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William Swanson, Victor Malinovsky, Mitchell Dul; Why ISNT Rim Area Greater for Superior Optic Disc?. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5919.
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© ARVO (1962-2015); The Authors (2016-present)
Histological data show greater retinal ganglion cell (RGC) density in superior than inferior retina (Curcio & Allen 1990 JCN 300:5), but neuroretinal rim area and retinal nerve fiber layer (RNFL) thickness are not greater for superior optic disc. The hypothesis was that this can be accounted for by macular visual field projection to inferior temporal (IT) but not superior temporal (ST) optic disc sectors (Hood et al, 2012 TVST 1:3).
One eye each of 61 patients with glaucoma and 71 age-similar control subjects was tested with HRT-III for rim area, Stratus OCT 3.4 for RNFL thickness, and 24-2 SITA Standard for visual field (VF) sensitivities. ST and IT optic disc sectors were defined by the 2002 Garway-Heath map (IOVS 43:2213), and estimates of RGCs from VF and RNFL values were derived with the 2007 Harwerth model (IOVS 48:763). Bivariate Gaussian ellipses at p = 0.90 characterized control data, and tangents yielded confidence limits for patients. Each sector maps to 14 VF locations: the first 13 locations are mirror images across the horizontal midline, and the 14th location is at 4° for IT and 21° for ST.
Controls had equal rim areas for ST and IT (0.21 mm2), and lower RNFL thickness for ST than IT (127 ± 26 vs. 136 ± 26 μm), yielding fewer RGCs for ST: 165 ± 38 vs. 176 ± 39 x 103 (t = -3.3, p < 0.002). With 14 locations, VF also yielded fewer RGCs for ST: 279 ± 38 vs. 352 ± 58 x 103 (t = -17, p < 0.0001). With 13 locations, ST yielded more RGCs than IT: 269 ± 36 vs. 239 ± 36 x 103 (t = +11, p < 0.0001). The 14th location provided an average of 10 x 103 RGCs for ST and 113 x 103 RGCs for IT. In both cases, patient data fell within the confidence limits, and above the line of equality (i.e., RGC estimates were greater for VF than RNFL).
Ganglion cell estimates from perimetry agreed with histological findings of greater ganglion cell density in superior retina, for 13 locations per sector but not 14 locations. This result underscores the role of mapping the macula to the optic disc in structure-function analyses.
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