Tonsillobiventral fissure approach to the lateral recess of the fourth ventricle

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OBJECTIVE

Surgical access to the lateral recess of the fourth ventricle (LR) is suboptimal with existing transvermian and telovelar approaches because of limited lateral exposure, significant retraction of the cerebellar tonsil, and steep trajectories near brainstem perforator arteries. The goal in this study was to assess surgical exposure of the tonsillobiventral fissure approach to the LR, and to describe the relevant anatomy.

METHODS

Two formaldehyde-fixed cerebella were used to study the anatomical relationships of the LR. Also, the tonsillobiventral fissure approach was simulated in 8 specimens through a lateral suboccipital craniotomy.

RESULTS

The pattern of the cerebellar folia and the cortical branches of the posterior inferior cerebellar artery were key landmarks to identifying the tonsillobiventral fissure. Splitting the tonsillobiventral fissure allowed a direct and safe surgical trajectory to the LR and into the cerebellopontine cistern. The proposed approach reduces cervical flexion and optimizes the surgical angle of attack.

CONCLUSIONS

The tonsillobiventral fissure approach is a feasible and effective option for exposing the LR. This approach has more favorable trajectories and positions for the patient and the surgeon, and it should be added to the armamentarium for lesions in this location.

ABBREVIATIONS CMF, tCMF = cerebellomedullary fissure, transcerebellomedullary fissure; LR = lateral recess of the fourth ventricle; PICA = posterior inferior cerebellar artery.

Article Information

Correspondence Arnau Benet, Department of Neurosurgery, Rm. M-779, University of California, San Francisco, 505 Parnassus Ave., San Francisco, CA 94143. email: arnau.benet@ucsf.edu.

INCLUDE WHEN CITING Published online October 28, 2016; DOI: 10.3171/2016.8.JNS16855.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    A: The fourth ventricle and rhomboid fossa viewed from an inferior trajectory. The tonsils are resected. The LR is located at the lateral corners of the rhomboid fossa. When followed laterally, it will open into the cerebellopontine cistern. The right LR is shown with a curved green arrow. Note the close relationship between the inferior cerebellar peduncle and the LR. B: Lateral view of the cavity of the fourth ventricle and the LR of the right half of the brainstem and cerebellum. The inferior cerebellar peduncle curves over the LR (green arrow) to connect the medulla oblongata to the cerebellum. The tonsillobiventral fissure (red dashed arrow) reaches the LR. C: Oblique view of the lower brainstem and cerebellum with the tonsils removed. The tela choroidea and the choroid plexus of the fourth ventricle are shown. The tela and the inferior medullary velum form the lower half of the roof of the fourth ventricle. The tela forms the posterior aspect of the LR. Blue probe shows the cavity and trajectory of the LR. D: The CMF is opened by elevation of the tonsil. This maneuver will expose the LR (yellow arrow). E: The tonsillobiventral fissure (green arrow) is opened in another specimen viewed from below. It is clear that the opening of the tonsillobiventral fissure leads to the LR. Please compare the different trajectories of the 2 approaches. F: The inferior view of the brainstem and the cerebellum. Note the relationship of tonsil to the biventral lobule and the vermis. The tonsillobiventral fissure (dashed green lines) runs inferolaterally from the uvulotonsillar junction to reach the cerebellomedullary cistern. The PICA emerges from the tonsillobiventral fissure to branch onto the suboccipital surface of the cerebellum, as denoted by the asterisks. G: Tonsillobiventral fissure approach. The right tonsillobiventral fissure is split to show the inferolateral trajectory to the LR of the fourth ventricle (green arrow). Note the exposed choroid plexus and the flocculus in the cerebellopontine cistern. Yellow arrow shows the trajectory of the tCMF approach to the LR. Cer. = cerebellar; fis. = fissure; Inf. = inferior; med. = medullary; Ped. = peduncle; plex. = plexus; T = tonsil; Tonsillobivent. = tonsillobiventral; Sup. = superior; vel. = velum; XI = 11th cranial nerve. Figure is available in color online only.

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    Schematic illustration and comparison of the CMF and tonsillobiventral fissure approaches to the LR. A and B: Patient positioning and craniotomy. C and D: Sagittal trajectory and manipulation of different segments of the PICA. E and F: Different trajectories in the operative view. G and H: Different trajectories from the inferior anatomical perspective. Copyright UCSF Skull Base and Cerebrovascular Laboratory. Published with permission. Figure is available in color online only.

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    Coronal FLAIR (A) and parasagittal contrast-enhanced T1-weighted (B) MR images obtained in a 24-year-old man with headaches after weight lifting showing a mixed-intensity lesion with a hypointense hemosiderin ring compatible with a cavernous malformation. After a suboccipital craniotomy in the prone position, the tonsillobiventral fissure was exposed (C) and opened (D) to expose the LR following the PICA branches, and also following the retrotonsillar and supratonsillar veins. Once the supratonsillar recess was opened, the hemosiderin staining on the cerebellar surface was identified, a small trajectory was made through the overlying cortex, and the malformation was immediately found. The malformation was dissected circumferentially and removed en bloc (E and F). Postoperative coronal (G) and sagittal (H) contrast-enhanced T1-weighted MR images show complete removal of the lesion. Cer. = cerebellar; Fis. = fissure; Inf. = inferior; Ped. = peduncle; Sup. = superior. V. = vein. Figure is available in color online only.

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    Comparison of the extent of exposure of the LR between the telovelar (left) and supratonsillar (right) approaches on the right side of a cadaveric head. The green arrow and the yellow arrow designate the approaches. bivent. = biventral; n. = nerve. Figure is available in color online only.

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    Depiction of the right LR. The fibers of the inferior cerebellar peduncle (partially resected) pass in close relationship to the dentate nucleus. nuc., nucleus. Figure is available in color online only.

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