Anatomical variations of the straight sinus (SS) were examined on magnetic resonance imaging (MRI) in 108 control cases and five cases with a pineal region tumor. The data of the latter were compared with the operative findings in the infratentorial supracerebellar approach. In the 108 control cases, the length of the SS was 51 ± 5.2 mm. The angle of the SS to the nasion-tuberculum sellae line, the tuberculum sellae-inion line, and the nasioninion line was 51 ± 6.7°, 44 ± 5.0°, and 47 ± 5.0°, respectively. The SS was inclined 1.4 ± 4.5° posteriorly to the bregma-inion line. The anatomical relationship between the SS and the corpus callosum was subgrouped into three types: in the common type (63%), the extended line of the slope of the SS is tangential to the upper aspect of the splenium of the corpus callosum; in the high-angle type (26%), the extended line is far above the tangent line; and in the low-angle type (11%), the extended line is far below the tangent line. The operative field was restricted by the steeply inclined tentorium in the case of the high-angle type, and the tumor was located much lower than the direction of the operative approach. In contrast, the tumor was easily exposed in the center of the wide operative field in the cases of low-angle type. The present study suggests that the preoperative MRI analysis of the angle of the SS is indispensable before taking the infratentorial supracerebellar approach to the pineal region tumors.
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