Fig. 2.1
Skull model with parietomastoid and occipitomastoid sutures identified and labeled
A burr hole placed just anterior to the asterion typically exposes the junction of the transverse and sigmoid sinuses, a key anatomic landmark that defines the superior and ventral extent of exposure, respectively. Tubbs et al. [8] further studied the relationship between the superficial bony landmarks and the location of the venous sinuses in 100 adult cadaver skulls and defined this keyhole relative to the intersection of the “zygomatic line” and a “mastoid line” (Fig. 2.2). The zygomatic line parallels the superior border of the zygomatic arch and extends posteriorly from the root of the zygoma to the inion. The mastoid line connects the mastoid notch to the squamosal suture. In 80/100 cases, irrespective of laterality, the transverse-sigmoid junction lies within 1 cm of a burr hole placed inferior to the zygomatic line and dorsal to the mastoid line [8].
Fig. 2.2
Horizontal “zygomatic line” and vertical “mastoid line” demonstrate the intersection where a burr hole placed at the inferior aspect of the zygomatic line and the dorsal aspect of the mastoid line will be within 1 cm of the transverse-sigmoid junction 80% of the time
Intradural Anatomy
The CP angle is bordered by the tentorium cerebelli; the superior and inferior limbs of the angular cerebellopontine fissure; the petrosal surface of the cerebellum, lateral pons, and middle cerebellar peduncle; the petrous temporal bone; and, at its deep extent, the prepontine cistern, the petrous apex, and Meckel’s cave. CNs V–XI are found within the CP angle, as well as branches of the superior cerebellar artery, anterior inferior cerebellar artery, and posterior inferior cerebellar artery. Rhoton et al. divided the contents of the CP angle into three neurovascular complexes, which have a relatively fixed position at the brainstem and skull base, and thus can be reliably identified even in the presence of distorting pathology such as tumors or vascular malformations [9–12].
The upper neurovascular complex consists of CN V, superior cerebellar peduncle, origin of the SCA (often duplicated), and superior petrosal venous complex (i.e., Dandy’s vein). CN V exits the lateral surface of the pons at its midportion and runs obliquely toward the petrous apex to enter Meckel’s cave. Typically, the SCA travels around the pons rostral to CN V but may send a caudal loop that comes in close proximity or contacts the CN V. The superior petrosal vein is often a complex of two to three veins that drain the lateral cerebellar surface before converging to insert into the petrosal sinus [7]. These veins may lie adjacent to CN V or may enter the tentorium separate from CN V, effectively tethering the lateral cerebellar hemisphere [7]. The middle neurovascular complex consists of the AICA, middle cerebellar peduncle, and cranial nerves VI–VIII. CN VII arises from the brainstem at the level of the pontomedullary junction 1–2 mm ventral to the vestibulocochlear entry point (Fig. 2.3). The facial and vestibulocochlear nerves course together as they travel laterally to the internal acoustic meatus. AICA typically forms a loop just below the CN VII–VIII complex with labyrinthine, recurrent perforating and subarcuate branches arising from this loop to course with CN VIII into the internal acoustic meatus [9–12]. The lateral recess of the fourth ventricle transitions into the foramen of Luschka, which is situated posteroinferior to the junction of CNs VII–VIII with the brainstem and is often not visualized, but may be identified by a tuft of choroid plexus protruding into the CP angle [11]. The flocculus , which projects from the lateral recess, forms a bulge of cerebellar tissue sitting superficial to CNs VII–VIII and adjacent to the choroid plexus [11].
Fig. 2.3
Cadaveric specimen demonstrating the right CN VII–VIII complex and CNs IX, X, and XI as they exit the brainstem
The lower complex consists of CNs IX–XI, the inferior cerebellar peduncle, and the PICA [10]. CNs IX–XI arise as rootlets along the posterior edge of the medullary olive in the groove of the postolivary sulcus (Fig. 2.3). CN IX is typically one or two rootlets arising from the upper medulla just caudal to CN VII, whereas CN X comprises a line of tightly packed rootlets just inferior to this. The rootlets of CN XI are more widely separated, running from the lower two-thirds of the olive to the upper cervical cord, its cranial roots often being difficult to distinguish from vagal fibers. CNs IX–XI exit at the jugular foramen with the glossopharyngeal exiting more anteromedially at the pars nervosa and CNs X–XI exiting posterolaterally at the larger pars vascularis. CN XII exits the medulla along the anterior margin of the caudal olive, its roots running anterolaterally to reach the hypoglossal canal. Both branches of PICA and the vertebral artery may pass through or contact rootlets of the lower cranial nerves [10].
Surgical Technique
Positioning
The three-quarter prone position is our position of choice because it maximizes the working space between the patient’s head and ipsilateral shoulder, thus providing adequate working space for the surgical corridor (Fig. 2.4a, b) [13]. Several other positions have been described for a retrosigmoid approach, each with its own relative advantages and disadvantages, including the sitting position [14], supine position with the head maximally rotated to the contralateral side [7], or the lateral position with the head turned 90° [15].
Fig. 2.4
(a, b) Photographs showing two patients in a three-quarter prone position, with the pressure points padded and the patients secured to the operating tables
A series of stepwise maneuvers are undertaken to place the patient in this position. If utilized, a vacuum positioner [i.e., “beanbag”] is prepositioned on the bed. Alternatively, pillows and bolsters may be used to support the torso. A minimum of four surgical personnel are required to safely perform these maneuvers, with the neurosurgeon typically in control of the head, assistants at each side controlling shoulders and hips, and one assistant in control of the legs and feet. Our preference is to pin the head with the skull clamp after the body is positioned and maintain direct manual control of the head.
Starting from a supine position, the patient is translated such that the shoulders are 2–4 cm above the end of the bed. The patient is then translated laterally such that the contralateral hip lies past the midline of the bed, often with the ipsilateral hip (temporarily) suspended by an assistant off the edge of the bed. Simultaneous with this translation, the torso is rotated to bring the ipsilateral shoulder up and over midline into the three-quarter prone position. Alternatively, if a lumbar drain is to be placed, the rotation may be paused at true lateral for insertion and then continued to three-quarter prone. An axillary roll is placed one handbreadth below the axilla, at the mid-nipple line. The dependent arm may be left extended and supported on a bed-rail arm board or may be slung in the armature of the head clamp, suspended hanging over the edge of the bed [16]. The independent arm is supported on pillows or an elevated arm board, with care to minimize pressure in the brachial fossa (Fig. 2.4a, b). The dependent leg is left extended, with care to pad the fibular head to prevent a peroneal nerve palsy. The independent leg is flexed at the hip and knee, allowing the iliac crest to roll over, and the pelvis is positioned with the same degree of rotation as the shoulders and torso.
The whole bed is then placed into a 10–15° reverse Trendelenburg position. This elevation increases venous outflow from the head. Combined with subarachnoid drainage, this should provide a significant working corridor without the need for hyperosmolar agents (e.g., saline or mannitol). If not already done, the head is placed into three-point skull clamp pin fixation, with one pin on the contralateral forehead (taking care not to pin the temporal branch of the facial nerve) and two pins at the contralateral asterion and inion. The head is then titled 5–10° toward the contralateral shoulder, opening the angle between the cardinal axis of the cranium and the torso. The head is rotated 10–15° to the contralateral shoulder, and the chin is flexed toward the sternum, taking care not to compromise the airway or contralateral jugular vein. Prior to locking the pin clamp to the bed, anesthesia should confirm the patency and position of the airway. Rotation and flexion of the head may dislodge a shallow intubation. If the head is over-rotated or over-flexed, the contralateral internal jugular vein may be compressed, leading to an increase in intracranial pressure and intraoperative cerebellar swelling.
The patient’s body is securely restrained so that the operating table can be rotated liberally to maximize the angle of approach for the microscope. This usually involves a vacuum positioner and reinforcement with nonperforated cloth tape across the shoulder, torso, and hips. In larger patients, the ipsilateral shoulder is taped down across the long axis of the arm to open up the angle between head and shoulders, maximizing the surgeon’s angles of attack and visualization of the operative space during the operation (Fig. 2.4a, b).
We utilize a C-shaped, retroauricular incision about two fingerbreadths medial to the mastoid process, which extends from the pinna to the tip of the mastoid (Fig. 2.5a, b). In most cases, we prefer to raise a single myocutaneous flap with a combination of electrocautery and sharp dissection. Although many authors prefer to raise a subgaleal flap and take the combined fascia/periosteum at the insertion of the sternocleidomastoid as a separate local periosteal flap, we harvest a separate free flap (temporoparietal fascia or fascia lata) as necessary. The subperiosteal dissection is carried down, and the asterion is identified (which may also be confirmed with intraoperative neuronavigation).
Fig. 2.5
(a, b) A C-shaped retroauricular incision two fingerbreadths medial to the mastoid process, extending from the pinna to the tip of the mastoid. Figure b has a red line drawn from the root of the zygoma to the inion, approximating the course of the transverse sinus
Craniotomy Technique
Prior to the marking of the skin incision, the positions of the transverse and sigmoid sinuses should be approximated with anatomic landmarks or confirmed with neuronavigation. A straight line drawn from the root of the zygoma to the posterior occipital protuberance (inion) defines the course of the transverse sinus (Fig. 2.5b) [8]. The asterior and digastric grooves are identified (Fig. 2.6a). The “zygomatic line” and a “mastoid line” as described by Tubbs et al. [8] and demonstrated in Fig. 2.6b are marked with a surgical marker at the time of surgery to approximate the transverse-sigmoid junction.
