Fig. 1
![Fig. 1](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2Fs12886-022-02288-z/MediaObjects/12886_2022_2288_Fig1_HTML.png)
A Colour image of the right optic nerve head from which major retinal vessels arise. The arterioles are light red in colour, the venules dark red. B Similar image of left optic nerve head. C Digitized image of A with concentric circles drawn around optic nerve to calculate the arteriovenous ratio. This ratio was 0.648. D Similar image of B. This ratio was 0.670