Biologia 72/2: 230—237, 2017 Section Zoology DOI: 10.1515/biolog-2017-0018 Prevalence of dorsal notch and variations in the foramen magnum shape in dogs of different breeds and morphotypes Marta Kupczy´ nska 1 , Norbert Czubaj 1 , Karolina Barszcz 1 , Wojciech Soko lowski 1 , Micha l Czopowicz 2 , Halina Purzyc 3 *, Ma lgorzata Dzierz˛ ecka 1 , Wojciech Kinda 4 & Zdzislaw Kie lbowicz 4 1 Warsaw University of Life Sciences – SGGW, Faculty of Veterinary Medicine, Department of Morphological Sciences, Nowoursynowska 159, 02-776 Warsaw, Poland 2 Warsaw University of Life Sciences – SGGW, Faculty of Veterinary Medicine, Laboratory of Veterinary Epidemiology and Economics, Nowoursynowska 159, 02-776 Warsaw, Poland 3 Wroclaw University of Environmental and Life Sciences, Faculty of Veterinary Medicine, Department of Biostructure and Animal Physiology, Ko˙ zuchowska 5, 51-631 Wroclaw, Poland; e-mail: halina.purzyc@up.wroc.pl 4 Wroclaw University of Environmental and Life Sciences, Faculty of Veterinary Medicine, Department of Surgery, Pl. Grunwaldzki 51, 50-366 Wroclaw, Poland Abstract: The study evaluated shape of the foramen magnum (FM) in dog with regard to its constitutional type (small, medium and large breeds) and morphotype (brachycephalic, mesaticephalic and dolichocephalic dogs). We used multi-slice CT scanner to examine occipital area of 138 purebreds dogs. Two studied groups could be distinguished – living animals undergoing standard diagnostic CT procedure (n = 47) and macerated skulls of euthanized dogs (n = 91). Morphometric analysis was focused on presence of dorsal notch within foramen magnum. This anatomical feature was present in 33.3% of examined dogs. It was more often observed in small size dogs (70%) and those of brachycephalic morphotype (94.1%). For the first time dorsal notch was noted in breeds such as French Bulldog, Fox Terrier, Keeshond, Standard Schnauzer, English Bulldog, Cocker Spaniel, Boxer and Rottweiler. We distinguished four different shape of FM: oval, pentagonal, rhomboid and circular. Most common FM shape was oval (41 dogs; 45%) whereas circular type was least observed (5 dogs; 5.5%). The pentagonal shape was noticed in 27 specimens (29.7%) and the rhomboid shape in 18 dogs (19.8%). Authors conclude that dorsal notch is not pathology and should be considered a morphological variation within the normal anatomy. Results presented in this study should be taken into account during interpretation of the CT images of the craniocervical junction. Key words: foramen magnum; occipital bone; dorsal notch; dog Introduction The foramen magnum is the biggest opening in the skull, located in the central part of the occipital bone. It is limited ventrally by the basilar part of occipi- tal bone, laterally by the lateral parts of the occipi- tal bone, and dorsally by the squama of the occipital bone (Evans & de Lahunta 2013). Many scientific pa- pers in the field of veterinary medicine focus on the description of the occipital bone and the caudal cra- nial fossa, located at its inner surface, which includes the metencephalon (Simoens et al. 1994a, b; Onar et al. 1997; Chrószcz et al. 2006; Rusbridge & Knowler 2006; Janeczek et al. 2008, Wiel˛ adek & Kupczy´ nska 2008; Cross et al. 2009; Baroni et al. 2011; Janeczek et al. 2011). Some races experience disorders in the develop- ment of this structure, which may result in neurologi- cal symptoms. Parts of the brain may bulge through the foramen magnum and move towards the vertebral canal (Czubaj et al. 2015). For this reason, the foramen mag- num has been of interest to researchers attempting to determine whether abnormalities in its morphology af- fect the incidence or severity of the symptoms. In some breeds of dogs, a dorsal notch occurs within the squama of the occipital bone, which is a dorsal extension of the foramen magnum, giving the structure an appearance of a characteristic “keyhole” (Parker & Park 1974; de Lahunta A. & Glass 2009; Evans & de Lahunta 2013). In the source literature, this condition is called occip- ital dysplasia (Parker & Park 1974; de Lahunta A. & Glass 2009). However, it has to be underlined that this term is debatable. Some researchers define the “dorsal notch” as pathology (Wright 1979; Sharp & Wheeler 2005) but others consider it a morphological variation that can be accepted as part of the normal anatomy of the domestic dog (Simoens et al. 1994a; Onar et al. 2013). There are also numerous publications in the field of human medicine that describe morphometry of * Corresponding author c 2017 Institute of Zoology, Slovak Academy of Sciences