Fig. 8.1
Suboccipital surface of the cerebellum
The fourth ventricle, tentlike in appearance, is located between the cerebellum and brain stem. Dorsally, the roof is divided into superior and inferior parts. The ventricular surface of the superior part of the roof of the fourth ventricle is further divided into a median part formed by the superior medullary velum and two lateral parts formed by the inner parts of the cerebellar peduncles. Access to the fourth ventricle is therefore either by the cerebellomedullary fissure described above or through the vermis.
Understanding posterior fossa vasculature, in particular the supply to the cerebellum and the brain stem, is crucial to the safety of median suboccipital approaches. Three neurovascular complexes in the posterior fossa have been described by Rhoton [9]: an upper complex related to the superior cerebellar artery (SCA) which contains the midbrain; cranial nerves III, IV, and V; the cerebellomesencephalic fissure; the superior cerebellar peduncle; and the tentorial surface of the cerebellum. The middle complex, supplied by the anteroinferior cerebellar artery (AICA), contains the pons; the cranial nerves VI, VII, and VIII; the middle cerebellar peduncle; the cerebellopontine fissure; and the petrosal surface of the cerebellum. The lower complex, related to the posterior inferior cerebellar artery (PICA), contains the medulla and lower cranial nerves IX, X, XI, and XII. The PICA has the most tortuous course of the posterior circulation arteries and is exposed through a median suboccipital craniotomy. It is closely related to the cerebellomedullary fissure; coursing around the cerebellar tonsils, it enters the cerebellomedullary fissure and traverses the lower half of the roof of the fourth ventricle to finally exit the cerebellomedullary fissure. It provides branches that are distributed to the vermis and hemispheres of the suboccipital surface.
Veins of the posterior fossa may become an issue when enlarged due to tumor growth. They are divided into four groups: superficial, deep, brain stem, and bridging veins [7]. Of relevance to the median suboccipital approach are the large inferior vermian veins, which give rise to inferior hemispheric veins. These need to be carefully divided in midline approaches through the vermis.
This drawing represents an enlarged view of the suboccipital surface of the cerebellum. The suboccipital fissure divides the cerebellum into a superior and inferior part. Three different parts of the vermis are represented here: the tuber lies above the suboccipital fissure, the pyramid and the uvula below. The foramen of Magendie is located below the uvula, in the midline. The cerebellar tonsils lie lateral to the foramen of Magendie.
8.3 Goals of the Surgery
The aims of surgery for posterior fossa tumors, associated with brain stem compression and hydrocephalus, are (1) to decompress the brain stem and cerebellum, (2) to restore normal cerebrospinal fluid pathways through the fourth ventricle and the foramina of Luschka or Magendie, and (3) to attempt complete resection, without new neurological deficit, in order to improve postoperative survival.
8.4 Surgical Planning and Types of Approaches
The median suboccipital approach provides access to most pediatric tumors in the posterior fossa. Careful assessment of preoperative imaging to determine tumor location within the fourth ventricle, the vermis, or cerebellar hemisphere is essential to determine the most appropriate surgical route and strategy. Depending on the tumor localization, approaches through a median suboccipital craniotomy can be classified into five major categories: (1) transcortical, (2) transvermian, (3) telovelar, (4) infratentorial supracerebellar, and (5) combined, involving a telovelar approach and a small inferior vermian incision or involving a telovelar approach and a supracerebellar approach as described recently [10]. Brain stem tumors can be resected either by a transvermian or telovelar approach (Figs. 8.2 and 8.3).
Fig. 8.2
Median suboccipital approaches to the posterior fossa. The main classical approaches to the posterior fossa via a median suboccipital approach are represented. (a) Sagittal view of the posterior fossa. Three surgical corridors are represented. In yellow, the infratentorial supracerebellar approach which gives access to pineal region tumors or to the superior part of voluminous posterior fossa tumors; in green, the transvermian approach, a classical route to access tumors from the vermis or within the fourth ventricle; in red the telovelar approach, which gives access to the fourth ventricle via the cerebellomedullary fissure after retraction of the cerebellar tonsils and dissection of the tela choroidea. (b) Axial view of the posterior fossa and the brain stem. Two surgical corridors are represented: in gray is the combination of the three approaches described above which are seen as one on this view. In blue is the corridor to access hemispheric tumors which have a median component
Fig. 8.3
(a, b) Tumors of the fourth ventricle with compression of the vermis but no extensive vermian infiltration. These cases are good candidates to telovelar approach. (c) Tumor of the fourth ventricle with limited dimensions (<3 cm) but with extensive infiltration of the vermis. In this case telovelar approach with limited incision of the lower vermis was sufficient to achieve complete removal. (d) Tumor of the fourth ventricle with largest diameter >4 cm and extensive compression/infiltration of the lower vermis. In similar cases incision of the lower vermis for traditional transvermian approach is usually necessary. (e) Large medulloblastoma of the upper vermis without involvement of the fourth ventricle. Transvermian approach with incision of the upper vermis is here the only option. (f, g) Medulloblastoma of the upper vermis with extensive lateral adhesion to tentorium. The tumor was completely removed through a supracerebellar approach
Initially, standard neurosurgical practice involved splitting the vermis to access “midline” posterior fossa tumors [11]. This approach was often associated with the known “cerebellar mutism syndrome” due to surgical incision of the vermis and lateral retraction to the dentate gyrus and its outflow tract [12–14].
The telovelar approach is now a well-accepted surgical route and should be performed alone or in combination with incision of the inferior vermis dependent on the tumor size and its superior extension [15]. The microsurgical anatomy of this approach has been well described [8]. It involves gentle retraction of the tonsils laterally, which exposes both the tela choroidea and the inferior medullary velum. The tela choroidea is a thin arachnoid membrane and forms the lower portion of the inferior half of the roof of the caudal wall of each lateral recess. The foramen of Magendie delimits the lowest part of the membrane. In its superior parts and laterally to the uvula, it prolongs into the telovelar junction followed by the inferior medullary velum (Fig. 8.4). Opening of the tela gives access to the floor of the fourth ventricle, from the obex to the aqueduct. Further rostral opening of the velum gives additional access to the superior half of the roof of the fourth ventricle, the fastigium and the superolateral recess, a technique well described by Mussi [8].
Fig. 8.4
Anatomy of the cerebellomedullary fissure
Although this surgical corridor appears restricted, it has been shown to be sufficient to remove large fourth ventricle tumors without splitting the inferior vermis [15]. In cadaveric dissection it has been quantified, using triangles from defined anatomical points, that the telovelar approach with the removal of C1 posterior arch provided a larger working area than the transvermian approach except for a limited angle to the rostrum of the fourth ventricle [16]. These findings have been confirmed by other studies [17]. For ependymomas with lateral extension near the brain stem or cranial nerves, this surgical corridor offers attractive possibilities, preventing potentially devastating complications associated with the transvermian approach [18]. For giant posterior fossa midline tumors, some surgeons have proposed a combined transventricular and infratentorial supracerebellar approach, highlighting that this was a safe technique, avoiding the incision of the vermis [10]. In contrast however, others have raised concerns regarding utilizing the telovelar approach for large fourth ventricular tumors, suggesting that a staged dissection of the tela and the uvulotonsillar cleft to be superior [19].
It is therefore imperative to consider on the basis of preoperative imaging the different approaches that are available and to define surgical strategy accordingly.
This drawing represents the surgical anatomy of the cerebellomedullary fissure. The tonsils have been retracted on both sides, and exposure of the tela choroidea on the left side of the drawing is seen. The inferior limit of the tela corresponds to the teania and the superior part to the inferior medullary vellum. On the right side of the drawing, the tela has been removed, showing exposure of the fourth ventricle with the foramen of Magendie on its lowest aspect and the lateral limits represented by the inferior cerebellar peduncle.
8.5 Preoperative Consideration
8.5.1 Surgical Consent
Often, the diagnosis of a posterior fossa tumor in children is unexpected. Special care therefore needs to be taken in relaying the diagnosis and potential surgical risks to the parents [20]. Although surgical morbidity is low in most cases, significant neurological deficits are possible [21]. While the radiological appearances of the tumor may point to distinct pathologies, explicit management strategy discussions, particularly with respect to adjuvant chemotherapy and radiotherapy, should be delayed until the formal pathology is known. Signed consent is obtained from the parents. Age-appropriate information should be provided to the child regarding the proposed surgical procedure.
8.5.2 Anesthetic Considerations
Ideally, a specialized pediatric neuroanesthetic team should be involved with children, undergoing posterior fossa craniotomy for tumor excision. A full description of neuroanesthetic principles is however beyond the scope of this chapter. Of critical importance during surgery is monitoring for air embolism and hemodynamic fluctuations due to brain stem irritation or blood loss. The latter is of particular relevance in young children where rapid hypotensive shock can ensue. Routine anesthetic practice in our institution would include central venous and invasive arterial pressure monitoring and an indwelling urinary catheter. Mannitol is not given routinely. Perioperative antibiotic prophylaxis (cephazolin) and dexamethasone is used routinely.
8.5.3 Management of Hydrocephalus
The presence of hydrocephalus is assessed on preoperative imaging, and the necessity for its rapid treatment is dependent on the clinical state of the patient. If hydrocephalus does not need to be treated urgently, surgery to remove posterior fossa tumors should be performed within 48 h of presentation and potentially on the day of admission if the child is critically unwell.
Different strategies are available to manage hydrocephalus in patients with posterior fossa tumors, including the insertion of an external ventricular drain (EVD), a ventriculoperitoneal shunt (VP) shunt, or performing a third ventriculostomy or the anticipated restoration of normal CSF flow secondary to tumor removal. There is no consensus yet as to which is the best option, often being dependent on surgical expertise and preference. The reader is referred to Chap. 7 for a full discussion regarding hydrocephalus management in posterior fossa tumors. Recognition should be given, however, that unless critical the author’s preference is to not treat the hydrocephalus separately to tumor removal.
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