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Limitations of the transcallosal transchoroidal approach to the third ventricle.

OBJECT
The aim of this study was to determine the anatomical limitations of the transcallosal transchoroidal approach to the third ventricle.


METHODS
Twenty-six formalin-fixed specimens were studied. Sagittal dissections were used to determine the anatomical relationships of the foramen of Monro, the angle of approach to landmarks, and placement of a callosotomy. Lateral ventricular dissections were performed to quantitate the forniceal anatomy.


RESULTS
The foramen of Monro was found 1.07+/-0.11 cm superior and slightly anterior to the mammillary bodies, 1.48+/-0.16 cm posterosuperior to the optic recess, and 2.26+/-0.16 cm anterosuperior to the aqueduct. Relative to the genu, a callosal incision 2.64+/-0.53 cm long and angled 37+/-4.3 degrees anterior was needed to access the aqueduct, and an incision 4.92+/-0.71 cm long and angled 49+/-7.4 degrees posterior was needed to access the optic recess. The fornix progressively widened within the lateral ventricle, from 1.25+/-0.63 mm at the foramen of Monro to >7 mm at 2 cm behind the foramen. Three zones of exposure were identified, requiring unique craniotomies, callosotomies, and angles of approach. The major limiting factors in the approach included the columns of the fornix anteriorly, the width of the fornix posteriorly, and the draining veins of the parietal cortex. The choroidal fissure opening was limited to 1.5 cm posterior to the foramen of Monro; this limited opening created an aperture effect that required an anterior-to-posterior angle, an anterior craniotomy, and an anteriorly placed callosotomy to access the posterior landmarks. In contrast, a posterior-to-anterior angle, posteriorly placed craniotomy, and posteriorly placed callosotomy were required to access anterior landmarks.


CONCLUSIONS
The transcallosal transchoroidal approach was ideally suited to access the foramen of Monro and the middle and posterior thirds of the third ventricle. Exposure of the anterior third ventricle was limited by the columns of the fornix and by the presence of parietal cortical draining veins.