Purpose: To evaluate optic nerve characteristics independent of systemic factors predisposing to parafoveal scotoma in normal tension glaucoma.
Methods: We included 40 patients with bilateral normal tension glaucoma with parafoveal scotoma (visual field defect in one hemifield within 10° of fixation with at least one point at p<1% lying at the two innermost paracentral points) in only one eye (Parafoveal group, PF, n = 40) identified from the hospital database in this observational cross sectional study. The other eye with no parafoveal scotoma constituted the control group (n = 32). Red free fundus photographs were evaluated using Image J software analyzing parameters including vertical and horizontal disc diameter, disc haemorrhage, location and angular width of the retinal nerve fibre layer depth and displacement of the central vessel trunk, CVT (vertical and horizontal). Clinical characteristics and disc parameters were compared in the two groups.
Results: The PF group had lower mean deviation(MD) and visual field index (VFI) and higher pattern standard deviation (PSD) than control group (p≤0.001) for similar untreated IOP, (p = 0.9). Disc haemorrhages were more frequent in the PF group, p = 0.01. The PF group had greater width of nerve fibre layer defects, p = 0.05 and greater vertical displacement of the central vessel trunk, p = 0.001. On multivariate logistic regression, parafoveal scotoma was significantly associated with increased vertical distance of the CVT, p = 0.0001.
Conclusion: Increased vertical displacement of the CVT is associated with parafoveal scotoma in normal tension glaucoma. Localising the vessel trunk may help clinicians in identifying patients at risk for parafoveal involvement.