Relevant microsurgical anatomy
The cerebellomedullary fissure separates the cerebellum from the medulla, extending upwards between these regions. This fissure is at the dorsolateral aspect of the inferior half of the roof of the fourth ventricle and has to be surgically enlarged in order to perform the telovelar approach. Its anterior wall is formed by the posterior surface of the medulla, the inferior medullary velum and the tela choroidea; its posterior wall is formed by the uvula and nodule of the vermis medially and the tonsils and biventral lobules laterally . (Fig. 3d).
The inferior medullary velum is a thin bilateral semitranslucent butterfly-shaped sheet of neural tissue that stretches from the nodule medially and to the flocculus laterally at each side. It is attached inferiorly to the tela choroidea at the telovelar junction. (Fig. 3d) It is continuous with the superior medullary velum at the level of the fastigium.
The tela choroidea forms the lower portion of the inferior half of the roof fourth ventricle and extends from the nodule medially to the lateral recess laterally. The tela choroidea does not completely enclose the forth ventricle and forms three openings which connect this cavity to the subarachnoid space: the paired foramina of Luschka, located at the outer margins of the lateral recesses, and the foramen of Magendie, located just inferior to the uvula . Caudally, the tela choroidea includes the inferolateral edges of the floor along narrow white ridges, the teenier, which meet at the obex (Figs. 1 and 2).
The posterior inferior cerebellar artery (PICA) originates at the vertebral artery and course within the cerebellomedullary fissure, forming its telovelotonsillar segment between the tonsil posteriorly and the tela choroidea and inferior medullary velum anteriorly (Fig. 2c). It loops around the superior pole of the tonsil, forming a convex curve referred as the supratonsillar loop, where it faces the inferior medullary velum. The PICA is responsible for supplying the inferior medullary velum, the inferior cerebellar peduncle and the suboccipital cerebellar surface .
The superior cerebellar peduncle (SCP) is the largest cerebellar efferent bundle, which is a group of fibres that emerge from the hilus of the dentate nucleus. These fibres pass rostrally into the upper fourth ventricle. The superior cerebellar peduncles emerge from the upper and medial part of the white substance of the hemispheres. They project in parallel and are placed under cover of the superior and inferior colliculi. They connect the dentate nucleus to the red nucleus—where they decussate, forming the dentatorubrothalamic tract, part of Guillain-Mollaret triangle (with the connection in between the inferior olivary nucleus with the red nucleus via the central tegmental tract and the dentate afferents from the same inferior olivary nucleus). The superior cerebellar peduncles are joined by the superior medullary velum, can be followed up to the inferior colliculi and disappear in the red nucleus (Figs. 1 and 2f). A comprehensive description of all the tracts passing through the SCP is provided in Table 1.
The superior medullary velum (valve of Vieussens; anterior medullary velum) is a thin lamina of white matter extending between the cerebellar peduncles and participates in the formation of the roof of the fourth ventricle. The dorsal surface of the lower half is covered by the lingula of the cerebellum, caudal to the exit of the fourth cranial nerve, and separating the lingula from the superior medullary velum creates a working corridor (the cerebellomesencephalic fissure) . The superior medullary velum does not appear to be a uniform structure when one moves from a mediolateral direction as it forms a larger angle with the floor. It connects both superior cerebellar peduncles. Besides the decussating fibres of these cerebellar peduncles which lie deep in relation to the superior medullary velum, there are few essential neural structures in the composition of the superior medullary velum, making its sacrifice, not clinically significant, especially caudal to the decussation of the trochlear nerves  (Fig. 2a). The superior medullary velum receives its arterial supply from the superior cerebellar artery via its precerebellar branch . Venous drainage is by the superior cerebellar peduncle veins, which unite superiorly at the emergence of the trochlear nerve from the brainstem, to form the vein of the cerebellomesencephalic fissure .
The uvulotonsillar telovelar approach is developed in between the structures of the paleocerebellum, mainly responsible for the muscle tonus. The medial tonsillar surface is displaced laterally away uvula, exposes the inferior half of the ventricular roof (tela choroidea and inferior medullary velum) and the opening the tela choroidea upward to the telovelar junction provides access to the floor of the fourth ventricle, from the aqueduct to the obex and to the medial portion of the lateral recesses  (Fig. 2c–f). The junction of the uvula and pyramid limits the lateral and medial retractions of the tonsil and the uvula at the uvulotonsillar space, limiting the exposure of the cisternal surface of the superior half of the ventricular roof. However, the inferior medullary velum is continuous at the level of the fastigium with the superior medullary velum, and the additional opening of the inferior medullary velum can improve the exposure of the ventricle roof, including the fastigium, superolateral recesses and the ventricular surface of the superior half of the roof  (Fig. 2c–f).
Diffusion MRI data and tractography dissection
Diffusion MRI data were obtained for one healthy young adult (male, age category 26–30 years) from the Human Connectome Project (Van Essen et al., 2013) (https://www.humanconnectome.org/) . The data were acquired on a 3T Siemens “Connectome Skyra” scanner using a 32-channel head coil and consisted of 90 diffusion directions; b value = 2000 s/mm2; 18 B0 volumes; 1.25 mm isotropic voxels; TE = 89.5 ms and TR = 5520 ms. Data were corrected for motion, eddy-current distortions and susceptibility distortions, according to the HCP minimal pre-processing pipelines . Diffusion tensor (DTI) modelling and tractography were performed in StarTrack (https://www.mr-startrack.com/) with a fractional anisotropy (FA) threshold of 0.2, step size of 0.5 mm and angle threshold of 30°.
The superior cerebellar peduncles were manually dissected in TrackVis (http://trackvis.org/) based on sphere regions of interest (ROI). The first ROI was placed in the cerebellum at the level of the dentate nucleus and the second in the brain stem at the level of the red nucleus. Only tractography streamlines terminating in both ROIs survived the filtering process, thereby effectively isolating the pathway of interest. To visualize the control tractography data on the patient’s brain, linear and non-linear image registration were performed using the Advanced Normalization Tools . Image registration was based on the T2w images as they provide good contrast in subcortical regions involving the dentate nucleus and red nucleus. The resulting affine matrix and warp field were applied to the density map of the control subject’s tracts. (Fig. 3).