Epidermoid cysts are slow growing and benign and do not respond to chemotherapy or radiotherapy, and asymptomatic small epidermoid cysts of the CPA can be monitored with serial imaging. However, for symptomatic or large asymptomatic cysts with significant mass effect on the brainstem, surgical removal is indicated. Complete surgical resection of the epidermoid tumor along with its capsule is ideal to reduce the risk of recurrence, but it is almost impossible to predict preoperatively how adherent the cyst capsule is to the brainstem and the traversing CPA neurovascular structures. Hence, the goal of surgical resection for these lesions should be maximum safe resection. Some studies have demonstrated that recurrence risk for CPA epidermoid cysts is the same with complete removal and incomplete removal [17,30]. However, these studies are limited by short follow-up periods. Epidermoid cysts are composed of soft white material and are often amenable to suctioning. In some cases, however, the lesion grows around the cranial nerves and blood vessels in the CPA, making it difficult to distinguish between the cyst contents and the cranial nerves, especially the sixth and fourth cranial nerves, which are small and fragile and can be easily injured during surgery. Identifying the cranial nerves where they enter the skull base foramina during surgery may be a helpful trick during surgery for safe resection.

Surgery is the mainstay of treatment for epidermoid cysts. Epidermoid cysts of thefourth ventricle are removed using a standardmidline suboccipital craniotomy . Most of the epidermoid cysts confined to theCPA are removed using a standardretrosigmoid approach [17,30,33], but epidermoids that extend into the middle fossa can be removed using a combined retrosigmoid and subtemporal transtentorial approach simultaneously or in separate stages [28,30] or an extended retrosigmoid transtentorial approach. More recently, the endoscopic-assisted retrosigmoid approach [28] or pure endoscopic retrosigmoid approach has been described [24]. In the following section, we describe the keyhole retrosigmoid transtentorial approach for epidermoid cyst of the CPA extending into the supratentorial (middle fossa) compartment.

After general anesthesia induction and patient intubation, the patient is positioned in a lateral decubitus position with the head held in a three-point fixation head clamp. Muscle paralytic agents are avoided, and neuromonitoring—including motor evoked and somatosensory evoked potentials, facial nerve and auditory brainstem evoked responses, and 9th, 10th, 11th, and 12th cranial nerve monitoring—is used routinely in these cases. The head is kept in a neutral position with slight lateral neck flexion toward the contralateral shoulder so that the mastoid tip is at the highest point in the surgical field. An axillary roll is placed under the dependent side; the ipsilateral shoulder is slightly flexed, and the arm is pulled down and secured with a tape over a bellow or an arm support fixed in the operating table. Undue traction of the ipsilateral shoulder and arm should be avoided to prevent brachial plexus injury. The operating table is reflexed, and the patient is secured onto the operating table using an adhesive tape for airplane positioning during the surgery. The pressure points are checked and padded appropriately. Neurophysiological baseline studies are obtained after positioning is complete so that undue traction of the brachial plexus or neck lateral flexion is detected and readjusted before starting the procedure. Aneuronavigation system can be used to help localize the position of the skin incision and the bony removal in relation to the sigmoid and transverse sinuses.

Anatomically, a straight line connecting the ipsilateral root of the zygoma and the inion along the superior nuchal line is a landmark for the ipsilateral transverse sinus. The asterion is located 4 cm posterior to the ipsilateral external auditory meatus and can usually be felt as a bony depression just superior and posterior to the mastoid notch. This point marks the inferior margin of the sigmoid-transverse sinus junction. A line extending from theasterion inferiorly just posterior to the mastoid tip marks the posterior margin of the sigmoid sinus. A curved skin incision is planned 0.5–1 cm posterior to the mastoid notch so that one third is above and two thirds are below the asterion. The hair above the ipsilateral pinna is clipped and draped in case a subtemporal bony window is necessary for resection of the supratentorial part of the lesion.

Once the skin incision and the bony exposure are complete, a small craniotomy or craniectomy is performed. It is very important to expose the posterior third of the sigmoid sinus and the inferior third of the transverse sinus to be able to reflect the dural edges or leaflet and to see along the posterior petrous and the inferior tentorial surface without difficulties. In cases wherehydrocephalus is present on preoperative imaging, a preoperative or intraoperative frontal or occipitoparietal external ventricular drain may be inserted and used to relax the brain during the surgery and to prevent postoperative CSF leak. Thedura is opened in a cruciate fashion or curved along the inferior sinus border leaving a small cuff of dura for watertight closure at the end. The dural margins are tacked up superiorly and anteriorly over the sinuses margins using dural stitches. Under microscopic vision, the prepontine and/or the pontomedullary cistern arachnoid membrane is opened sharply, and CSF is allowed to egress to aid cerebellar relaxation. The cyst should be visible at this stage, and the arachnoid membrane covering the cyst is dissected sharply A small, self-retaining retractor with a brain spatula may be used with minimal retraction over the lateral part of the cerebellum to allow better visualization. Care must be taken not to cause traction injury of the seventh and eighth cranial nerves or petrosal veins during this stage. The seventh and eighth nerve complex should be identified early. Attempts should be made to preserve all petrosal veins, but in certain cases, some petrosal veins may have to be sacrificed to prevent accidental tears or to provide better visualization for the deeper portion of the CPA cistern. At this stage, the cyst contents are debulked using careful microdissection techniques. It is important to remember that the cranial nerves and posterior circulation blood vessels and the perforators may be embedded within or displaced by the cyst and should be preserved at all costs. Careful attention to theabducens and trochlear nerves is important because these two nerves are small and can be difficult to visualize during the debulking of epidermoids. The abducens nerve can be identified below the seventh and eighth cranial nerve complex from its brainstem origin before going back up to its clival intradural fold before it enters Dorello’s canal. The trochlear nerve should be sought just medial to the dural free margin at the incisura. Enough debulking is achieved once the 4th, 5th, 6th, 7th, and 8th complexes; the 9th, 10th, 11th, and 12th cranial nerves; the basilar artery; and the anterior inferior cerebellar artery are identified and decompressed. At this stage, the arachnoid-epidermoid cyst wall adhesions are identified and divided sharply, and the cyst wall is removed. In cases where the cyst wall is very adherent to the brainstem, cranial nerves, or blood vessels, the adherent part of the cyst is left behind to prevent neurological or vascular injury.

Attention now is turned toward the supratentorial middle fossa portion of the cyst. The tentorial surface posterior to the superior petrosal sinus and medial to the petrosal vein is divided sharply using a sharp blade and extended toward the tentorial free margin using a microscissor. The trochlear nerve is at risk during this step and must be visualized and protected before completing the tentorial cut, which is performed posterior to where the trochlear nerve enters the tentorial edge. Bleeding from the tentorium can be controlled with hemostatic agent and bipolar cautery. The supratentorial portion of the cyst can now be debulked, and the third cranial nerve, posterior cerebellar artery, and posterior communicating artery can be identified and freed using microsurgical techniques. The endoscope can be brought at this stage and 30-, 45-, and 70-degree angled scopes can be introduced into the surgical field so the remaining pieces of the lesion can be visualized and removed using curved suction and microinstruments.

In cases where the supratentorial portion of the cyst proves difficult to resect microscopically or endoscopically from the retrosigmoid window, the skin incision can be extended superiorly by curving forward and then inferiorly to the level of the root of the zygoma just anterior to the tragus, and a separate subtemporal bone window is made. Once thebone window is elevated, the inferior margin of the temporal bone is drilled away until the root of the zygoma is flush with the middle fossa floor. The dura is opened in an inverted U shape and reflected inferiorly. The temporal lobe is elevated, and the tentorium is followed medially to the free margin. The arachnoid over the ambient cistern is divided sharply, and the lesion should be evident at this stage. In a similar fashion, the epidermoid is debulked, and the oculomotor and trochlear nerves, the posterior cerebellar artery, and the posterior communicating artery should be identified and freed (Fig.12.3).

The dura is closed in a watertight fashion either primarily or using a dural substitute. Bone wax is used to seal the middle fossa and mastoid bone to ensure obliteration of any opened air cells. Muscle and fascia, galea, and skin are closed in a multilayered fashion.

Surgical position-related neuropathy is not uncommon, especially in an obese patient with a short neck when lateral positioning is used. In our experience, somatosensory and motor evoked potentials may provide invaluable information after final positioning and may be used to adjust the extremity position of neck position before starting the procedure.

To avoid sigmoid or transverse injury during surgical exposure and to ensure adequate bony removal before opening the dura, a neuronavigation system is a useful surgical adjunct when available. Careful attention to the sigmoid and transverse dominance in the preoperative imaging is important in all posterior fossa approaches. When sinus injury occurs during surgery, every effort is made to repair the injured sinus primarily using a vascular stitch; depending on the extent of the injury, postoperative antiplatelet treatment using aspirin may be considered. When a subtemporal window is planned, the superficial sylvian vein drainage patterns and vein of Labbé, especially in the dominant temporal lobe, should be identified in the preoperative imaging and protected during surgery.

When preoperative hydrocephalus or significant perilesional edema is evident preoperatively, insertion of an external ventricular drain before starting the procedure can be of clinical benefit to avoid intraoperative intracranial hypertension and postoperative acute deterioration. The drain is inserted preoperatively and opened at high pressure or kept closed until the bone is removed. Once this is done, the drain is opened, and CSF is drained to allow for cerebellar relaxation just before dural opening. The drain can be kept in place to prevent postoperative CSF leak for few days, and then a weaning trial is attempted. If the trial fails, then a shunt should be inserted.

Most of these surgical cases can be done with dynamic brain retraction using the suction instrument in the nondominant hand; however, when a fixed brain retractor is used, care must be taken not to apply too much retraction on the cerebellum to prevent cranial nerve (especially seventh and eighth nerve complex) traction injury or petrosal vein tears. During cyst wall dissection off of the cranial nerve, manipulation of the seventh and eighth nerve complex should be avoided to prevent postoperative facial nerve weakness or hearing loss. On the other hand, the trigeminal nerve is more resilient to manipulation and can be maneuvered with impunity.

Approaching the supratentorial portion of the cyst from the posterior fossa can be challenging. Care must be taken not to injure the trochlear nerve when performing the tentorial cut by identifying the nerve before completing the cut. When dissecting the middle fossa portion, care must be taken not to injure the oculomotor nerve, posterior cerebral artery, or posterior communicating artery. The adherent portion of the cyst wall should be kept in place, with no attempt to pull or dissect it, because these lesions are benign and slow growing. Any residual tumor can be monitored with serial imaging postoperatively.