4 Approaches to Thalamic, Pineal, and Brainstem Lesions

Figure 4.1. Anterior skull base approaches. (a) The various anterior and anterolateral skull base approaches that can be used to access lesions in the brainstem. These approaches constitute a continuum, and they include unilateral and bilateral subfrontal, eyebrow/supraorbital or orbitopterional, pterional, orbitozygomatic, and Kawase approaches. With improvements in endoscopy, anterior transclival approaches can be used for ventral pontine and pontomedullary lesions.

Figure 4.1. (b) The various skin incisions used to perform the anterior skull base craniotomies in (a). (c) The various working trajectories afforded by anterior skull base approaches to the top of the brainstem. The extent of the lesion plays an important role in the choice of actual approach used and the degree of bony removal necessary to achieve the exposure. All of the various approaches to the brainstem are summarized at the end of the section (Table 4.1).

Figure 4.2. Patient positioning used for various anterior skull base approaches. The exact degree of head rotation depends on the approach selected, the specific target of interest, and the extent of the pathology. The patient is positioned to enable gravity to retract the frontal and temporal lobes from the floor of the anterior and middle fossae. When possible, head rotation enables the surgeon to obtain the necessary view down the longest axis of the lesion. The head should be placed above the level of the heart, and kinking of the neck resulting in venous congestion should be avoided.

Figure 4.3. Unilateral subfrontal or frontolateral approach. (a) The patient’s head is rotated 30 degrees off the vertical axis, with the neck slightly extended. The skin incision should be placed behind the hairline from the midline to the zygomatic process. This craniotomy is performed along the floor of the frontal fossa, extending to the pterion. Drilling the anterior fossa floor flush provides a flat and unhindered exposure to the basal cisterns. (b) The relationship between the craniotomy and the underlying neural structures.

Figure 4.4. Orbitopterional approach. (a) An alternative to the unilateral subfrontal approach is the orbitopterional approach. The patient is positioned with the head rotated 30 degrees off the vertical axis with the neck slightly extended. Extension of the neck enables gravity to displace the frontal lobe off the floor of the anterior fossa, minimizing the need for fixed retraction. The skin incision is similar to that used for the unilateral subfrontal approach and can be placed behind the hairline. Alternatively, the incision can be made above or within the eyebrow (dotted line) of patients with generous eyebrows or a receding hairline, and it can be extended laterally to the corner of the eye for a more lateral approach. The orbitopterional craniotomy (solid circular outline) is performed along the floor of the frontal fossa, and the frontal fossa floor is drilled flush to enhance the exposure. The amount of pterional bony removal (circular dashed line) depends on the lateral extent of pathology and the amount of lateral visualization and the working trajectory necessary to approach the lesion. (b) The orbitopterional craniotomy is outlined (orange line). The pterional portion of the craniotomy can be extended as necessary to accommodate the lesion and site of involvement. The direction of the last cut across the frontal fossa floor is demonstrated (arrow).

Figure 4.4. (c) The orbitopterional craniotomy extends into the orbit and provides access to the ventral mesencephalon and the sellar/parasellar region. (d) The extent of the removal of the orbital wall can be better appreciated from an anterior view. Removal of this bony prominence minimizes the need for frontal lobe retraction while providing the surgeon an improved view for high-riding lesions at the level of the basilar apex and the mesencephalon. (e) When orbital bar removal is combined with tailored removal of the pterion, the orbitopterional approach (orange arrow) can be used for aneurysms in the proximal circle of Willis and lesions in the midbrain. The removal of the orbital bar provides a greater degree of working angle than a simple pterional approach (purple arrow), notably in the superior direction. When properly closed, this exposure provides a cosmetically outstanding outcome for the patient.

Figure 4.5. Bilateral subfrontal approach. (a) The head is often placed in the midline with some extension of the neck to enable gravity retraction of the frontal lobes. When enhanced superior exposure is necessary, the orbital bandeau (shaded area) may be removed to increase the surgical working space. (b) The relationship of the craniotomy relative to the underlying neural structures.

Figure 4.6. Pterional and mini-pterional approaches. (a) The pterional craniotomy is the mainstay skull base approach for lesions in the anterior cranial fossa and for ventral mesencephalic lesions. The skin incision and craniotomies for the standard (solid line) and mini-pterional (dashed line) approaches are illustrated. (b) The relationship between the craniotomy and the underlying neural structures.

Figure 4.6. (c) The scalp is elevated to expose the zygomatic root posteroinferiorly and the keyhole anteriorly. The fat pad superficial to the temporalis fascia should not be entered because the frontalis branch of the facial nerve (CN VII) lies in the interfascial plane and can be injured by manipulation. The temporalis muscle is incised and reflected anteroinferiorly, leaving a cuff of fascia and muscle along the superior temporal line for eventual suturing at closure. (d) Extradural drilling of the sphenoid wing can enhance working room.

Figure 4.6. (e) When the pterional bone flap is removed, the working trajectory to the middle fossa is steep (thick arrow). Removal of the lesser wing of the sphenoid bone medially to the superior orbital fissure and the anterior clinoid process provides a flat surface over the orbit connecting the anterior and middle cranial fossae (thin arrow). (f) The dura is opened in a semicircular fashion, extending from the floor of the middle cranial fossa at the posterior-inferior aspect of the exposure to the floor of the anterior cranial fossa at the anterior-inferior aspect of the exposure. This opening provides an unobstructed view of the carotid cistern.

Figure 4.6. (g–k) Opening the sylvian fissure. (g) With a suction tip, the frontal lobe and adjacent temporal lobe are retracted to place the arachnoid membrane over the fissure under tension. The arachnoid membrane is then incised using a knife. (h) Sharp dissection is used to cut the arachnoid membrane to gradually open the sylvian fissure. Large sylvian veins must be preserved; if venous pruning is necessary, veins should be preferentially mobilized with the temporal lobe.

Figure 4.6. (i) Removal of cerebrospinal fluid allows for brain relaxation. However, removal of excess cerebrospinal fluid early in the splitting of the sylvian fissure may complicate opening of the fissure by allowing the frontal and temporal opercula to adhere to each other. The judicious use of saline irrigation can aid with the dissection of the sylvian fissure. (j) The arachnoid membrane overlying the internal carotid artery is opened and arachnoid bands connecting the internal carotid artery and anterior cerebral artery to the optic nerve (CN II) are sharply cut to enable mobilization of the frontal and temporal lobes to arrive at the deep cisterns used to access the brainstem. For exposure of the basilar apex and ventral brainstem, the carotid and optic cisterns must be maximally opened to enable cerebrospinal fluid release.

Figure 4.6. (k) The membrane of Liliequist is opened to release the cerebrospinal fluid from the interpeduncular cistern. In total, three working corridors could be utilized to visualize the contents of the interpeduncular cistern. These corridors include the carotico-oculomotor, opticocarotid, and supracarotid working corridors.

Figure 4.7. Cadaveric dissection demonstrating the mini-pterional craniotomy. (a) A curvilinear incision is centered on the extension line of the sphenoid groove (blue dashed line) 1 cm behind the hairline. (b) The subcutaneous tissue, frontal branch of the superficial temporal artery, and superficial fat pad are exposed after the galeal flap is reflected toward the temporal fossa.