| Summary: | Chiari-like Malformation (CM) and secondary Syringomyelia (SM) is a complex, debilitating abnormality which compromises the normal cerebrospinal fluid movement of the central nervous system culminating in the development of fluid-containing cavities within the spinal cord and associated with behavioural signs of pain and neurological deficits. The prevalence of asymptomatic CM dogs suggest that cerebellar indentation and impaction may be normal anatomical variations and unsuitable as a definition of CM. Magnetic Resonance Imaging (MRI) remains the definitive means of diagnosing CM/SM and a morphometric technique of quantifying CM and SM on mid-sagittal MRI has been successfully applied and validated in previous studies to a cohort of Griffon Bruxellois (GB) dogs with and without CM and a mixed breed GB family crossed with a mesaticephalic breed (Australian Terrier). Using a refined technique which took account of recent research findings, morphometries using a triangulation of circles, lines and angles were used to ‘map’ MRIs of the whole brain and cervical region in order to quantify the severity of the CM and SM phenotype in the Cavalier King Charles (CKCS). A further morphometric analysis was undertaken to explore brachycephaly and miniaturization as risk factors for CM and SM by comparing their impact in the CKCS, Affenpinscher and Chihuahua breeds. The collective framework of lines and angles generated a unique ‘signature’ for the dog, characterised by “concertina” type flexures demonstrating the combined nature of segregated traits towards the severity in the phenotype. Compared to controls, CKCS with CM pain are characterised by increased brachycephaly and airorhynchy, while significant traits for SM in the three dog breeds included those reported for the GB, suggesting a common aetiology. The characterisation of the CM phenotype provides the possibility of a diagnostic tool for veterinarians and means to assist breeders with mate selection to reduce symptomatic prevalence of CM/SM.
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