Assessing Vascular and Structural Hemiretinal Asymmetry at Different Stages of Normal Tension Glaucoma Using Optical Coherence Tomography

Précis: Hemiretinal asymmetry in different stages of normal tension glaucoma (NTG) may be helpful in diagnosis and staging. Purpose: To investigate hemiretinal asymmetry in peripapillary capillary vessel density (pcVD), macular vessel density (macVD), circumpapillary retinal nerve fibre layer (RNFL) thickness, and macular ganglion cell-inner plexiform layer (GCIPL) across various stages of normal tension glaucoma (NTG) and determine its potential as a staging tool. Subjects and Methods: A total of 199 subjects were included, categorized into the preperimetric, early, moderate, and advanced stage based on mean deviation (MD). Vessel density was measured using optical coherence tomography angiography, while peripapillary RNFL and macular GCIPL thickness were assessed with spectral-domain optical coherence tomography scans. The asymmetry parameter was defined as the absolute difference between superior and inferior values in the same eye. We analysed the characteristics of these parameters across glaucoma stages and evaluated the distribution of structural damage among the groups. Results: Hemiretinal asymmetry parameters showed significant statistical differences across the 4 groups, following a trend of moderate > early > late > preperimetric stages. Furthermore, hemiretinal difference in pcVD (hd pcVD) ( P= 0.009) and hemiretinal difference in RNFL (hd RNFL) ( P= 0.003) have statistical differences between preperimetric and early stage. Besides, 50% to 80% of the participants in each stage exhibited smaller structural parameters in the inferior hemisphere. Conclusions: The hemiretinal asymmetry parameter exhibits a clear trend across different stages of NTG, aiding in the staging of NTG before reaching the moderate stage. Furthermore, it has advantages in differentiating between the preperimetric and early stages. In addition, the intraocular inferior hemisphere structure damage is more severe.