Foramen Magnum in Dogs of Small and Toy Breeds - Morphometric Study
Authors: Júnior Emílio Leite de Sousa , Medeiros Gildenor Xavier , Gomes Iara Macedo de Melo , De Oliveira Suelton Lacerda , Dias Rafael Gonçalves , Marinho Paulo Vinícius Tertuliano 
Published: December 2024

Background: The foramen magnum is located in the occipital bone and communicates the cranial cavity with the spinal canal of the vertebral column. Variations in the shape and size of this foramen, such as the presence of a notch in its dorsal contour, characterize occipital dysplasia and may occur due to a defect in the supraoccipital bone ossification process during the gestational period. Occipital dysplasia has been reported primarily in small, toy, and brachycephalic breeds, and its clinical relevance remains controversial. The aim of the present study was to evaluate the size of the foramen magnum in asymptomatic dogs of small and toy breeds.
Materials, Methods & Results: The study was conducted at the Veterinary Hospital of the Center for Rural Health and Technology of the Federal University of Campina Grande (UFCG), Patos Campus, located in Paraíba state, Brazil. Twelve (n = 12) asymptomatic, small and toy breed dogs, with variable sex and over 10 months of age, were referred to the Veterinary Hospital for elective surgical procedures and were used in this study. All dogs underwent complete neurological examination to confirm the asymptomatic status and were radiographed in rostrocaudal position, with their mouths closed and the hard palate at an angle of approximately 70º to 80º with the table of the X-ray apparatus. The foramen magnum of each specimen was evaluated in the radiographs using a precision caliper. The analyzed parameters included height (H), the height of the dorsal notch (N), total height (H+N), and width (W), and the obtained data were used to establish the degree of occipital dysplasia and determine the shape of the foramen magnum. Of the 12 animals studied, 75% (n = 9) exhibited a dorsal notch in the foramen magnum, which varied between 3.00 and 10.00 millimeters (mm) in height, characterizing occipital dysplasia. Among the affected animals, 77.77% (n = 7) were classified as grade 1 for the alteration, 11.11% (n = 1) as grade 2 and one animal (n = 1; 11.11%) as grade 3. The prevalent shape observed for the foramen magnum was oval (83.3%).
Discussion: Although occipital dysplasia has been associated with the occurrence of nonspecific neurological clinical signs, such as tremors, ataxia, and epileptic seizures, the presence of this alteration in asymptomatic dogs indicates that the formation alone is just an anatomical variation, as demonstrated herein and in previous studies conducted over the past few years. This hypothesis has been increasingly supported by scientific evidence through publications that portray occipital dysplasia in dogs of various breeds and sizes without clinical manifestations. The clinical signs attributed to occipital dysplasia may originate in situations where there is a coexistence of other conditions. Occipital dysplasia has been reported several times in conjunction with other pathologies, such as occipital hypoplasia and syringomyelia, in symptomatic dogs. The dorsal notch-shaped occipital defect is covered by a fibrous tissue membrane in dogs affected by occipital dysplasia. The presence of this soft tissue membrane has been related to the late onset of syringomyelia due to the decompressive effect that it provides to the flow of cerebrospinal fluid. When occipital dysplasia is identified in symptomatic dogs, it is suggested that the lesion be correctly located within the nervous system and that, according to its neurolocalization, a thorough investigation of other underlying causes for the occurrence of the neurological clinical manifestation be carried out. To date, there is no evidence characterizing occipital dysplasia as a single entity causing neurological deficits.
Keywords: anatomy, morphology, occipital bone, skull, occipital dysplasia, neurology.