Miniaturization in Domestic Dogs: Relationships Among Cranial Shape, Head Indexes, and Body Weight.

Diagnostic interpretation is complicated by normal anatomic variation in canine heads. This retrospective cross-sectional study examined how miniaturization-proxied by body weight-relates to four cranial shape categories (archetypal, intermediate, round, and extreme round) and three quantitative head measures (cranial index, skull index, and facial index) in 852 domestic dogs using archived medical records, including CT images. Dogs were also grouped for further analysis: Group A, breeds predisposed to syringomyelia; Group B, breeds not at risk. Head indexes correlated positively with each other and inversely with weight, except for skull and facial indexes in Group A. Cranial shape, ranging from archetypal to extreme round, was significantly (p < 0.0001) associated with a lower median body weight in all dogs, indicating that cranial morphology does not always scale isometrically with miniaturization: Smaller dogs more frequently had rounder crania. The distribution of cranial shapes also differed significantly between groups (p < 0.0001): Group A dogs more frequently had rounder crania. Group A dogs also had lower median body weight and higher median cranial, skull, and facial indexes (all p < 0.0001). Logistic regression revealed that each 1.0 kg reduction in body weight increased the odds of belonging to Group A by 25%, whereas each 1.0 increase in cranial, skull, and facial indexes raised odds by 28%, 16%, and 3%, respectively. These findings underscore the complex interplay among cranial shape, head indexes, and body weight in domestic dogs, highlighting miniaturization's influence on skull morphology and providing context for interpreting normal variation in clinical imaging.