Comparing optic nerve head parameters in two Spanish dog breeds using digital planimetry

The optic discs of dogs exhibit considerable size, shape, and colour variations, depending on the degree of myelination. This variability makes the interpretation of lesions difficult, owing to the lack of reference patterns. This study aimed to compare optic nerve head (ONH) parameters determined by digital planimetry (DP) in two pure-bred dogs, Spanish Greyhound (SG) and Spanish Water Dog (SWD). The vertical and horizontal diameters, area, circularity, and the proportion of the ONH at the tapetal or non-tapetal zone (top height and bottom height) were calculated using image treatment software and compared between breeds. Significant between-breed differences were detected for all parameters, with SWD exhibiting greater height, width, area, top height, and lower circularity of the ONH than SG. Linear regression revealed that age significantly influenced mean disc height (R² = 0.310; p = 0.0001), mean disc width (R² = 0.280; p = 0.0001), mean disc area (R² = 0.281; p = 0.0001), and circularity (R² = 0.243; p = 0.0001). The multiple regression model significantly predicted mean disc height, width, and area [(S) (R² = 0.715; p = 0.001), (R² = 0.742; p = 0.001), and (S) (R² = 0.736; p = 0.001), respectively], based on age and breed. Excellent concordance was observed between the measurements of experienced and novice researchers, and there were no differences between the parameters measured by the researcher and those obtained by the software. Planimetric ONH measurements can be easily performed by novice operators using a portable fundus camera and digital computer software. These results may be of considerable clinical value, but further studies are required because of the great variability of the ONH in dogs.