Evaluation of Optic Disc and Retinal Nerve Fibre Layer Changes and its Association with Visual Field Defects in Patients with Normal Tension Glaucoma: A Hospital Based Interventional Study

Introduction: Normal Tension Glaucoma (NTG) is a spectrum of primary open angle glaucoma, characterised by optic nerve head change in the form of glaucomatous cupping, visual field defects despite Intraocular Pressure (IOP) <21 mmHg. Patients remain asymptomatic until significant visual field loss occurs. An early sign of NTG is Retinal Nerve Fiber layer (RNFL) thinning. Although IOP is within normal range, further reduction can slow the disease progression. Early diagnosis of NTG is challenging due to absence of raised intraocular pressure, thus structural evaluation by optic disc, RNFL and functional evaluation by visual field indices helps in earlier diagnosis and disease progression. Aim: The aim of this study was to assess the relationship of optic nerve head and RNFL changes with visual field findings in NTG patients. Materials and Methods: The present prospective, interventional, hospital based study included 40 eyes with NTG conducted at SRM Medical College Hospital and Research Centre, Kattankulathur, Chennai, Tamil Nadu, India and duration of study was one year and six months. Thorough history, ophthalmic examination was done which includes visual acuity, anterior and posterior segment, IOP, Central Corneal Thickness (CCT), visual fields and Optical coherence Tomography (OCT). Patients started on 0.2% brimonidine eye drops twice a day and progression was tracked by repeat visual fields six months later. Data was analysed using independent samples t test and correlation was assessed using Pearson’s rank correlation. Statistical Package for Social Sciences (SPSS) software version 26 was used for the statistical analysis. Results: Forty eyes were studied. Mean age was 49.75 years with majority males 15 (56%). Average IOP was 14.825 mmHg, Average CCT was 528.58 micrometres. Correlation between IOP and Average RNFL was statistically insignificant (p=0.857). CCT had weak positive correlation with average RNFL, which was not significant statistically (p=0.659). Average CDR was 0.61. Average RNFL thickness was 81.4 μm. CDR weakly correlated to RNFL thickness (r=-0.179: p=0.269) but strongly correlated to visual field indices (Average Mean Deviation (MD) r=+0.955: p<0.00001). RNFL correlated with Mean Sensitivity (MS). Followup after six months showed that only two eyes (7%) had visual field progression and was changed to latanoprost 0.005% eye drops once daily. Conclusion: CCT and Diurnal IOP variation tests play an important role in diagnosis of NTG. Hypoperfusion might be an etiological factor in NTG. OCT aids in identifying RNFL thinning even before field defects. The progression of field defects in NTG was reduced by lowering IOP.