Anatomical Step-by-Step Dissection of Midline Suboccipital Approaches to the Fourth Ventricle for Trainees: Surgical Anatomy of the Telovelar, Transvermian, and Superior Transvelar Routes, Surgical Principles, and Illustrative Cases

Abstract

Introduction  Safe, effective access to the fourth ventricle for oncologic resection remains challenging given the depth of location, restricted posterior fossa boundaries, and surrounding eloquent neuroanatomy. Despite description in the literature, a practical step-by step dissection guide of the suboccipital approaches to the fourth ventricle targeted to all training levels is lacking.

Methods  Two formalin-fixed, latex-injected specimens were dissected under microscopic magnification and endoscopic visualization. Dissections of the telovelar, transvermian, and supracerebellar infratentorial-superior transvelar approaches were performed by one neurosurgery resident (D.D.D.), under guidance of senior authors. The dissections were supplemented with representative clinical cases to highlight pertinent surgical principles.

Results  The telovelar and transvermian corridors afford excellent access to the caudal two-thirds of the fourth ventricle with the former approach offering expanded access to the lateral recess, foramen of Luschka, adjacent skull base, and cerebellopontine angle. The supracerebellar infratentorial-superior transvelar approach reaches the rostral third of the fourth ventricle, the cerebral aqueduct, and dorsal mesencephalon. Key steps described include positioning and skin incision, myofascial dissection, burr hole and craniotomy, durotomy, the aforementioned transventricular routes, and identification of relevant skull base landmarks.

Conclusion  The midline suboccipital craniotomy represents a foundational cranial approach, particularly for lesions involving the fourth ventricle. Operatively oriented resources that combine stepwise neuroanatomic dissections with representative cases provide a crucial foundation for neurosurgical training. We present a comprehensive guide for trainees in the surgical anatomy laboratory to optimize familiarity with fourth ventricle approaches, mastery of relevant microsurgical anatomy, and simultaneous preparation for learning in the operating room.

Introduction

Pathologies situated in the fourth ventricle, potentially involving the dorsal brainstem, adjacent cerebellopontine angle, cerebellar vermis and peduncles, and/or craniocervical junction, represent some of the most challenging neurosurgical entities to treat. The deep, midline location of the fourth ventricle, the restricted surgical corridor required for its access, and this region's density of eloquent neuroanatomical structures constitute significant limitations to operating in this area. ¹ Common lesions encountered in the fourth ventricle include ependymomas, medulloblastomas, pilocytic astrocytomas, hemangioblastomas, epidermoid cysts, and cavernous malformations that may or may not be accompanied by craniospinal anomalies. ² Tumors are often concealed by cerebellar structures, including the cerebellar hemispheres, peduncles, tonsils, and vermis. They may involve adjacent subarachnoid spaces through lateral extension via the foramen of Luschka into the cerebellomedullary and cerebellopontine cisterns. Further, infiltration throughout the remainder of the ventricular system can occur via superior extension through the cerebral aqueduct or inferiorly through the obex into the spinal subarachnoid space and potentially into the central canal of the spinal cord, thus contributing to supratentorial disease or drop metastases, respectively. The posterior inferior cerebellar artery (PICA) and lower cranial nerves are often intimately related to and frequently encased or displaced by tumors in this area. Due to tumor growth or infiltration, many lesions are adherent to the dorsal brainstem that give rise to the floor of the fourth ventricle, including the facial colliculus, vestibular area, hypoglossal trigone, and vagal trigone. ^{3 4}

To resect large lesions that expand the fourth ventricle, wide ventricular access must be obtained longitudinally along the craniocaudal length of the lesion and laterally toward the lateral recess and apertures. A schematic overview detailing the presently accepted inferior and superior trajectories to the fourth ventricle is represented in Fig. 1 .

Fig. 1.

Schematic sagittal overview depicting transcranial approaches to the fourth ventricle, including three superior routes ( green arrows ) and two inferior routes ( purple arrows ). The superior transvelar approach includes all trajectories that traverse the superior medullary velum and may be performed by an interhemispheric precuneal, occipital transtentorial, or supracerebellar infratentorial (modified midline suboccipital) craniotomy. Inferior routes to the fourth ventricle include the telovelar and transvermian approaches both performed via midline suboccipital craniotomy.

The two most common operative corridors to the fourth ventricle used today are the transvermian and telovelar approaches, two inferior trajectories performed via midline suboccipital craniotomy with or without C1 laminectomy. ⁵ The transvermian approach transgresses the vermis, while the telovelar approach implicates dissection of the telovelar junction, or the intersection of the inferior medullary velum with the tela choroidea of the fourth ventricle. A superior transventricular trajectory via access through the superior medullary velum, termed the superior transvelar approach, may provide a reasonable alternative for certain indications. The superior transvelar approach may be performed through one of the following three transcranial routes: supracerebellar infratentorial, occipital transtentorial, and posterior interhemispheric (precuneal). Similar to the aforementioned transvermian and telovelar corridors, the superior transvelar route via supracerebellar infratentorial approach utilizes a midline suboccipital craniotomy with few modifications. ⁶

Selection between approaches to the fourth ventricle should be tailored to the location and direction of growth of the fourth ventricular, cerebellar, or brainstem lesion for which the surgery is being designed. Although several preceding publications and neuroanatomical atlases have described these approaches, the present literature is not optimized for the education of neurosurgical residents and skull base fellows via a dissection-based neuroanatomic model. Accordingly, the objective of the present study was to create a novel educational resource using high-quality, cadaver-based, operatively oriented dissections. We set out to provide a step-by-step surgical guide for laboratory dissection of the three routes (transvermian, telovelar, supracerebellar infratentorial–superior transvelar) that utilize a midline suboccipital craniotomy for fourth ventricular access that parallels the operative experience. Furthermore, we supplemented the core neuroanatomy with contextualizing details, illustrative case examples, and other insights that will assist trainees at any level in learning or mastering the common suboccipital approaches to the fourth ventricle.

Methods

All aspects of this study were approved by the Mayo Clinic institutional review board and biospecimens committee, as required by standard protocols. Two formalin-fixed specimens, injected with colored latex using a six-vessel technique previously published by the study staff, were utilized to perform the anatomical dissections presented herein. ⁷ Suboccipital approaches to the fourth ventricle were performed and led by one neurosurgery resident (D.D.D.). The suboccipital craniotomy with telovelar and transvermian approaches were performed in one specimen, while a modified suboccipital craniotomy with a supracerebellar infratentorial-superior transvelar approach was completed in the second specimen. All dissections were completed under microscopic magnification with a Zeiss S100/OPMI microscope (Zeiss, Jena, Germany) as well as endoscopic visualization using a Stryker Precision S 4K Sinuscope (Stryker Endoscopy, San Jose, California, United States). Standard microsurgical equipment and instrumentation were used for all approaches (Medtronic Midas Rex electric system; Medtronic USA Inc., Jacksonville, Florida, United States). The dissections were documented in a stepwise fashion using three-dimensional photographic and endoscopic image acquisition techniques, also previously described. ⁷ Following successful dissection, illustrative case applications were reviewed under the guidance of the senior authors (M.J.L., D.J.D., and M.P.C.) to highlight important principles of approach selection and surgical planning.

Results

Common Steps of Midline Suboccipital Craniotomy

Positioning and Skin Incision

The specimen was placed in a simulated sitting position. In the operating room, the patient may be positioned in either the prone, sitting, or park-bench position. In adult patients, the skull is placed in three-point pinion fixation in a Mayfield head holder. A single pin is placed roughly 3 cm above the pinna on the right side, while two pins are affixed with their midpoint directly opposite of the single pin and angled in a manner that leaves the suboccipital area unobstructed. In the case of very young children or pediatric patients with long-standing hydrocephalus, a foam-padded horseshoe head holder is alternatively used with the child in the prone position. With the specimen successfully positioned, attention was turned to planning the occipitocervical skin incision, which was oriented in the midline, initiated at the inion, and extended to the spinous process of C2 ( Fig. 2A ). When performing the supracerebellar infratentorial approach, this incision was extended cephalad by approximately 3 cm to uncover the entirety of the external occipital protuberance and expose the underlying torcula.

Fig. 2.

Step-by-step midline suboccipital craniotomy and C1 laminectomy in an anatomical specimen. C1 laminectomy is not required for a superior transvelar approach. ( A ) Midline skin incision (dashed line) extends from the inion to the spinous process of C2. ( B ) After myofascial posterior cervical dissection, subperiosteal dissection of the occiput and C1 lamina reveals osseous landmarks of the craniocervical junction. ( C ) Burr holes are placed slightly inferior to the inion and superior nuchal line on either side of the midline. To expand this approach for a supracerebellar infratentorial route, additional bone removal can be performed to expose the inferior edge of the transverse sinuses. After extensive dural dissection, a craniotome is used to connect the burr holes across midline and turn a craniotomy laterally and inferiorly toward the foramen magnum at its junction with the occipital condyles. ( D ) The suboccipital bone flap is elevated revealing posterior fossa dura. ( E ) C1 laminectomy is performed with lamina elevated to reveal cervical spinal dura in preparation for the durotomy. ( F ) A Y-shaped durotomy is created extending from the midline cervical spinal dura and turning bilaterally toward the superolateral apices of the posterior fossa dura. ( G ) The three leaflets of the durotomy are separated from the arachnoid overlying the cisterna magna and secured to the myocutaneous flap. The arachnoid is subsequently dissected to relieve cerebrospinal fluid. ( H ) After dural reflection, the posterior cervical spinal cord and the suboccipital surface of the cerebellum is fully exposed to facilitate inferior or superior transventricular access to the fourth ventricle. Arter, arterious; Bivent, biventral; Br, branches; Cerebel, cerebellar; Colum, columns; Craniocer, craniocervical; Gang, ganglion; Horizon, horizontal; Inf, inferior; Nuch, nuchal; PICA, posterior inferior cerebellar artery; Post, posterior; Proc, process; Semil, semilunar; Sup, superior; Tuber, tubercle; VA, vertebral artery; Vert, vertebral.

Scalp, Myocutaneous, and Subperiosteal Dissection: Anatomical Landmarks for Suboccipital Craniotomy and C1 Laminectomy

The midline fascial planes extending from the inion to the C2 spinous process were subsequently developed to separate the muscles of the posterior cervical region. Superiorly, the trapezius, splenius capitis, semispinalis capitis, and rectus capitis posterior minor and major attaching to the superior and inferior nuchal lines were dissected subperiosteally and retracted laterally. After carefully elevating the muscles, the occipital bone from the inion to the foramen magnum was exposed ( Fig. 2B ).

For full exposure of the telovelar and transvermian approaches in this specimen, the posterior arch of C1 was bluntly dissected along its inferior edge with caution to prevent vertebral artery injury within the suboccipital muscles over the sulcus arteriosus within the lateral arch of C1. When C1 laminectomy is indicated to optimize intradural tumor access, lateral dissection over C1 should be continued until the posterior arch of C1 is exposed to the width of the spinal dura ( Fig. 2B ).

Burr Holes, Suboccipital Craniotomy, and Bone Flap Elevation

For the telovelar and transvermian approaches, the suboccipital craniotomy was initiated by fashioning two burr holes inferior to the edge of the superior nuchal line below the inion ( Fig. 2C ). After burr hole placement using the high-speed drill, a Penfield 1 dissector was used to carefully strip the dura mater from the overlying occipital bone away from the venous sinuses. For a superior transvelar route, the burr holes were placed on the superior nuchal line, and the craniotomy was enlarged superiorly to find the inferior edge of the transverse sinuses and torcula to facilitate optimal dural reflection and supracerebellar infratentorial access. Using an angled curette, the posterior atlantooccipital membrane was stripped from the foramen magnum. Small notches were made on either side with a Kerrison rongeur to create “landing spots” for the craniotome footplate. These notches were spaced at an equal width to the underlying spinal dura and did not extend to the occipital condyles.

Placement of burr holes was followed by utilization of the matchstick drill bit across the midline keel to protect a possible occipital sinus. The craniotome footplate was then used to connect each burr hole to its ipsilateral “landing spot” at the foramen magnum, thereby completing a one-piece suboccipital craniotomy ( Fig. 2C ). The underlying dura was dissected from the inner table of the craniotomy, the bone flap was elevated, and the posterior fossa dura was now exposed from the inferior edge of the transverse sinus to the foramen magnum ( Fig. 2D ).

C1 Laminectomy

The laminectomy was performed from the medial edge of the sulcus arteriosus bilaterally to include the C1 tubercle, a width that typically correlates to the width of the foramen magnum. These osteotomies were conducted using the spiral bit and footplate attachment, taking care to carefully strip the underlying connective tissue and cervical dura before attempting to remove the posterior arch of C1. A transverse band of thick connective tissue needed to be released first to expose the spinal dura at the craniocervical junction ( Fig. 2E ).

Y-Shaped Durotomy with Exposure of the Craniocervical Junction

Under microscopic view, the dura was opened sharply at the inferior edge of the exposure by making a single, linear, 15-mm cut below the level of C1 in a caudal to cranial direction ( Fig. 2F ). This incision allowed for immediate access to the arachnoid overlying the cisterna magna for cerebrospinal fluid (CSF) drainage ( Fig. 2G ). In the operating room, care is taken to incise the arachnoid separately after sufficient hemostasis of a large circular or occipital sinus if present; CSF egress then facilitates posterior fossa relaxation and thus, completion of the durotomy safely ensues. In this specimen, the durotomy was then extended superiorly toward the foramen magnum, and the two upper limbs of the “Y” shape were fashioned by turning scissors laterally in each direction over the cerebellar hemispheres toward the superolateral corners of the craniotomy. The durotomy terminated below the inferior edge of the transverse sinuses to facilitate safe closure in all approaches as well as optimal dural reflection and sinus elevation in the superior transvelar approach ( Fig. 2F , 2G ). While not appreciated in the specimens, a circular or occipital sinus is commonly present in children and may require hemostatic control with suture or clip ligation.

The three dural leaflets were reflected superiorly and laterally and secured in place using 3–0 silk sutures. The exposed transverse sinus should first be protected with a moistened surgical pledget prior to its elevation with dural reflection in the superior transvelar approach that aids in mitigating risk of iatrogenic venous thrombosis. Exposure of the suboccipital surface of the cerebellum from the infratentorial surface to the tonsillomedullary junction completed the transcranial approach required to perform all three transventricular trajectories to the fourth ventricle described herein ( Fig. 2H ).

Inferior Corridors to Access the Fourth Ventricle

Telovelar Route

Since transgression through normal cerebellar parenchyma is not required in the telovelar approach, this route was performed on the specimens first to optimize operative simulation for both inferior approaches. The arachnoid overlying the cerebellar tonsils was sharply incised with an arachnoid knife to facilitate bilateral opening of the uvulotonsillar and medullotonsillar spaces ( Fig. 3A ). Careful incision of the arachnoid between the inferior edge of the tonsils and the adjacent edge of the uvula advanced the dissection in a lateral and deep direction around the surfaces of the tonsils and biventral lobules ( Fig. 3B , 3C ). The plane between the tonsils and the vermis was opened one side at a time. With deeper dissection and dynamic superolateral retraction of the medial side of the tonsils, the ventral limit of the tonsils was visualized, exposing choroid plexus, choroidal vessels, as well as the telovelar junction and inferior roof of the fourth ventricle ( Fig. 3B , 3C ). Special attention was directed to the location and bifurcation of PICA into medial and lateral trunks at this point. The vessels were gently mobilized laterally with dynamic retraction to protect them on each side, together with all of the perforating arteries. From the medial attachment at the nodulus and proceeding in an inferolateral direction toward the lateral recess, the inferior medullary velum was divided from the tela choroidea bilaterally ( Fig. 3C , 3D , 4A , 4B ). Elevating the uvula superiorly facilitated access into the rhomboid fossa and provided immediate visualization of the key fourth ventricular structures ( Fig. 3E , 3F , 4E-H ).

Fig. 3.

Step-by-step dissection of inferior corridors to the fourth ventricle in an anatomical specimen, including telovelar (B-G) and transvermian (H) approaches. ( A ) Anatomical overview of the exposed suboccipital cerebellar surface required for inferior dissection to the fourth ventricle, including the vermis and tonsils. An incision ( dashed yellow line ) in the uvula facilitates the transvermian route, while arachnoid dissection within the uvulotonsillar plane ( green arrow ) initiates access to the telovelar junction. ( B ) Telovelar approach : Retractors are placed within the uvulotonsillar plane to the depth of the telovelar junction with the vermis kept intact. ( C ) Progressive lateral retraction of the tonsils further exposes the telovelar junction and lateral recesses bilaterally in preparation for fourth ventricle entry ( green arrow ). ( D ) Sharp dissection divides the inferior medullary velum and the tela choroidea bilaterally on either side of the uvula with inferolateral extension along the lateral recesses. ( E ) The uvula is retracted superiorly to expose the floor of the fourth ventricle with care to avoid injury to the telovelotonsillar segment of PICA. ( F, G ) The choroid plexus is divided to fully expose the floor of the fourth ventricle from the cerebral aqueduct to the obex. The lateral recess is followed toward the foramen of Luschka further exposing the cerebellopontine angle and cranial nerves VII to XI. ( H ) Transvermian approach : A midline incision is performed in the uvula extending through the nodule. The halves are bilaterally retracted in a superolateral direction to expose the floor of the fourth ventricle and cerebral aqueduct. The vermian incision can be modified according to the size and epicenter of the corresponding tumor. Aqued, aqueduct; Br, branches; Cerebel, cerebellar; Chor, choroid; CN, cranial nerve; Collic, colliculus; Emin, eminence; Fasc, fascicle; Hypogl, hypoglossal; IAC, internal acoustic canal; Inf, inferior; Lusch, Luschka; Med, medial; Medul, medullaris; Pedunc, peduncle; PICA, posterior inferior cerebellar artery; Sup, superior; Trig, trigone; Tuber, tubercle.

Fig. 4.

Endoscopic-assisted views of the inferior corridors to the fourth ventricle. ( A ) The telovelar junction (left side) is identified between the plane of the inferior medullary velum and the tela choroidea immediately deep and adjacent to the uvula and nodule of the vermis. ( B ) Upon dissection of the telovelar junction, entry to the rhomboid fossa and lateral recess is facilitated. ( C, D ) Extension of telovelar dissection in an inferolateral direction along the rhomboid lip of the flocculus exposes both foramina of Luschka (C, left side) (D, right side). ( E ) Superior retraction of the uvula provides access to the floor of the fourth ventricle. ( F ) Division of the choroid plexus within the midline further exposes the floor of the fourth ventricle to the level of the obex. ( G, H ) Inferior anatomical views of fourth ventricle including dorsal midbrain landmarks, cerebral aqueduct, and superior roof of the fourth ventricle as visualized with a 30-degree endoscope. Aqued, aqueduct; Cereb, cerebral; Cerebel, cerebellar; Chor, choroid; CN, cranial nerve; Coch, cochlear; Collic, colliculus; Emin, eminence; Hypogl, hypoglossal; Inf, inferior; Lusch, Luschka; Med, medial; Medul, medullaris; Oblong, oblongata; Pedunc, peduncle; PICA, posterior inferior cerebellar artery; Rhomb, rhomboid; Sup, superior; Trig, trigone; Vent, ventricle; Vestib, vestibular.

To access the lateral apertures and adjacent cerebellopontine angle, the telovelar incision was extended more inferolaterally within the tela choroidea along the rhomboid lip of the flocculus ( Fig. 3D-3G , 4C , 4D ). To access the caudal end of the fourth ventricle and foramen of Magendie, the tela choroidea and choroid plexus were divided in the midline and gently mobilized laterally ( Fig. 3F , 4F , 4G ). Opening of the velum tela choroidea can occur in any sequential order as described above and is often predetermined by the extent of tumor infiltration into these structures. To obtain access to the lower cranial nerves IX to XII, increasing superolateral retraction of the tonsils and further arachnoid dissection along the flocculus was performed ( Fig. 3F , 3G , 5A , 5B ). Subsequent subfloccular retraction and the use of an angled endoscope facilitated visualization and access to cranial nerves V to VIII ( Fig. 5C–F ).

Fig. 5.

Endoscopic-assisted dorsal views of the adjacent skull base and cerebellopontine angle. ( A ) Posterolateral overview of cranial nerves VII–XII exiting the brainstem into their respective foramina are visualized in the adjacent lateral posterior fossa with retraction of the tela choroidea and flocculus. ( B, C ) Closer inspection of the relationship between the IAC and jugular foramen is assessed with and without retraction of the flocculus. ( D, E ) Progressive ventral dissection reveals the lateral pons, superior rim of the IAC, motor root of the trigeminal nerve, and petrous apex dura. ( F ) Complete ventrolateral dissection using a 45-degree endoscope to the depth of the petroclival dura provides a medial view of the ventral pons, trigeminal nerve, and SCA. CN, cranial nerves; Hypogl, hypoglossal; IAC, internal acoustic canal; Inf, inferior; Lusch, Luschka; Petros, petrosal; Sup, superior; SCA, superior cerebellar artery; Tuber, tubercle.

Transvermian Route

Similar to the telovelar approach, the arachnoid overlying the suboccipital surface of the cerebellum was opened with sharp dissection, and the plane between the tonsils and the vermis was carefully developed. Any remaining arachnoid adhesions were dissected away and the inferior portion of the vermis, including the uvula, pyramid, and tuber were exposed ( Fig. 3A ). As previously mentioned, both PICAs were encountered in this area as they coursed around the lower tonsillar pole to reach the inferior margin of the cerebellomedullary fissure. A transvermian incision was performed through sharp dissection of the inferior vermis, followed by iterative deepening of the dissection to expose the underlying nodule, the final component of the vermis that faces the fourth ventricle and must be incised to access the inferior roof. Lateral retraction of the two halves of the incised vermis provided approximately 2 cm of working space for access into the fourth ventricle ( Fig. 3H ). The midline incision through the tela choroidea previously performed during the telovelar approach further expanded transventricular access attained by lateral retraction of the two halves of the nodule, providing access to the full length of the fourth ventricular floor from its entry into the cerebral aqueduct to the obex preceding the central canal of the spinal cord. The endoscope was brought into the field to better access and visualize the fourth ventricle ( Fig. 4 ) and cerebellopontine angle ( Fig. 5 ).

Superior Corridor to Access the Fourth Ventricle

Supracerebellar Infratentorial–Superior Transvelar Route

To access the fourth ventricle using a supracerebellar infratentorial approach through the superior medullary velum, modifications to the extradural steps included the following in a second specimen: (1) extended skin incision 2 to 3 cm superior to the inion to the C2 spinous process, (2) full exposure of the transverse sinus at the superior edge of the suboccipital craniotomy via additional bone removal, and (3) termination of the Y-shaped durotomy closer to the inferior edge of the transverse sinuses. C1 laminectomy was not performed. Retraction sutures anchored the dural flap in place, and the transverse sinus was rotated and mobilized superiorly, thereby expanding the operative corridor within the narrow supracerebellar infratentorial space ( Fig. 6A ).

Fig. 6.

Step-by-step dissection of the superior transvelar approach to the fourth ventricle in an anatomical specimen, performed via midline suboccipital craniotomy with supracerebellar infratentorial dissection. ( A ) Anatomical overview of the superior end of the suboccipital cerebellar surface with the dural venous sinuses gently elevated and retracted with sutures to facilitate infratentorial dissection. ( B ) Retractors are placed on the infratentorial cerebellar surface with gentle, inferior retraction to expose the superior apex of the vermis. ( C, D ) Progressive, anterior advancement of the retractors toward the midline along the anterior lobe of the cerebellum brings the pineal gland and tentorial incisura into view. ( E ) Magnified view of the junction of the lingula and dorsal midbrain with emphasis on the deep venous anatomy, pineal gland, tectum, trochlear nerves, and the cerebellomesencephalic fissure below which the superior medullary velum can be visualized with an angled endoscope ( not shown ). ( F ) Microscopic overview of the extent of fourth ventricular access provided by the supracerebellar infratentorial approach after incision of the superior medullary velum has been made with endoscopic assistance ( Figure 7 ). Cereb, cerebral; CN, cranial nerve; Collicul, collicular; Horizon, horizontal; Inf, inferior; Int, Internal; Lunogr, lunogracile; Quadrig, quadrigeminus; Rosent, Rosenthal; SCA, superior cerebellar artery; Semil, semilunar; Sup, superior; Vent, ventricle.

Dissection under direct microscopic vision was performed in the subarachnoid space along the superior surface of the cerebellum as the trajectory was deepened toward midline upon which the anterior lobe of the cerebellum, primary fissure, the pineal gland, and superior vermian vein were progressively exposed ( Fig. 6B–E ). Midline vermian bridging veins, including the precentral vein, were divided to provide an unobstructed working corridor. Of note, as the tectum and lingula of the cerebellum came into view, the operative trajectory was directed inferiorly because of the steep, acute angle between the superior medullary velum and the cerebellum. Careful retraction separated the lingula from the superior medullary velum to expose the point for transventricular access: the cerebellomesencephalic fissure ( Fig. 6E , 7A ). A 1-cm infratrochlear midline incision was fashioned in the superior medullary velum using a sickle knife ( Fig. 7B–C ). The superior portion of the superior medullary velum, in between the two emerging trochlear nerves, was left intact to avoid damage to their decussating fibers.

Fig. 7.

Endoscopic-assisted supracerebellar infratentorial view of the superior transvelar corridor to the fourth ventricle. ( A ) Upon access to the cerebellomesencephalic fissure and quadrigeminal cistern, a 45-degree endoscope is utilized to facilitate adequate visualization of the superior medullary velum from its junction with the trochlear nerves to the fastigium. ( B ) A sickle knife is used to sharply incise the superior medullary velum in the midline below the level of the trochlear nerves inferiorly toward the fastigium. ( C, D ) Superior views of an infratrochlear midline incision in the superior medullary velum to expose the rostral third of the floor off the fourth ventricle. ( E, F ) Overviews of the rostral fourth ventricle as seen from a magnified and demagnified supracerebellar infratentorial corridor, respectively. ( G ) Magnified view of the inferior extent of the fourth ventricle floor from the perspective of the superior roof. ( H ) Magnified view of the rostral end of the floor of the fourth ventricle and the cerebral aqueduct utilizing a 45-degree endoscope. Ant, anterior; Aqued, aqueduct; Cereb, cerebral; Cerebel, cerebellar; CN, cranial nerve; Collic, collicular; Horizon, horizontal; Inf, Inferior; Int, internal; Lunogr, lunogracile; Pedunc, peduncle; Semil, semilunar; Sup, superior; Rosent, Rosenthal; SCA, superior cerebellar artery; Vent, ventricle.

With the incision complete, a narrow window into the fourth ventricle was exposed, and the endoscope was employed for further intraventricular visualization ( Fig. 7D ). This exposure was ultimately limited inferiorly to the rostral third of the fourth ventricle, but further provided an excellent view of the cerebral aqueduct and dorsal midbrain ( Fig. 6G , 7E–H ).

Representative Case Review

Case 1. Telovelar Approach (Pediatric)

A 2-year-old male with no significant past medical history presented with anorexia, intermittent fevers, stiff posture, and gait instability. Computed tomography of the head demonstrated significant obstructive hydrocephalus secondary to a large cerebellar mass centered predominantly within the caudal two-thirds of the fourth ventricle. Magnetic resonance imaging (MRI) redemonstrated the 3.6 × 2.9 × 3.1 cm enhancing intracranial mass involving the brainstem, interpeduncular cistern, tectum, optic tracts, and lateral cerebral fissures ( Fig. 8A , 8B ). MRI of the remaining neuraxis revealed an additional 1.4 cm metastatic focus within the infundibular recess, diffuse extramedullary metastases from T9 to L1, conus, and cauda equina, and leptomeningeal dissemination suggestive of medulloblastoma.

Fig. 8.

Representative case studies of fourth ventricular tumors resected via midline suboccipital craniotomy with telovelar (A–D), transvermian (E–H), or combined inferior and superior transvelar approaches (I–L). Case one : Preoperative postcontrast sagittal ( A ) and T2 axial ( B ) MRIs demonstrating a homogenously enhancing cystic tumor ( yellow arrowheads ) centrally located within the inferior two-thirds of the fourth ventricle. Postoperative postcontrast sagittal ( C ) and axial T2 ( D ) magnetic resonance imagings (MRIs) following a telovelar approach demonstrating gross total resection of a medulloblastoma. Case two : Preoperative T1-weighted postcontrast sagittal ( E ) and axial ( F ) MRIs demonstrating a cystic, nonenhancing mass ( red arrowheads) with restricted diffusion ( F, inset ) expanding the fourth ventricle. Postoperative postcontrast sagittal ( G ) and axial ( H ) MRIs following a telovelar approach demonstrating gross total resection of an adult epidermoid cyst. Case three : Preoperative postcontrast sagittal ( I ) and axial ( J ) MRIs demonstrating a heterogeneously enhancing tumor ( green arrowheads ) predominantly centered within vermis with infiltration of the fourth ventricle floor. Postoperative postcontrast sagittal ( K ) and axial ( L ) MRIs following a transvermian approach demonstrating gross total resection of the vermian medulloblastoma. Case four : Preoperative postcontrast sagittal ( M ) and axial ( N ) MRIs demonstrating a large, heterogeneously enhancing tumor ( blue arrowheads ) centered within the pineal region with extension into the fourth ventricle, superior cerebellum, and vermis with hydrocephalus and mass effect on the dorsal midbrain. Postoperative postcontrast sagittal ( O ) and axial ( P ) MRIs following a combined supracerebellar infratentorial–superior transvelar and transvermian approach demonstrating near-total resection of an atypical teratoid/rhabdoid tumor.

The patient underwent surgical resection of the fourth ventricular mass via midline suboccipital craniotomy with C1 laminectomy and telovelar transventricular approach. Intraoperatively, the tumor infiltrated the obex and displaced the tonsils laterally thereby creating a natural subtonsillar working corridor. Molecular and histopathological analysis established the diagnosis of a non-Wingless/Integrated (WNT)/non-Sonic Hedgehog (SHH) medulloblastoma. Gross total resection of the fourth ventricular mass was confirmed by postoperative MRI ( Fig. 8C , 8D ).

The patient's immediate recovery was uncomplicated without new neurological deficits. He underwent subsequent debulking of the spinal metastasis, after which he developed signs of increased intracranial pressure, dysconjugate gaze, as well as pupillary changes without electroencephalography correlate. He ultimately required a ventriculoperitoneal shunt. Follow-up for evaluation of recurrence is presently pending due to recent surgery.

Case 2. Telovelar Approach (Adult)

A 26-year-old male presented with severely ataxic spastic gait, dysarthric speech, and dysphagia in addition to wide amplitude large nystagmus in all directions of gaze. Neuroimaging revealed a 5.5 cm multilobulated cystic mass consistent with CSF signal and restricted diffusion at the cerebellar vermis, displacing the pons and medulla anteriorly and the cerebellum posteriorly, and extending caudally into the foramen magnum ( Fig. 8E , 8F , inset ). No hydrocephalus was clinically apparent at the time of presentation.

A suboccipital craniotomy with C1 laminectomy employing a bilateral telovelar approach to the fourth ventricle was performed for resection. To improve intraoperative visualization, the suboccipital craniotomy was extended laterally and superiorly nearly adjacent to both transverse and sigmoid sinuses. The tumor consistency was pearly, white with grumous contents. The cyst contents were internally debulked, and the thin capsule resected. Gross total resection was achieved and confirmed by postoperative MRI ( Fig. 8G , 8H ). Histopathology confirmed the diagnosis of an epidermoid cyst.

Postoperatively, the patient had no new neurological deficits. With enrollment in physical, occupational, and speech therapy, his coordination and speech were markedly improved one year after surgery. He demonstrated no aspiration on repeat swallow studies, and his diplopia was completely resolved. No recurrence of the epidermoid is known to date.

Case 3. Transvermian Approach

A previously healthy 6-year-old male presented with new onset progressive gait imbalance, headaches, nausea and vomiting, and decreased activity. Neuroimaging revealed mild obstructive hydrocephalus secondary to a large, heterogenous 3.5 × 3.6 × 4.2 cm cystic and solid mass within the fourth ventricle ( Fig. 8I , 8J ). Preoperative external ventricular drain (EVD) was placed to relieve increased intracranial pressure in preparation for surgery.

A midline suboccipital craniotomy with C1 laminectomy and transvermian transventricular approach was selected for tumor resection. To enlarge the operative view, an additional 2 cm of occipital bone was drilled on either side of the foramen magnum. Once intradural, a gray, hemorrhagic tumor with a clear tumor-cerebellar interface was encountered. The transvermian corridor was naturally created by tumor infiltration, and the deep, cephalad apex of the tumor at its origin was subsequently identified with progressive tumor removal and retraction of the vermis. Gross total resection was achieved and confirmed by postoperative MRI ( Fig. 8K , 8L ). Pathology revealed a SHH-activated medulloblastoma.

No adverse sequelae were evident in the postoperative period. Adjuvant therapy included proton beam radiation and chemotherapy. The patient had no tumor recurrence after seven years of regular follow-up.

Case 4. Combined Inferior and Superior Transventricular Approaches

A 10-month-old male with no significant past medical history presented to the emergency department with persistent vomiting complicated by lethargy and loss of consciousness. Neuroimaging showed significant obstructive hydrocephalus secondary to a large 4.5 × 5.2 × 6.3 cm heterogeneous, poorly enhancing, cystic mass centered within the superior vermis with significant extension into the rostral fourth ventricle, intratumoral hemorrhage, and calcification ( Fig. 8M , 8N ). Pronounced mass effect caused tonsillar ectopia, pontine flattening, and transtentorial herniation prompting urgent EVD placement.

Surgery via a midline suboccipital craniotomy with C1 laminectomy employing both supracerebellar infratentorial and subtonsillar approaches to the fourth ventricle was performed. Durotomy first revealed congested tonsils and vermis, and a highly vascular tumor with a plane that could not be easily developed. An inferior transventricular approach was performed first to perform debulking of the most easily accessed areas of tumor, in which neither a purely telovelar nor transvermian route could be defined due to significant tumor infiltration and mass effect on the tonsils and vermis, The tumor was substantially hemorrhagic upon internal debulking, and the patient required a blood transfusion. The fourth ventricle was opened and expanded from this inferior trajectory, and the cerebellar tonsils were subsequently retracted to resect significant amounts of tumor within the caudal end of the fourth ventricle.

With a significant amount of tumor resected inferiorly, the cerebellum naturally fell away from the tentorium, allowing for utilization of a supracerebellar infratentorial-superior transvelar approach to address rostral ventricular pathology. Anteriorly, within the cerebellomesencephalic fissure, the vein of Galen and the right basal veins of Rosenthal were challenging to identify, and some adherent tumor was ultimately left on these structures. Tumor was resected from the rostral ventricle, and an angled endoscope verified lack of residual within the cerebral aqueduct in an effort to improve CSF flow. All macroscopic nonadherent tumor was resected and confirmed by postoperative imaging ( Fig. 8O , 8P ). Histopathology was diagnostic for atypical teratoid/rhabdoid tumor.

Postoperatively the patient developed bilateral hygromas due to altered CSF absorption and required ventriculoperitoneal shunt placement. He received adjuvant chemotherapy and a stem cell transplant. There is no evidence of recurrence on neuroimaging in over six years of close follow-up.

Discussion

The midline suboccipital craniotomy with the three transventricular approaches presented herein (telovelar, transvermian, superior transvelar) represent a versatile set of tools for accessing the fourth ventricle, dorsal brainstem, and the relevant surrounding skull base anatomy, which continue to pose a surgical challenge due to the deep location and abundance of eloquent neurovascular structures. ^{1 3 4} Between the transventricular approaches presented, the transvermian route represents the most traditional means to access pathologies located in and adjacent to the fourth ventricle. ^{3 5 8} Historically, this approach was established by partial removal of the cerebellar hemispheres and via disruption of the cerebellar cortex by unrestricted opening of the vermis. In light of morbidity associated with vermian disruption, it has since been further modified or abandoned. ^{9 10} The tenets of modern skull base surgery endorse the use of natural cleavage planes in approaching fourth ventricular lesions, without incising or resecting normal parenchyma. ¹ Consequently, an alternative approach directed through the cerebellomedullary fissure to the telovelar junction has gained increasing popularity. ^{11 12} Several reports in the literature have compared the microsurgical anatomy of the telovelar approach with the more traditional transvermian approach, highlighting distinguishing features pertaining to the trajectory of the approaches, the route to assess surgical targets, the anatomical structures retracted and sacrificed, the direction of retraction, the areas exposed, the structures limiting the exposure, and the clinical applications. ^{8 12 13} These reports do not demonstrate nor compare these approaches in a stepwise fashion nor with context that parallels the three-dimensional operative orientation; further the literature largely lacks concomitant discussion of the inferior transventricular trajectories with consideration of superior surgical routes. Approaching the fourth ventricle from its superior roof may add yet another alternative or supplementary route to the arsenal of skull base and transventricular approaches. ⁶ Our principal objective in the current study was to provide trainees with an operatively-oriented educational resource for the midline suboccipital craniotomy for fourth ventricular access along its entire sagittal plane, to guide education in the neuroanatomy laboratory, to enhance decision-making regarding surgical approach selection, and to optimize preparation for intraoperative participation in surgical cases.

The transvermian route contemporarily involves incising no more than the inferior third of the cerebellar vermis and retracting the two halves in opposite lateral directions to “split” the tissue. Dandy advocated that splitting the vermis could be performed without causing a disturbance in function, provided that the dentate nuclei are carefully avoided. ¹⁴ Nevertheless, the transvermian technique is a “destructive” approach that requires manual disruption of cortical and functional areas of the cerebellum. Pathologies located within the inferior vermis, iatrogenic neural injury caused by a vermian incision, or retraction of the two halves of the incised vermis may result in caudal vermis syndrome, which manifests as dysequilibrium with staggering gait, truncal ataxia, cervical oscillation, and nystagmus on assuming the erect position. ^{15 16} Subsequent damage to the dentate nucleus may exacerbate symptoms observed with vermian lesions alone and additionally cause intention tremor during voluntary movement of the extremities. ¹⁷ Although the exact anatomical substrate for transient cerebellar mutism remains unknown, splitting the inferior portion of the vermis has been implicated in the pathogenesis of this syndrome in several studies. ^{18 19 20 21} This postoperative condition is characterized by a temporary lack of speech with intact comprehension, oral pharyngeal apraxia, emotional lability, ataxia, and hypotonia and constitutes a highly morbid and challenging syndrome to manage following the removal of posterior fossa tumors. ^{9 10 22 23}

When contemplating the use of an inferior route to the fourth ventricle, the transvermian route provides a greater sagittal working angle in comparison to the telovelar approach. ¹² Indeed, the transvermian approach affords enhanced access to the rostral half of fourth ventricle just proximal to the aqueduct, including better visualization of the midline inferior portion of the superior medullary velum and the fastigium. ⁸ However, the narrow width of exposure and working room afforded by the transvermian approach are limited to midline lesions with minimal lateral extension, whereby the lateral limit of dissection is demarcated by the medial portion of the vestibular area. ^{1 12} Unless sections of the vermis or tonsils are intentionally removed or destroyed by exophytic tumor growth, the lateral recess cannot be easily accessed. ^{8 12} For this reason and the inherently high potential for complications, the most common and compelling application of the transvermian approach today is to approach lesions that grow from the superior vermis and extend secondarily into the middle and/or rostral third of the fourth ventricle with limited lateral extension. ¹ As seen in case 3 in the above series, the most common malignant pediatric tumor arising in this location is medulloblastoma, in which the non-WNT/non-SHH subtype stems directly from the nodulus of the vermis. This anatomic location remains one of the more challenging areas to visualize, as the nodulus is situated at apex of the tent-shaped ventricular roof, where the inferior and superior medullary vela join, and requires greater retraction of the vermis to achieve the necessary line of sight.

The telovelar approach, first described by Matsushima et al and further outlined by Mussi and Rhoton, is arguably the most contemporary technique for approaching the fourth ventricle as it takes advantage of the natural anatomical planes in the cerebellomedullary fissure and is therefore a nondestructive alternative to its transvermian counterpart. ^{11 24 25} By retracting the tonsils superolaterally, elevating the uvula, and dividing the tela choroidea from the inferior medullary velum in a noneloquent plane, a corridor is created through the entire width of the inferior roof of the fourth ventricle with a similar sagittal trajectory to the transvermian approach, yet without passing through functional neural tissue. As such, this approach provides more optimal lateral exposure to structures along the lateral recess of the fourth ventricle, such as the foramen of Luschka and lower cranial nerves. ^{13 25 26} Growing evidence, including different anatomical studies and several clinical reports, supports the front-line role of the telovelar approach for most fourth ventricular lesions, including those extending to the cerebellopontine angle. ^{1 8 11 12 13 17 18 24 26 27 28 29}

Tumors located on the ependymal surface of the fourth ventricle and in the dorsal brainstem often do not invade the cerebellum and are best accessed using the telovelar approach. ¹² For fourth ventricular tumors that extend into the lateral recess, such as ependymomas, elevating the ipsilateral tonsil and using the telovelar route provides adequate surgical exposure and favorable working angles. ⁸ Larger tumors frequently stretch and thin the tela choroidea and inferior medullary velum, thus facilitating easier and wider access to the cerebellomedullary fissure. ^{5 25} In such cases, the subtonsillar perspective offers early visualization of the floor of the fourth ventricle, which allows for prompt understanding of the interface between the lesion and the dorsal brainstem. ²⁵ Further, this emphasis on superolateral retraction of the tonsils facilitates safe mobilization of posterior circulation vasculature, unique dorsal entry point into the foramen of Luschka and cerebellopontine angle, and optimal access to the lower cranial nerves and their skull base foramina. The versatility of this operative corridor is particularly significant for infiltrative lesions occurring within a naturally small fossa.

The telovelar approach can be conceptually divided into three main parts. ¹² Adequate ventricular exposure is attained in most cases by opening the tela choroidea alone either in the midline starting at the obex or superolaterally along the vertical portion of the taenia. This opening is targeted for lesions confined to the lower half of the floor. Extending the dissection laterally toward the foramen of Luschka opens the lateral recess and exposes the peduncular surfaces bordering the recess. Opening the inferior medullary velum ultimately grants entry into the rhomboid fossa, providing improved access to the floor and ventricular cavity including the superolateral recess and the superior half of the roof, except for the most rostral end proximal to the cerebral aqueduct, which can prove difficult to reach. A more favorable working angle to the rostral fourth ventricle is achieved via C1 laminectomy, thereby allowing for a more inferior starting point of dissection and thus a steeper sagittal working angle. ⁸

Although gentle tonsillar retraction and wide dissection of the cerebellomedullary fissure may significantly reduce the occurrence of cerebellar mutism or other major cerebellar deficits more common to transvermian dissection, the risks of a telovelar approach for fourth ventricular tumor resection remain inherently high, as gross total resection of common oncologic pathologies is often required to prolong survival. ^{1 30 31} The risk for iatrogenic injury of neural structures along the floor of the fourth ventricle and the neighboring vasculature, in addition to lower cranial nerve dysfunctions, CSF leak, and the risk for postsurgery shunt dependency should be discussed with all patients and their family prior to undergoing fourth ventricular surgery. ^{1 26}

Large rostral lesions of the fourth ventricle remain challenging even when approached through a generous transvermian corridor. ^{12 26} Both inferior approaches are ultimately limited in terms of their exposure of the superior-most aspect of the roof of the fourth ventricle and its conduit to the cerebral aqueduct. ^{8 13} Large tumors or lesions deeply embedded in the rostral third of the fourth ventricle are at considerable risk of subtotal resection and postoperative complications such as shunt dependency. ²⁶ For such indications, a superior trajectory to the fourth ventricle via access through the superior medullary velum, termed the superior transvelar approach, may provide a reasonable alternative. While this approach may be performed through a variety of transcranial approaches, the precise epicenter of the primary pathology, its extension superior to the tentorium, within the third ventricle, or into adjacent neural tissue such as the occipital lobe or corpus callosum will primarily dictate the optimal corridor for safe maximal resection and, thereby, which craniotomy to perform. Inclusion of the supracerebellar infratentorial corridor in this study demonstrates the versatile ability to combine both inferior and superior transventricular trajectories to the fourth ventricle through one craniotomy with few minor modifications.

Even though the superior transvelar approach is limited to the rostral third of the fourth ventricle floor due to restricted working angles in the infratentorial compartment, elevation of the inferior edge of the torcula and transverse sinuses from minimal cephalad extension of the suboccipital craniotomy can expand the angle of access between the tentorium and cerebellum, thus facilitating a larger exposure. ⁶ To ensure safe, maximal mobilization of the cerebellar hemispheres to further widen this corridor, the inferior and lateral extent of suboccipital craniotomy should be fashioned similarly to those using an inferior transventricular trajectory, with termination at the foramen magnum caudally and the medial surface of the bilateral occipital condyles laterally. Furthermore, tumors in the upper aspect of the fourth ventricle may stretch the superior medullary velum superiorly and posteriorly to an extent that minimizes the need for additional cerebellar retraction. Fourth ventricular tumors often cause obstructive hydrocephalus requiring CSF diversion. ^{3 32} Besides providing direct surgical access to the lesion, fenestration of the superior medullary velum opens up an additional pathway for CSF flow via the quadrigeminal cistern, potentially reestablishing physiologic CSF circulation and managing concomitant hydrocephalus. ^{6 33}

Overall, the number of anatomical studies and clinical reports on the supracerebellar infratentorial route for fourth ventricular access is limited, which renders definite conclusions on the feasibility and applicability of this technique in routine clinical practice difficult. Both the occipital transtentorial approach and the interhemispheric precuneal approaches constitute additional options for accessing lesions in the fourth ventricle and dorsal brainstem through the superior medullary velum, albeit they are performed via different transcranial approaches than the midline suboccipital craniotomy described herein. ^{6 34 35} Despite the choice of craniotomy, considerable intraoperative risks in this region include iatrogenic injury to the tectal plate, pineal gland, trochlear nerves, as well as the deep cerebral venous system.

A succinct set of surgical techniques and considerations should be utilized upon performance of fourth ventricular surgery through a midline suboccipital craniotomy. First, the extent of boney removal in the craniotomy should be customized to each patient's individual anatomy to promote optimal visualization and mobilization of the posterior fossa components, to enhance access to the various planes of dissection, and to allow for improved dynamic retraction. When designing the transcranial and transventricular approach, consideration must also be given to the natural anatomical planes disrupted by the tumor, including its exophytic growth, to mitigate injury to healthy parenchyma. Similarly, meticulous opening and releasing of arachnoid adhesions will help mobilize the tonsils and vermis in inferior corridors as well as facilitate CSF release from accessible cisterns, thereby allowing for cerebellar relaxation and facile tumor resection. To further achieve a safe operative window, the vertebral arteries, PICAs, and choroid plexus should be identified early and gently mobilized away from view and protected. Likewise, accurate anticipation of the cisternal courses of the lower cranial nerves and the location of their nearby foramina is imperative to avoid injury, particularly when they are displaced or encased by tumor; therefore, mastery of the microsurgical anatomy of the fourth ventricle and adjacent skull base should be requisite for neurosurgical trainees. Finally, the use of angled endoscopes in posterior fossa surgery facilitates improved access to deep ventral structures and optimizes direct visualization of the intended transventricular approach.

Conclusion

The midline suboccipital craniotomy is a foundational skull base approach that provides access to pertinent structures in the posterior fossa, including fourth ventricular access through three main surgical routes. The most traditional route to the fourth ventricle, the transvermian approach, provides access to the full extent of the floor, but is limited in its lateral exposure and is associated with several postoperative neurological sequelae. Although the telovelar approach may have a modest decrease in sagittal working angles, this more contemporary technique takes advantage of natural anatomical clefts that also facilitate access to lesions situated in the cerebellopontine angle. For a subset of lesions located in the rostral fourth ventricle, cerebral aqueduct, and/or dorsal mesencephalon, superior approaches traversing the superior medullary velum provide another feasible option. In the current cadaveric study, we described a comprehensive, operatively oriented step-by-step approach to mastering the midline suboccipital craniotomy with telovelar, transvermian, and superior transvelar approaches to the fourth ventricle and relevant surgical anatomy. Furthermore, we highlighted illustrative cases that emphasize their indications, and provided contextualizing conceptual and technical details to aid trainees in understanding approach selection and optimizing their performance in the operating room.

Funding Statement

Funding The Rhoton Neurosurgery and Otolaryngology Surgical Anatomy Program within which this work was completed received the funding from Department of Neurosurgery, Mayo Clinic, Rochester, Minnesota, Joseph and Barbara Ashkins Endowed Professorship in Surgery and Radiology, Mayo Clinic, Rochester, Minnesota and Charles B. and Ann L. Johnson Endowed Professorship in Neurosurgery, Mayo Clinic, Rochester, Minnesota.