Medial-tonsillar telovelar approach for resection of a superior medullary velum cerebral cavernous malformation: anatomical and tractography study of the surgical approach and functional implications

Cerebral cavernous malformations of the superior medullary velum represent a challenge in terms of safest surgical approach. A very good understanding of the anatomical structures surrounding the superior medullary velum and the efferent pathways of the cerebellum at risk of being damaged is mandatory to approach the superior medullary velum region. Moreover, access to this deep region requires knowledge on how to enhance the advantages offered by opening microsurgically the uvulotonsillar compartment of the cerebello-medullary fissure and consequently the tela choroidea and the inferior medullary velum to gain access to the cavernous of the superior medullary velum.

A 46-year-old man presented with a progressively deteriorating 1-year history of cerebellar ataxia and left superior arm intentional tremor. An MRI scan showed a round lesion in the superior medullary velum suggestive of a cerebral cavernous malformation. (Fig. 4) The patient had a right unilateral uvulotonsillar telovelar approach and successful resection of the lesion, proved to be cerebral cavernous malformation after histopathology report. Post-operative course was uneventful. At 3 months follow-up, the patient experienced a complete remission of symptoms, and he is currently under regular follow-up.

Superior cerebellar peduncle syndrome was first described in monkeys by Ferrier and Turner in 1894 [11]. Other groups replicated their experiments in monkeys (Keller and Hare, 1934 [16]; Ferraro and Barrera, 1936 [10]; Walker and Boterrel, 1937 [33]). The main consequences of the complete surgical lesion of the SCP were ataxia (improved with time but do not resolve completely), tremor, dysmetria and decomposition of movement in the ipsilateral extremities. Nystagmus and hypotonia were temporary and not reproducible in all cases. The incomplete lesion of the SCP produced a mild and temporary syndrome.

Girard et al published in 1950 [13] the first description of this syndrome in humans in a context of superior cerebellar artery infarct. The link between the SCP and palatal myoclonus was initially performed by Trelles (1935) [29] and posteriorly described by the work performed by Lapresle and Ben Hamida (1965–1971) [17] who showed the pathway between the dentate nucleus, contralateral inferior olive and the red nucleus involving the SCP, the inferior cerebellar peduncle and the central tegmental tract. The hypertrophic olivary degeneration is a rare but well-known MRI sign of transsynaptic degeneration apparent 3–4 weeks after injury of Guillain-Mollaret triangle. Cerebral cavernous malformations and posterior fossa tumours are among the most common lesions in the neurosurgical literature [21].

Great attention has been given to the SCP in the last decade as a major player in the cerebellar mutism. This condition is characterized by reduced or entirely absent speech with onset generally within the first post-operative week, and multiple case reports in both children [3, 5, 20] and adult [31] with supporting DTI studies have documented unilateral atrophy of the SCP in relation with the cerebellar mutism.

These symptoms related SCP injury render crucial the preservation of this structure during the surgical approaches to SMV lesions. Both this function-preserving approach and the fact that the SCP is not known to be epileptogenic supported the decision of not removing the haemosiderin deposits around the lesion. This is supported by the most recent literature that advocates a careful risk-benefit approach to the haemosiderin deposits in eloquent cerebral cavernous malformations [8]. The classic surgical corridors are hampered by natural anatomic barriers: the cerebellum and the tentorium. When considering a space occupying lesion that enlarges and displaces the boundaries of the SMV—as a tumour—the growth pattern may facilitate a particular surgical approach [28]. However, the surgical approach for a lesion that grows within the SMV without enlarging it is difficult and hazardous when considering the corridor with the least anatomical disruption despite proper exposure for resection. The most common approaches are summarized in Fig. 7 and Table 2.

Even though telovelar, transvermian and supracerebellar infratentorial are the most common approaches to posterior fossa pathology, different groups described new surgical approaches to lesions within mesencephalum and fourth ventricle [4, 6, 7, 9, 26] exploring the natural fissures and CSF spaces within the posterior fossa. The fissures around the Tonsils have been extensively explored, and different names have been given to similar approaches via similar spaces and fissures [4]. These can be divided into two large groups: medial-tonsillar approaches and lateral-tonsillar approaches. The medial-tonsillar approaches include all the surgical corridors in between the tonsil and the uvula and pyramid of the vermis: tonsilouvular, telovelar and cerebellomedullary fissure approaches. The lateral-tonsillar approach includes all the surgical corridors in between the tonsil and the biventral lobule of the cerebellar hemisphere: supratonsillar and uvulobiventral approaches. The criteria underlying the choice of one of these routes lies in the location of the lesion: medial-tonsillar approach to midline (floor of the fourth ventricle) [27] and superior located lesions (tegmentum and aqueductal lesions) [9] and lateral-tonsillar approach to lateral recess [26] and inferior cerebellar peduncle [18].

For the first time, we describe a medial-tonsillar approach to a SMV lesion. This approach is safer for preservation of both SCP and dentate nucleus when compared with lateral-tonsillar approach. The inferior cerebellar peduncle is an anatomic reference to dentate nucleus presentation as it runs anterior to it [1]. This allows a direct corridor to the SMV region with no need for cerebellar retraction or parenchymal resection. As it is appreciated in the DTI study (Fig. 3), the lesion was located medially to the ipsilateral dentatorubrothalamic tract. Therefore, an approach coming from the fourth ventricle is the only available to preserve this crucial pathway within the SCP. When compared with the transvermian approach, it provides a better access to the foramen of Luschka, lateral and supralateral recesses which is crucial for laterally and inferior located lesions in the SMV. When compared with the previous approaches, it has a decreased exposure of the superior half of the SMV, near the aqueduct (> 75% the distance between the obex and the aqueduct). It is technically demanding considering the exposure of the craniovertebral junction and the close relation with the PICA along the exposure and the vertebral artery during the initial bone exposure. Nevertheless, the vascular intimal relationship is the surgical endeavour of the posterior interhemispheric approach as well, which balance both these approaches in term of the vascular risk. Even though the craniocervical junction is exposed and resection of the posterior arch of C1 and a suboccipital craniotomy is performed, the craniovertebral instability is an uncommon event. The risk of hydrocephalus can be minimized by the bilateral exposure the cerebellomesencephalic fissure and the augmentation of the fourth ventricle (communication with the aqueduct and the cisterna magna) [27]. Even though the resection of the tonsils can provide a wider corridor to the lateral (Luschka foramina) and superior (Sylvian aqueduct) structures [15], the preservation of the non-involved anatomical structures should be attempted as they can provide support to the cerebellar retraction if needed and provide anatomical orientation if redo surgery is required.