THE TRANSCALLOSAL-TRANSFORAMINAL APPROACH TO THE 3RD-VENTRICLE WITH REGARD TO THE VENOUS VARIATIONS IN THIS REGION

Surgical approaches to lesions located in the anterior and middle port ions of the third ventricle are challenging, even for experienced neur osurgeons. Various exposures involving the foramen of Monro, the choro idal fissure, the fornices, and the lamina terminalis have been advoca ted in numerous publications. The authors conducted a microsurgical an atomical study in 20 cadaveric brain specimens (40 hemispheres) to ide ntify an exposure of the third ventricle that would avoid compromising vital structures. An investigation of the variations in the subependy mal veins of the lateral ventricle in the region of the foramen of Mon ro was performed, as these structures are intimately associated with t he surgical exposure of the third ventricle. In 16 (80%) of the brain specimens studied, 19 (47.5%) of the hemispheres displayed a posterior location of the anterior septal vein-internal cerebral vein (ASV-ICV) junction, 3 to 13 mm (average 6 mm) beyond the foramen of Monro withi n the velum interpositum, not adjacent to the posterior margin of the foramen of Monro (the classic description). Based on this finding, the authors advocate opening the choroidal fissure as far as the ASV-ICV junction to enlarge the foramen of Monro posteriorly. This technique a chieves adequate access to the anterior and middle portions of the thi rd ventricle without causing injury to vital neural or vascular struct ures. The high incidence of posteriorly located ASV-ICV junctions is a significant factor influencing the successful course of surgery. Prec ise planning of the surgical approach is possible, because the locatio n of the junction is revealed on pre-operative neuroradiological studi es, in particular on magnetic resonance venography. It can therefore b e determined in advance which foramen of Monro qualifies for posterior enlargement to gain the widest possible access to the third ventricle . This technique was applied in three patients with a third ventricula r tumor, and knowledge of the venous variations in this region was an important resource in guiding the operative exposure.