Introduction

The selection of an approach is one of the most critical parts of planning an operation for lesions in the brainstem or deep locations in the brain. The choice of approach is heavily influenced by surgeon experience and comfort with the approach. Therefore, for optimal results, surgeons should be familiar and comfortable with a variety of approaches. Key factors to consider for management of intrinsic lesions are the availability of a safe entry zone, the path of least morbidity, and the approach that allows the best exposure to the lesion yet minimizes the need to traverse the brainstem structures. A lesion can often be approached equally well from several different approaches. In these cases, a combination of the path of least morbidity, surgeon experience, and patient habitus dictates the choice of approach. In this chapter, we review the common approaches to the dorsal brainstem and thalamus and highlight the key steps of each approach. A discussion of approach selection that is based on safe entry zones is provided to guide surgeons in their management of intrinsic brainstem and deep-seated pathologies.

Approaches

Supracerebellar Infratentorial Approach

The supracerebellar infratentorial (SCIT) approach is a workhorse for pathology in the pineal region, ^{1 , 2 , 3} the dorsal thalamus, and the dorsal mesencephalon down to the pontomesencephalic junction. ⁴ The SCIT approach was first utilized in 1911 by Oppenheim and Krause ⁵ and subsequently popularized by Stein ⁶ for lesions in the pineal region. The SCIT approach has been described with several variations: midline, lateral, and extreme lateral approaches ( Fig. 9.1 ). The midline SCIT approach provides a robust route to the pineal gland and pineal pathology. Lateral SCIT and extreme variants ⁷ can be used to gain access to dorsal and dorsolateral mesencephalic and pontomesencephalic pathology and to posterior thalamic lesions, notably cavernous malformations (CMs). ^{4 , 8}

The SCIT space has the potential to be readily developed after the disconnection of the arachnoid membrane and the sacrifice of small tentorial veins. This space provides a direct posterior approach to the pineal region and the posterior incisura. The pineal region can be exposed after traversing the arachnoid membrane covering the pineal region in a lateral to medial direction and ensuring preservation of the superior cerebellar vein, when possible. The view into the pineal region can be obstructed by the vein of Galen and the splenium of the corpus callosum, but these structures can be readily mobilized to approach pineal pathology and are at times mobilized by the pathology itself. Lateral variants of the SCIT displace the vein of Galen complex and provide a robust approach to intrinsic lesions. The anatomy of the pineal, thalamic, and mesencephalic regions is covered in Chapter 2 (“Anatomy of the Brainstem, Thalamus, Pineal Region, and Cranial Nerves”) and will not be further discussed here.

Midline Supracerebellar Infratentorial Approach

The patient should be placed in the prone position with the head flexed ( Fig. 9.2a ). Alternatively, the patient can be placed in the sitting position ( Fig. 9.2b ). The sitting position allows for gravity retraction of the cerebellum and drainage of blood from the surgical field, but it is less comfortable for the surgeon, and it has been associated with an increased potential for air embolism. The surgeon should perform a linear midline skin incision that extends from the occipital protuberance to the upper cervical spinous processes. The rostrocaudal length of the incision can be extended, depending on the habitus of the patient. The neuronavigation system should be used to identify the location of the transverse sinus and torcula. Although exposure of the transverse sinus is not necessary, it should be blue-lined so that it can be retracted with the tentorium. The dura mater is opened with one pedicle based on the transverse sinus and retracted to expose the cerebellum ( Fig. 9.2c ). The superior extent of the craniotomy dictates how low on the craniocaudal axis the surgeon can see. This view is maximized by using tack-up stitches to superiorly retract the transverse sinus and torcula ( Fig. 9.2d ). Next, depending on whether the procedure is performed microscopically or endoscopically, a piece of Telfa (Covidien) is placed on the cerebellum to protect it while the SCIT potential space is developed. To do so, the surgeon coagulates and cuts the small arachnoid bands and the occasional bridging vein between the superior surface of the cerebellum and the tentorium. Avulsion of these bridging veins can result in bleeding, which can be stopped by placing hemostatics, such as Surgicel Nu-Knit (Ethicon), over the bleeding site. The dissection proceeds to the quadrigeminal cistern, where the vein of Galen and the complex of veins draining into it must be identified and protected.

The midline SCIT approach provides robust access to the superior colliculus. Opening the cerebellomesencephalic fissure allows visualization of the inferior colliculus down to the frenulum of the superior medullary velum and visualization of the safe entry zones associated with these structures ( Fig. 9.2e ). ⁹ The posterior wall of the third ventricle is located anterior to the superior colliculi and the pineal gland ( Fig. 9.2f ). The pulvinar of the thalamus is lateral to these structures.

For improved exposure of the pineal region, the veins of the cerebellomesencephalic fissure and the precerebellar vein can be coagulated and cut without risk of avulsion injury to the veins. Case 1 ( Fig. 9.3 ) illustrates the use of the midline SCIT approach for a brainstem CM.

Lateral Supracerebellar Infratentorial Approach

For the lateral SCIT, the patient is placed in a position similar to that for the midline SCIT, but the head is rotated to the side ipsilateral to the craniotomy ( Fig. 9.4a ). Alternatively, the patient can be placed in the park bench position. The more lateral the position of the craniotomy, the more the slope of the tentorium can be used, and the less need there is for retraction on the cerebellum ( Fig. 9.4b ). A craniotomy is placed lateral from the midline. The size of the craniotomy should be tailored to the size and depth of the lesion. To enhance exposure, the surgeon opens the dura with a pedicle based on the transverse sinus to allow for retraction of the sinus and the tentorium ( Fig. 9.4c ). There is rarely any need for the placement of permanent retractors, and the cerebellum can often be retracted dynamically using gravity.

The lateral SCIT approach provides enhanced exposure of the ipsilateral tectal plate and the superior cerebellar peduncle, and it uses the slope of the tentorium to minimize cerebellar retraction ( Fig. 9.4d, e ). Case 2 ( Fig. 9.5 ) illustrates the use of the lateral SCIT approach for the resection of a brainstem CM.

Extreme Lateral Supracerebellar Infratentorial Approach

The extreme lateral SCIT approach is performed with the patient in the park bench position ( Fig. 9.6a ). Alternatively, this approach can be performed with the patient supine and the head maximally turned to the contralateral side. The key to the extreme lateral SCIT approach is to extend the head toward the floor to allow for the cerebellum to dynamically retract and to increase the surgeon’s ability to develop the SCIT space. This maneuver places the mastoid at the highest point of the field. A linear skin incision is placed over the transverse sinus at the level of the sigmoid sinus, down to the mastoid tip. A retrosigmoid craniotomy is performed, taking care to ensure that the edge of the transverse sinus is exposed so that it can be retracted by the dura ( Fig. 9.6b ). The transverse-sigmoid junction is exposed to allow access to the cerebellopontine angle, as needed, for the release of cerebrospinal fluid (CSF) for cerebellar relaxation. The dura should be opened with one pedicle at the transverse sinus and with another pedicle at the sigmoid sinus. Retraction of the tentorial surface allows for exposure of the ambient cistern. Care should be taken during this approach to preserve Dandy’s vein when entering the cerebellopontine angle to release CSF. Dissection of the superior cerebellar surface allows visualization of the lateral mesencephalon and identification of the trochlear nerve (cranial nerve [CN] IV), branches of the superior cerebellar artery, the tectal plate, the superior cerebellar peduncle, and the lateral mesencephalic safe entry zone ( Fig. 9.6c, d ). ⁹ Case 3 ( Fig. 9.7 ) demonstrates the use of the extreme lateral SCIT approach for resection of a brainstem CM.

Supracerebellar Transtentorial Approach

The supracerebellar transtentorial (SCTT) approach was first reported in 1976 by Voigt and Yaşargil, ¹⁰ and it was later popularized by Yonekawa et al ¹¹ for resection of lesions in the posteromedial temporal lobe. Yonekawa and colleagues subsequently used this approach to resect a CM of the thalamus. ¹² Türe et al ¹³ and de Oliveira et al ⁴ described modifications of this approach that provided access to the entire mesial temporal structure. Endoscopic-assisted variants of this approach have been reported for the resection of posteromedial temporal lesions and thalamic lesions.

The patient’s position for the SCTT approach is supine, with the head maximally turned to the contralateral side, the neck flexed toward the contralateral shoulder, and the head extended toward the floor ( Fig. 9.8a ). The ipsilateral shoulder should be elevated, especially for patients who do not have a supple neck. Alternatively, the SCTT approach can be performed with the patient in the sitting, ¹¹ park bench, or prone position. ¹⁴ A linear skin incision is made so that the craniotomy allows for exposure of the transverse sinus in the upper one-fourth of the opening. The optimal placement of the craniotomy allows the surgeon to retract the transverse sinus dynamically to increase working space. Additional considerations r egarding the craniotomy location include optimal placement to make use of the angle of the tentorium to minimize cerebellar relaxation. In general, the more paramedian the craniotomy, the shallower the angle of the tentorium and the more working space the surgeon will have access to. The dural opening should be performed with a pedicle at the transverse sinus, and stitches should be used to assist with retraction and mobilization of the sinus ( Fig. 9.8b ). In cases where the craniotomy is placed at the junction between the transverse and sigmoid sinuses, CSF can be released from the cerebellopontine angle. Alternatively, the craniotomy can be extended so that CSF can be released from the foramen magnum cistern to achieve brain relaxation. Like the SCIT approach, the SCTT approach develops the potential space between the cerebellum and the tentorium until the surgeon arrives at the optimal point for resection of the lesion ( Fig. 9.8c ). The optimal point for tentorial disconnection can be identified using neuronavigation. In general, we do not advocate opening the entire tentorium and instead place a small opening in the tentorium immediately adjacent to the lesion, making sure to prevent injury to CN IV. The tentorium is coagulated and incised using a No. 11 blade. Scissors are used to expand the opening, which is then retracted to improve visualization of the lesion. We do not repair the tentorial opening. The craniotomy is repaired in the usual fashion. Case 4 ( Fig. 9.9 ) illustrates the use of the SCTT for resection of a thalamic or posterior temporal CM.

Related posts:

8 Surgical Approaches to the Ventral Brainstem and Thalamus 10 Skull Base Approaches to the Lateral Brainstem and Cranial Nerves 12 Safe Entry Zones to the Brainstem Section IV Surgical Approaches to the Brainstem, Thalamus, and Pineal Region 4 Pathology of the Brainstem 13 Adult Brainstem Gliomas

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