Rhoton

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Satoshi Matsuo, Serhat Baydin, Abuzer Güngör, Erik H. Middlebrooks, Noritaka Komune, Koji Iihara, and Albert L. Rhoton Jr.

OBJECTIVE

A postoperative visual field defect resulting from damage to the occipital lobe during surgery is a unique complication of the occipital transtentorial approach. Though the association between patient position and this complication is well investigated, preventing the complication remains a challenge. To define the area of the occipital lobe in which retraction is least harmful, the surface anatomy of the brain, course of the optic radiations, and microsurgical anatomy of the occipital transtentorial approach were examined.

METHODS

Twelve formalin-fixed cadaveric adult heads were examined with the aid of a surgical microscope and 0° and 45° endoscopes. The optic radiations were examined by fiber dissection and MR tractography techniques.

RESULTS

The arterial and venous relationships of the lateral, medial, and inferior surfaces of the occipital lobe were defined anatomically. The full course of the optic radiations was displayed via both fiber dissection and MR tractography. Although the stems of the optic radiations as exposed by both techniques are similar, the terminations of the fibers are slightly different. The occipital transtentorial approach provides access for the removal of lesions involving the splenium, pineal gland, collicular plate, cerebellomesencephalic fissure, and anterosuperior part of the cerebellum. An angled endoscope can aid in exposing the superior medullary velum and superior cerebellar peduncles.

CONCLUSIONS

Anatomical findings suggest that retracting the inferior surface of the occipital lobe may avoid direct damage and perfusion deficiency around the calcarine cortex and optic radiations near their termination. An accurate understanding of the course of the optic radiations and vascular relationships around the occipital lobe and careful retraction of the inferior surface of the occipital lobe may reduce the incidence of postoperative visual field defect.

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Satoshi Matsuo, Serhat Baydin, Abuzer Güngör, Koichi Miki, Noritaka Komune, Ryota Kurogi, Koji Iihara, and Albert L. Rhoton Jr.

OBJECTIVE

A common approach to lesions of the pineal region is along the midline below the torcula. However, reports of how shifting the approach off midline affects the surgical exposure and relationships between the tributaries of the vein of Galen are limited. The purpose of this study is to examine the microsurgical and endoscopic anatomy of the pineal region as seen through the supracerebellar infratentorial approaches, including midline, paramedian, lateral, and far-lateral routes.

METHODS

The quadrigeminal cisterns of 8 formalin-fixed adult cadaveric heads were dissected and examined with the aid of a surgical microscope and straight endoscope. Twenty CT angiograms were examined to measure the depth of the pineal gland, slope of the tentorial surface of the cerebellum, and angle of approach to the pineal gland in each approach.

RESULTS

The midline supracerebellar route is the shortest and provides direct exposure of the pineal gland, although the culmen and inferior and superior vermian tributaries of the vein of Galen frequently block this exposure. The off-midline routes provide a surgical exposure that, although slightly deeper, may reduce the need for venous sacrifice at both the level of the veins from the superior cerebellar surface entering the tentorial sinuses and at the level of the tributaries of the vein of Galen in the quadrigeminal cistern, and require less cerebellar retraction. Shifting from midline to off-midline exposure also provides a better view of the cerebellomesencephalic fissure, collicular plate, and trochlear nerve than the midline approaches. Endoscopic assistance may aid exposure of the pineal gland while preserving the bridging veins.

CONCLUSIONS

Understanding the characteristics of different infratentorial routes to the pineal gland will aid in gaining a better view of the pineal gland and cerebellomesencephalic fissure and may reduce the need for venous sacrifice at the level of the tentorial sinuses draining the upper cerebellar surface and the tributaries of the vein of Galen.

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Noritaka Komune, Satoshi Matsuo, Koichi Miki, and Albert L. Rhoton Jr.

OBJECTIVE

The application of the endoscope in the lateral skull base increases the importance of the middle ear cavity as the corridor to the skull base. The aim of this study was to define the middle ear as a route to the fundus (lateral end) of the internal acoustic canal and to propose feasible landmarks to the fundus.

METHODS

This was a cadaveric study; 34 adult cadaveric temporal bones and 2 dry bones were dissected with the aid of the endoscope and microscope to show the anatomy of the transcanal approach to the middle ear and fundus of the internal acoustic canal.

RESULTS

In the middle ear cavity, the cochleariform process is one of the key landmarks for accessing the fundus of the internal acoustic canal. The triangle formed by the anterior and posterior edges of the overhang of the round window and the cochleariform process provides a landmark to start drilling the bone to access the fundus of the internal acoustic canal.

CONCLUSIONS

The external acoustic canal and middle ear cavity combined, using endoscopic guidance, can provide a route to the fundus of the internal acoustic canal. A triangular landmark crossing the promontory has been described for reaching the meatal fundus. This transcanal approach requires an understanding of the relationship between the middle ear cavity and the fundus of the internal acoustic canal and provides a potential new area of cooperation between otology and neurosurgery for accessing pathology in this and the bordering skull base.