Three-dimensional cranial geometric morphometrics of syringomyelia in toy breed dogs.

Syringomyelia (SM) is a neurological disorder characterized by fluid accumulation within the spinal cord and is frequently observed in toy breed dogs. Cranial malformations associated with Chiari-like malformation are considered a major cause; however, previous studies have relied mainly on midsagittal imaging, and breed-specific cranial differences have not been fully examined. This study aimed to identify breed-specific cranial abnormalities associated with SM using three-dimensional (3D) geometric morphometrics (GM) based on cranial landmarks obtained from computed tomography (CT) scans. Chihuahuas, Toy Poodles, Pomeranians, and Cavalier King Charles Spaniels (CKCS) were included. Procrustes analysis was applied to normalize CT coordinate data, followed by principal component analysis, canonical variate analysis, discriminant function analysis, and Procrustes analysis of variance (ANOVA) to assess differences between the SM and control groups. Shape differences were visualized using wireframe diagrams. In Chihuahuas, Toy Poodles, and Pomeranians, cranial shape differences clearly distinguished SM from controls (p < 0.001 for all three breeds). Although statistical differences were present in CKCS (p < 0.001), classification accuracy was low. Across all breeds, consistent abnormalities were detected in the foramen magnum, including not only the enlargement reported in previous linear morphometric studies but also novel 3D deformities, particularly rostral displacement of the lateral margins. Displacement of rostral cranial structures, such as the frontal sinus and cribriform plate, was observed in Toy Poodles and Pomeranians but not in Chihuahuas or CKCS, suggesting that SM may develop through breed-specific mechanisms. These findings demonstrate that GM can detect subtle 3D cranial abnormalities and provide insights into potential breed-specific pathways in the pathogenesis of SM.