The anatomy of the veins of the SCVS is of interest to clinicians. Special focus lies in the anatomy of the AV, since these veins are regarded as ‘dangerous veins’ in the neurosurgical field, due to the high risk of postoperative infarction after an otherwise successful surgery7. However, there is a significant paucity of literature focusing on the AV, especially in the context of the SCVS entirely. This study aimed to fill the gap in the literature regarding the anatomy and anatomical variations of the AV to assist clinicians in preoperative planning. Therefore, familiarity with the variant anatomy of these AV will elucidate the SCVS since precise information regarding these veins would be able to form a network of reference points that are relevant during surgical planning and surgical procedures9,17.
Presence
Krings et al. stated that the VL or VT may be absent, but that the SSV would be invariably present4. We reported a 75.5% presence of the SSV in the present study. The SSV had a presence of 79.0% and 72.0% on the left and right cerebral hemispheres, respectively (Table 2). The occurrence of the SSV in the present study is lower than that reported by Kawamata et al. who reported a 99.2% presence in the left hemisphere and 95.1% in the right cerebral hemisphere20. Wangaryattawanich et al. stated that the SSV may be absent in 9.0–10.0% of individuals; this statement is corroborated by the study conducted by Kazumata et al., who reported an absent or hypoplastic SSV in 10.0% of the sample12,21. In the absence of the SSV, the territory is drained by the deep system of cortical veins, or the adjacent AV12,22. The SSV was reported to have a higher percentage of presence in females (77.5%) than males (72.5%), as well as a much higher presence in White ethnic group (88.9%) when compared to the Indian ethnic group (70.0%) (Table 2).
The VL had the highest occurrence, with a 96.5% presence in the total sample size. We also report 97.0% and 96.0% presence of the VL in the left and right cerebral hemispheres, respectively (Table 2). These results are much higher than those reported by Kawamata et al., who reported 64.8% in the left hemisphere and 41.8% in the right hemisphere20. The VL was determined to have a 100% occurrence in the 20–29 years age group, as well as the Coloured and Indian ethnic groups (Table 2). The high occurrence of the VL must be noted by clinicians since a sacrifice of this vein in a poorly anastomosed SCVS would lead to a swollen temporal lobe and brainstem compression, as well as posterior haemorrhagic infarct5.
We report a presence of the VT in 64.5% of the total sample size, with 61.0% and 68.0% presence in the left and right cerebral hemispheres, respectively (Table 2). These results are higher than those reported by Kawamata et al., with 59.0% in the left hemisphere and 54.9% in the right hemisphere20. In the absence of the VT, blood is drained by the superior cortical veins into the superior sagittal sinus or drained by an adjacent AV. Kilic and Akakin stated that on lateral angiograms, 8–12 superior cortical veins can be visualised, while Appaji et al. stated that there are 10–15 superior cortical veins; however, in the present study, a minimum of three and maximum of seven superior cortical veins draining the superolateral cerebral convexity in the absence of the VT was identified11,18. This study reports a low occurrence of the presence of VT in the 30–39 year age group (47.5%), a higher presence of VT in males (71.2%) in comparison to females (60.0%), and the lowest occurrence in the African ethnic group (62.8%) (Table 2).
Each of the three AV have been reported to have double veins present, in a study conducted by Tanriverdi et al. it was reported that 9.9% of patients had a duplication of any of the three AV8. A vein was considered as duplicate if the AV had more than one distinct vein that drained the respective territory of the AV into its’ respective dural venous sinuses, as seen in Figs. 1C-1F. We report a presence of 12% of double SSV (left: 13.0%; right: 11.0%); this is lower than the results of the study conducted by Bisaria, who reported a double bilateral SSV in 37.5% of their sample10. Double VL was also recorded in the present study, with an occurrence of 22.0% (left: 19.0%; right: 25.0%); this is corroborated by the studies conducted by Koperna et al. and Avci et al. who reported presence of double VL in 18.0% and 20.0% of their samples, respectively15,23. We also report a double VT in 19.5% (left: 15.0%; right: 24.0%) of the cases in this study.
Furthermore, this study includes the presence of triple AV, as seen in Figs. 1G-1L. The triple SSV and triple VT were present in right cerebral hemispheres, whereas both occurrences of the triple VL were present in left cerebral hemispheres. The triple VT was present in a male patient, while the triple SSV and triple VL were present in female patients. Although no statistically significant relationships were determined between sex and presence of the AV (p > 0.05), since the triple AV is a novel finding this would be important to note. Knowledge of the triple AV variant is imperative for neurosurgical perspectives since this could affect the consequence of sacrifice of some of these AV and must be considered during preoperative planning.
Diameter
Sindou et al. stated that any of the veins of the SCVS with a certain calibre can be presumed to have a functional role16. We reported mean diameters for the SSV (1.99 ± 0.500mm), SSV2 (1.92 ± 0.477mm), SSV3 (2.68mm), VL (2.18 ± 0.579mm), VL2 (1.96 ± 0.388mm), VL3 (1.52 ± 0.0424mm), VT (2.14 ± 0.472mm), VT2 (2.19 ± 0.604mm) and VT3 (1.63mm) (Table 3).
Kawamata et al. classified the calibre of the AV into three groups based on their diameter: (i) small: < 1.00mm, (ii) medium: 1.00–3.00mm and (iii) large: > 3.00mm20. The results of the present study indicate that the mean diameters fall within the medium calibre group. Silva et al. reported mean diameters of 3.2mm and 3.32mm for the VL and VT, respectively; these diameters are greater than those reported in the present study and could be attributed to the difference in population since Silva et al. studied the AV in a Portuguese cohort13. However, the results of the present study do not report any statistically significant results between the different ethnic groups within the South African cohort. Statistically significant results were reported among age groups, for the SSV (p < 0.001), VL (p = 0.025), VT (p = 0.015) and VT2 (p = 0.003) (Table 3). This relationship should be noted by clinicians since the 50–59 year and ≥ 60 years age groups had relatively smaller diameters of the AV. A statistically significant relationship was also determined between females and males for the diameter of VL (p < 0.001), with the mean diameter of the VL in females (2.06 ± 0.542mm) being lower than that of males (2.35 ± 0.590mm) (Table 3). Avci et al. stated that double VL were of equal size, whereas Krings et al. stated that if a double VL is present, the posterior VL tends to be larger4,23. The present study disagrees with both statements since the mean diameters of VL, VL2 and VL3 are vastly different; furthermore, the posterior VL, in this case VL2 and VL3, have smaller diameters than the anterior VL (Table 3).
Dominant patterns
Pertaining to the dominance patterns of the AV of the SCVS, the literature has reported the system being in haemodynamic balance4,7; furthermore, studies undertaken by Kawamata et al. and Tanriverdi et al. report variant patterns of dominance of these AV8,20. The present study reports seven types of AV dominance, viz. Equilibrium, dominant SSV, VL, VT, SSV/VL, SSV/VT as well as dominant VL/VT (Table 4, Fig. 2).
Dominant patterns of the AV were ascertained by the territorial drainage of the veins, and in some cases the absence of a vein played little to no role in the dominance of the AV. For example, in the absence of the VT, in some cases the territory of the VT was drained by the superior cortical veins and not the accompanying AV, i.e SSV and VL. The system was determined to be an SSV/VL dominant drainage pattern if the SSV and/or VL crossed into the territory of the VT.
The Equilibrium dominant pattern of drainage had the highest occurrence (54.5%) in the present study and is aligned with the theory of haemodynamic balance of the SCVS, due to each AV draining their respective territories in a well-anastomosed system, as seen in Fig. 2A and 2B. However, the studies conducted by Kawamata et al. and Tanriverdi et al. report this type of dominant pattern of the AV in only 23.8% and 11.2% of the sample size, respectively9,20. Kawamata et al. had a predominance of the SSV/VT drainage pattern in 27.0% of the cohort studied, and Tanriverdi et al. reported a majority of the SSV/VL dominance pattern (19.5%); however, the dominance patterns of SSV/VT and SSV/VL were reported as 3.0% and 2.5% in the present study, respectively4,20. Co-dominance of VL and VT was seen to have the second highest occurrence in the present study with 28.0% of the cohort presenting with the VL/VT dominant type; furthermore, this dominant type was determined to have a statistically significant relationship with sex (p = 0.034) with males (36.2%) having a higher presence of the VL/VT type than females (22.5%) (Table 4, Fig. 2). Dominant patterns that presented with a statistically significant relationship with age were the VL (p = 0.037) and SSV/VL (p = 0.027) groups, the age group of 30–39 years had the highest occurrence of the respective dominant types (20.0% VL dominant; 10.0% SSV/VL dominant) (Table 4). These dominant patterns of drainage are important in determining the safety of sacrifice of one of the AVs since this is largely based on collateral circulation and alternate drainage routes which would be available in well-anastomosed systems7.
Previous studies have hypothesised an inverse relationship between the dominant patterns of VL and VT based on laterality, stating that the presence of a VT dominance on the left hemisphere would lead to a VL dominance on the right hemisphere, and vice versa2,8,24,25. A Cramers V correlation was carried out to determine the correlation of the types of dominant patterns of the AV, based on laterality (Table 5). The results of the present study indicate a weak correlation between the VL dominant pattern on the left hemisphere and VT dominance on the right (0.03), as well as a VL dominant AV on the right and a VT dominant AV on the left hemisphere (0.04). Results from the Cramers V correlation reported the highest correlation between dominant patterns of the AV, based on laterality, with a VL dominant AV on the left and right hemisphere having a medium correlation of 0.51 (Table 5).