15 Keyhole Surgery of the Tectum and Pineal Region

10.1055/b-0035-104227

15 Keyhole Surgery of the Tectum and Pineal Region

Michael E. Sughrue and Charles Teo

15.1 Introduction

15.1.1 The Use of the Endoscope Is Recommended in Every Single Pineal Region Case

The pineal region, the tectum, and the posterior third ventricle are deep regions surrounded by numerous critical structures. Much of the pathology in this region mandates aggressive resection, most notably of pineocytomas but also pineoblastomas, cavernomas, symptomatic pineal cysts and low grade glial neoplasms, as adjuvant therapies are not very effective for these tumors, and repeat surgery in this region is unpleasant and dangerous, to say the least.

Given these challenges, some may question whether it is wise to perform minimal craniotomies in these patients. Based on the keyhole principle, we would answer that these are among the best tumors to address using small approaches, as they are deep targets and the potential viewing and working angles obtainable from a small craniotomy are therefore extensive. By the time the target is reached, significant arachnoidal dissection has been performed, and brain relaxation is not an issue. The endoscope, however, is critical for visualizing some of the difficult angles in this region, such as underneath the vein of Galen, under the splenium, or into the third ventricle, where residual tumor can usually be found if you look. As noted above, many of these diseases really require gross total resection, and thus leaving tumor behind due to a lack of visualization is not optimal.

15.2 Approach Selection

There are essentially four basic approaches available to access tumors of the pineal region: the midline/paramedian infratentorial supracerebellar, the lateral supracerebellar, the occipital transtentorial, and the anterior interhemispheric transcallosal. The anterior interhemispheric transcallosal approach (Fig. 15.1, see Videos 15.1 and 15.2) is our approach of choice for tumors of the posterior third ventricle, and thus the first step in choosing an approach is determining if the tumor is actually in the posterior third ventricle or in the pineal region. The location of the vein of Galen relative to the tumor on a midsagittal cut provides a helpful clue to answering this question, as posterior third ventricular tumors generally push the vein downward and backward, while pineal tumors push it upward and forward. This point is important because it is prudent not to have the vein of Galen in the line of approach, if at all possible.

**Fig. 15.1** **a–e** Anterior interhemispheric transcallosal approach to a posterior third ventricular tumor. **(a)** Preoperative imaging demonstrating a gangliocytoma of the posterior third ventricle. Note that the vein of Galen is inferior and posterior to the tumor. **(b)** Postoperative imaging. **(c)** Anterior interhemispheric transcallosal skin incision. **(d)** Endoscopic image taken from the lateral ventricle demonstrating the suprachoroidal, subfornical approach to this tumor. **(e)** The endoscope in this image is directed posteriorly and the thick arachnoid over the Galenic complex is visible.

The lateral supracerebellar approach (Fig. 15.2) is helpful for tectal lesions that arise at the junction of the tectum and tegmentum, or that need an entry point at the posterolateral portion of the midbrain, as discussed in Chapter 12.

**Fig. 15.2** **a–k** Lateral supracerebellar approach to a brainstem cavernoma performed in the sitting position. **(a–e)** Preoperative imaging demonstrating a large brainstem cavernoma which has bled multiple previous times despite radiosurgery. **(f)** Postoperative image. **(g)** Image demonstrating the position for this approach. The head is slightly rotated and flexed so that the trajectory is as flat and centered as possible. **(h)** Image guidance is used to plan a small craniotomy which exposes the edge of the transverse sinus. **(i)** A craniotomy approximately the size of the mini-retrosigmoid approach is performed. **(j)** After opening the dura and dissecting the supracerebellar arachnoid, the cerebellum descends just as it does with the traditional larger sitting supracerebellar approach, and the working view is excellent. **(k)** The final incision size.

As for the midline posterior approaches, the infratentorial supracerebellar and occipital transtentorial, these both have their merits and both can be performed through small craniotomies. Determining the better approach is dependent on many variables such as the angle of the tentorium, the long axis of the tumor and venous anatomy. We generally perform the occipital transtentorial approach when the long axis of the tumor takes you to a point above the inion and the supracerebellar/infratentorial approach when the point is below the inion.

15.2.1 The Keyhole Occipital Transtentorial Approach

Fig. 15.3, Fig. 15.4, Fig. 15.5, see Videos 15.3, 15.4, 15.5, and 15.6

**Fig. 15.3** **a–c** Occipital transtentorial approach to tectal tumor. (a) Preoperative imaging demonstrating a high grade glioma of the tectum. Note that the vein of Galen is superior to the tumor. (b) Postoperative imaging. (c) Image demonstrating the skin incision and positioning for the mini-occipital transtentorial approach.

**Fig. 15.4** **a–h** Steps of the mini-occipital transtentorial approach. **(a)** Skin incision and positioning. Note that while we prepare to make a longer skin incision, if mandated by the bridging vein anatomy, we typically do not need to open this entire incision **(b).** Two bur holes are made next to the sinus, and **(c)** a narrow long bone flap is made paralleling the sinus. The dura is opened in a curved flap with its base facing the sinus, and CSF is allowed to flow out to slowly relax the brain while the microscope is brought in. **(d)** The occipital lobe is gently retracted until the posterior tentorial incisura is exposed. Gradual CSF drainage is necessary until the cisterns can be accessed and opened to drain more CSF, **(e)** the falcotentorial junction and the straight sinus are identified, and the tentorium is divided sharply. Bleeding is usually encountered, and this is usually due to tentorial sinuses which should be stopped with bipolar coagulation. **(f)** The internal occipital vein is usually seen bridging from the occipital lobe to the Galenic system and this can be sacrificed with impunity. **(g)** At this point the thickened arachnoid of the quadrigeminal cisterns is opened to mobilize the Galenic system and expose the neural elements of the tectal region.

**Fig. 15.5** **a–f** Mini-occipital transtentorial approach for a tectal tumor. **(a)** Preoperative imaging demonstrating a tectal tumor. **(b)** Skin incision and positioning for this approach. **(c)** Identification of the straight sinus prior to tentorium division. **(d)** Arachnoidal dissection and division of the precentral cerebellar vein provides access to the involved tectum. **(e)** Tumor removal proceeds as normal. **(f)** Inspection of the resection cavity with the endoscope and the combined angled bipolar/sucker.

**Fig. 15.6** **a–c** Use of the endoscope in the tectum and pineal region. The endoscope is invaluable in this region for ensuring complete tumor removal. These schematics demonstrate the use of the endoscope for inspecting **(a)** underneath the vein of Galen, **(b)** into the third ventricle behind the tectum, and **(c)** downward below the cut tentorial incisura.

**Fig. 15.7** **a-i** Supra-cerebellar, infra-tentorial approach to the pineal region. This 29-year-old female presented with visual disturbance and was found to have a pineal region tumor **(a).** The provisional diagnosis was a germ cell tumor so she was taken to the operating room for a complete resection via an infratentorial approach given that the long axis clearly projects to an entry point below the inion. She was of slim build and was therefore placed prone **(b)** with the head flexed **(c).** If she had been obese then we prefer to operate on these patients in the lateral decubitus position **(d).** Any bridging veins encountered on approach may be sacrificed **(e)** until one identifies the Galenic complex after which all veins need to be preserved. The thick arachnoid **(f)** may be disconcerting at first but gentle and deliberate dissection will reveal the anatomy **(g, h).** This patient had an uneventful postoperative recovery and her postoperative MRI revealed a complete resection **(i).** The pathology returned as a mixed germ cell tumor. She is alive without recurrence 9 years later.

The patient is positioned prone with the head neutral (i.e., not rotated and not flexed or extended; Fig. 15.3). A neutral head position makes it easier for the surgeon to avoid becoming disoriented at deeper levels. After mapping out the approach angle, the head should be repositioned if necessary so the long axis of the pineal tumor is straight up and down. The incision is linear and slightly paramedian to provide a long thin craniotomy with its medial border in line with the lateral edge of the superior sagittal sinus. The long of the craniotomy should be oriented parallel to the sinus as, while very little lateral extension of the bone flap is required, it is wise to have some room along the sinus to be able to avoid bridging veins if they are present.

After opening the dura, the goal is to relax the occipital lobe in order to gain access to the falcotentorial junction. Patience is critical, and it is essential not to panic and injure the occipital lobe in the process of gaining access to the cistern. As with the retrosigmoid approach, most surgeons fear that the occipital lobe will externally herniate, as it is prone to do in large approaches. However, this really does not happen in the keyhole version of approach, so there is time to work. Good anesthesia conditions are critical, and it is important to make sure the abdomen is not compressed as this will make the procedure more difficult. During the approach a piece of Telfa™ wound dressing is slowly advanced along the medial cortical surface and the interhemispheric arachnoidal bands are divided. Any bridging vein from the interhemispheric occipital surface is safe to sacrifice, and should be pre-emptively divided to avoid tearing it later, and then having to find and cauterize it. These veins can often insert underneath the edge of the bone flap, making them hard to address. Once the tentorial incisura is reached, cerebrospinal fluid (CSF) should be drained from any possible source, and after a short time the occipital lobe will relax. If necessary, the lateral tentorium can be divided and the supracerebellar cisterns tapped. In two patients to date, we have had to tap the occipital horn to achieve relaxation, but most of the time this is unnecessary.

Once the brain is relaxed, the straight sinus inside the falcotentorial groove should be identified and the tentorium divided. Brisk venous bleeding is usually encountered during the tentorial division; however, persistent cautery will stop this, and there is no need to worry when it occurs. During the approach to the posterior incisura, an internal occipital vein is usually encountered, and this can be sacrificed with impunity to untether the falcotentorial junction as required. The tentorium is shrunk back with cautery to make more room.

The arachnoid overlying the Galenic complex is very thick and extensive, and prior to making any definitive moves this needs to be completely dissected outward, both to release more CSF and to define the Galenic anatomy. In most patients, the precentral cerebellar vein descending downward from the Galenic complex to the cerebellum should be divided, and this will provide access to the pineal region.

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