OPERATIVE TECHNIQUE Infradentate Approach to the Fourth Ventricle Ali O. Jamshidi, MD BACKGROUND: The use of minimally invasive transcranial ports for the resection of deep- Blake Priddy, BS seated lesions has been shown to be safe and effective for supratentorial lesions. The Andre Beer-Furlan, MD routine use of this surgical modality for posterior fossa masses has not been well estab- Downloaded from https://academic.oup.com/ons/article/16/2/167/5057030 by OUP site access user on 03 February 2021 Daniel M. Prevedello, MD lished in the literature. In particular, fourth ventricular tumors are not the typical target for neuro-port surgery because of potential injury to the dentate nucleus. Department of Neurological Surgery, The OBJECTIVE: To describe the use of a tubular retractor system to reach the fourth ventricle Ohio State University Medical Center, while sparing the cerebellar vermis and the dentate nucleus. Three cases illustrations are Columbus, Ohio presented. Correspondence: METHODS: Surgical access to the fourth ventricle was developed sparing the cerebellar Daniel M. Prevedello, MD, vermis and the dentate nucleus. The authors reviewed 3 cases to illustrate the feasibility Wexner Medical Center at The Ohio State of minimal access transcerebellar port surgery for the resection of these lesions using an University, Department of Neurological Surgery, infradentate access. N-1049 Doan Hall, RESULTS: None of the patients developed new neurological deficits and the pathology th 410W10 Avenue, was successfully resected in all cases. There were no major complications related to surgery Columbus, OH 43210-1240. E-mail: [email protected] and no mortalities. CONCLUSION: The infradentate approach obviates the need for traditional approaches Received, November 1, 2017. to the fourth ventricle, thus making this challenging target in the posterior fossa more Accepted, July 1, 2018. Published Online, July 21, 2018. accessible to neurosurgeons. The authors observed successful removal of lesions involving the fourth ventricle while avoiding any associated morbidity or mortality. Copyright C 2018 by the KEY WORDS: Fourth ventricle, Infratentorial, Posterior fossa tumors, Transcerebellar, Port surgery, Telovelar Congress of Neurological Surgeons approach, Minimally invasive, Vermis, Dentate nucleus Operative Neurosurgery 16:167–178, 2019 DOI: 10.1093/ons/opy175 ntoward injury to the deep cerebellar technique to access the fourth ventricle without nuclei, particularly the dentate, can transgressing the vermis or the dentate nuclei. U increase morbidity associated with the surgical management of cerebellar and fourth ventricular tumors. Furthermore, resection of Vermis these masses can be complicated by cerebellar The vermis is posterior to the fourth ventricle. mutism in the postoperative period. The It is formed by 4 structures oriented from incidence of this complication is most common superior to inferior: the folium, tuber, pyramid, in the pediatric population after surgical and the uvula as well as the nodule, which treatment of medulloblastoma; however, this is found anterior to the uvula. The prepyra- syndrome is not unique to this population.1,2 midal fissure splits the vermis into superior and Many authors believe that splitting of the vermis inferior halves between the tuber and pyramid. places patients at risk for this type of mutism, The fastigial nucleus rests in the anterior portion which is characterized by its typical self-limited of the pyramid forming the apex of the fourth course and possible persistent dysarthria.3 In ventricle roof. The dentate is located in the order to avoid this morbidity, the authors superolateral portion of the fourth ventricle. describe their use of a minimally invasive port Historically, neurosurgeons accessed the fourth ventricle by removing a segment of the cerebellar hemisphere or dividing the vermis3.Asaresult ABBREVIATIONS: EVD, external ventricular drain; of the morbidity related to these approaches, MRI, magnetic resonance imaging; PICA, posterior strategies that maintain the integrity of these inferior cerebellar artery structures have developed. OPERATIVE NEUROSURGERY VOLUME 16 | NUMBER 2 | FEBRUARY 2019 | 167 JAMSHIDI ET AL Dentate Nucleus of the tela choroidea, which is the foramen of Magendie. The Because the dentate receives afferent connections from the opening of the fourth ventricle begins at the foramen of Magendie supplemental motor area, this nucleus is involved with the coordi- and extends laterally through the tela choroidea towards the nation, initiation, and timing of volitional movement as well lateral recess rather than superiorly toward the inferior medullary as balance and posture.4 Additionally, connections have been velum in order to avoid disconnecting the nodule from the discovered between the dentate nucleus and the premotor, oculo- flocculus. motor and posterior parietal areas of the cerebral cortex.4 The At the level of the fastigium, the inferior medullary velum vermis has dorsal and ventral components related to motor and blends medially into the superior medullary velum, which forms nonmotor functions, respectively. the medial portion of the rostral half of the fourth ventricle roof. The vermis and the cerebellar peduncle bundle encompass the The lateral portion of the superior fourth ventricle roof is formed Downloaded from https://academic.oup.com/ons/article/16/2/167/5057030 by OUP site access user on 03 February 2021 3 dentate and have important associations. The pyramid’s extension by the superior and inferior cerebellar peduncles. Opening the laterally to the superolateral recess is a reliable surgical landmark superior medullary velum further exposes the floor and lateral that leads to the dentate nucleus. The inferior cerebellar peduncle recess of the fourth ventricle; the nuclei for the sixth and eighth has fibers that pass anteriorly and superiorly to the dentate and cranial nerves are embedded in the floor at this level. The dentate carry dorsal spinocerebellar, cuneocerebellar, olivocerebellar, and nucleus forms an impression in the superolateral recess of the roof, vestibulocerebellar pathways.4 The middle cerebellar peduncle known as the dentate tubercle, which is found superior and lateral has 2 sets of fibers that pass over the dentate. The first group to the superior vestibular nucleus. is the pontocerebellar fibers that run parallel to the dentate and There are situations that the telovelar approach is not possible the second group is the corticocerebellar fibers that travel parallel due to anatomic variations or peculiarities. For example, a branch to the midline.4 The superior cerebellar peduncle connects of the posterior inferior cerebellar artery (PICA) can divide early the dentate to the red nucleus via the dentatorubrothalamic to supply the vermis. This branch can hinder the possibility of tract, decussates, and ultimately projects to the ventrolateral full exposure of the fourth ventricle by obstructing visualization thalamus; disruptions of these connections are thought to cause of the tela choroidea. Patients who have had previous surgery the symptoms of cerebellar mutism.4 In order to avoid neuro- or radiation can have prohibitive scar overlying the foramen of logical morbidity, the surgeon must take every precaution to Magendie limiting direct dissection into the forth ventricle as respect the integrity of the vast network of white matter tracts well. that run along the dentate. Minimally Invasive Port Surgery The neurosurgeon should understand the relationship of the dentate to readily identified structures along the suboccipital Port technology has been promoted in order to reduce potential surface. The medial aspect of the dentate is approximately operative morbidity related to the resection of intra-axial lesions. 10.9 mm superior from the tip of the tonsil; the supratonsillar area This modality has been in existence since the 1980s when cylin- where the tonsil is attached to the cerebellum is 0.6 mm lateral to drical retractors were used instead of retractor blades because less 5 the dentate.4 damage was done to the surrounding cortex. This technique has since gained popularity, particularly for accessing deep-seated lesions.6 The port approach for infratentorial pathologies is not SURGICAL APPROACHES TO THE FOURTH well established in the literature. However, there are published VENTRICLE cases describing the use of ports for the resection of tumors from the cerebellar peduncles.7 Telovelar Approach In the authors’ opinion, the main advantage of the cannulated The telovelar approach is favored to access tumors of the tubular system is the cylindrical shape of the port that allows for fourth ventricle because it provides a corridor spanning from tissue to be displaced around the tubes. The minimal disruption the obex to the aqueduct of Sylvius and between the paired of brain tissue using this system has been supported by postoper- foramina of Luschka, while avoiding injury to the deep cerebellar ative magnetic resonance imaging (MRI) findings, which show nuclei. The first step of this approach is directed through minimal radiographic changes; similar findings are seen in the the cerebellomedullary fissure, which is the cleft that allows described cases.6 access to the vermis and the roof of the fourth ventricle after the medial tonsillar surfaces are lateralized. After the tonsils Infradentate Port Approach to the Fourth Ventricle are retracted laterally, the pyramid and uvula of the vermis The premise of