Introduction

Epidermoid tumors are congenital and benign lesions, which typically are present between the third and fifth decades of life, that account for 0.2 to 1.8 % of all intracranial tumors [2, 5, 19]. They preferentially locate in the cerebellopontine angle (CPA) that can be involved in 40 to 60 % of the cases [6] and constitute approximately 4.6 to 6.3 % of all CPA lesions [10, 30]. The clinical signs and symptoms of CPA epidermoids are associated with the presence of adhesions and compression of adjacent neurovascular structures and brainstem. They tend to spread widely along the subarachnoid space and encompass adjacent neurovascular structures, occasionally extending through the tentorial incisura and occupying multiple cisterns [19]. Besides the growth of the tumor, it can result in a chronic inflammatory reaction that promotes their adherence to neurovascular structures [31].

In the premicrosurgical era, the surgical mortality of patients with epidermoids ranged from 20 to 57 % [4, 5, 8, 15]. This rate has significantly decreased with dramatic improvement of microscopes and microsurgical instruments. However, the rate of surgery-related complications is still rather high [2, 24, 29]. These associated complications have prompted the investigative use of endoscopy in the application of CPA epidermoid tumors. In fact, the use of an endoscope as a sole tool for visualization during surgical treatment of CPA epidermoids is rarely mentioned in the relevant literature [7]. The aim of this paper is to present the advantages and limitations of full endoscopic excision of CPA epidermoids.

Clinical material and methods

Patient population

After informed patient consent, six patients harboring an CPA epidermoid tumor underwent pure endoscopic removal of the tumor in our institution between 2008 and 2013. The characteristics of these patients are presented in Table 1. The series consisted of three male and three female patients whose mean age was 37 years (range 18–59 years). Two patients had facial numbness, two suffered from neuralgia, one from imaging test after craniocerebral trauma, and hemifacial spasm was present in one patient. MR imaging was performed in all patients preoperatively and postoperatively (Figs. 1 and 2).

Table 1 Summary of clinical characteristics in eight patients with epidermoid tumors
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Fig. 1
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Preoperative MRI scans showing epidermoid tumors located in the left or right cerebellopontine angle (CPA) with different tumor extension (cases 2–5)

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Fig. 2
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Radiographic studies in case 1 and case 6. a–b Preoperative MRI revealing a hypointense lesion located in the left CPA extending into prepontine cisterns. c–d Postoperative MRI confirming radical excision of the tumor. e–f Preoperative MRI showing a right CPA epidermoid mass dislodging the brain stem. g–h Postoperative MRI demonstrating no tumor remnants

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Endoscopic equipment

The entire surgical procedure was accomplished using a rigid 0° or 30° endoscope (ZEPPELIN, Germany). The endoscope was connected to a light source through a fiberoptic cable and to a digital camera, which was connected to a 21-in. flat screen monitor supporting the high resolution of the images. The endoscopic image on the screen could be enlarged by adjusting the zoom lens of the camera. The endoscope was fixed with the aid of an endoscope-holding device (mechanical arm or pneumatic arm) in order to bimanually handle a tumor, and tumor removal was performed using reconstructed microsurgical instruments (curved tip instruments, Fig. 4i, j).

Surgical procedure

The main operative procedures are shown in Figs. 3 and 4. The patient was placed in the park bench position for the retromastoid approach after general anesthesia. Ten milligrams of dexamethasone was given every 8 h for 2 days, and antibiotic agents were not administered after surgery. A sagittal 4 cm incision of sufficient size was made to allow a 30-mm craniotomy (Figs. 3a, b and 4a, b). The craniotomy for the retrosigmoidal approach was made to provide the optimal path to the CPA region and minimal retraction of the cerebellum as well as avoiding bleeding of the transverse and sigmoid sinuses. Then, the dura is bipolar cauterized and opened to expose the junction of the transverse and sigmoid sinuses. First of all, the lateral cerebellomedullary cistern was opened to allow sufficient drainage of cerebrospinal fluid (CSF, Figs. 3c and 4c); afterwards, the CPA cistern and the cisterna magna were also opened to release more CSF aiming at reducing the intracranial pressure and cerebellar drop, thus yielding a wide and straight working channel to the CPA (Figs. 3d–f and 4d, e). The endoscope was then introduced in the CPA, where the tumor with pearly appearance was usually located in the space between the tentorium and the cranial nerve bundle VII/VIII, which was always surrounded by the lesion (Figs. 3f and 4d–f). Sometimes, the tumor could have significant ventral and caudal extension so that it was firmly adherent to the surrounding neurovascular structures such as the cranial nerves V, VI, IX, X, and XI, the superior petrosal vein, vertebrobasilar artery, the branches of the anterior-inferior cerebellar artery, and the superior cerebellar artery. The tumor was removed in a piecemeal fashion after its thin capsule was incised under dynamic endoscopic view (Figs. 3g–i and 4f–i). The mass was carefully dissected from the surrounding neurovascular structures; nevertheless, when it was firmly adherent to these structures or the brainstem, parts of its capsule were left in place to prevent neurological complications. After all solid tumor had been extirpated completely, the subarachnoid spaces were repeatedly irrigated to remove any tumor remnants and avoid aseptic meningitis. No external ventricular drains were placed. After watertight closure of the dura, the bone flap was replaced, and the wound was then closed by a continuous intradermic suture with absorbable thread. All the patients were monitored overnight in the neurosurgical intensive care unit.

Fig. 3
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Intraoperative endoscopic view of case 1. a–b The incision and burr hole compared with a brain puncture needle; c opening the cerebellomedullary cistern to allow sufficient drainage of cerebrospinal fluid; d–e cranial nerve (CN) VII/VIII, IX, and X are seen after cerebellar drop; f the tumor is located between the superior petrosal vein (SPV) and the CN VII/VIII; g bimanual dissection of the tumor under the endoscope which is fixed with a self-retaining holding device; h–i resection of the capsule; j–k removal of tumor remnants located in surgical anatomic corners with a curved tip suction catheter; l CN VII/VIII, IX, and X are shown again after the total removal of the tumor. 1 CN X; 2 CN IX; 3 CN VII/VIII; 4 cotton; 5 superior petrosal vein; 6 tumor; 7 cerebellar cortex; 8 CN V

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Fig. 4
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Intraoperative endoscopic view of case 6. a–b The incision and burr hole; c the cerebellomedullary cistern is opened and the CN XI is shown; d–e the tumor is located between the SPV and the CN VII/VIII; f the tumor capsule is cut bimanually; g–h removal of the “pearly” tumor; i–j removal of tumor remnants located in surgical anatomic corners with curved tip instruments; k resection of the tumor capsule; l endoscopic views of the CPA after the total removal of the tumor. 1 CN XI; 2 cerebellar cortex; 3 CN IX/X; 4 CN VII/VIII; 5 tumor; 6 superior petrosal vein; 7 CN V

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Results

Clinical presentations, tumor extension in MR imaging, complications, therapeutic outcomes, and follow-up periods are displayed in Table 1. The follow-up periods ranged from 14 to 50 months. All the cyst contents were completely removed. In five patients (cases 1, 2, 3, 5, and 6), a total removal of the capsule was performed, resulting in permanent cranial nerve deficits in one of them. In case 4, the majority of the capsule were resected while small portions of the capsule that were tightly adherent to the adjacent neurovascular structures were left in place.

The symptoms caused by mass effect of the lesion resolved postoperatively. There were no deaths, but permanent deficits occurred in one patient. In case 5, the CN VII/VIII complex was cut accidentally while dissecting the tumor from the CN IV and CN V because as the endoscope moved deeper into the CPA, visualization of the caudal structures was lost. The accidental damage resulted in facial nerve paralysis as well as permanent significant hearing loss. MR imaging after surgery revealed no residual tumor in any patients. There have been no recurrences to date.

Discussion

According to Obrador and Lopez-Zafra [20], with irregular lobulated surface and pearl-like shine epidermoid tumor was also known as the “pearly tumor.” The tumor consists of a thin capsule filled with epithelial cells consisting predominantly of keratin in concentric layers, water, and cholesterol from cell membrane degradation as desquamation occurs [19, 22]. Meticulous evacuation of the cyst contents is easy because of their waxy consistency and avascular nature. Nevertheless, the capsule may be firmly adherent to the adjacent neurovascular structures or the brainstem, making any attempt at total removal risky [25]. Furthermore, because small parts of the tumor may extend into the narrowest spaces, posterior and anterior to the nerves in the surrounding cisterns, a considerable amount of manipulation and phases of enhanced retraction in operation may put the critical neurovascular structures at high risk.

A microscope for visualization is commonly used in the traditional retrosigmoidal approach to the CPA, which has necessitated a larger bone window to allow a direct view of the targeted neurovascular structures as well as to view deep into the midline of the posterior fossa. Retraction of the cerebellum is usually necessary to achieve optimal visualization [28]. Excessive manipulation and retraction of the cerebellum may increase the risk of infarction and other morbidities because the tumor compression leads to decreased cerebral perfusion [1, 9, 32].

Recent developments in neuroendoscopy have facilitated the visualization of deep anatomical structures directly through narrow surgical corridors with no need for highly invasive craniotomy. In particular, advances in endoscopic technology have developed high-quality rod endoscopes of different lengths and small working channel diameters with variably angled views, irrigation sheaths for cleaning the lenses when operating inside the deep field, and endoscope holders that enabled the surgeons to keep on using their microsurgical operative techniques bimanually. These significant improvements allowed the endoscope, with its wide panoramic, multi-angled, and close observation of the target structures, to perfectly exhibit its best properties in dealing with deep-seated and narrow space-located lesions such as CPA lesions.

The use of endoscope in the operation of the CPA or pineal region, either with endoscope-assisted [3, 12, 16–18, 21] or endoscope-controlled [11, 13, 14, 26, 27] techniques, has played a crucial role in the treatment of hemifacial spasm or trigeminal neuralgia resulting from vascular compressions. Yet, there was comparatively little literature on the endoscopic treatment of CPA epidermoids. Schroeder et al. [25] reported a series of eight patients with CPA epidermoids who underwent endoscope-assisted resection. The authors attributed the less complications to the use of the endoscope and stressed better visual control of the structures without excessive retraction and requirements of enlarged craniotomy. Furthermore, depending on their clinical experience with endoscopic transsphenoidal surgery, de Divitiis et al. [7] performed purely endoscopic removal of a CPA epidermoid tumor using the previously described freehand “dynamic” technique that required two surgeons to operate in close collaboration: the “navigator,” who held the endoscope and made precise position adjustments coordinately, and the “pilot,” who performed the procedure itself bimanually under dynamic endoscopic view. However, in the comments of this article, Michael R. Gaab doubted that this was a better solution than the endoscopy-assisted way of performing the surgery under the microscope; it seemed to be a stopgap solution because of an obvious drawback of the endoscope which could not see the instruments used before they passed the front lens. This resulted in greater risk of injuring structures with the endoscope remaining in place unless the handling of instruments was controlled by the microscope before reaching the front lens or the anatomy had to be protected with other instruments. Even Safavi-Abbasi and colleagues [23] did not believe that purely endoscopic removal was superior to microsurgical technique, and they did not advocate endoscopic removal of CPA epidermoids. They argued that the endoscope was a reliable tool for evaluating the extent of tumor resection, but no attempt should be made to remove either the cyst contents or its capsule with the endoscope alone because endoscopic resection of epidermoid tumors which tended adhere to adjacent neurovascular structure was considerably difficult and hazardous and the high-fat content of the tumor adhering to the lens could blur the view.

In our series, full endoscopic removal of epidermoid tumors involving both the posterior and middle cranial fossa was performed, and satisfactory results were obtained. In fact, there are some especial advantages in the pure endoscopic removal of CPA epidermoids. First of all, a properly placed small craniotomy permits less tissue damage, the minimization of cerebellar retraction and a direct path to the CPA. Secondly, the endoscope can be advanced into the cerebellomedullary cistern and the cisterna magna through the single small craniotomy to allow fast cerebrospinal fluid drainage and cerebellar drop, thus yielding a wide and straight working channel to the CPA. Thirdly, by using optics with 30° of view and modified microsurgical instruments with slender and curved tip, tumor parts hidden behind delicate neurovascular structures can be safely visualized and removed. In the end, because of minimal tissue damage of the operation and no need to take out stitches, the period of hospitalization was greatly shortened (3–5 days in our series) as well as reduced hospital costs.

Of course, it is of great importance to take into account the surgical techniques. Adequate cerebellar collapse can be obtained by proper head positioning and CSF drainage from the surrounding cisterns. Besides, the endoscope can be fixed with a self-retaining holding device permitting bimanual skilled movements repertory of microsurgery to dissect the adherent capsule and its contents from the normal neurovascular structures (Figs. 3g and 4f). In addition, it is important to understand the limitations of the technology which include lack of stereoscopy, obscuration of vision, and loss of control in event of bleeding. These limitations can be overcome by improving surgical techniques. However, the foremost limitation is lacking visualization of structures behind the tip of the endoscope resulting in injury to important neurovascular structures that are not in direct view during surgery. In one patient of our series, the CN VII/VIII complex was cut accidentally while dissecting the tumor in the deeper operative area. The surgeon should be responsible for the visual control of the instruments, which constantly remain under direct endoscopic view; the endoscope should follow the linear in-and-out movements of the instruments, thereby minimizing the risk of injury to the neurovascular structures. After we noticed the potential risk, accidental injury to the neurovascular structures by endoscope or instruments never happened again. Certainly, one must have extensive experience in endoscope use and microsurgical techniques before applying this technique to other applications.

Conclusions

The endoscope-controlled microsurgical technique enables a safe tumor removal even when parts of the lesion are not visible in a straight line in CPA epidermoids. By angled endoscopic lenses, tumor extending into adjacent cranial compartments or surgical anatomic corners can be removed through a single small craniotomy without retracting neurovascular structures. Although this technique has proved in our experience to be effective in our series, to evaluate its true effectiveness in the CPA a larger patient series is necessary.