The skull base is deeply located and adjacent to critical neurovascular structures, making it one of the most difficult areas to access. The best treatment for skull base lesions is to excise the tumor while causing minimal collateral damage. To minimize cerebellar retraction and reduce the risk of injury to vital neurovascular structures, skull base surgeons prefer to dissect as much soft tissue and bone as possible. However, the enlarged skull base approaches also may prolong the procedure, increase blood loss, and damage important vessels, such as the internal carotid artery and vertebral artery.6, 7 Numerous surgical approaches have been developed to address skull base pathologies in a minimally invasive manner, but endoscopic technology is revolutionizing the industry.
The operating microscope provides an excellent stereoscopic view with superb resolution and high manipulability by allowing two-handed operation. However, when operating in deep surgical corridors, there is a considerable decrease in light intensity at the entry point, and the field depth is low under high magnification. Furthermore, lesions that are not visible in a straight line cannot be explored. With a lateral view and small size, the endoscope can bypass superficial structures to expose deep configurations while increasing illumination and showing clear details of the neurovascular tissue at close range. However, the endoscopic image quality is inferior compared to the microscope due to a smaller lens diameter. The lack of stereoscopic vision and motion parallax effect contributes to the inferior image quality.8 Therefore, the advantages of microscopes and endoscopes should be combined during neurosurgery.
Many studies have attempted to apply endoscopic techniques to neurosurgical procedures, and one important modality is known as the “endoscopic-assisted techniques.” In this procedure, the endoscope is used as a supplementary visualization tool while the primary surgical operation is performed under a microscope.9, 10 Previous research reports that endoscopic-assisted techniques have significant advantages in the treatment of a wide range of intracranial lesions. Endoscopic techniques have been utilized to inspect the trigeminal or facial nerves during microvascular decompression surgery,11, 12 to verify completion of clipping during aneurysm surgery13, 14 and to visualize the internal auditory canal during vestibular schwannomas surgical resection.15 However, the endoscope cannot visualize the area behind the lens and may damage important neurovascular structures when in a deep and narrow skull base surgical corridor.
Few studies have examined the application of endoscopic-assisted techniques in complex skull base tumor surgery, especially involving the cavernous sinus, petroclival region, and the foramen magnum. In this study, we used a combined microscopic-endoscopic technique in complex skull base surgery. This technique integrates microscopic and endoscopic images in a picture-in-picture mode, overcoming the disadvantages of endoscopic-assisted techniques and utilizing the efficiency of the endoscope during skull base surgery.
This combined microscopic endoscopic technique increases the visualization of residual tumors.16, 17 In skull base surgery, tumors obscured by bone and nerves cannot be detected because the microscope can only visualize in a straight line. In our series, total resection was achieved under the microscope in 23 patients, but the endoscope detected residual tumors in 19 patients. The microscopic blind spots are unique to different regions of the skull base. For example, the anterior transpetrosal approach is often used for petroclival region tumor resection in this series. However, even with extensive bone drilling, the lower cranial nerves, the central clival depression, and the contralateral side of the prepontine cistern may not be exposed adequately with a microscope. The endoscope significantly expands the surgical field with a lens that can penetrate deep into narrow surgical corridors, providing superior illumination and excellent surgical field depth. Furthermore, angled endoscopes allow one to look around and visualize the surgical space behind the neurovascular structures (Fig. 2). When dealing with foramen magnum tumors, the endoscope provided better visualization of the pontomedullary portion of the AICA, bilateral VAs, the origin of PICA, ASA, and the lower cranial nerves (Fig. 3). In addition, the endoscope allowed us to look behind the jugular tubercle and remove tumor tissue deep into the hypoglossal canal.18
Another advantage of this technique is a reduction in cranial nerve stretching. The endoscope can expand the surgical field without retracting the cranial nerves.19 In cavernous sinus surgery, the microscope can only observe the dorsal surface of the neurovascular structures, but the angled endoscope offers a wider field of view, allowing us to visualize every corner of the cavity. As shown in Fig. 4A, an incision was made between the trochlear nerve and the ophthalmic branch of the trigeminal nerve. An endoscope was introduced through this (VI-V1) corridor. We were able to visualize the anterior surface of the cranial nerves and detect residual tumors without nerve stretching.
This technique also allows for a reduction in skull base drilling and bone resection (Fig. 4B, C). The endoscope minimized bony exposure in skull base approaches. In the far lateral approach with angled endoscopes, removal or exposure of the C1 arch is unnecessary and condyle and jugular tubercle resection could be avoided.18, 20 In the anterior transpetrosal approach, the endoscope can reduce drilling of the petrous apex, reducing the risk of cochlea and internal carotid artery damage. In the present series, several tumors located in the petroclival region can be completely resected via a simple retrosigmoid approach.21, 22
The combined microscopic-endoscopic technique used in this study also overcomes the disadvantages of endoscopic-assisted techniques. Endoscope insertion and movement in skull base corridors must occur under direct visualization to prevent damage to critical neurovascular structures. When the microscope and endoscope are combined in the same setting, the surgeon has to switch between the microscope ocular view and the endoscope monitor view. This nonintegrated visual information can disrupt the flow of the operative procedure. Few studies have attempted to integrate endoscopy and microscopy.17, 23 Here we applied a picture-in-picture mode in which microscopic and endoscopic images are merged on the same screen. With simultaneous microscope monitoring, the surgeon can safely place and move the endoscope and easily transition between the two modalities. The endoscope is a powerful assistant in skull base surgery that enhances recognition of the surgical landscape.
Patients with cholesteatoma are the most common in this series. Cholesteatoma are benign, slow-growing congenital neoplasms of the central nervous system with a poor blood supply. Cholesteatoma generally occur at the skull base, expanding through cisterns and the subarachnoid space, and may adhere firmly to critical neurovascular structures. We found the combined microscopic-endoscopic technique especially suitable for the removal of intracranial cholesteatoma. The endoscope could easily detect tumors that were often left behind with the microscope, significantly reducing tumor recurrence.