Optic nerve head size and cognitive function

Editor, The optic disc area with an interindividual variability of about one to seven (Jonas et al. 1988) is associated with the optic nerve fibre number (Jonas et al. 1988), area of neuroretinal rim and optic cup (Bengtsson 1976) and other parameters such as refractive error and body height (Xu et al. 2011). Factors responsible for the optic disc size variability have been unclear so far. It has been discussed that the number of embryologically formed primitive retinal ganglion cell axons influences the size of the optic disc and that the number of embryological retinal ganglion cell axons that eventually regress during fetogenesis influences the size of the future optic cup (Rakic & Riley 1983; Jonas & Naumann 1992). Although the optic nerve as cerebral fascicle is a part of the brain, it has not been assessed yet for an unselected population whether the disc size is associated with cerebral or intellectual functions (Hellström 1999). We therefore examined the potential associations between disc size and level of education and time necessary to perform a standardized automatic visual field test as surrogates of cognitive function. The population-based Beijing Eye Study included 4439 subjects with a mean age of 56.2 ± 10.6 years (Jonas et al. 2009). All study participants underwent a comprehensive eye examination and an interview with questions on their socioeconomic background and their level of education. The visual field was examined by frequency-doubling threshold perimetry (program C-20-1; Zeiss-Humphrey, Dublin, CA, USA). The disc size was measured on photographs (Wang et al. 2006). Only one eye per subject was taken for statistical analysis. The study included 4089 (92.1%) subjects for whom the assessable disc photographs and data of education level and perimetric test time were available. In univariate analysis, disc size (mean: 2.61 ± 0.50 mm2) increased significantly with education level (p < 0.001; correlation coefficient r = 0.14), shorter perimetric test time (p < 0.001; r = −0.05), higher age (p < 0.001; r = 0.08), higher body height (p = 0.006; r = 0.05), urban versus rural region of habitation (p < 0.001) and refractive error (p = 0.002; r = 0.05) (Table 1). In multivariate analysis, disc area remained to be significantly associated with higher education level (p < 0.001) and shorter test duration (p = 0.004), in addition to higher age (p < 0.001), female gender (p = 0.004), higher best-corrected visual acuity (p = 0.01) and refractive error (p < 0.001) (Table 2). A similar result was obtained if the amount of visual field defects was added and if highly myopic eyes were excluded. In a reverse manner, education level was significantly (multivariate analysis) associated with increasing disc size (p < 0.001) and shorter perimetric test duration (p < 0.001), in addition to lower age (p < 0.001), male gender (p < 0.001) and higher best-corrected visual acuity (p < 0.001) (Tables 1 and 2). If eyes with an intraocular pressure >21 mm Hg, glaucomatous optic nerve damage, macular or retinal diseases or nuclear cataract were excluded, disc size was still significantly associated with higher level of education (p < 0.001) and shorter test duration (p = 0.002), after adjustment for age (p = 0.001), gender (p = 0.007) and refractive error (p = 0.01). Within the Chinese population, optic nerve head size was associated with education level and perimetric test time, after the adjustment for age, gender, refractive error and best-corrected visual acuity. If one considers education level and perimetric test time as surrogates of cognitive or intellectual function, one may speculate that optic disc size was related to cognitive function. One may speculate that the optic nerve as cerebral fascicle and the correlation between the disc size and optic nerve fibre count suggest that the optic nerve and thus the optic nerve head reflects aspects of the brain. This has already been shown by Hellström (Hellström 1999) for patients with adverse event during their prenatal and perinatal life.