Wide opening of the uvulo-tonsillar space to facilitate early intraoperative control of feeding arteries in fourth ventricular tumor surgery: How I do it

Abstract

Background

The feeding arteries of fourth ventricular tumors often arise from the posterior inferior cerebellar artery (PICA), particularly the tonsillomedullary and telovelotonsillar segments. Achieving early control of these feeders is critical to minimize bleeding.

Method

We present a surgical technique that facilitates early control of feeders from the telovelotonsillar segment, which courses dorsally to the inferior medullary velum and tela choroidea, by wide and deep opening of the uvulo-tonsillar space before tumor manipulation.

Conclusion

This technique enables early interruption of PICA feeders, allowing safe and effective devascularization of fourth ventricular tumors with abundant arterial supply.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00701-026-06798-3.

Relevant surgical anatomy

The fourth ventricle is ventrally bordered by the pons and medulla oblongata and dorsally by the cerebellar vermis, superior and inferior medullary velum, and tela choroidea. Surgical access to fourth ventricular lesions commonly requires a wide incision of the tela choroidea, and in some cases, the inferior medullary velum from the dorsal aspect [4, 6]. To expose these structures, opening the cerebellomedullary fissure is required [4].

This fissure consists of two compartments: the medullo-tonsillar space, between the medulla and cerebellar tonsil, and the uvulo-tonsillar space, between the uvula and tonsil. Wide opening of the medullo-tonsillar space exposes the tela choroidea, and lateral extension of its incision allows superolateral retraction of the tonsil, ensuring wide exposure of the fourth ventricular cavity. From a surgical perspective, adequate opening of the medullo-tonsillar space is essential for direct and safe access to the fourth ventricle.

The posterior inferior cerebellar artery (PICA) consists of five segments: anterior medullary, lateral medullary, tonsillomedullary, telovelotonsillar, and cortical [3]. Among these, the tonsillomedullary and telovelotonsillar segments lie closest to the fourth ventricle and consequently supply its tumors. Within the cerebellomedullary fissure, the tonsillomedullary segment runs through the medullo-tonsillar space, while the telovelotonsillar segment traverses the uvulo-tonsillar space. The former is naturally exposed during the opening of the medullo-tonsillar space as part of the fourth ventricle approach. However, because the telovelotonsillar segment lies dorsal to the inferior medullary velum and tela choroidea, a wide uvulo-tonsillar space opening is required for adequate exposure of this segment.

Description of the technique

Preoperative evaluation

Magnetic resonance imaging (MRI) is used to assess hydrocephalus secondary to fourth ventricular obstruction, the extent of tumor extension into the lateral recess, and possible adhesion of the fourth ventricular floor. Time-of-flight magnetic resonance angiography (TOF-MRA) or, when necessary, digital subtraction angiography is performed to delineate the detailed course of the PICA and identify the side and segment of the feeding arteries supplying the tumor.

Position and craniotomy

The patient is positioned prone with the neck flexed and the head secured in a Mayfield three-pin head holder. A midline occipital incision and standard suboccipital craniotomy, including foramen magnum opening, are then performed.

Opening of the cerebellomedullary fissure and management of feeding arteries

The dura is opened in a Y-shaped manner. At this stage, the proximal portion of the tonsillomedullary segment of the PICA can be identified. The medullo-tonsillar space of the cerebellomedullary fissure is first opened. As this space is relatively loose, it can be easily accessed through sharp arachnoid dissection, allowing visualization of the distal course of the tonsillomedullary segment (Fig. 1a). When tumor feeders arise from the telovelotonsillar segment of the PICA, the uvulo-tonsillar space should be widely opened. Although a partial caudal opening is sufficient for entry into the fourth ventricle, a wider dissection is required to identify and manage feeding arteries from the telovelotonsillar segment, which lies deep within the field (Fig. 1b). Despite being narrower than the medullo-tonsillar space, the uvulo-tonsillar space can be safely expanded through meticulous arachnoid dissection. As the tela choroidea exposed through the medullo-tonsillar space is incised laterally, feeding arteries from the tonsillomedullary segment entering the tumor are simultaneously coagulated and transected (Fig. 1c). Furthermore, by coagulating and transecting the feeders entering the tumor through the tela choroidea and inferior medullary velum from the fully exposed telovelotonsillar segment (Fig. 1 d), most of the PICA-derived arterial supply can be managed before direct tumor manipulation begins.

Fig. 1.

Intraoperative photographs. a The right medullo-tonsillar space before opening. b The right uvulo-tonsillar space before opening. c Opening of the right medullo-tonsillar space, exposing the tonsillomedullary segment of the PICA. d Wide opening of the right uvulo-tonsillar space down to its deepest portion, demonstrating the entire course of the telovelotonsillar segment of the PICA. e The right tela choroidea before incision. f Coagulation of feeding arteries arising from the tonsillomedullary segment of the PICA. g The right inferior medullary velum before incision. h Coagulation of feeders arising from the telovelotonsillar segment of the PICA. i Sufficient opening of the left lateral recess, showing the tumor extension into this region

Tumor removal

Since most feeding arteries have been managed, intraoperative bleeding during tumor debulking is minimal. After both cerebellar tonsils are retracted superolaterally to widely expose the fourth ventricular cavity, the tumor is circumferentially dissected. In cases of medulloblastoma, which typically arise from the inferior medullary velum [8], adhesion to the ventricular floor is usually minimal or absent. Conversely, ependymomas originating from the ventricular floor often exhibit strong adhesions in certain regions [1, 7]. Forceful dissection in these adherent areas should be avoided to prevent serious neurological complications; in such cases, leaving a thin layer of residual tumor on the ventricular floor should be considered.

For lesions extending laterally through the lateral recess into the cerebellomedullary cistern, a continuous opening of the tela choroidea toward the foramen of Luschka allows direct visualization [2, 5] (Fig. 1e).

Video 1 demonstrates medulloblastoma removal. In this case, the main feeders originated from the telovelotonsillar segments of both PICAs, with additional smaller feeders from the tonsillomedullary segments bilaterally (Fig. 2).

Fig. 2.

Pre- and postoperative MRI findings. a, b Preoperative T2-weighted images reveal a hyperintense tumor occupying the fourth ventricle. A thin hyperintense layer indicative of cerebrospinal fluid is observed between the tumor and the ventricular floor (a). The tumor extends laterally through the left lateral recess toward the cerebellomedullary cistern (b). c Preoperative T1-weighted gadolinium-enhanced image demonstrating partial enhancement within the tumor. d TOF-MRA reveals the feeding arteries arising from the telovelotonsillar segments of both PICAs. e, f Postoperative T2-weighted images confirm complete tumor removal, including the portion that extends laterally

Postoperative management

Early postoperative management includes blood pressure control to prevent hemorrhage and assessment of swallowing function before initiating oral intake when the lateral cerebellomedullary cistern is manipulated.

Indications

This approach is applicable to tumors with abundant blood supply located within the fourth ventricle, particularly when their feeding arteries arise from the telovelotonsillar segment of the PICA.

Limitations

In tumors with poor vascularity, such as epidermoid cysts, the wide opening of the uvulo-tonsillar space is not required for vascular control. Even in tumors with abundant blood supply, when most feeding arteries arise from the tonsillomedullary segment of the PICA, extensive opening of the uvulo-tonsillar space is generally unnecessary.

How to avoid complications

For tumors adherent to the floor of the fourth ventricle, total removal should be avoided; instead, a thin residual layer should be left on the ventricular floor, where the critical nuclei and tracts are densely located.

For tumors extending laterally from the lateral recess toward the cerebellomedullary cistern, microsurgical dissection should be performed under lower cranial nerve monitoring to ensure safe manipulation near the nerves.

Specific information for the patient

Dissection of the tumor from the floor of the fourth ventricle poses a significant risk of complications, including disturbances in ocular movement, facial nerve palsy, and respiratory dysfunction. Surgical manipulation around the cerebellomedullary cistern and lower cranial nerves may also result in dysphagia or hoarseness. In addition, patients should be counseled regarding general surgical risks.

Ten key points

1. Preoperative MRI should determine whether the tumor originates from the inferior medullary velum, as in medulloblastoma, or from the floor of the fourth ventricle, as in ependymoma, and determine any lateral extension into the lateral recess.

2. During preoperative evaluation, it is crucial to identify the segment of the PICA that gives rise to the feeding arteries of the tumor.

3. To achieve wide exposure of the fourth ventricle, the medullo-tonsillar space of the cerebellomedullary fissure should be adequately and laterally opened.

4. The feeding arteries arising from the tonsillomedullary segment of the PICA can be coagulated and transected concurrently during incision of the tela choroidea.

5. To expose the telovelotonsillar segment of the PICA, the uvulo-tonsillar space should be opened to its deepest portion.

6. These arteries can be managed before direct tumor manipulation by coagulating and transecting feeders entering the tumor through the inferior medullary velum and tela choroidea from the telovelotonsillar segment.

7. Achieving early control of the major feeding arteries before tumor debulking minimizes intraoperative bleeding and shortens operative time.

8. When the tumor is firmly adherent to the floor of the fourth ventricle, aggressive dissection should be avoided to prevent complications; instead, a thin layer of the tumor may be intentionally left on the adherent area.

9. For tumors extending laterally from the lateral recess toward the cerebellomedullary cistern, continuous opening of the tela choroidea up to the foramen of Luschka allows direct visualization and safe removal under microscopic control.

10. Intraoperative neurophysiological monitoring should be employed as appropriate, based on the degree of tumor extension and ventricular floor involvement.

Supplementary Information

Below is the link to the electronic supplementary material.

Author Contributions

M.M. wrote the manuscript, prepared the figures and video. A.Y. revised the manuscript. E.I. revised the manuscript.

Funding

No funding was received for this work.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethical approval

Not applicable.

Competing interests

The authors declare no competing interests.