Supracerebellar Transtentorial Approach

48 Supracerebellar Transtentorial Approach

Jean G. de Oliveira, Paulo A. S. Kadri, Aziz Rassi-Neto, Renan M. Lovato, José Carlos Esteves Veiga, and Luis A. B. Borba

Abstract

The supracerebellar transtentorial (SCTT) approach has important advantages that include preserving the lateral and basal cortex, preserving optical radiation, and preventing retraction of the temporal lobe. However, a simple incision in the tentorium, although it improves the exposure, still presents a deep and narrow surgical field, requiring a greater working distance. Consequently, a broad tentorial opening or ressection allows wider exposure and different work angles so that neurosurgical instruments can reach the mediobasal temporal region in all its extension, navigating through the natural spaces, also known as brain cisterns.

In this chapter, the authors provide a stepwise description of the SCTT approach.

Keywords: approach, craniotomy, infratentorial, microsurgery, supracerebellar, tentorium, transtentorial

48.1 Introduction

Lesions located in the medial and basal region of the temporal lobe are usually treated neurosurgically through subtemporal, transtemporal (transcortical), transsylvian, or inter-hemispheric parieto-occipital approaches.1 ^, ²

The subtemporal approach usually requires retraction that can cause injuries to the temporal lobe by direct contusion or venous infarction due to eventual damage to the Labbé vein complex. The transtemporal approach naturally causes injury to the temporal cortex, including damage to the optical radiation, mainly when the temporal horn is opened in the posterior aspect of its lateral wall. The transsylvian approach with a transinsular variant may also damage optical radiation when the inferior insular groove is opened posteriorly. Likewise, the transcisternal variant of the transsylvian approach provides limited access to the posterior part of the medial region of the temporal lobe. Similar to the subtemporal access, the inter-hemispheric parieto-occipital approach also requires cortical retraction of these lobes, being limited by the cortical veins that drain into the superior sagittal sinus (SSS), causing both an eventual cortical lesion as well as a venous infarction, being responsible for visual deficits.

The supracerebellar infratentorial (SCIT) approach is commonly used for lesions located in the midline, posterior incisural space, and pineal region.³ The tentorium’s opening (transtentorial approach), by incision⁴ or resection,⁵ provides surgical access to paramedian lesions located or extended laterally into the mediobasal region of the temporal lobe.

This chapter describes the supracerebellar transtentorial (SCTT) approach for the microsurgical treatment of lesions located in the mediobasal temporal region.

48.2 Historical Landmarks

The pioneering surgeon to perform an SCIT approach was Horsley in 1910.⁶ Later, Oppenheim and Krause in 1913 and Krause in 1926 also published their early experience of the approach.⁷ ^, ⁸ However, Stein⁹ was the one who revisited and popularized this approach for pineal region tumors. Consequently, the SCIT approach became the first choice for neurosurgical management of lesions located in the midline at the posterior incisural space, an area also called the pineal region.³ ^, ¹⁰ ^, ¹¹ ^, ¹² ^, ¹³ ^, ¹⁴ ^, ¹⁵ In addition, lesions located in the posterolateral region of the midbrain and upper portion of the pons can also be surgically removed by SCIT approach using one of its three variants, namely, median, paramedian, or extreme lateral, as described by de Oliveira and Spetzler.³

The birth of the SCTT approach was in 1976 when Voigt and Yaşargil¹⁶ published the removal of cavernous malformation in the parahippocampal gyrus through a paramedian variant of the SCIT approach, where they describe a 4-cm incision laterally in the tentorium to gain a better view of the mediobasal parts of the temporal and occipital lobes.

Since then, several authors have also published their experience using the SCTT for different kinds of lesions.² ^, ⁴ ^, ¹⁷ ^, ¹⁸ ^, ¹⁹ ^, ²⁰ ^, ²¹ Besides, some authors, such as de Oliveira et al⁵ and Türe et al,²² ^, ²³ ^, ²⁴ ^, ²⁵ evolved this access performing a broad opening or even the resection of the tentorium, allowing a corridor through the ambient cistern up to the most anterior aspect of the mediobasal temporal region.

48.3 Indications

Lesions arising from the posterior incisural space, and pineal region with lateral and upper extension could be accessed with a lateral cut in the tentorium. Also, those lesions located in the mediobasal region of the temporal lobe could be reached through an SCTT approach with a broad opening or resection of the tentorium. In addition, the atrium of the lateral ventricle can be also accessed to achieve surgical removal of lesions in this region, through the collateral sulcus.²⁰

Any kind of neoplastic⁵ ^, ¹⁸ ^, ²² ^, ²⁵ and cerebrovascular lesions,⁵ ^, ²² including aneurysms of the posterior cerebral artery (PCA),¹⁷ cavernous malformations,²⁶ and arteriovenous malformations (AVMs),²³ can be neurosurgically treated through SCTT approach. This route also may serve as a corridor to perform an amygdalohippocampectomy for epilepsy treatment.²⁴ ²⁷

48.4 Preoperative Assessment

The preoperative assessment should cover a detailed neuroradiological evaluation, including computed tomography (CT) scans, magnetic resonance imaging (MRI), and venous phase of angio-MRI to analyze the venous sinuses and the eventual tentorial venous lakes, which may limit or contraindicate the tentorial opening.5 In addition, all patients should be referred to preoperative anesthesiology examination, including echocardiography to exclude any atrial or ventricular communication, especially patency of foramen ovale, which may increase the risk of air embolism, contraindicating a semi-sitting position.⁵ ^, ²²

48.5 Surgical Technique

48.5.1 Anesthesia and Intraoperative Neurophysiological Monitoring

Regardless of the nature of the lesion, the SCTT approach is performed under general anesthesia. Due to the risk of gas embolism during the surgical procedure, a central venous catheter is placed in all patients to allow the eventual need for air aspiration. Besides, a Doppler ultrasound probe is placed over the precordium for the accurate diagnosis of any embolism. If the semi-sitting position is performed, the transesophageal echocardiography helps provide real-time information about the heart, great vessels, and air embolism into the blood circulation during surgery. Intermittent compression stockings are used on the lower extremities to prevent deep vein thrombosis.

The intraoperative neurophysiological monitoring, including somatosensory evoked potentials (SSEPs), motor evoked potentials (MEPs), and cranial nerves monitoring, is extremely recommendable.

48.5.2 Positioning

Although the semi-sitting position has been described for the SCTT approach by some authors,⁵ ^, ²² the risk of air embolism should not be neglected and requires an experienced anesthesiology team to be very well integrated with the neurosurgical team. In addition, the semi-sitting position may be exhaustive for the neurosurgeon, who has to keep both arms extended in time-consuming procedures.

Consequently, the prone position is preferable because it is comfortable for the neurosurgeon and provides enough exposure to the space between the cerebellum and the tentorium. The head is fixed using a three-pin Mayfield head holder with the head flexed and tilted laterally. Care must be taken to avoid any compression over the jugular veins, which may cause swelling in the cerebellum by impairment of the venous outflow circulation. The surgical table should be elevated, like in the reverse Trendelenburg position, and the surgeon works on the patient’s side (Fig. 48.1).

Fig. 48.1Patient positioning with the head fixed by a three-point Mayfield head holder (a), with the head elevated and tilted laterally (b). The surgical table is placed in a reverse Trendelenburg position, and the surgeon works on the patient’s side.

48.5.3 Skin Incision and Muscle Management

The trichotomy is usually performed along the posterior cervical midline, approximately 5 cm above the external occipital protuberance. A linear cutaneous incision is made in the median surgical plane ( Fig. 48.1b) and extends from the inion to the spinous process of the second cervical vertebra (C2), providing wide exposure of the posterior fossa. The skin, subcutaneous, and muscle layers are dissected as a single plane and retracted laterally using surgical hooks (Fig. 48.2).

Fig. 48.2The incision extends from the inion to the spinous process of the second cervical vertebra (C2), with the skin, subcutaneous, and muscle layers retracted laterally, providing wide exposure of the posterior fossa.

48.5.4 Craniotomy

The suboccipital craniotomy should be broad enough to expose the transverse sinus and the confluence of the sinuses to allow a retraction of the tentorium superiorly and expose the junction of the transverse and sigmoid sinuses. The craniotomy is usually performed after multiple burr holes, under continuous irrigation. Any eventual opening of the intraosseous sinus or tributary diploic veins must be occluded with bone wax, which reduces the risk of air embolism. The dura mater and the venous sinuses are separated from the bone using a blunt dural dissector so that the bone flap can be removed, exposing one-third above the transverse sinus and two-thirds below the transverse sinus (Fig. 48.3).

Fig. 48.3The suboccipital craniotomy. Multiple burr holes facilitate dural dissection, mainly over the venous sinuses (a, b). Bone flap: outer table (superior) and inner table (inferior) (c). Wide exposure of the transverse sinus and the confluence of the sinuses (d) is paramount to achieving enough surgical field and different work angles.

48.5.5 Dural Opening

The dura mater opening should be preferably performed under microscope view. The dural incision is usually made in a U-shaped fashion, reaching the level of cisterna magna to release the cerebrospinal fluid (CSF), to achieve brain relaxation, and a wider space between the cerebellum and the tentorium (Fig. 48.4). Care must be taken to avoid inadvertent lesions on the transverse sinus when cutting the dura on the superior aspect. Dural retraction using stitches should be performed to make a wider corridor under the tent (Fig. 48.5).

Fig. 48.4 (a–c) After dura mater opening, proceeding to release the cerebrospinal fluid of the cisterna magna to achieve brain relaxation.

Fig. 48.5Dura mater upward retraction with stitches to enlarge the surgical corridor under the tentorium.

Cutting arachnoid adhesions and sacrificing some small bridging veins commonly found between the tentorial surface of the cerebellum and the tentorium is enough to create a broad surgical field toward the posterior incisural space (Fig. 48.6). The veins should be coagulated and cut closest to the cerebellum instead of the entry point into the tent, where the hemostasis may be more difficult.

Fig. 48.6 (a–f) Cutting arachnoid adhesions and sacrificing some small bridging veins commonly found between the cerebellum’s tentorial surface and the tentorium is a valuable maneuver to make the surgical field wider.

The neurosurgeon must be aware of the veins’ anatomical variations to mobilize them and modify the tentorial incision whenever necessary. The preoperative magnetic resonance angiography (MRA) is paramount in this planning. Avoid sacrificing the lateral petrosal vein or the tentorial veins in the midline, which are the main veins draining the posterior fossa.

48.5.6 Microsurgical Dissection

Through the SCIT corridor it is possible to reach and open the posterior aspect of the quadrigeminal cistern and the ambient cistern, where the superior cerebellar artery branches can be found. Besides, paramount anatomical landmarks must be identified, such as the trochlear nerve, quadrigeminal plate, the pineal gland, the vein of Galen, and the basal vein of Rosenthal.

The arachnoid dissection from the quadrigeminal cistern toward the ambient cistern usually exposes the trochlear nerve, which arises from the dorsal aspect of the crus cerebri and runs between the PCA and superior cerebellar artery. It is important to identify the trochlear nerve’s entry point into the tent before the tentorial opening. The posterior aspect of the parahippocampal gyrus commonly extends medially over the posterior part of the free border of the tentorium (Fig. 48.7).