength index, vessel density, pseudoexfoliation syndrome