Editor, I write to extend discussion on a paper on optic disc size in India, published recently in your journal and entitled: ‘Optic disc size in a population-based study in Central India: the Central India Eye and Medical Study (CIEMS)’ (Nangia et al. 2008). Jost B. Jonas, who appears as the last author of this paper, is to be thanked for initiating and backing thorough studies with a focus on optic disc morphology over several decades. Optic disc images in 1000 subjects aged ≥ 30 years in a rural area of central India were morphometrically examined, using confocal laser scanning tomography. Disc sizes were given according to Heidelberg retinal tomography (Heidelberg Engineering GmbH, Heidelberg, Germany). This technique was also used in a previous study with a similar aim, although only as a form of control (in 52 eyes) for the classical method employed. This earlier study, the Beijing Eye Study (Wang et al. 2006), comprised 4439 subjects aged ≥ 40 years, from rural and urban parts of northern China, and was based on Canon fundus camera photographs (Canon fundus camera, CR6-45NM; Canon Inc., New York, NY) and the ‘manual’ algorithm presented by Littmann (1982), which at the time represented the gold standard or best approximation. Chinese disc sizes were reported to be in the range of 1.03–7.75 mm2, which can be compared with recent findings for Indian disc sizes of 1.12–4.61 mm2. Corresponding mean values were 2.65 mm2 (standard deviation [SD] 0.57) and 2.25 mm2 (SD 0.51), respectively. As the test populations were adult and mainly rural, we might wonder why both samples appear to be skewed towards high(er) myopia, as indicated by mean refractive values of − 0.33 D and − 0.20 D, respectively, although the median value in both samples was 0.00 D. The refractive distributions are possibly important to one of the main conclusions of both studies: that there is a highly significant negative correlation between optic disc size and refraction, and, correspondingly, a positive correlation between disc size and axial length. This result is contrary to findings in German samples (Jonas et al. 1988a, 1988b) and elsewhere (Britton et al. 1987; Varma et al. 1994). Thus, no correlation was indicated within the central normative block of ametropia, comprising eyes with refraction ranging between − 8 D and + 6 D, with pathology excluded. Extremely myopic outliers in a subsequent study (Jonas 2005), however, showed a slight trend, because macrodisc size was more often calculated at this end of the refractive continuum. The same impression is given by the scattergram of Chinese cases (Wang et al. 2006), which included 48 eyes with myopia between − 8 D and − 20 D. These outliers represent 1.2% of all included eyes in the series. If we consider the original algorithm (Littmann 1982), which was elaborated to correct for the magnification of standard fundus photographs, among the ocular parameters corneal curvature radius and axial length appear to influence calculations markedly, particularly that for keratometry value. It would appear that an averaging shortcut was made in the recent Chinese study, using a standard curvature radius value of 7.8 mm, not individual scores. It is also interesting that Littmann’s correction factors are valid down to − 15 D myopia, but hardly beyond this figure. Out of ignorance, I therefore ask whether calculation factors per se might have contributed to the macrodisc values and to the reported correlations with refraction and axial length. I am also ignorant as to how exactly the Heidelberg equipment makes allowance for such factors, and how it safeguards against including peripapillary changes in optic disc size. Finally, I shall stress that making a true assessment of actual optic disc size versus gaining an ophthalmoscopic impression is an everyday issue in the clinical evaluation of glaucomatous or other disc changes and, consequently, we look forward to having such a means of clarification to hand. The two studies under discussion, however, suggest enormous variability, which seems beyond biological likelihood, to which methodological factors might have contributed. Thus there is a factor of × 4–7 when comparing smallest and largest disc sizes in the two Asian population-based samples, which are expected to be normative. With a calculated variability factor of < × 2, classical textbook size estimates outline a much closer fit around average disc size, with mean diameter values mainly of 1.5−1.8 mm (Duke-Elder 1938; American Academy of Ophthalmology 1997). I request the authors of the recent article to comment on my remarks and questions. Is the final solution to the optic disc size issue just around the corner, or do we have it already?
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Open Access
Sep 13, 2023
Eun Hye Jung + 3
