Microanatomical Study Of Transcerebellomedullary Fissure Approach

Objective:Because of the complexity of anatomical relationship, insufficient operative space,and adjacent to vital center, it is difficult to deal with the tumor located in fourth ventricle and dorsal brain stem. To obtain adequate exposure,conventional approach to the fourth ventricle is by splitting the vermis or partial removal of the cerebellar hemisphere. Excessive retraction the cerebellum can damage the dentate nuclei and dentatonucleocortial projections causing cerebellar mutism syndrome; And even if splitting vermis, fourth ventricle lateral exposure is still not satisfied.In the course of anatomizing the fourth ventricle, We have taken note of the importance of cerebellomedullary fissure at the fourth ventricle surgery. In this article,by anatomizing cerebellomedullary fissure, we study an approach to the fourth ventricle and the lateral brainstem without splitting vermis, and study how to free this fissure to obtain the necessary spaces and the best vision in surgery. Observe the composition, morphology, taking shape of cerebellomedullary fissure as well as the relationship with adjacent structure. Research the microanatomy of posterior inferior cerebellar artery—the important vessel of this approach. This study provide more anatomic base to clinicians in resecting the tumors of dorsal brainstem and the fourth ventricle via transcerebellomedullary fissure approach.Method: Five cadaveric adult heads fixed with formalin, soaked in hydrogen peroxide and alcohol successively, anatomized the neck blood vessel, intubation and washing, perfused with color latex, and analog operation. Anatomizing by median suboccipital craniotomy, observation the distribution of posterior cervical muscles and the morphology of vertebral artery's horizontal segment(V3h), and measuring the distance between inner margin of V3h and the midline. Observation of the distribution and taking shape of cerebellomedullary fissure. Dissecting this fissure, observe the composition, distribution and taking shape of the tela choroida and inferior medullary velum—the key structure of the transcerebellomedullary fissure approach, and study anatomic base of this approach. Discuss the relationship between the incision site of the tela choroida and inferior medullary velum and the exposed extent of the fourth ventricle. Observation the appearance of the rhomboid fossa, measuring the length and width of the fourth ventricle floor, and analyzing the structure's guiding role of the fourth ventricle floor in transcerebellomedullary fissure approach. To view the course, distribution,and the branch of posterior inferior cerebellar artery(PICA), to measure PICA's origin to the point from which the vertebral artery turns into Intracal, to compare both sides(the left and the right) of the distance which were in line with the small sample paired t-test, and according to significance level ofα=0.05, to determine whether there were differences on both sides. Focusing on observation the tonsil- medulla segment and the telovelotonsillar segment of PICA which are closely related to transcerebellomedullary fissure approach,and researching measures to avoid vascular injury in operation.Result: Cerebellomedullary fissure is a natural anatomical gap which is located between cerebellar tonsil,biventral lobule and medulla oblongata. Its ventral wall is formed by the posterior surface of the medulla oblongata, the tela choroida and the inferior medullary velum; The dorsal wall is composed of the uvula and vermis nodule in the midline, and the tonsils and biventral lobules laterally. This fissure extends superiorly to the roof of the fourth ventricle, and communicates around the superior poles of the tonsils with the cisterna magna, through the foramen of Magendie with the fourth ventricle, and around the foramina of Luschka with the cerebellopontine fissures and the cerebellomedullary cistern laterally. Sharp separation the surrounding arachnoid of the tonsils, we discover the tonsils are attached to the cerebellum at the superolateral margin, and the remaining surfaces are free. Connecting the inferior edge of cerebellar hemisphere, the tonsils, the inferior border of vermis with the dorsal medulla oblongata are cistern arachnoid, the choroid, and the telovelar junction which is the junction between the tela choroida and the inferior medullary velum. The inferior medullary velum is a semitranslucent sheet which connects the flocculi and the nodule. It blends into the ventricular surface of the nodule medially,is attached to the flocculus cerebelli and middle cerebellar peduncle laterally, continuous at the level of the fastigium with the superior medullary velum upward, and caudally attached to the tela choroidea. The tela choroidea sweeps inferiorly from the telovelar junction around the superior pole of each tonsil to its attachment to the inferolateral edges of the floor, which is like a"tent"covering the inferior part of the fourth ventricular roof, and forms an orifice at the caudal tip of the fourth ventricle that is the Magendie foramen. The tela choroidea and the inferior medullary velum separate the anteosuperior margin of the tonsil from the fourth ventricle. It is necessary to cut this layer of membrane organization into the fourth ventricle by cerebellomedullary fissure approach. The usual practice is to expose the Magendie foramen and the Luschka foramen and to cut the taenia between the two holes. The straight-line distance of incision is (20.1±1.6)mm. If extending the telar opening upward to the inferior medullary velum or laterally toward the posterior wall of the lateral recess, the fourth ventricular floor can be exposed, superior to the undercut of aquaeductus mesencephali, inferior to the Magendie foramen, and laterally to the the Luschka foramen. The length of rhomboid fossa is 38.1±0.8mm, which is the distance from the aqueduct to the obex; the width is 17.3±0.6mm, the distance between the narrowest of both lateral recesses. In the fourth ventricular floor, striae medullares have great variation, facial colliculus are sometimes difficult to identify.The posterior inferior cerebellar artery(PICA) originates in the vertebral artery, through the inferior part of olive, upward coursing in the lower cranial nerves, and folding back to the cerebellar fissures. The distance of PICA's origin to the point from which the vertebral artery turns into Intracal is (18.3±5.5) mm, the left side of the distance is (19.3±4.6) mm, and the right side is (17.4±6.5) mm; both sides of paired t-test: t=0.540, P>0.50. Distance of the left and the right sides cann't be believed that there are differences. The posterior inferior cerebellar artery is tortuous.The posterior inferior cerebellar artery is generally divided into 5 segments: the anterior segment of medulla oblongata, the lateral segment, the tonsil- medulla segment, the telovelotonsillar segment, and the cortical segment.The tonsil- medulla segment and the telovelotonsillar segment are the most important blood vessels of the cerebellomedullary fissure approach. The tonsil- medulla segment courses in the cleft between the medulla oblongata and the tonsils, parallel with the centerline from the inferior to the superior.It comes into being the caudal-loop in the tonsil's inferior pole whose location has great variability.The telovelotonsillar segment is is the most complex, coursing in the superolateral of the cerebellomedullary fissure.It emerges the rostral-loop between the superior pole of the tonsils with the tela choroidea and the inferior medullary velum, which processes to the lateral recess. Most bifurcation of the posterior inferior cerebellar artery originate from the telovelotonsillar segment. It usually issue two trunks, the medial one supplying the vermis and paravermian area and the lateral supplying.most of the surface of cerebellar hemisphere and tonsil. Some branches also supply blood for the dentate nucleus, choroid, and choroid plexus. Horizontal segment of vertebral artery (V3h) means the segment lying between atlas transverse foramen and duramater. The left distance between inner margin of V3h and midline is (14.5±0.6) mm,and the right distance is (14.8±0.8) mm.Conclusion: By transcerebellomedullary fissure approach, it is possible to provide adequate exposure to the fourth ventricle without splitting the vermis. The tonsil is attached to the cerebellum at its superolateral margin, and the other surfaces are free, which forms the anatomical space with the cerebellum. Connecting the inferior edge of cerebellar hemisphere, the tonsils, the inferior border of vermis with the dorsal medulla oblongata are cistern arachnoid, the choroid, and the telovelar junction which don't contain nerve tissue. Transcerebellomedullary fissure approach takes advantage of the natural space to expose the fourth ventricle floor by opening the tela choroidea and the inferior medullary velum. Depending on the location of the affection and the exposure extent of the ventricle, methods of opening the cerebellomedullary fissure can be classsified into three types: extensive (aqueduct type) , lateral wall, and lateral recess. Quantification of the length and width of the the fourth ventricular floor provides a two-dimensional data of operation space for transcerebellomedullary fissure approach. The striae medullares are greatly variable which cann't be taken as intraoperative locating mark, and the situation of facial colliculus are relatively constant, but sometimes difficult to identify. The tonsil- medulla segment and the telovelotonsillar segment of PICA are the crucial blood vessels for transcerebellomedullary fissure approach. The tonsil-medulla segment can be adequately exposed at the beginning of the appoach, which make the approach have unique advantage in dealing with tumor-associated blood vessels (PICA). When separating of the two vessels,we should sharply dissect the perivascular arachnoid to increase vascular activity and to avoid injury in different form of vascular loops. When opening cerebellomedullary fissure, the two vessels and their branches, together with the cerebellar tonsil,should be retracted laterally to avoid injury. To protect the vertebral artery, it is safe to grind posterior arch of atlas not far than 14mm away from the midline.