## 4 Discussion
In vivo imaging studies are more appropriate to analyze the vascular anatomy in animals because of vein collapse in cadaver specimens. In humans, epidural venous engorgement in the lumbar spine, including epidural varices, may cause pain in the lower back and radiiculopathy secondary to nerve root impingement [27]. The diagnosis of IVVP engorgement is often mistaken for a herniated disc on radiologic interpretation, and the true diagnosis is finally made in the surgical intervention. In dogs, vertebral venous system abnormalities have been identified on MRI in 12% of all sighthounds, but the underlying cause is unknown [14]. Recently, severe dilation of the right IVVP causing significant compression of the spinal cord and nerve roots was identified in an adult Scottish Deerhound [13]. Bilateral vertebral venous sinus thrombosis and dilation causing cervical spinal cord compression were also recently detected in a 10-year-old male mixed dog [28]. In another study, the visibility of the IVVP was significantly (\(p<0.001\)) different between Great Danes with and without clinical signs of cervical spondylopathy [29]. Variation in the size of the IVVP in the central nervous system (CNS) can be explained by the Monro-Kellie doctrine, which establishes an inverse relationship between cerebrospinal fluid (CSF) volume and intracranial blood volume [30]. Hence, as CSF is removed from the intracranial compartment, more blood enters the intracranial compartment. Because the intracranial compartment and vertebral canal are nearly a closed system, the principles of the Monro-Kellie doctrine can be extended to the vertebral canal. As expected, the size of the dural sac increased with increased intracranial CSF or blood volume and decreased with decreased intracranial CSF or blood volume [30]. In the brain, the dura mater is closely opposed by bone, whereas in the vertebral canal, the dura mater has the IVVP and fat separating it from the fixed bony vertebral canal. As the epidural fat is constant and likely has little movement in and out of the neural foramen, the IVVP is most likely to change in size with changes in dural sac volume [30]. In relation to the dural sac, the increase in the average area recorded at the L4 verteba level is associated with medullary expansion of the L4-S3 segment, known as lumbar enlargement (_intumescentia lumbalis_) [1]. From this lumbar enlargement, the nervous roots of the lumbosacral plexus emerge, which can be seen in the transverse images from the L5 verteba caudal. The dural sac area values recorded for each vertebral segment, with the exception of L4, are smaller than those published by [PERSON] et al. (2005) for the cervical segment in dogs, where an average area of 6.40 + 1.0 mm\({}^{2}\) was recorded. A CT study of the dural sac in humans indicated that between the L4 and L5 segments, the average area of the dural sac was >100 mm\({}^{2}\), which was higher than that recorded in the present study for the same vertebral segment in dogs [31]. In humans, a dural sac cross-sectional area < 100 mm\({}^{2}\) and dural sac anteroposterior diameter <10 mm are frequently considered to assess the severity of spinal canal stenosis [31; 32]. The percentage of the cross-sectional area of the dural sac found in our study was between 50 and 72% of the vertebral canal between L1 and L4. These values were similar to those reported by [PERSON] et al. (2018) [33], with values of 64% in a group of 12 normal Beagle dogs. In this study, a significant reduction in the ratio cross-sectional area of the dural sac (40%) in patients with different spinal disorders (IVDD, spinal tumors, hematomas, etc.) to that of the vertebral canal was observed [33]. The vertebral canal area measured 8.94 mm\({}^{2}\) between the L1 and L7 vertebral segments on average. Similar results were observed in a morphometric CT study of the thoracic spine performed in 13 German Shepherd dogs for the T2-T13 vertebral level (8.36 \(\pm\) 4.03 mm\({}^{2}\)) [34].