Relation between bilateral differences in internal jugular vein caliber and flow patterns of dural venous sinuses

Measurement of the sizes of the dural sinuses and evaluation of the drainage patterns of the SSS and RS showed that most or all SSS flow enters the right TS in most cases, as has been previously reported (e.g., Gibbs and Gibbs 1934; Woodhall 1936). Compared with the previous studies, the incidence of SSS draining into the right TS in our study was marked, while the SSS joining or bifurcating in the torcular Herophili and draining into the bilateral TSs was low (Table 3). In this study, the torcular Herophili region was cut and observed, and the sizes on the left and right sides at the bifurcation and flow due to the septum in the lumen were also taken into consideration for classification. Therefore, left or right predominance may have become clearer.

To evaluate the influences of the SSS drainage pattern on the TS and IJV sizes, their relationship in the order of venous flow is shown in Table 4. In cases showing SSS draining into the right TS, the right TS and IJV sizes were large. In cases showing SSS draining into the left TS, the left TS and IJV sizes were large. These results suggest that the SSS drainage pattern markedly affects IJV caliber.

Although the SSS drainage pattern was consistent with the IJV caliber in most cases, a reverse relationship was observed in 5 of the 91 cases (Table 4). This suggests that the IJV caliber is also affected by draining veins after the TS (such as the superior and inferior petrosal sinuses and facial vein). In addition, certain diseases may also cause poor flow in the unilateral IJV and its surrounding veins, resulting in an increase in the contralateral IJV caliber.

The RS showed a slightly higher incidence of drainage into the left TS, although no significant difference was present. The RS drainage pattern may not have sufficient influence to determine the left–right difference in IJV caliber.

Our results showed that the SSS frequently drains into the right TS, and, as a result, the right IJV becomes thicker. Why does the SSS frequently drain to the right? Since Padget (1957) and Masaki (1959) showed that the right TS tends to be larger in fetuses aged 4–10 months, the SSS drainage pattern may be determined at a relatively early stage of the prenatal period. Figure 11 shows a schematic diagram of the development of the dural sinuses and brachiocephalic vein, with reference to the reports of Streeter (1915) and Moore (1988). Based on these courses, the SSS, derived from the sagittal plexus, forms after the left anterior cardinal vein has anastomosed the right anterior cardinal vein. We speculate that the high incidence of SSS draining into the right TS is associated with the anastomosis of the left and right anterior cardinal veins (brachiocephalic veins later), in other words, disappearance of the left superior vena cava. Concerning the pathway from the dural sinuses to the heart through the IJV, the pathway through the right brachiocephalic vein is slightly shorter than that through the left brachiocephalic vein. The pathway through the right brachiocephalic vein shows linear flow into the heart, while the pathway through the left brachiocephalic vein causes a mild angle at the confluence. Thus, the vascular resistance is lower for drainage into the right TS than that into the left TS, and, therefore, the sagittal plexus flow may shift gradually to the right. Due to their relationships, the predominance of the right TS may increase, resulting in gradual increases in the right TS size and right IJV caliber.

To confirm this speculation, the absence of the predominance of the right IJV in cases of persistent left superior vena cava should be shown. However, this anomaly is very rarely observed, and time is necessary for the evaluation of substantial data. In addition, to clarify the embryological flow in the dural sinuses and IJV further, more detailed autopsy studies on the prenatal venous system are necessary.

In the internal occipital protuberance on the inner surface of the occipital bone, the SSS continues to the TS; a groove forms along the TS flow. However, there have been no studies in which the degree of consistency between the groove arrangement and sinus flow was confirmed. In this study, this consistency was also evaluated, and, as a result, the SSS drainage pattern was mostly consistent with the groove running type (Fig. 10).

In physical anthropology, the SSS groove that continues to the left TS groove is called the sagittal sinus groove to the left, and has frequently used been as a minor cranial variant (cranial non-metric variant) for the analysis of the lineage of races and groups (Dodo and Ishida 1990, Yamaguchi 1977, Saiki et al. 2000, Wakebe et al. 2012, etc.). The incidences of this variant in various groups, mainly in East Asia, are shown in Table 5. The incidence in each group is similar in the same region or period. Since the SSS drainage pattern is determined at an early stage of the prenatal period, this pattern and the groove formed based on this pattern may be affected more markedly by genetics than by the postnatal environment. The similar tendency in each region (Table 5) might be due to genetic influences. Although only the incidence of the sagittal sinus groove to the left has been evaluated in many previous studies, we classified the SSS groove arrangement into types, allowing application of the estimation of the SSS drainage pattern and the calibers of the left and right IJVs to ancient people.

The right IJV is used most frequently as the site for central venous catheterization since the catheter can be advanced linearly to the superior vena cava, and the risk of pneumothorax as a complication is low. In general, the catheter is inserted into the right IJV for central venous catheterization (e.g., Yamayoshi 1996), but the right IJV is smaller than the left IJV in some patients, as was shown by the results of this study. This fact should be taken into consideration for this procedure. For accurate and safe central venous catheterization, the size and arrangement of the IJV should be confirmed using ultrasonography before puncture, as is recommended in the anesthesiological field (e.g., Karakitsos et al. 2006; Kunisawa 2009).

In the dural sinuses, thrombosis or stenosis sometimes occurs. Dural sinus occlusion due to thrombi is observed most frequently in the SSS, followed by the TS (Ferro et al. 2004; Saposnik et al. 2011). Multiple sinuses were reported to be involved in about one-third of patients (Bousser 1999). For the diagnosis of dural sinus thrombosis, CT and MRI have been used frequently in recent years. However, the interpretation of imaging findings is difficult, and misdiagnosis often occurs (Provenzale and Kranz 2011). Studies on the SSS drainage pattern in the torcular Herophili have shown individual differences and difficulty in determining whether the blood flow is original or due to thrombosis or occlusion when the flow is thin on the images obtained. In patients with dural sinus diseases such as sinus thrombosis, it should be taken into consideration that the drainage pattern of each sinus shows a certain tendency, but is not the same among patients, and appropriate diagnosis and treatment are necessary.

Moreover, Zamboni et al. (2006, 2009) recently proposed a new hypothesis for pathomechanisms of multiple sclerosis (MS): chronic cerebrospinal venous insufficiency (CCSVI). Research reports to verify this hypothesis have also increased (Doepp et al. 2010; Baracchini et al. 2011; Tanaka et al. 2011). The IJV and dural sinus are considerably associated with CCSVI. Although this study does not reveal the details of this relationship, we should conduct an investigation based on this hypothesis in future studies.

This study was performed in cadavers. After patient death, about 10 l preservative (containing formalin, ethanol, and water) was injected into the femoral and radial arteries. Therefore, the possibility that blood returned to the venous side, and the veins were dilated compared with the premortem state cannot be excluded. There is a possibility that the measurement results regarding veins in this study slightly differ from the premortem in vivo state.