Surgical anatomy of the upper clivus, subchiasmal region, and floor of the third ventricle in the suboccipital retrosigmoid approach

The surgery of pathologies located in the upper clivus, subchiasmal region and the floor of the third ventricle is difficult due to the critical neurovascular structures in the region. The detailed anatomy of the region should be well known to avoid complications that may develop during the surgical treatment of these pathologies. In this study, the anatomy of the region and safe surgical mobilization techniques were examined. Twenty adult cadaveric specimens were dissected in this study. Dural dissections were performed in accordance with the retrosigmoid approach, and anatomical relationships of the cranial nerves were observed in the upper clivus region. Intradural courses of the oculomotor, trochlear and abducens nerves were revealed and mobilized. In addition, structures supporting the pituitary stalk were revealed on 8 specimens, and evaluations were made with regards of mobilization of the pituitary stalk and gland. The bilateral abducens nerves were mobilized by intradural dissection and could be separated by an average of 4.21±1.02 mm. We observed that the trochlear nerve proceeded in a straight line along its intradural course and underwent changes in direction at the entry site of dural porus and the lateral wall of the cavernous sinus. The intradural course of the trochlear nerve in the petroclinoid dura was revealed by dissection. Intradural course of the nerve was 9.07±1.83 mm on the right and 9.29±1.77 mm on the left. There was no obvious barrier on the lateral wall of the cavernous sinus between the oculomotor and trochlear nerve. When the arachnoid membranes surrounding the pituitary stalk are dissected and the inner arachnoid membranes are examined; Two different thick arachnoid band structures were seen surrounding the pituitary stalk and superior pituitary arteries. Cranial nerve pairs located in this critical region can be mobilized by intradural dissection, considering their anatomical course. Larger manipulation areas can be provided to avoid deterioration during surgery and to perform total resections. Although the internal arachnoid bands that fixate the pituitary stalk are thought to establish an anatomical protective system against neuroendocrinological deficits such as diabetes insipidus and hypothalamic damage that may develop in head trauma, it appears as an anatomical structure that should be treated extremely carefully in the mobilization of the pituitary gland. Key words: Upper clival region, Subchiasmal region, floor of the third ventricle, pituitary stalk.