ORIGINAL ARTICLE

Endoscopic telovelar approach to the fourth ventricle:anatomic study

Antonio Di Ieva & Mika Komatsu & Fuminari Komatsu &Manfred Tschabitscher

Received: 18 May 2011 /Revised: 24 August 2011 /Accepted: 8 October 2011 /Published online: 15 December 2011# Springer-Verlag 2011

Abstract The telovelar approach allows reliable access to thefourth ventricle and avoids the splitting of the vermis and itsassociated posterior vermal split syndrome. Our objectivewas to describe the endoscopic topographical anatomy of thetelovelum approach to the fourth ventricle as accessed by thecerebellomedullary corridor. A series of 20 fresh and fixedinjected anatomical specimens were used. The endoscopicequipment consisted of rigid endoscopes with different lensangles, while the extradural step required the use of themicroscope and/or the exoscope. All the anatomical land-marks and relationships within the fourth ventricle and thecerebellomedullary fissure were identified by means of theendoscopic microscope/exoscope-assisted telovelar approach.In conclusion, we showed that the endoscope is a valid tool togain an anatomic understanding of the fourth ventricle reachedby means of the telovelar approach.

Keywords Anatomy . Cerebellomedullary fissure .

Endoscopy. Exoscopy . Telovelar approach . Fourth ventricle

Introduction

In the last few years, the telovelar approach has been dem-onstrated as a reliable approach to access the fourth

ventricle. This method avoids splitting the vermis or remov-ing part of the cerebellum [26, 27, 29, 31, 37], thereforeavoiding the associated posterior vermal split syndrome[2, 5, 9, 19]. The anatomical key to this approach is thetelovelum, the sheet formed by the tela choroidea and theinferior medullary velum that covers the lower part of theroof of the fourth ventricle. The telovelum can be reachedand explored also endoscopically through the cerebellome-dullary fissure, the natural cleft between the tonsils, thevermis, and the medulla. Within this study, we describethe endoscopic topographical anatomy of the telovelumapproach to the fourth ventricle accessed via the cerebello-medullary corridor.

Materials

This anatomic study was performed in the Microsurgicaland Endoscopic Laboratory of the Department of SystematicAnatomy at the Medical University of Vienna, Austria. Aseries of 20 fresh and fixed anatomical specimens wereused. The arteries were injected with red silicon; in somespecimens the veins were also injected with blue silicon.One specimen was not injected in order to study thesyntopy of the telovelum, focusing on its endoscopicrelationship with the surrounding anatomical structures.The endoscopic equipment consisted of 2.7- or 4-mm-diameter rigid endoscopes with various viewing angles(0, 30, 45, and 70) (Karl Storz Endoscopy, Tuttlingen,Germany). For the extradural macroscopic step, an exoscopewas used (VITOM SPINE, model E10511, Karl Storz,Tuttlingen, Germany) fixed to a mechanical holder 2040 cm far from the field. The exoscope was left in situ for thedescribed maneuvers so that the intracranial steps wereexoscope assisted. In some cases, an operative microscope

A. Di Ieva (*) :M. Komatsu : F. Komatsu :M. TschabitscherCenter for Anatomy and Cell Biology, Department of SystematicAnatomy, Medical University of Vienna,Waehringerstrasse 13,1090 Vienna, Austriae-mail: diieva@hotmail.com

A. Di IevaDepartment of Neurosurgery, Medical University of Vienna,Vienna, Austria

Neurosurg Rev (2012) 35:341349DOI 10.1007/s10143-011-0371-0

was used for the extradural step and to follow the endoscopicprocedure, even if the exoscope was preferred for the imageacquisition, in a special way because it was fixed far enoughfrom the operative field to not interfere with the surgicalmaneuvers. For the illumination, we used a 300-W xenonfiber optic light source (Xenon Nova300, Karl Storz Endos-copy, Tuttlingen, Germany). A digital high-definition (HD)video camera with a camera control unit was used tovisualize the images on an HD wide flat screen (twomillion pixels). The AIDA compact HD System (Karl Storz,Tuttlingen, Germany) was used to record the images andvideo sequences.

Methods

The heads were positioned to achieve a surgical suboccipitalapproach. Under microscopic or exoscopic magnification, a3-cm midline skin incision was made above the craniocer-vical junction. The dissection was performed by splitting themuscles mediolaterally in order to access and expose the

inferior portion of the occipital squama, the inferior edge ofthe foramen magnum, the craniocervical junction, the atlan-tooccipital membrane, and the posterior arch of the atlas(Fig. 1a). Inserting the craniotome into the inferior edge ofthe foramen magnum, a 2-cm occipital craniotomy wasperformed (Fig. 1a). Removing the posterior arch of theatlas was never considered necessary. After opening andremoving the atlantooccipital membrane, the dura materoverlying the craniocervical junction was exposed and in-cised longitudinally for approximately 2 cm (Fig. 1b), alsoto create the space to insert the spatulas for the eventualretraction of the cerebellar tonsils. Through the durotomy itwas possible to insert various angled endoscopes (0, 30, 45,and 70) into the cisterna magna (Figs. 1c, d and 2e). Theendoscopic approach to the cisterna cerebellomedullaris(cisterna magna) and the fourth ventricle was microscopeor exoscope assisted. The exoscope was often used for theacquisition of the video images of the endoscopic procedure(Fig. 1d). After inspection of the cisterna magna, the arach-noid was removed to approach the fourth ventricle via amedial transcysternal route [30, 33] (Fig. 1e, f). To visualize

Fig. 1 Exo-endoscopic approach to the fourth ventricle. a Exoscopicsuboccipital approach to the median craniocervical junction, withexposure of the occipital bone, the posterior arch of the atlas, and theatlantooccipital membrane. The dotted circular lines show the site ofthe craniotomy. b Opening the dura mater in the C0-C1 region and thearachnoid of the cerebellomedullary cistern (cisterna magna). Lateralreflection of the meningeal layers and exposure of the medulla andcerebellar tonsils is also demonstrated. c Introduction of the endoscopeonto the field. d The endoscopic procedure can be exoscope assisted,

as shown in the picture. Introduction of the tip of the endoscope intothe foramen of Magendie by the transforaminal median route. e Theintroduction in the foramen of Magendie of an endoscope with a 70-angled optic oriented towards the roof of the fourth ventricle allowsvisualization of the ventricular surface of the tela choroidea with therelated choroidal plexi and choroidal arteries. f The transforaminalapproach allows the visualization of the rhomboid fossa and the floorof the fourth ventricle

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the roof of the fourth ventricle and the telovelum, the fol-lowing methods were used: (a) mediolateral displacement of

the cerebellar tonsils using spatulas and (b) tonsillar resec-tions, even if the latter was used only to show the anatomic

Fig. 2 The cerebellomedullary fissure, the fourth ventricle and thesynthopy of the telovelum, demonstrated via exoscopic anatomicaldissection. a The elevation of the tonsil reveals the attachments ofthe tela choroidea (arrows), which borders the medial sides of thefourth ventricle and continues superiorly in the arachnoid coveringthe mesial surfaces of the cerebellar tonsils. b The vallecula can besplit to allow access to the inferior vermis. Here the superior part of thearachnoid that form the tela choroidea, like a tent (showed by thearrows), is visible covering the mesial surfaces of the tonsils. c Thepartial resection of the right tonsil reveals its relationships with thecerebellar biventral lobe and with the telovelum, allowing visualizationof its anatomical affiliations (nidus avis, marked with an asterisk). dThe complete resection of the tonsil allows visualization of the retro-and sovratonsillar segments of the PICA. e The introduction of theendoscope in the fourth ventricle allows, by means of transillumina-tion, the visualization of the attachments of the tela choroidea inferiorly

and superiorly to the telovelar junction. f The removal of both tonsilsand the complete resection of the telovelum allow inspection of thefourth ventricle structures. The four corners of the rhomboid fossa arevisible (obex/foramen of Magendie, opening of the aqueduct of Syl-vius, and the two lateral recesses with the foramen of Luschka). Thestriae medullares divide the floor in a superior pontine triangle and aninferior medullary triangle, which ends in the calamus scriptorius. gThe inferior triangle's base is on the striae medullares and the vertex ison the obex. In this location, one can visualize the funiculus separans, athickening of the ependyma bordering the area postrema, and theligula, which is the medullary insertion of the membrana tectoria. ctcuneate tubercle; Fac. Col. facial collicus; Fovea inf. fovea inferiorwith the vagal trigone overlying the dorsal motor nucleus of the vagusand the nucleus of the solitary tract; gt gracile tubercle (clava); Med.vestib. area medullary vestibular area; St. med. striae medullares; Trig.Hypog trigonum of the hypoglossal nerve

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relationships but not considered strictly necessary for theendoscopic telovelar approach.

Results

The introduction of the endoscope into the cisterna magnaoffers a panoramic view of the inferomedial part of thecerebellar hemispheres, the inferior portion of the vermis,the branches of the posterior inferior cerebellar artery (PI-CA), the posterior cerebellar and medullary veins, the cer-ebellomedullary fissure, and the foramen of Magendie(Figs. 1c and 2bd). A preliminary inspection of the fourthventricle can be performed by means of the classic medialtranscysternal route [30, 33]; the introduction of the endo-scope into the foramen of Magendie gives a wide panoramicview of the fourth ventricle (Fig. 1e, f). The endoscope isintroduced into the foramen of Magendie, obtaining a widepanoramic view of the floor of the rhomboid fossa, thesuperior velum, the superior and middle cerebellarpeduncles, the opening of the aqueduct in the fourth ventri-cle, and the interior aspect of the telovelum with the relatedchoroid plexus. The insertion of the angled endoscope (par-ticularly 45 and 70) into the foramen of Magendie allowsvisualization of the ventral (ventricular) surface of the telachoroidea where it is possible to identify the choroid plexusand its related vessels (Fig. 1e). A broad panoramic view ofthe fields can be obtained by rotating the scope on its axis.

The lateral displacement of the tonsils (unilaterally orbilaterally) allows for endoscopic navigation of the cerebel-lomedullary fissure (Fig. 2b). The distance of the displace-ment ranged from 0 to 10 mm, depending on the anatomicalconditions (e.g., the size of the tonsils and the intertonsillardistance). In six cases (30%), in fact, the intertonsillar dis-tance was inferior to 5 mm, and it was necessary to displacethe tonsils 1 cm laterally, in order to achieve the cerebello-medullary fissure. In ten cases (50%), the required displace-ment was less than 5 mm, while in four cases (20%) theintertonsillar distance was quite wide (10 mm), requiring nolateralization. The introduction of the endoscope in themedullotonsillar space allowed the exploration of the cere-bellomedullary fissure and tela choroidea. The tela choroi-dea was cut unilaterally or bilaterally by starting the incisionat the level of the foramen of Magendie and following itlaterally to the foramen of Luschka. The inferior margin ofthe tela choroidea was found at a distance from the obexranging 212 mm, forming in the middle the foramen ofMagendie (average diameter, 5 mm). Opening the telovelumbilaterally provided complete access to the floor and body ofthe fourth ventricle, from the aqueduct to the obex, rostro-caudally, reaching the lateral recesses mediolaterally(Fig. 2f). Opening the telovelum unilaterally allowed theintroduction of the endoscope and the opportunity to

visualize, by means of the 30, 45, and 70 angled scopes,the contralateral structures (Fig. 3f). To achieve good visu-alization of the complete intraventricular structures, it wasdeemed unnecessary to cut the telovelar junction, limitingthe resection to the lateral attachments of the tela choroidea.Moreover, a partial opening of the tela choroidea was con-sidered sufficient to introduce and move the endoscope andany surgical tools within the fourth ventricle (Fig. 3).

The endoscopic panoramic view of the cerebellomedul-lary fissure makes it possible to observe the branches of theposterior cerebellar arteries, the foramen of Magendie, theobex, and the tela choroidea (Figs. 1b, c and 2ad). Themedial anatomic landmarks are the pyramid and the uvula(inferior portion of the vermis) in the posterior cerebellarincisura (Fig. 1d). The nodule of the vermis faces the lowerhalf of the roof of the fourth ventricle. The vallecula is theinferior continuation of the posterior cerebella fissure, sep-arating the two tonsils. The depth of the vallecula dependson the anatomical conformation of the tonsilsvirtual insome cases (very tied tonsils) or very widewhen themesial tonsillar portions face each other at a distance ofabout 1 cm. The resection of the tonsils reveals the so-called nidus avis (bird's nest) [35] and the relationshipsbetween the tonsillar faces and the inferior medullary velum,tela choroidea, uvula and biventral lobules, in the spacecalled the telovelotonsillar cleft, where the portions of thePICA (retrotonsillar and subtonsillar portions of the telove-lotonsillar segment of the PICA) run (Fig. 2c, d, f).

Starting at the foramen of Magendie, the taenia along theinferior cerebellar peduncles can be cut to reach the lateralrecess, on a single side or bilaterally (Fig. 2f). The telachoroidea can be elevated rostrally (unilaterally or bilateral-ly) in order to obtain access to the fourth ventricle; thearachnoid of the vermian cistern can be dissected, splittingthe uvulotonsillar cleft.

The telovelum is formed by two thin, membranouslayers: the tela choroidea and the inferior medullary velum[17, 20, 31, 32]. The site where the two membranes attach toeach other is the telovelar junction (Fig. 2e). The cranialextension of the tela choroidea is the uvulotonsillar space;according to Matsushima's definition [27], this space is atent formed by two layers of arachnoid, which cover themesial and posterior faces of the tonsils. In some cases,when the tonsil was tied tightly, the vallecula became avirtual space and these two arachonidal layers seemed tobe fused in a kind of tonsillar falx. However, carefuldissection always revealed the existence of the two separat-ed layers, covering the mesial tonsillar surfaces, reachingrostrally to the uvula and continuing into the arachnoidcovering the pyramid (Fig. 2a, b). On the ventricular surfaceof the tela choroidea, one can recognize the choroid plexuswith its associated choroidal arteries, which originate fromthe supratonsillar segment of the PICA (Fig. 1e) [11]. Some

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veins originate from the lateral edge of the nodule anduvula, crossing the telovelar junction and taking some out-flows from the telovelum [28].

The entire floor of the fourth ventricle is visible in itstypical rhomboid shape. The four corners of the rhom-bus, formed by the opening of the aqueduct proximally,the foramen of Magendie caudally and medially, and thetwo foramina of Luschka laterally are endoscopicallyvisible (Fig. 3). The median sulcus divides the floor intotwo symmetrical halves; this sulcus serves as a good cor-ridor to follow when advancing the endoscope along thefloor of the fourth ventricle. It is also a very visible andconstant landmark to be used to avoid losing orientationintraventricullary, especially when the angled optics areused (Figs. 2f, g and 3ae). The striae medullares (the dorsalpontocerebellar fibers and parts of the dorsal acoustic striae)[32] are very well visualized in their mediolateral variablecourse; in our specimens we found an average of 52 striaeon each side, in accordance with some previously publisheddata (Fig. 2f) [45]. The striae divide the floor into a superiorpontine triangle and an inferior medullary triangle, whichends in the calamus scriptorius. The striae medullares are theinferior triangle's base while the obex is the vertex of the

inferior triangle; here the funiculus separans, a thickening ofthe ependyma bordering the area postrema, is visible(Fig. 2g). The median sulcus is easily visible crossing me-dially in the triangle, bordered paramedially by the twopaired sulci limitantes (Figs. 2g and 3e). The sulcus limitansis discontinuous; it is more prominent in the pontine andmedullary portions of the floor. The points in which thesulcus limitans deepens correspond to two endoscopicallyvisible depressions: the superior and the inferior fovea(Fig. 3b, e) [31]. Lateral to the sulci limitans, the endoscopedemonstrates the vagal trigone (corresponding to the dorsalmotor nucleus of the vagus and the nucleus of the solitarytract) and its lateral depression, the fovea inferior, in theinferior portion (Figs. 2g and 3e). More laterally, the vestib-ular area is visualized, corresponding to the vestibular nuclei(Fig. 3c, f). The visible prominence in this area is theauditory tubercle, which overlies the dorsal cochlear nucleusand the cochlear part of the vestibulocochlear nerve. Thedark, triangular field between the trigonum hypoglossi andthe lower part of the area acoustica is the ala cinerea,corresponding to the sensory nucleus of the vagus andglossopharyngeal nerves. At the lower end of the ala cinera,the funiculus separans is visible as a narrow, translucent

Fig. 3 Endoscopic telovelarapproach. By using an appropri-ate insertion point for theendoscope, angle of visualizationand angled lens, it is possible tovisualize the anatomical struc-tures within the fourth ventricle.AS Opening of the aqueduct ofSylvius; ChPl choroid plexus;FColl facial colliculus; inf CPinferior cerebellar peduncle;infFov inferior fovea; LR lateralrecess; me medial eminence;mVA medullary vestibular area;ms median sulcus; pSL parame-dian sulcus limitans; pVApontine vestibular area; SMVsuperior medullary velum; supCP superior cerebellar peduncle;supFov superior fovea

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ridge; more inferiorly, closely related to the clava, there isthe area postrema.

The sulcus medianus continues in the superior triangle,ending at the superior angle of the rhomboid fossa where theopening of the aqueduct is visible (Fig. 3). The sulcus limi-tans divides the superior triangle in a similar fashion with amediana area, where the median eminence (containing thefacial colliculus) and a lateral pontine vestibular area. Agood landmark for endoscopic navigation through the lateralrecesses is the superior fovea (fovea trigemini), which cor-responds to the motor trigeminal nucleus (Fig. 3b). Moresuperiorly, at the upper end of the sulcus limitans, lies thelocus coeruleus although it is not endoscopically (or even bythe operating microscope) identifiable.

On the roof of the fourth ventricle lies the superiormedullar velum medially and the cerebellar pedunclesmore laterally, each one separated by a very well-defined sulcus: the superior cerebellar peduncle mediallyand the inferior cerebellar peduncle laterally (Fig. 3). Thedentate tubercle, formed in the fourth ventricle by the im-pression of the overlying dentate nucleus, is visible in thesuperolateral portion of the roof (Fig. 3e). The lateral por-tions of the cerebellar peduncles are very well visualizedusing the angled endoscopes. The angled lenses, especiallythe 45 and 70 lenses, permit visualization of the lateralrecesses to the foramina of Luschka, which open in theinferior cerebellopontine cisterns. If the tela choroidea isopened only on one side, the introduction of the endoscopewith the angled lens allows for visualization of the contralateralrecesses (Fig. 3f).

Discussion

It is known that the cerebellomedullary fissure is a usefulroute by which to approach the fourth ventricle and avoidsplitting the vermis [6, 16, 18, 19, 26, 27, 31, 43, 44, 46].The opening of the tela choroidea and eventually the inferiormedullary velum (telovelar approach) allows for completevisualization of the fourth ventricle also including the mostproximal and lateral portions, from the aqueduct to thelateral regions around the lateral recesses. This has beendemonstrated in several previous surgical reports and inqualitative and quantitative anatomic studies [8, 19, 26,27, 31, 36, 37, 40].

The fourth ventricle can also be approached endoscopi-cally. The paradigm of modern neurosurgery, to offer ther-apies by means of minimal surgery and less traumaticapproaches, has developed into the philosophy of minimallyinvasive neurosurgery [9, 33, 34] or, more appropriately,minimally traumatizing neurosurgery [42]. It should beemphasized that endoscope is an additional tool which canbe used in neurosurgical procedures as well in anatomical

studies, even if no randomized controlled studies have neverbeen performed to prove its clinical advantages when com-pared to the operations performed by means of microscope.

In the 1990s, the first reports detailing endoscopic caudalexploration of the fourth ventricle and the aqueduct werepublished [3, 15, 30, 33, 39]. Subsequently, other surgical oranatomical reports were published detailing the anatomy ofthe fourth ventricle as visualized by the endoscope using atransaqueductal approach [10, 13, 22, 23, 38, 41].

From the first anatomic reports that the fourth ventricleand aqueduct could be approached endoscopically, not onlyfrom the third ventricle but also through a tailored cranio-cervical approach [30, 33], other applications of this surgicalapproach were investigated. In some selected cases, thisendoscopic approach has been used in the management ofobstructive hydrocephalus due to aqueduct obstruction, re-establishing the free communication between the third andfourth ventricle [4, 14, 41]. However, this approach via theforamen of Magendie limits the introduction and manipula-tion of other surgical tools within the lateral recesses of thefourth ventricle; these limits are overcome with the telovelarapproach. Matsushima et al., who pioneered the techniqueof the telovelar approach, emphasized that a detailed under-standing of the anatomy of the fissure and its surroundingsis required to perform this kind of approach [26, 27].

The microsurgical exploration of the fourth ventricle bymeans of the telovelar approach can also be performedendoscopically. We chose to demonstrate this for two rea-sons: (a) to show the feasibility of the technique and (b) tooffer a different perspective on the same anatomical struc-tures, improving the anatomical orientation of the surgeonswho perform this type of surgery in this very complexregion. It is important to emphasize that this manuscriptdoes not advocate for the superiority of the endoscopicapproach over the microsurgical method. In the best scenario,endoscopic assistance in the microsurgical treatment oflesions involving the fourth ventricle could allow visualizationaround corners including those opposite to the microsur-gical field, allowing a less invasive and traumatic sur-gery. The topographical relationships between theanatomic structures and landmarks are fundamental inneuroendoscopy. Working in an anatomical training labis essential to develop a sense of spatial orientation; sucha lab allows the surgeon to compare different images ofthe same anatomical area and to form a three-dimensionalmental image of the fourth ventricle. This type of training isvery helpful in understanding pathoanatomic topography.There is no microsurgical or endoscopic anatomy: theanatomy stays the same although it is visualized differentlythrough a microscope and an endoscope. It is important todevelop spatial orientation to understand surgical anatomy,bearing in mind that the apparent differences in the samestructures when they are seen laterally or medially, rostrally

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or caudally, or through the operative microscope or through anendoscope [42].

In our study, we found a large variability in the insertionpoints of the inferior limits of the tela choroidea. As dem-onstrated by Barr in 1948 [1], the outlets of the fourthventricle have a highly variable shape and dimension. Atthe level of the foramen of Magendie, the different inser-tions of the caudolateral margins of the telovelum couldjustify this variability. Thus, even if no neurologic deficitsare associated with the opening of the telovelum of thefourth ventricle, in cases in which the foramen of Magendieis very small, a wide resection of the attachment of the telachoroidea is required for the introduction and manipulationof the endoscope and any surgical tools. In cases in whichthe foramen is very wide, the resection of the tela can belimited. The opening of the tela choroidea on the lateralmargins allows the introduction of the endoscope tovisualize all of the intraventricular anatomical structures.The additional opening of the inferior medullary velumoffers a more cranial visualization of the fourth ventricle,even if it is not generally required. It is interesting thatthe sense of three dimensionality in the two-dimensionalendoscopic images can be suggested by the shadowsand intensity of colors caused by the protuberances anddepressions, as on the floor of the fourth ventricle at thelevel of the foveae (Fig. 3b, e).

The intracranial and intraventricular endoscopic approachcan be microscope or exoscope assisted. It has recently beensuggested that exoscopes could be used in neurosurgicalprocedures, particularly in spinal operations [24, 25].Exoscopes are telescopes that, like microscopes, are notintroduced into the surgical field. They offer very highquality images with very good illumination and, as theyare fixed to a holder remote from the operating field, theydo not interfere with the handling of surgical instruments.However, their technical limitations include the fact that theimages are focused centrally (and are therefore less focusedperipherally) and, like endoscopes, they lack true stereopsis.We have recently begun using exoscopy for anatomicaldissections and image acquisition [7, 42], suggesting itsuse especially in anatomical laboratories where the purchaseof an operative microscope is limited by some economicalreasons.

A relative advantage of the endoscopic telovelar approachis that it requires less space than the normal microscopicapproach so that the opening of telovelum, the resection ofsome choroidal vessels, and the lateralization of the tonsils canbe limited. The latter is very important as excessive stretchingof the tonsils can cause a compression on the related dentatenuclei and cerebellar peduncles. It is known that the neuro-logic deficits related to dentate nuclei injury are disturbancesmuch more serious than the ones caused by vermis splitting[12, 21]. Moreover, the craniotomy performed in our study

was relatively small and no specimens required laminec-tomy of the atlas.

Using the described approach, the visualization of theanatomical structures of the fourth ventricle is optimal andthe space to maneuver surgical tools seems to be adequate.However, the surgeon must recall that the movements of thetip of the endoscope within the fourth ventricle should be veryaccurate; a soft pressure of the tip of the endoscope againstthe anatomical structures can cause injuries, particularly onthe protruding structures (e.g., cerebellar peduncles, dentatetubercle, and facial collicus).

Conclusion

The natural cleft of the cerebellomedullary fissure, betweenthe vermis, the tonsils, and the medulla, can be the corridorfor the introduction of the endoscope into the fourth ventri-cle by means of the telovelar approach, allowing completevisualization of the ventricular cavity without splitting thevermis. Considering that the telovelar approach is recom-mended for the treatment of lesions occupying the cerebel-lomedullary fissure and the fourth ventricle, especially itslateral recesses, the endoscope can be a valid tool for gain-ing a better anatomic understanding of this complicatedneuroanatomic region; it may also be a potential tool to beused for microsurgical endoscope-assisted operations.

Acknowledgments The authors wish to thank the FMEA (Societyfor the Promotion of Research inMicrosurgical and Endoscopic Anatomy)for paying the costs related to this research.

Disclosure The authors have no personal financial or institutionalinterest in the devices described in this article.

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Comments

Toshio Matsushima, Saga, JapanThe authors studied the surgical anatomy of the fourth ventricle

through the telovelar approach using endoscopy. We call this approachtranscerebellomedullary fissure approach.

However, surgical indications using this method will be limited. Itseems difficult to remove large fourth ventricular tumors such asmedulloblastomas or ependymomas by this method. This endoscopic

348 Neurosurg Rev (2012) 35:341349

less invasive surgery seems good for small tumors or biopsy, especiallylesions around the fastigium and lateral recess, which are difficultregions for observation in the microsurgery. As the authors mention,this endoscopic surgery had better be performed with microsurgery.

The authors point out that excessive stretching of the tonsils cancause a compression on the related dentate nuclei and cerebellarpeduncles. However, in our experiences of microsurgery the retractionof the tonsils did not cause any neurological deficits when the cere-bellomedullary fissure is sufficiently opened. This is a well-writtenpaper on endoscopic study with detailed description and will highlycontribute to the surgical treatment of the fourth ventricular lesions inthe future.

Dattatraya Muzumdar, Mumbai, IndiaDi Ieva et al. report a cadaveric study in 20 fresh and fixed injected

anatomical specimens exploring the endoscopic anatomy of the fourthventricle through a telovelar approach. They conclude that the endo-scope can be a valid tool for gaining a better anatomic understanding ofthis complicated neuroanatomic region; it may also be a potential toolto be used for surgical endoscopeexoscope-assisted operations. Themanuscript is well written, elaborate, and informative. The dissectionsare noteworthy and reveal the finer aspects of the anatomy. Microsur-gical anatomy of the fourth ventricle is described in the literature butthere are no elaborate articles about the use of endoscope in this area.They discuss the limitations of the study was well.

Fourth ventricle is a limited space harboring critical neurovascularstructures as well as the brain stem. The anatomy is complex and thelesions occurring in this limited space are formidable. Exploration ofthe fourth ventricle through the telovelar approach using the endoscopecan be an aid or adjunct to the resection of the complex tumors in thisregion. The telovelar approach allows reliable access to the fourth

ventricle. It avoids the splitting of the vermis which is usually associ-ated with cerebellar mutism. Endoscopic telovelar approach is truly amimimally invasive surgery since it requires less space than the normalmicroscopic approach and limits the opening of telovelum. A digitalHD video camera provides a panoramic view of the fields allowing forcomplete visualization of the fourth ventricle including the most prox-imal and lateral portions, from the aqueduct to the lateral regionsaround the lateral recesses.

Anatomical laboratory training in endoscopy is indispensable andparamount to develop a sense of spatial orientation. The laboratoryallows the surgeon to compare different images of the same anatomicalarea allowing formation of a three-dimensional mental image of thefourth ventricle. It is very helpful in understanding pathoanatomictopography. As a futuristic procedure of minimal access surgery andless traumatic approaches, it can be correctly termed as minimallyinvasive neurosurgery or minimally traumatizing neurosurgery.

Richard Lochhead, Robert F. Spetzler, Phoenix, USADi Ieva et al. present a cadaveric anatomical study entitled Endo-

scopic Telovelar Approach to the Fourth Ventricle: Anatomical Study.The authors use a rigid endoscope with different lenses to describe thefourth ventricular anatomy via the telovelar approach. They discussvarious endoscopic approaches to the fourth ventricle with endoscopicphotographs to demonstrate the fourth ventricular anatomy that can bevisualized with minimal dissection and no brain retraction. This articleaddresses a need for increased understanding of fourth ventricularanatomy through minimally invasive techniques, and the authors areto be commended. Further work in this field may include quantificationof the endoscopic access through the different approaches to improveunderstanding of the anatomical landmarks that can be visualized ineach approach without tissue retraction.

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Endoscopic telovelar approach to the fourth ventricle: anatomic studyAbstractIntroductionMaterialsMethodsResultsDiscussionConclusionReferences