Computed tomographic characteristics of anatomical variations of external and internal jugular veins in dogs.

The external jugular vein (EJV) has been commonly used for clinical procedures in veterinary medicine, while the internal jugular vein (IJV) has been used mainly for research. Limited literature regarding anatomical variants in both EJV and IJV is available. The objectives of this descriptive retrospective study were to identify the number of dogs with variations of the jugular veins in canine patients undergoing CT of the head and neck and characterize these variations. Of 1000 dogs, 193 dogs (19.3%) had anatomical variations. Six types of anatomical variations were identified. The most common Type I was the absence of the linguofacial vein (72.0%, n = 139), with the lingual, facial, and maxillary veins entering the EJV at a common branching point. Type II described a variable course of the EJV and IJV, which in some dogs resulted in the absence of the EJV (14.0%, n = 27). Type III was a variable anastomotic loop formed by the linguofacial or maxillary vein cranial or at the junction of the EJV (4.2%, n = 8). In Type IV, the EJV was either suspected to be aplastic or markedly reduced in diameter compared with the contralateral side just distal to its formation (3.6%, n = 7). Type V was found in only six dogs (3.1%) with unilateral differences in the diameter of the IJV. Six of 193 dogs had a combination of more than one variation (Type VI, 3.1%). Type I was significantly observed more frequently in meso- and dolichocephalic breeds. Type II was significantly observed more frequently in brachycephalic breeds.