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As in the case with the pterional craniotomy for supratentorial parasellar lesions, the retromastoid craniotomy is the workhorse of infratentorial approaches for reaching the cerebellopontine (CP) angle and ventrolateral brainstem.
Based on the definition of the extended pterional approach, lateral sphenoid wing is resected to the level of the superior orbital fissure and the roof of the orbit is flattened to provide an unobstructed operative corridor toward the subfrontal and parasellar area. Similarly, I define the extended retromastoid approach as a modification of the standard retromastoid craniotomy that includes partial removal of the bone over the sigmoid sinus.
This “untethering” of the sigmoid sinus allows its lateral mobilization using retraction sutures after dural opening. This maneuver expands the lateral operative trajectory toward the CP angle while reducing the retraction on the cerebellar hemisphere.
This extended retromastoid craniotomy must be tailored to the specific target pathology. The following steps describe the general principles of this approach, whereas the individual chapters on specific lesions and disorders (ie cranial nerve compression syndromes) review tailored exposures via this route.
Indications for the Approach
This approach is flexible, efficient and familiar to all neurosurgeons. This versatile and adaptable exposure can reach almost all lesions within the CP angle and ventrolateral brainstem with gentle retraction and mobilization of the forgiving lateral cerebellar hemisphere.
The different petrosectomy exposures to the ventrolateral and anterior brainstem are most likely overutilized as the extended retrosigmoid route offers many of the same advantages with minimal morbidity. Although the location and size of the tumors dictate the appropriate operative approach, the texture of the tumor-fibrous (meningioma), soft and suckable (epidermoid)-also plays a crucial role in selection of a more expanded pathway to the anterior brainstem. For example, a large epidermoid tumor crossing the ventral brainstem can be removed through the retrosigmoid approach while a similar fibrous ventralateral meningioma with vascular encasement may require petrosectomy.
In simple terms, the operative corridor for the extended retrosigmoid approach extends from the ventrolateral midbrain and upper clivus to the ventrolateral medulla and lower clivus.
Monitoring brainstem auditory evoked responses (BAERs) during microvascular decompression (MVD) for hemifacial spasm and other lesions that potentially require dynamic retraction parallel to and affecting the CN VII/VIII complex is recommended. Latency of peak V is considered the best electrophysiologic indicator for signaling cochlear nerve injury. Contralateral hearing loss may be a contraindication for retromastoid craniotomy.
The transverse and sigmoid venous sinuses may have a slightly variable location along the temporal and occipital bones. Patients with short necks or mild brachycephaly may also have a short sigmoid sinus and their transverse-sigmoid junction may be displaced inferiorly, disrepecting anatomical landmarks. These variations can be studied on preoperative scans and considered during craniotomy. These anatomical variations can easily disorient the surgeon and lead to venous sinus injury or restricted bony exposure leading to ineffective lesional access.
Generous mastoid air cells increase the risk of postoperative CSF fistula.
The park-bench position prevents the non-physiologic neck posture that would occur if the patient were in a supine position. The supine position requires a significant turn in the patient’s neck, often leading to venous congestion. For these reasons, I believe the park-bench patient position decreases postoperative neck pain.
The patient’s ipsilateral shoulder is allowed to fall forward and is mobilized caudally away from the incision, therefore providing additional space within the surgical field. This maneuver is ergonomically important as it prevents a bulky shoulder, especially among obese patients with short necks, from obstructing the operative working space in the suboccipital area. Lack of attention to this detail can substantially limit operative movements.
The patient should be well secured to the operating room table to prevent risk of his or her displacement. To avoid risk of common peroneal nerve palsy, I do not place tape directly over the head of the fibula.
The head of the patient is slightly flexed and turned toward the floor. The head of the bed may be flexed 15 degrees upward, and then the entire table is slightly angled in the Trendelenburg position to lower the patient’s head 10 degrees. The latter maneuver brings the operative area into a horizontal plane.
For tumors approaching the middle aspect of the clivus and displacing the brainstem, I do not rotate the head toward the floor so that the operative viewing angle toward the medial brainstem is not compromised by the overhanging lateral cerebellar hemisphere.
I use the modified reverse "U" incision, originally described by Walter Dandy. This incision has certain advantages over the more commonly used variations of the linear incision.
The reverse "U" incision:
- Obviates the need for muscle dissection caudal to the posterior fossa floor and, in my experience, leads to less postoperative suboccipital pain.
- Reflects and mobilizes the myocutaneous scalp flap inferiorly, out of the surgeon’s working zone. The linear incision, in contrast, often accumulates the scalp and muscle layers under the scalp retractor and increases my working distance.
This burr hole often barely exposes the medial and inferior walls of the junction of the dural venous sinuses. The asterion is not a constant finding (especially in older crania), and a burr hole over the asterion often exposes the entire width of the transverse sinus, placing this structure at risk of injury. Please note the use of a cerebellar retractor and fishhooks to enhance bone exposure through the retraction the myocutaneous flap and scalp, respectively.
I place a single burr hole at the edge of the junction of dural sinuses to orient myself regarding the desired location of the planned craniotomy/ectomy. The exact location of these dural sinuses is slightly variable, and the initial burr hole should be placed with caution; it may be enlarged in the correct direction when the initial small pilot burr hole is done.
I dissect the dura from the inner surface of the skull bone, paying special attention to detaching the edge of the dural sinuses, especially the sigmoid sinus. The sigmoid sinus may be embedded into the inner cortical bone and should be left alone to avoid a tear in its wall; the roof of the sigmoid sinus may be ‘egg-shelled” and removed in a second step after elevation of the bone flap. In the case of microvascular decompression operations, the size of the craniotomy or craniectomy is often small, about 1.5-2 times the size of a quarter coin.
The first limb of the osteotomy extends posteriorly, inferiorly, then anteriorly, and stops short of the sigmoid sinus’s posterior border. The second bony cut is performed over the posterior edge of the sigmoid sinus in a cranio-caudal direction. A craniotomy should be avoided in older patients and in those with an adherent dural surface to the inner skull bone. When in doubt, plan a craniectomy as it is a safer option.
After I complete the craniotomy and elevate the bone flap, I drill a portion of the mastoid bone, exposing the posterior edge of the sigmoid sinus along its length. I also bevel out the outer bony edge in the area and remove any overhanging bone to improve lateral mobilization or rotation of the sigmoid sinus during reflection of the dura. This maneuver allows a more anterolateral operative trajectory toward the CP angle, potentially minimizing the need for cerebellar retraction while allowing more flexible operative working angles.
The bone over the sigmoid sinus is best removed by careful drilling and “egg-shelling” since the wall of this dural sinus is often very adherent to the inner aspect of the calvarium. The drill’s handle should move in the direction of its rotating burr to avoid its inadvertent slippage over bony edges.
OPERATIVE CORRIDORS: THE SUPRALATERAL AND INFRALATERAL ROUTES
As discussed previously, purely medial retraction of the cerebellar hemisphere parallel to CN VIII places this sensitive nerve at risk of retraction injury. Therefore, I use two operative corridors initially to reach the CP angle: the supralateral and the infralateral routes.
The Supralateral Route for Lesions Posterior and Superior to the CN’s VII/VIII Complex
During opening of the dura, if a venous sinus tear is evident or a venous lake is encountered, this laceration or opening into the sinus may be sutured primarily over a small piece of muscle. Bipolar coagulation of the tear usually retracts the dural edges further, increasing the size of the laceration and compounding the problem.
A piece of glove (cut slightly larger than the cottonoid) acts as a rubber dam and protects the cerebellar hemisphere from potential injury caused by friction from the cottonoid’s surface while “going around” the hemisphere. I advance the cottonoid parallel to the junction (groove) of the tentorium and petrous apex, toward the petrous side. Identification of the petro-tentorial junction prevents
- an unintentional exposure (and resultant tear and bleeding) of the supracerebellar bridging veins superiorly, and
- a retraction vector directly parallel to the CN VII/VIII complex, which would place the patient’s hearing at risk.
Strategic placement of the suction apparatus will allow dynamic retraction of the lateral hemisphere and selective exposure of the operative field at the level of CN V’s root entry zone, obviating a need to employ fixed rigid retractors and to sacrifice the superior petrosal vein.
The Infralateral Route for Lesions Anterior and Inferior to the CN's VII/VIII Complex
The dura is incised parallel to the sigmoid sinus and the floor of the posterior fossa. The edges of the dura are tacked up against the craniotomy margins using three silk sutures, and the same techniques are applied as mentioned above in the section related to the supralateral route.
The vector of retraction is parallel to CN IX. The shaft of the suction apparatus is used to mobilize the cerebellar hemisphere in a dynamic fashion during the dissection process. Along with generous sharp dissection of the regional arachnoid membranes within the inferior CP angle, this maneuver minimizes the risk of cranial nerve and cerebellar injury.
The operator can easily get disoriented and early identification of petrous-tentorial junction (supralateral route) and petrous-posterior fossa floor junction (infralateral route) avoids any surprise or confusion.
The dura is approximated primarily. I do not perform a watertight dural closure and have experienced a very low rate of CSF leak for microvascular decompression operations. These procedures are pristine and associated with a small risk of increased CSF pressures postoperatively.
However, a watertight dural closure is mandatory for operations involving resection of tumors. Mastoid air cells are rewaxed thoroughly (“wax in, wax out”), and the bone flap is replaced or a methyl methacrylate cranioplasty is performed. The muscle and scalp are closed in anatomical layers.
After surgery, patients are usually admitted to the ICU for an overnight stay for observation and then transferred to the regular ward for a couple of days before they can be discharged home. Special attention should be paid to hemodynamic parameters, the neurologic examination, and wound care. Steroids are administered to prevent aseptic meningitis and minimize postoperative nausea and headaches.
Occasionally, delayed facial palsy and hearing loss may occur, especially after MVD for hemifacial spasm and resection of epidermoid tumors. These deficits are most often temporary and respond well to a tapered dose of dexamethasone of 1 week in duration.
Other Considerations to Expand the Operative Corridor: The Transtentorial Route
The retromastoid and supracerebellar routes may be combined to expose the tentorium. A wide excision of the tentorium would allow access to the ventrolateral petroclival area and posterior basal temporal lobe (posterior parahippocampus) as well as middle fossa floor. Moreover, this pathway would facilitate resection of posterior fossa tumors extending into the middle fossa in one stage while avoiding a second operation through the supratentorial corridor. I will discuss these modifications in the chapter related to supracerebellar craniotomy.
Alternative Incisions for Retromastoid Craniotomy
Pearls and Pitfalls
- The extended retromastoid craniotomy is the flexible workhorse approach for resection of lesions within the CP angle.This approach can sometimes obviate the need for more involved petrosal osteotomies.
- The cerebellum should be retracted only parallel to the direction of CN’s V and IX to avoid direct traction on CN VIII. Sharp arachnoid dissection and strategic dynamic cerebellar retraction will facilitate cerebellar mobilization without placing the cranial nerves at risk.
For additional illustrations of a retrosigmoid approach to the internal auditory canal and cerebellopontine angle, please refer to the Jackler Atlas by clicking on the image below:
For additional illustrations of a retrosigmoid approach to the jugular foramen, please refer to the Jackler Atlas by clicking on the image below:
Cohen-Gadol AA. Microvascular decompression surgery for trigeminal neuralgia and hemifacial spasm: Nuances of the technique based on experiences with 100 patients and review of the literature. Clin Neurol Neurosurg. 2011;113:844-853.
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