SURGICAL ANATOMY AND APPROACHES TO THE PINEAL REGION

Background: pineal gland is a diencephalic structure related to the posterior wall of the third ventricle and lying between the superior colliculi of the quadrigeminal plate. It gives the name to a specific topographic region called pineal region. Lesions involving the pineal region are considered very difficult to treat mainly due to the deep site of the gland, as well the intimate relationships these lesions have with the deep venous system. A perfect knowledge of the microneurosurgical anatomy of this region is of utmost importance to deal with pineal tumors and, at the same time, to make safer all the approaches to the pineal region. Materials and Methods: the authors review in detail the microneurosurgical anatomy of the pineal region focusing on those anatomical aspects related to the different pattern of tumor growth and playing a pivotal role in the choice of the most suitable and advantageous approach among others. The occipital trans-tentorial approach and the supra-cerebellar infratentorial approach have also been reviewed in detail with reference to some surgical cases. Results: pineal region is bounded laterally by the tentorial edge and the thalami, rostrally by the posterior third ventricle, caudally by the vermis of the cerebellum, ventrally by the quadrigeminal plate and dorsally by the splenium of the corpus callosum. Pineal gland is oval in shape and measures, on the average, 7.4 mm in longitudinal length, 6.9 mm in transverse width, and 2.5 mm in thickness. Its arterial supply belongs to the pineal artery, one of the branches of the medial posterior choroidal artery (MPChA), whereas the venous outflow is realized by means of the pineal vein that empty into the Galenic system. The distance between the pineal gland and the tentorial notch ranges from 10 to 30 mm. The arteries that run within the pineal region and posterior incisural space are as follow: P2 posterior and P3 segments of posterior cerebral artery (PCA), circumflex artery from P1 segment of PCA, thalamo-geniculate artery from P1 segment of PCA, MPChA from P2 anterior segment of PCA, lateral posterior choroidal artery from P2 posterior segment of PCA, cortical branches of PCA, that are the inferior temporal artery, parieto-occipital artery, calcarine artery, and splenial artery and distal segment of superior cerebellar artery. Regarding the venous structures, the pineal region holds the vein of Galen, formed by the union of the paired internal cerebral veins, the paired basal veins of Rosenthal, the inferior ventricular vein, lateral and posterior mesencephalic veins, internal occipital vein, posterior pericallosal vein, straight sinus and a number of bridging veins from the tentorial surface of the cerebellum, especially in the midline. The fourth cranial nerve emerges posteriorly below the inferior colliculi of the quadrigeminal plate and passes trough the quadrigeminal cistern in the pineal region in its forward direction. Occipital trans-tentorial approach is mainly indicated for those cases where the distance between the pineal gland and the tentorial notch is less than 20 mm and/or the lesion encroaches the posterior part of the third ventricle. On the other hand, the supra-cerebellar infra-tentorial approach offers the best view and working space for those lesions having a caudal infratentorial extension, even though it should be always considered the unavoidable sacrifice of the bringing veins from the tentorial surface of the cerebellum. A combined approach is also very useful for those lesions having a supra and infra-tentorial extension. Conclusion: the majority of pineal region tumors can be safely approached either via the supra-cerebellar infra-tentorial approach or the occipital trans-tentorial approach. An extremely detailed knowledge of the anatomy of this region is mandatory in the selection and execution of all of these approaches.