How Does spinal rigid endoscope Work?

Spinal endoscopy, especially using rigid endoscopes, represents a significant advancement in the treatment of spinal diseases. The endoscope has been increasingly used to treat various spinal diseases, including those affecting the intradural space. This review highlights the advancements in spinal endoscopy and its application in treating intradural pathologies. Importantly, available literature indicates the safety and feasibility of spinal endoscopy for intradural lesions, such as subarachnoid cystic lesions, tethered cord syndrome, extramedullary tumors, spinal arteriovenous malformations, and cordectomy for intractable pain. The results of this review enhance further development and broader application of endoscopic techniques for various intradural pathologies.

The concept of direct spinal cord visualization via intradural endoscopy originated in the 1930s. However, early endoscopes were too large and unreliable to safely insert into the intradural space. Instead, spinal endoscopic techniques evolved to treat extradural pathology, such as herniated discs and spinal canal stenosis, through minimally invasive methods. Recent advancements have produced smaller and more flexible endoscopes, rekindling interest in applying this technology to intradural pathologies. Cadaveric studies verified the usefulness and safety of manipulating endoscopes in the intradural space. Recent clinical reports show how spinal intradural endoscopy provides direct, magnified visions inside the spinal canal from minimal exposure. To discuss its usefulness, safety, and limitations, we categorized available literature into the following: subarachnoid cystic and inflammatory diseases, tethered cord syndrome, intradural extramedullary tumors, spinal arteriovenous malformations (AVM), and intractable pain.

A sufficiently small, flexible fiberscope can be passed through the dura into the intradural space with a minimum incision. Based on anatomical studies, the sizes of the ventral and dorsal subarachnoid space around the spinal cord range from 1-3 mm and 2-6 mm, respectively. Thus, an endoscope with an external diameter of fewer than 2 mm can pass safely along the spinal cord. Spinal endoscopy has proven its usefulness in treating spinal arachnoid cysts. In surgeries, following several levels of hemilaminectomy, an endoscope was inserted into the cyst cavity through the dura and bone window. The endoscope moved in cranial and caudal directions to penetrate or remove the cyst wall. Intraoperative fluoroscopy helps surgeons confirm the endoscope's position relative to the vertebral levels, providing critical intraoperative feedback. These maneuvers allowed establishing the communication of the cyst cavity with the subarachnoid space, improving neurological symptoms. Although long-term follow-up is required to estimate recurrence rates, endoscopic treatment can serve as a crucial surgical intervention option.

The same strategy applies to subarachnoid inflammatory diseases, where observation and biopsy are necessary. Surgeons can observe a relatively long range along the dorsal and ventral spinal cord surface with an endoscope. In a case of suspected neurosarcoidosis, a spinal endoscopic biopsy of a nodular lesion in the lumbar spinal cord established the diagnosis. Torres-Corzo et al. reported a rare case of neurocysticercosis. In this case, the flexible spinal endoscope explored the entire subarachnoid space and removed parasites and cysticerci. Thickened adhesive arachnoid membranes were cleared, and cerebrospinal fluid (CSF) flow was restored under direct vision through an endoscope. Cases of anterior sacral meningocele and superficial siderosis were also successfully treated using intradural spinal endoscopy. These cases indicate that various intraspinal subarachnoid lesions, including arachnoid cysts and arachnoiditis, can benefit from spinal endoscopy. The flexible endoscope, inserted and advanced through a small opening, allows less invasive approaches, especially when pathologies extend over multiple vertebral levels in the subarachnoid space.

Endoscopy has shown diagnostic and therapeutic potential for tethered cord syndrome. In the lower lumbar vertebral level, an endoscope can be advanced through the cauda equina to observe the filum terminale. Yörükoglu et al. described a percutaneous fully endoscopic interlaminar approach to the filum terminale in cadaveric studies. Clinical cases demonstrated that endoscopic observation is useful for visualizing the filum terminale in patients with tethered cord syndrome. A laminectomy and a 2-mm dural incision are sufficient to insert a flexible endoscope, confirming posterior displacement of the filum terminale, one diagnostic criterion of tethered cord syndrome. Endoscopic untethering techniques for the syndrome have also been reported. By the interlaminar approach and a 1-cm durotomy, a rigid endoscope could open the dura, coagulate, cut the filum terminale, and close the dura with continuous sutures. Although case numbers are small, percutaneous endoscopic interlaminar approaches may become viable options for untethering the filum terminale.

Barami et al. reported their first experience using an endoscope to remove ventrally located intradural extramedullary tumors. Spinal endoscopy effectively provides views of the ventral spinal cord without retraction, which is difficult through a microscope. Parihar et al. confirmed that endoscopic surgery applies to spinal tumors located at any spinal vertebral level. They successfully removed tumors with maximal sagittal and axial diameters not exceeding 4.1 cm and 1.8 cm, respectively. Zhu et al. demonstrated the feasibility of endoscopic removal of intradural extramedullary tumors through an interlaminar approach. Vital structures, including arteries and affected spinal nerve roots, were safely dissected and coagulated using a bipolar flexible radiofrequency probe. Combining endoscopy with the interlaminar approach causes minimal bone destruction, benefiting patients with less postoperative pain, blood loss, and shorter recovery periods. Individualized surgical planning and satisfactory dural closure techniques are key to success. Watertight CSF closure methods can increase the application of spinal endoscopy for these surgeries.

Endo et al. proposed other examples of using angled endoscopes in direct surgeries for spinal AVMs. Perimedullary arteriovenous fistulas in the cervical spine are often located on the ventral surface of the spinal cord near the anterior spinal artery. Visualizing these under a microscope via a common posterolateral approach is difficult without extensive spinal cord rotation. Instead, combining posterolateral exposure with endoscopy provides sufficient ventral spinal cord views without rotation. Following open microsurgical procedures, hemilaminectomy and paramedian 2-cm dural incision is required to secure the rigid angled endoscope's insertion space from posterolateral exposures. With assisted use of an endoscope, 360° circumferential views of the spinal cord surface can be achieved. When combined with indocyanine green (ICG) fluorescence endoscopy, it further enhances visibility. Surgeons gain detailed information about vascular anatomy and blood flow, crucial for properly managing spinal cord vascular lesions.

Technical advancements have yielded smaller-diameter spinal endoscopes, making percutaneous use practical. Tanaka et al. used a percutaneous endoscope to perforate a spinal arachnoid cyst and restore CSF flow. The endoscope, similar in size to a needle, punctures and provides magnified views of the spinal cord surface. Fonoff et al. contributed to developing percutaneous endoscopic procedures for intractable pain. Using percutaneous dual channels, the endoscope provides clear views of the spinal cord's pial surface and the radiofrequency cordotomy probe's tip. Surgeons appreciate real-time views, making the determination of electrode insertion targets more reliable and reducing the likelihood of spinal cord vessel or nerve root injury. Procedures achieve sufficient pain control without CSF leak complications. Recent reviews suggest cordotomy as an optional method to treat cancer pain. Considering the increasing number of cancer pain patients and the minimal invasiveness and safety of endoscopic procedures, percutaneous endoscopic cordotomy may have wider clinical applications.

This review focused on intradural spinal endoscopy's usefulness, highlighting that both flexible and rigid angled endoscopes can provide views otherwise difficult to obtain through a microscope. However, manipulating endoscopic instruments presents limitations. Recent achievements in endoscopic procedures were confined to basic maneuvers. The risk of bleeding and difficulty in achieving hemostasis must be considered. Technical difficulties encountered in full endoscopic procedures may necessitate converting to microsurgical procedures as a backup option in spinal intradural endoscopic surgery.

A crucial issue is preventing postoperative CSF leaks. The rate of dural tears in endoscopic lumbar spinal surgery was reported as high as 8.6%. Although no gold standard exists for managing dural tears or closure in endoscopic spine surgery, direct dural suturing techniques from endonasal and minimally invasive spine surgeries could be helpful. If surgeons become more comfortable achieving watertight dural closures under endoscopy, the application of endoscopic spine procedures for both intradural and extradural pathologies would expand.

Existing literature confirms increased utilization of spinal endoscopy for various intradural pathologies. An endoscope provides direct views of the spinal cord from a small incision. Moving a fiberscope along the spinal cord allows approaching longitudinal lesions and surgical management over multiple vertebral levels. An endoscope also offers magnified views from different angles than a microscope. Using a rigid angled endoscope, even ventral spinal cord tumors or vascular lesions can be managed surgically through a posterolateral approach. Technical advancements make percutaneous procedures possible in select clinical cases. As experiences and technical advancements accumulate, intradural endoscopy will likely be applied to treat other diseases in this region.

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Acknowledgments: The authors thank Enago (www.enago.jp) for the English language review. Funding: None.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Open Access Statement: This article follows the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0). See: https://creativecommons.org/licenses/by-nc-nd/4.0/. Provenance and Peer Review: This article was commissioned by the Guest Editors (Hisashi Koga and Alf Giese) for the series “Full-endoscopic Spine Surgery,” published in the Journal of Spine Surgery. It underwent external peer review organized by the Guest Editors and the editorial office. Conflicts of Interest: Both authors have completed the ICMJE disclosure form (available at http://dx.doi.org/10.21037/jss.2020.01.06), indicating no other conflicts of interest related to this work.