Foveal cone function in normal-tension glaucoma

Purpose. To study the foveal cone function in patients with normal-tension glaucoma (NTG). Methods. Seventeen eyes of 9 patients with NTG were studied. All had clear media, a normal macula on ophthalmoscopy, and visual acuity of 20/20-20/50 (mean: 20/25). Fo\eal cone electroretinograms (ERGs) were elicited with a stimulatorophtlialmo.seope with a 41" stimulus (retinal illuminance: 4.S log td) flickering at 42 11/ and centered within a 12' .steady surround (retinal illuminance: 5.5 log td). Results were aualy/ed in relation to Humphrey 30-2 visual field "mean central (4°) deviation" (MCDi. and optic disc relative cup area, using simple and multiple regression and paired t-test analysis. Results- Foveal cone ERGs were subnormal in 6 (356/O of the 17 eyes, and in one or both eyes of 4 (44<#) of the 9 patients. Ampliiude was inversely correlated with MCD (p=O.OH)7) while relative cup area was directly correlated with MCD (p=().(KX) I ). MCD was additively affected by the foveal ERG amplitude and hy the relative cup area (p=0.(X)34 and p=0.0001, respectively). Furthermore, when fellow eyes with asymmetric relative cup area were compared, foveal ERG implicit time was significantly slower in the eyes with the larger relative cup area <p=0.0132). Conclusions. Central visual field loss in NTG may he caused by outer retinal foveal cone dysfunction in addition to optic nerve axonal loss. Since both the pre-laminar optic disc and the foveal cones are nourished by the choroidal circulation, our results may support a significant role for vascular insufficiency causing both foveal cone dysfunction and optic nerve axonal loss in NTG.