Abstract
PurposeTo evaluate optic nerve characteristics independent of systemic factors predisposing to parafoveal scotoma in normal tension glaucoma.MethodsWe 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.ResultsThe 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.ConclusionIncreased 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.
Highlights
(1) Such descriptive mechanisms fail to explain the mechanism of glaucomatous cupping in low or normal tension glaucoma (NTG) patients where vascular factors are believed to predominate among many other risk factors [2,3]
All indices including mean deviation (MD), pattern standard deviation (PSD) and visual field index (VFI) were significantly better in the control group than the PF group with lower MD and VFI and higher PSD in the latter group for similar untreated intraocular pressure (IOP) in the two groups
Visual field defects in the PF group included superior (n = 29) or inferior (n = 5) arcuate defects involving parafoveal area in 35 eyes and isolated defects involving the parafoveal area in 6 eyes, Table 2
Summary
Glaucomatous optic nerve damage is the end point of pressure dependent and independent glaucoma. [1,2] Factors leading to glaucomatous cupping in high tension glaucoma are well described in literature with mechanical compression and vascular factors being some postulated mechanisms for glaucomatous optic nerve damage. (1) Such descriptive mechanisms fail to explain the mechanism of glaucomatous cupping in low or normal tension glaucoma (NTG) patients where vascular factors are believed to predominate among many other risk factors [2,3].Several studies have tried to identify possible systemic risk factors which include vascular abnormalities like migraine, Raynaud’s phenomenon, cardiovascular insufficiency and female gender. [3] Local ocular factors like differential pressures across the optic nerve, termed a translaminar pressure gradient and low cerebrospinal fluid pressure are increasingly recognised as predominant risk factors for optic nerve damage at ‘‘normal’’ intraocular pressure (IOP). [4] Presumably, optic nerve head response to different pressure differentials across the lamina cribrosa may partly explain presence of visual field (VF) defects which are deeper and closer to fixation in NTG as compared to high pressure glaucoma or primary open angle glaucoma (POAG) eyes. Since central field is affected in advanced stages, evaluating causes of central involvement in earlier stages in some eyes with NTG would be useful To identify these local optic nerve head parameters determining the stress response, we attempted to study differences in bilateral NTG eyes with involvement of parafoveal area in one eye only despite similar systemic risk factors (like hypertension, cardiovascular disease) influencing both eyes. This makes it easier to evaluate disc related factors responsible for early central involvement in one eye in a patient with NTG with similar systemic factors at play in both eyes
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