Comparison of the transvermian and telovelar approaches to the fourth ventricle

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Object. The two most common surgical routes to the fourth ventricle are the transvermian and telovelar approaches. The purpose of this study was to compare the microanatomy and exposures gained through these approaches.

Methods. Ten formalin-fixed specimens were dissected in a stepwise manner to simulate the transvermian and telovelar surgical approaches. Stealth image guidance was used to compare the exposures and working angles obtained using these approaches.

The transvermian and telovelar approaches provided access to the entire rostrocaudal length of the fourth ventricle floor from the aqueduct to the obex. In addition, both approaches provided access to the entire width of the floor of the fourth ventricle. The major difference between the two approaches regarded the exposure of the lateral recess and the foramen of Luschka. The telovelar, but not the transvermian, approach exposed the lateral and superolateral recesses and the foramen of Luschka. The transvermian approach, which offered an incision through at least the lower third of the vermis, afforded a modest increase in the operator's working angle compared with the telovelar approach when accessing the rostral half of the fourth ventricle.

Conclusions. The transvermian approach provides slightly better visualization of the medial part of the superior half of the roof of the fourth ventricle. The telovelar approach, which lacks incision of any part of the cerebellum, provides an additional exposure to the lateral recesses and the foramen of Luschka.

Article Information

Address correspondence to: Albert L. Rhoton Jr., M.D., Department of Neurological Surgery, McKnight Brain Institute, University of Florida, P. O. Box 100265, Gainesville, Florida 32610–0265. email: rhoton@neurosurgery.ufl.edu.

© AANS, except where prohibited by US copyright law.

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Figures

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    Photographs demonstrating posterior views of cadaveric brain specimens. The suboccipital surface of the cerebellum and the cerebellomedullary fissure are shown. Panels A and B show one specimen and C and D another. A: The suboccipital surface is located below and between the sigmoid and transverse sinuses and is the surface exposed during a wide bilateral suboccipital craniotomy. The vermis sits in a depression, the posterior cerebellar incisura that lies between the hemispherical surfaces. The cerebellomedullary fissure extends superiorly between the cerebellum and the medulla oblongata. The vallecula extends upward between the tonsils and communicates through the foramen of Magendie with the fourth ventricle. The left PICA courses below the lower pole of the tonsil to reach the lower margin of the cerebellomedullary fissure and supplies part of the suboccipital surface. B: The left tonsil has been retracted laterally and the uvula medially to expose the inferior portion of the roof of the fourth ventricle. The inferior portion of the roof is formed by the tela choroidea and the inferior medullary velum. The bifurcation of the PICA into the medial and lateral trunks has been retracted to show the lateral recess. Some branches of the medial trunk, which supplies the vermis, course along the uvulotonsillar space along with the vein of the cerebellomedullary fissure. C: The right tonsil has been retracted medially from the biventral lobule. The tonsils, the most prominent structures blocking access to the caudal portion of the fourth ventricle, are attached to the cerebellar hemisphere along their superolateral borders. The fissure between the tonsil and the biventral lobule is called the tonsillobiventral fissure. D: Enlarged posterior view of the same hemisphere. Both tonsils have been retracted laterally to expose the inferior medullary velum and the tela choroidea. The choroidal arteries course along the tela choroidea, from which the choroid plexus projects into the roof of the fourth ventricle. The inferior vermian surface within the posterior cerebellar incisura has a diamond shape, the upper portion of which is called the pyramid. The lower half projects downward between the tonsils and is called the uvula. The nodule is hidden deep in relation to the uvula. The medial margin of the rostral portion of the tonsils borders the tapering edges of the uvula. The strip of the vermis within the incisura is broadest at the junction of the pyramid and uvula. The right PICA bifurcates into lateral and medial trunks, which course along the uvulotonsillar space. The lateral recess, a narrow pouch formed by the union of the roof and floor, is covered caudally by the tela choroidea. A. = artery or arteries; Cer. = cerebellar or cerebello; Chor. = choroidal, Fiss. = fissure; For. = foramen; Inf. = inferior; Lat. = lateral; Med. = medial or medullary; Post. = posterior; Tons. Bivent. = tonsillobiventral; Tr. = trunk.

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    A–C: Coronal (A and C) and sagittal (B) MR images of a cadaveric brain obtained from the Stealth workstation, demonstrating the location of the telovelar junction (red dots indicated by yellow arrows in A and B) and the superolateral recess (red dot indicated by yellow arrow in C). D and E: Photographs of cadaveric brain specimens showing the stepwise dissection of the suboccipital surface of the cerebellum and the inferior portion of the fourth ventricle. D: The left biventral lobule has been retracted to show the tonsillobiventral fissure. The right tonsil has been removed by dividing the peduncle of the tonsil located along the superolateral margin of the tonsil. The uvula projects downward between the tonsils, mimicking the anatomy in the oropharynx. The uvulopyramidal junction, the broadest strip of vermis within the posterior cerebellar incisura, limits the lateral retraction of the tonsils. The telovelar junction is the line of attachment of the tela to the velum. E: Both tonsils and part of the floor have been removed to expose the superior medullary velum and the superolateral recess. The inferior medullary velum is a thin bilateral layer of neural tissue that extends from the nodule medially and blends into the dorsal margin of each lateral recess, forming the peduncle of each flocculus laterally. The incision through the inferior portion of the vermis during the transvermian approach exposes the underlying nodule, which must be incised to gain access to the fourth ventricle. This incision through the vermis is limited by the position of the superior medullary velum, which is closely related to the anterior portion of the nodule. The superior medullary velum forms the medial part of the superior half of the roof and expands laterally and posteriorly from the aqueduct to the level of the fastigium and lateral recess. The superolateral recess, located within the fourth ventricle and lateral to the uvula and nodule, is medial to the superior cerebellar peduncles and lies anterior to the upper edge of the inferior medullary velum. Ped. = peduncle; Sup. = superior.

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    A–E: Stealth MR images demonstrating the anatomical landmarks (red dots indicated by yellow arrows) along the floor and roof of the fourth ventricle. Coronal (A) and sagittal MR images (B) demonstrating the location of the obex. Sagittal MR images (C and D) revealing the fastigium and aqueduct. Sagittal MR image (E) depicting the floor of the fourth ventricle and its relation to the superior medullary velum and fastigium. (This brain section is located at approximately the same level as the sagittal section of a cadaver shown in panel G.) F: The cerebellum has been cut at the midline and the left half has been sectioned further in an oblique coronal plane to show the relationship of the rostral pole of the tonsil to the dentate nucleus. The dentate nucleus appears to wrap around the tonsil. The fibers of the superior cerebellar peduncle arise in the dentate nucleus and ascend in the medial side of the superior cerebellar peduncle. The striae medullaris cross the midportion of the floor. The vestibular area, the portion of the floor that is lateral to the median eminence and the sulcus limitans, is located at the lateral limit of the floor of the fourth ventricle. The inferior fovea is a depression in the sulcus limitans located lateral to the hypoglossal triangle. The median eminence contains the facial colliculi in its upper part and the hypoglossal and vagal triangles and the area postrema in its lower portion. The median eminence is crossed by the funiculus seperans. The trochlear nerves arise below the inferior colliculus. G: Sagittal section of a hemisphere. The full length of the floor of the fourth ventricle from the inferior edge of the aqueduct to the obex is approximately 4 cm. The roof of the fourth ventricle is a tent-shaped structure. The apex of the roof, the fastigium, is the site of the roof's greatest height and width. The superior half of the roof expands posteriorly from the aqueduct to the level of the fastigium. This sagittal section corresponds to the Stealth MR image in panel E. CN = cranial nerve; Coll. = colliculi or colliculus; Tri. = triangle; Vest. = vestibular.

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    A–C: Coronal (A), sagittal (B), and axial (C) MR images of a cadaveric head obtained from the Stealth workstation, demonstrating the lateral recess (red dots indicated by yellow arrows). The point marked on all three images is identical and was obtained from the same cadaver. D and E: Photographs of another cadaveric specimen showning the stepwise dissection of the suboccipital surface of the cerebellum. Both tonsils have been retracted superolaterally and the left half of the tela choroidea has been opened (D). The choroid plexus projects from the ventricular surface of the tela choroidea into the fourth ventricle. The tela choroidea extends laterally over the inferior cerebellar peduncles and forms the posterior wall of the lateral recesses. View of structures shown in panel D (E). The tela choroidea has been retracted to the right side to expose the entire floor of the fourth ventricle and the aqueduct. The fourth ventricle is continuous with the cerebellopontine angle through the foramen of Luschka at the lateral recess. The superior half of the roof is formed medially by the superior medullary velum and laterally by the inner surfaces of the superior cerebellar peduncles. The inferior medullary velum blends into the peduncles of the flocculus laterally and the surface of the nodule medially. F: The left cerebellar hemisphere, the vermis, and the medial portion of the right cerebellar hemisphere have been removed to expose the full length of the floor of the fourth ventricle. The trochlear nerves arise inferior to the inferior colliculus. The superior and inferior cerebellar peduncles face the surface of the ventricle. The median sulcus divides the floor vertically in the midline. The sulcus limitans divides each half of the floor of the fourth ventricle longitudinally into a medial strip, the median eminence, and a lateral portion, the vestibular area. The sulcus limitans is most prominent at the level of two dimples, the superior and inferior fovea. The facial colliculus is medial to the superior fovea and median eminence. The locus ceruleus, a darkened area of cells, is located at the rostral end of the sulcus limitans. G: Anterolateral view of the brainstem and petrosal surface of the cerebellum, corresponding to the area demonstrated by the Stealth MR image in panel A. The choroid plexus protrudes from the lateral recess and foramen of Luschka behind the glossopharyngeal and vagus nerves. The trigeminal nerve arises from the midpons. The abducent nerve arises in the medial portion and the facial and vestibulocochlear nerves at the lateral end of the pontomedullary sulcus. The hypoglossal nerves arise anterior to the olives and the glossopharyngeal, vagus, and accessory nerves arise posterior to the olives. The anterior inferior cerebellar artery arises at the pontine level and courses by the abducent, facial, and vestibulocochlear nerves. The right PICA arises at the level of the vertebrobasilar junction, courses in relation to the lower cranial nerves, and passes dorsally around the medulla oblongata near the lower portion of the tonsil. H: Enlarged view of the foramen of Luschka with the glossopharyngeal and vagus nerves retracted to expose the foramen and the structures forming the lateral recess. The flocculus protrudes laterally from the inferior margin of the middle cerebellar peduncle dorsal to the facial and vestibulocochlear nerves. The rhomboid lip, a sheet of neural tissue attached to the lateral margin of the floor of the fourth ventricle, has been retracted to show the choroid plexus protruding from the lateral recess and the foramen of Luschka. A.I.C.A. = anterior inferior cerebellar artery; Bas. = basilar; Mid = middle; Plex. = plexus; Sul. = sulcus; V. = vein; Vert. = vertebral.

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    A–D: Photographs of cadaveric specimens showing the transvermian approach to the fourth ventricle. The suboccipital surface of the cerebellum (A). The oldest and most widely used surgical approach to the fourth ventricle consists of splitting the inferior vermis on the suboccipital surface. Enlarged view of some of the anatomical structures shown in panel A (B). Both tonsils have been retracted laterally to expose the inferior portion of the vermis, the pyramid, and the uvula. The incision used for the transvermian approach into the fourth ventricle is most commonly described as being through the inferior portion of the vermis (dotted line). Both PICAs course around the lower pole of the tonsil to reach the lower margin of the cerebellomedullary fissure. The inferior portion of the vermis has been incised, and the vermis and tonsils have been retracted laterally (C). The incision through the vermis exposes the underlying nodule, which must be incised to expose the inferior portion of the roof. Retraction of the two halves of the lower vermis provides approximately 1 to 2 cm of working space between the two retracted edges of the inferior vermis. Following the retraction of the vermis and tonsils laterally, it is possible to follow and work between the right and left telovelotonsillar segments of the bilateral PICAs, without sacrificing any of the major branches of the vessels. The vein of the cerebellomedullary fissure courses transversely along the inferior medullary velum. Retraction of the two halves of the nodule and the opening of the tela choroidea and inferior medullary velum through a midline incision provide access to the full length of the floor (mean length 4 cm) from the aqueduct to the obex (D). The transvermian approach provides a wide exposure of the midline superior half of the roof of the fourth ventricle and the superior medullary velum. E–H: Stealth MR images of the transvermian dissection demonstrating the superior, inferior, and lateral limits of exposure (red dots indicated by yellow arrows). The superior limit of the transvermian exposure is at the inferior edge of the aqueduct (E). The inferior limit of the transvermian exposure is at the obex (F) and the right and left lateral limits are at the level of the striae medullaris (G and H). The medial edge of the vestibular area is the lateral limit of exposure. I and J: Sagittal (I) and coronal (J) Stealth MR images obtained during the dissection, demonstrating the trajectory of the transvermian approach (yellow rectangles leading to red dots). The fastigium and the inferior portion of the superior medullary velum are the main anatomical structures that limit the working angle to the rostral one fourth of the floor of the fourth ventricle during the transvermian approach and this is demonstrated in the sagittal plane. App = approach.

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    Photographs of a cadaveric brain specimen showing the telovelar approach to the fourth ventricle. A: After the exposure of the tonsil and biventral lobule, the uvulotonsillar and medullotonsillar spaces on both sides are opened. B: The tonsil has been retracted laterally to expose the right uvulotonsillar space with special attention directed to preservation of the PICA and its bifurcation into the medial and lateral trunks. The PICA bifurcation is located near the inferior end of the uvulotonsillar space, close to the telovelar junction, in aproximately 30% of the specimens. C: The right tonsil has been retracted superolaterally and the uvula medially to expose the right uvulotonsillar space. The tip of a right-angled nerve hook has been placed beneath to the inferior medullary velum in the superolateral recess. The tip of the hook can be seen through the thin inferior medullary velum. D: The uvula has been retracted superomedially and the right tonsil superolaterally to achieve maximum visualization. The PICA bifurcation has been retracted laterally without sacrificing any of the branches of the vessel. If the PICA bifurcation is present at the uvulotonsillar space along the site of the telovelar incision, it is easier to retract it laterally before creating an incision along the tela than to retract it medially in most hemispheres. The incision along the tela choroidea in the right half of the roof has been extended to the telovelar junction. The entire floor of the fourth ventricle from the obex to the aqueduct has been exposed. E: The right half of the inferior medullary velum has been divided to expose the superolateral recess and maximize the exposure along the medial superior half of the roof and superior cerebellar peduncle. Opening the inferior medullary velum increases the exposure of the lower portion of the superior medullary velum near the fastigium. The vein of the cerebellomedullary fissure, which crosses the upper end of the inferior medullary velum, has been preserved. Retracting the right tonsil superolaterally provides access to the lateral recess and exposes the rhomboid lip. Hem. = hemispherical; Ver. = vermian.

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    Telovelar approach. Sagittal, coronal, and axial Stealth MR images of the limits of the surgical exposure (red dots indicated by yellow arrows). A: The superior limit of exposure along the floor of the fourth ventricle is the aqueduct. B: The telovelar approach provides access to the entire length of the floor of the fourth ventricle from the aqueduct to the obex, which is the inferior limit of exposure. C and D: The lateral limits of exposure along the floor of the fourth ventricle are located at the level of the striae medullaris, near the medial edge of the lateral recesses. E: Elevating the tonsils superolaterally provides exposure of the full length of the lateral recess and the foramen of Luschka at the inferolateral end of the lateral recess. F: Superolateral limit of exposure is the superolateral recess, which can be exposed only by incising the inferior medullary velum. G and H: Coronal and axial Stealth MR images obtained during the dissection demonstrating the trajectory of the telovelar approach (yellow rectangles leading to red dots). In the telovelar approach, the fourth ventricle is approached off the midline through the uvulotonsillar space by opening the tela choroidea and the inferior medullary velum lateral to the uvula.

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    Photographs of cadaveric brain specimens showing a comparison of the transvermian and telovelar exposures in a stepwise dissection. A and B: Transvermian approach. The transvermian incision directed through the inferior portion of the vermis exposes the underlying nodule, which must be incised to gain access to the fourth ventricle (A). The incision through the vermis was limited by the position of the superior medullary velum, located at the anterosuperior edge of the nodule. Retraction of the paired halves of the lower vermis provides an exposure between the edges of the incised vermis that is approximately 1 to 2 cm wide. Enlarged view (B). Following the opening of the tela choroidea and the inferior medullary velum through a transvermian incision, the full length of the floor (mean length 4 cm) from the aqueduct to the obex, can be exposed. The superior medullary velum limits the upper portion of the exposure. The transvermian approach provides better exposure of the midline portion of the superior half of the roof of the fourth ventricle. The lateral limit of the transvermian exposure extends to the striae medullaris and the medial edge of the vestibular area. C–F: Telovelar approach. The tonsils have been retracted superolaterally to expose the tela choroidea, inferior medullary velum, and both lateral recesses (C). The lateral limit of the exposure achieved is the foramen of Luschka, where the choroid plexus protrudes behind the glossopharyngeal and vagus nerves. The uvula has been retracted medially and the right tonsil laterally, and the tela and inferior medullary velum have been opened to expose the entire length of the floor of the ventricle (D). The aqueduct and the obex are the superior and inferior limits of the exposure along the floor. View of structures shown in panel D (E). The lateral limits of the exposure along the floor of the fourth ventricle are similar to those provided by the transvermian approach and located at the level of the striae medullaris, near the medial edge of the lateral recesses. Incising the inferior medullary velum provides access to the superolateral recess and most of the midline superior half of the roof of the fourth ventricle formed by the the superior medullary velum. View of another specimen (F). Both tonsils have been elevated to expose the full length of the lateral recesses. The rhomboid lip at the lateral end of the lateral recess has been exposed on the right side. The left PICA bifurcates into medial and lateral trunks at the inferior end of the uvulotonsillar space.

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    Diagrams superimposed over photographs of a section of cadaveric brain, demonstrating the comparison of operative angles of the transvermian and telovelar approaches along the floor of the fourth ventricle. The angles were measured along the floor of the fourth ventricle in the sagittal plane. The apex of the angle was placed at three sites along the floor of the fourth ventricle: the aqueduct, the midpoint (50% point) of the distance measured from the obex to the inferior border of the aqueduct, and the 75% point of that distance. A: A sagittal section demonstrating the vermian parts of the suboccipital surface of the cerebellum. The highlighted portions of the vermis are the declive (orange), folium (violet), tuber (blue), pyramid (yellow), uvula (green), and nodule (light red). The fastigium, the apex of the roof of the fourth ventricle, divides the roof into superior and inferior portions and is located between the midpoint and the 75% point, near the midsagittal level. The upper limit of the incision (dotted line) extends through the uvula, pyramid, tuber, and the lower portion of the folium to the fastigium, but not into the inferior edge of the superior medullary velum. B: The transvermian and telovelar approaches provide excellent exposure of the entire floor of the fourth ventricle from the obex to the aqueduct. The mean operative angles of the transvermian and telovelar approaches measured at the aqueduct were 38° (Angle A) and 30° (Angle B), respectively. In the sagittal plane the exposure gained at the aqueduct when the telovelar approach is used is shaded green. The mean difference between the operative angles of the two approaches at the aqueduct did not exceed 8° in favor of the transvermian approach (shaded light red). In both the transvermian and telovelar approaches, the operative angle of approach was limited by the superior medullary velum in the sagittal plane. The limitation was most significant when exposing the rostral end of the fourth ventricle near the aqueduct. The working angle decreases when moving away from the midline. In the telovelar approach, the fourth ventricle is approached off the midline, which accounts in part for the decrease in operative angle. The uvulopyramidal junction, the broadest strip of vermis within the posterior incisural space, limited the lateral and medial retractions of the tonsil and the uvula at the uvulotonsillar space, respectively. C: The mean operative angles of the transvermian and telovelar approaches measured at the 75% point were 72° (Angle A) and 67° (Angle B), respectively. In the sagittal plane, the exposure at the 75% point for the telovelar approach is shaded green. The mean difference in the working angle gained through the transvermian approach at the 75% point is shaded light red. The lower edge of the superior medullary velum limited the working angle at the 75% point in both approaches and the uvulopyramidal junction accounts for the decrease in the operative angle in the telovelar approach. D: The operative angles of the transvermian and telovelar approaches measured at the midpoint along the length of the floor of the fourth ventricle were identical, 84° (Angle A). At the midpoint, the working angle gained in the sagittal plane is shaded blue. At the midpoint along the floor of the fourth ventricle, the working angle increased dramatically compared with the working angle at the aqueduct.

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