The bregmatic fontanelle alone is an unreliable indicator of health in Pomeranian and other toy-sized dogs.

The Swiss Animal Welfare Ordinance prohibits the breeding of dogs with persistent fontanelles. Especially in toy-sized dogs, closed fontanelles are important for breeding selection. In such dogs, other alterations at the cranio-cervical junction, hydrocephalus and syringomyelia are frequently observed in magnetic resonance imaging (MRI). Whether such changes are associated with persistent fontanelles is unclear. The aim of the study was to investigate whether persistent fontanelles are associated with other abnormalities of the head and spinal cord. For this cross-sectional study, 41 toy-sized dogs were clinically and neurologically examined and underwent 3T MRI. Dogs were then grouped into cases and controls, based on ultrasonographic bregmatic fontanelle status. Findings in the two groups were compared using the unpaired t-test, linear regression and correlation as well as Chi-square and Fisher's exact tests. In result, 53,7 % of the dogs had closed fontanelles, while 46,3 % had persistent bregmatic fontanelles. Persistent fontanelles were more commonly observed in dogs with lower body weight and a shorter skull but showed no association with age. There was no evidence of an influence of fontanelle status on clinical and neurological presentation. MRI revealed Chiari-like malformation in 78,1 % of dogs, dilated lateral ventricles in 56,1 %, and syringomyelia in 41,5 %. Dogs with persistent fontanelles were 3,7 times more likely to have syringomyelia (Fisher's exact test; p-value 0,047). Dilated lateral ventricles increased by 15 times the odds of syringomyelia (Fisher's exact test; p-value < 0,01). No association was found between persistent fontanelles and Chiari-like malformation, other cranio-cervical abnormalities, or ventriculomegaly. Toy-sized dogs in this study frequently exhibited brain and spinal cord abnormalities on MRI, regardless of their fontanelle status. These findings suggest that presence or absence of bregmatic fontanelles may not be a reliable criterion for breeding selection aimed at maintaining a healthy dog population.