Supracerebellar Infratentorial Approach: Microsurgical Perspective

46 Supracerebellar Infratentorial Approach: Microsurgical Perspective

A. Karim Ahmed, Samuel Kalb, Jean G. de Oliveira, and Robert F. Spetzler

Abstract

The supracerebellar infratentorial approach is an invaluable technique for microsurgical exposure of lesions located medially in the posterior pineal or incisural region and lesions located laterally in the posterolateral mesencephalon and ambient cistern. This surgical technique allows extra-axial exposure to deep intracranial structures with a surgical trajectory beneath the deep venous system while avoiding the surgical morbidity from parenchymal dissection and minimizing ischemia from vascular compromise. Preoperative evaluation using a variety of imaging modalities is key to the success of this surgical technique. Part of the reason for the success of this procedure is the two-point method, which allows the surgeon to reach the lesion while avoiding incisions into vital brainstem parenchyma and reducing additional retraction. With adequate patient positioning, which can be performed in several different ways, the approach to deep lesions can be conducted from multiple angles, the techniques for which are discussed in this chapter.

Keywords: brainstem, brain tumors, cavernous malformations, developmental venous anomaly, infratentorial, microsurgery, supracerebellar

46.1 Introduction

Neurosurgery pioneer Victor Horsley 1 was the first to attempt the supracerebellar infratentorial (SCIT) surgical approach for a pineal region tumor resection in 1910. However, it was not until 1913 that the first successful surgical resection using the SCIT approach was performed by Hermann Oppenheim and Fedor Krause.² ^, ³ This midline surgical approach to the pineal region was eventually revised and popularized by Bennett Stein⁴ ^, ⁵ in 1971.

The SCIT approach is an invaluable technique for the microsurgical exposure of lesions located medially in the posterior pineal or incisural region and lesions located laterally in the posterolateral mesencephalon and ambient cistern.⁶ ^, ⁷ ^, ⁸ ^, ⁹ ^, ¹⁰ For appropriate lesions, the SCIT technique enables extra-axial exposure to deep intracranial structures with a surgical trajectory beneath the deep venous system while avoiding the surgical morbidity from parenchymal dissection and minimizing ischemia from vascular compromise.¹⁰ Alternative approaches to this region include the transtentorial supracerebellar, the occipital transtentorial, and the transcallosal interhemispheric variations.⁶ ^, ⁷ ^, ⁸ ^, ⁹ ^, ¹⁰ ^, ¹¹ ^, ¹²

To best localize symptomatology and effectively treat applicable lesions using the microsurgical SCIT exposure, one must be aware of the vital anatomical structures in this region and the limitations of the working corridor. The deep narrow operative field, the superior border of the tentorium, and the inaccessible lateral and anterior mesencephalon may limit the size and location of lesions that can be approached.⁹ ^, ¹⁰ ^, ¹¹ ^, ¹² This chapter describes the surgical decision-making, operative techniques, anatomical considerations, and key concepts involved in the microsurgical approach to lesions using the SCIT approach.

46.2 Preoperative Evaluation

The SCIT approach, including its variants, may be useful in treating neoplastic or vascular lesions localized to the superior cerebellar hemispheres, superior vermis, medial thalamus, pineal region, posterior and posterolateral midbrain and upper pons, posterior third ventricle, tentorial incisura, and parahippocampal gyrus. The World Health Organization reports that germ cell tumors, pineal parenchymal tumors, and gliomas account for most pineal region neoplasms.¹³ On MRI, germ cell tumors may be T2-hyperintense and T1-isointense or T1-hypointense, commonly with cystic areas. Among pineal parenchymal tumors, pineocytomas appear sharply demarcated with abundant cytoplasm, are hypointense on T1-weighted and T2-weighted MRIs, and are hyperdense on CT images, with cysts and calcifications in 50% of patients. In contrast, pineoblastomas are often lobulated on MRIs, isointense to gray matter on T2-weighed MRIs, and hyperdense on CT images with no associated calcifications. Pineal glial tumors mostly arise from the adjacent tectal plate with superiorly displaced calcifications.¹⁴ Surgical resection is indicated for most pineal region neoplasms, with the exception of radiosensitive pure germinomas, as patients with the latter commonly present with obstructive hydrocephalus.¹⁰ ^, ¹³ ^, ¹⁴ ^, ¹⁵ When endoscopic third ventriculostomy is performed for symptomatic flow diversion required because of a pineal neoplasm, an endoscopic biopsy of the lesion may be performed during the procedure in the absence of elevated blood alpha-fetoprotein, cerebrospinal fluid (CSF) alpha-fetoprotein, or β-human chorionic gonadotropin.¹⁴ About one-third of pineal region neoplasms are benign, for which gross-total resection is curative.⁵ ^, ⁹ ^, ¹⁵ Gliomas, metastases, ependymomas, epidermoid tumors, meningiomas, and arachnoid cysts of this region have also been successfully removed using the SCIT approach.⁹ ^, ¹⁰ ^, ¹¹ ^, ¹² ^, ¹³ ^, ¹⁴ ^, ¹⁵

Vascular lesions, including arteriovenous malformations and cavernous malformations (CMs) of the midbrain, upper pons, and thalamus, have been successfully treated with this approach, with good reported outcomes.⁹ ^, ¹⁶ ^, ¹⁷ ^, ¹⁸ Depending on the location of the lesion, most patients present with multiple neurologic symptoms resulting from mass effect, hemorrhage, or a combination of the two.⁹ ^, ¹⁰ Grossly, CMs have a characteristic “mulberry” appearance beneath a thin layer of arachnoid membrane.⁹ MRI, particularly T2-weighted gradient echo, is the imaging modality of choice to evaluate these lesions, which demonstrate a rim of signal loss because of surrounding hemosiderin.¹⁹ ^, ²⁰ In patients with CMs of the brainstem, associated developmental venous anomalies (DVAs) are reported in 8 to 26% of cases, and these DVAs may play a role in the recurrence of brainstem CMs.²¹ ^, ²² ^, ²³ In our clinical series of 45 patients with brainstem CMs treated with the SCIT approach, the senior author (Robert F. Spetzler) intraoperatively identified associated DVAs in all 45 (100%) patients, although the DVAs were identified on preoperative imaging in only 12 (26.6%) patients.⁹ Thus, the incidence of a DVA associated with a brainstem CM is likely to be underestimated. When a DVA is identified on preoperative imaging, the surgical approach should be modified to preserve the DVA. When a DVA is encountered during surgery, attention should be paid to preserve the main trunk to avoid ischemic complications.⁹ The senior author (Robert F. Spetzler) previously reported indications for the surgical treatment of patients with brainstem CMs using the SCIT approach (Table 46.1).⁹

Table 46.1 Surgical indications for resection of cavernous malformations of the brainstem using the supracerebellar infratentorial approach

Indication*	No.(%) (N = 45)
Lesion abuts the pial surface or is exophytic	17 (38)
Repeated hemorrhages resulting in progressive neurologic deficits	24 (53)
Acute hemorrhage extending outside the lesion capsule	1 (2)
Significant mass effect due to intralesional hemorrhage	3 (7)
*Surgery indicated if one or more indications are met.

46.3 Surgical Approach and Operative Techniques

There are four variants of the SCIT approach, and the most appropriate SCIT surgical trajectory is determined by the location of the lesion and by the optimal trajectory from the surface of the skin incision (Fig. 46.1). This trajectory may be assessed on preoperative imaging using the two-point method, which allows the surgeon to reach the lesion while avoiding incisions into vital brainstem parenchyma and reducing additional retraction required for a tangential approach (Fig. 46.2).⁹ ^, ²¹ Many surgically indicated lesions abut, or are close to, a pial or an ependymal surface.²¹ A trajectory line is drawn, using preoperative sagittal, coronal, and axial MRIs, from the center of the lesion to the surface of the cranium to intersect the superficial surface of the lesion. As is frequently the case, lesions with unilateral extension should be approached with a surgical trajectory aligned with the longest diameter of the lesion.⁹ ^, ¹⁰ ^, ²⁴

Fig. 46.1Artist’s illustration demonstrating surgical trajectories of the median (left arrow), paramedian (middle arrow), and extreme lateral (right arrow) supracerebellar infratentorial approaches. (Used with permission from Barrow Neurological Institute, Phoenix, Arizona.)

Fig. 46.2Application of the two-point method. Axial MRIs are used to determine the optimal surgical trajectory for (a) the median supracerebellar infratentorial approach, (b) the paramedian SCIT approach, and (c) the extreme lateral SCIT approach. (Used with permission from Barrow Neurological Institute, Phoenix, Arizona.)

There are several options for patient positioning with the SCIT approach, including the sitting, prone, park-bench, and supine positions. The sitting position facilitates gravity-dependent retraction of the cerebellum and minimizes venous pressure, although it increases the risk of air embolism and can be physically uncomfortable for the surgeon, who must operate with arms extended.²⁵ The Concorde position, which is a modified prone position, decreases the risk of air embolism and tension pneumocephalus, but unlike the sitting position, it does not provide gravity-assisted retraction or decreased venous pressure.²² ^, ²³ The angle of the straight sinus on preoperative imaging may also influence patient positioning.²⁴

Somatosensory evoked potentials, compressed spectral analysis or electroencephalography, and brainstem auditory evoked potentials should be monitored routinely during surgery. These measures may also be useful during the positioning of the patient to determine the amount of tolerable neck flexion or rotation. Neuronavigation should be used throughout the operative procedure to plan the skin incision and craniotomy, to confirm the planned trajectory, and to localize the lesion.

Intraoperative fenestration of the subarachnoid cisterns, including the cisterna magna, facilitates the release of CSF, which allows the cerebellum to relax and displace inferiorly. Gradual CSF release may also be accomplished using a lumbar drain that is placed after the induction of anesthesia.¹ ^, ² ^, ³ ^, ⁴ ^, ⁵

46.4 Median SCIT Approach

The median SCIT approach is ideal for lesions in the posterior midline incisural region (Fig. 46.3). We consider the prone position, with the patient’s head flexed, to be the most comfortable for the median SCIT approach. A midline skin incision is made from the occipital protuberance to the spinous process of C2. Superior retraction of the tentorium can be maximized by extending the standard suboccipital craniotomy above and below the transverse sinus to expose the junction of the torcular Herophili and transverse sinus. The dura is opened in an inverted Y shape, with the open end of the Y at the edges of the transverse sinuses.

Fig. 46.3Artist’s illustration demonstrating the median supracerebellar infratentorial approach. (a) Patient positioning, skin incision (dashed vertical line), and planned craniotomy (shaded area); (b) craniotomy and Y-shaped dural incision (dashed lines); and (c) surgical window (shaded triangle). (d) Photograph of a cadaveric dissection showing working corridor. 3rd Vent., third ventricle; IC, inferior colliculus; M.P.Ch.A., medial posteriorchoroidal artery; PCA, posterior cerebral artery; Pi, pineal; SCA, superiorcerebellar artery; SC, superior colliculus; Tent., tentorium. (Used with permission from Barrow Neurological Institute, Phoenix, Arizona.)