Application of neuroendoscopy in the surgical treatment of complicated hemifacial spasm

Hemifacial spasm (HFS) is a common neurological condition. Traditional microvascular decompression (MVD) has been the main treatment for patients with HFS.1 However, in some HFS patients with more complex anatomy (for example small posterior fossa volume, abnormal fullness of the cerebellar flocculus, petrous bone block, local thickening of arachnoid adhesions, and unidentiied ofending vessels), simple MVD effect is not good2 because their congenitally limitations of viewing angle and lighting. Therefore, minimally invasive endoscopic surgical techniques have been increasingly used in recent years to treat HFS. In contrast to the microscope, the neuroendoscope provides excellent lighting and a panoramic viewing angle, and gradually become an effective complement to MVD for HFS.3,4 In our research we retrospectively analyzed the indication, the surgical technique, the advantages, and the disadvantages of the neuroendoscopy in 42 patients with HFS with complicated local anatomy during the period from January 2008 to January 2012 in the Department of Neurosurgery, Hospital Affiliated with Nanjing Medical University of Wuxi, Wuxi, China.

Limitations of traditional MVD in complex HFS patients

At present, MVD treatment modified by Jannetta5 is mostly used for treatment of primary HFS; that is, under the concept of keyhole surgery, the traditional sigmoid sinus approach used for MVD treatment of primary HFS is modified so that the skull open window is set at 2–3 cm, eliminating excessive and unnecessary craniotomy for surgical exposure and thus reducing surgical trauma and postoperative complications. Such surgical methods have a good effect for most HFS patients but can be limited in patients with anatomical variations or complex offending vessels. First, the key to HFS surgery is correct evaluation of the offending vessels to avoid any omission, and all neurovascular contact areas must be screened for adequate nerve decompression. A good surgical field and excellent lighting are essential to a successful operation. At present, the small bone window in MVD surgery will limit the multi-angle viewing range of the microscope. In cases of development of a cerebellar flocculus and poorly exposed facial nerve roots, the operative space is further limited. Second, some HFS patients have complex local neurovascular anatomy, with abnormal local vascular courses or even entwined branches, while offending vessels are located deep in the abnormal vascular plexus. A dead spot in the microscopic view can be created when the cerebellum pompon is so full, and significant abnormal petrosal protrusion into the choroid plexus, enveloped by arachnoid, is present.1,2 In this case, the cerebellum should be further retracted or excessive separation of the arachnoid will be required to achieve better exposure, which will inevitably result in local excessive neurovascular disruption, cerebellar traction injury, and even avulsion of the petrosal vein, causing serious complications.6 Additionally, small offending blood vessels that do not create a pressure trace on the facial nerve or those located on the dorsal side will also impede microscopic evaluation.

Advantages of neuroendoscopic surgery for complex HFS

Surgical microscope illumination is prone to having a dead corner in the surgical field and does not reveal structures outside the microscope field well.7 The neuroendoscope provides excellent lighting and a panoramic viewing angle, as the lens can be inserted into the cranial cavity for full-angle observation of the nerves, brainstem, and blood vessels. This allows identification of the presence or absence of contact with the nerve surface and indentation or presence of blood vessels. At the same time, the entire length of the facial nerve can be viewed as well as the dorsal side,8 making it difficult to overlook offending vessels, thus reducing postoperative recurrence. Additionally, endoscopic surgery requires an opening just slightly wider than the width of the lens barrel. The smaller space required prevents stretch irritation to the brain stem, cerebellum, and peripheral nerves and blood vessels, thus greatly reducing surgical complications. Finally, endoscopic surgery allows pre-closure confirmation of appropriate location of the Teflon pads, adequate decompression, and secure placement of pads, providing immediate feedback to the surgeon and significantly improving surgical outcomes.9

Indications for endoscopy

Surgical exposure in the present cohort of 42 patients was challenging because of various anatomic abnormalities that made it difficult to view or recognize the offending vessels. A 0-degree endoscope with no refraction is no different from a microscope (often applied in the Otolaryngology Department, less applied in Neurosurgery), whereas a 70-degree endoscope has such large refraction that it is likely to accidentally injure deep structures (seldom used in the Department of Neurosurgery). In this case, 30-degree-endoscope assistance not only clarifies the offending vessels, but also avoids failure of surgery or iatrogenic injury, thus greatly improving the success rate of surgery in complex HFS patients.

Operation key points

1. Detailed preoperative planning, including brain MRA or CT angiography, is needed to show the presence of offending vascular compression and rule out location in the cerebellopontine angle and other secondary factors. 2. The head should be correctly placed to be more conducive to revealing the facial nerve REZ region. 3. The anterior border of the bone window should reach the sigmoid sinus, and the lower edge should reach the skull base level. 4. The optical axis must always be kept in line with the microscope field of view and CSF slowly released after opening the dura. When exposure is difficult because of petrosal bone protrusion and fullness of the cerebellar flocculus, the cerebellum must not be forcibly retracted. In such cases, the endoscope can be used to assist the operation in the intraoperative zone structure. 5. Before any sharp cut of the arachnoid, endoscopic observation of the local vasculature is necessary to avoid subsequent relocation of offending vessels that may affect evaluation. 6. Complex HFS patients tend to have a small surgical space and complex internal structures, in particular, when the offending vessel is located in a medial position or hidden, making it difficult to distinguish offending vessels by microscope. The combination of microscopic and endoscopic techniques is important for careful multi angle observation of the facial nerve. 7. Currently, in order to achieve full decompression, we often lift the offending vessels on both sides of the facial nerve root like the shape of a bridge. The use of Teflon pledgets alone is rare nowadays. After surgery, endoscopic confirmation of full and complete padding decompression should be performed.

At the early stage, the most serious complications following MVD in our hospital were cerebellar infarction, edema, tinnitus, hearing loss, and even death, which are associated with over-stretch of the cerebellum.10 In recent years, with the assistance of neuroendoscopic techniques, these complications have been significantly reduced and no case of secondary brain contusion, cerebral infarction, or hearing loss has ever occurred.11,12 However, the rates of facial paralysis, CSF leakage, and delayed healing are similar to those in simple MVD.

Limitations of endoscopy

All HFS patients in this study underwent neuroendoscopic-assisted separation and decompression, but we believe that while neuroendoscopy is an effective tool,13-15 not all HFS patients require endoscopy. The three-dimensional view and excellent depth of field of the microscope have their inherent advantages. It is still preferred for those with a large posterior fossa volume, no significant cerebellum pompons or petrosal shelter, fully released CSF, and clearly identified offending vessels. Although neuroendoscopy has its obvious advantages, there are disadvantages that cannot be ignored. First, the endoscope takes up most of the bone window, making it impossible to see bilateral and posterior structures after entry into the head, which may risk damage to the surrounding structures when changing positions or angles. Second, endoscopy can provide only a two-dimensional image, which lacks a sense of depth. Most neurosurgeons are not familiar with endoscopic imaging and manipulation techniques, as opposed to entirely endoscopic MVD. Many physicians prefer endoscopic-assisted MVD. Third, surgeons need to support the mirror while performing the surgery, which can lead to operational difficulties. At the same time, there is still no specific surgical instrument for endoscopic treatment of subarachnoid lesions. Although a 30-degree endoscope can be used to observe some lesions, it does not allow operation. Additionally, bleeding can easily contaminate the endoscope lens, affecting image quality and increasing operational difficulty, thus affecting the coherence of the procedure. In addition, the heat generated by the light source and the sharp edges of the endoscope are also likely to injure nearby structures.

In conclusion, endoscopy is a useful complement to traditional nerve MVD surgery.16 The advantage of endoscopy lies in its excellent lighting and panoramic perspective, while microscopy provides three-dimensional stereoscopic advantages and excellent depth of field. Combining the 2 with their respective advantages, may facilitate and improve surgery in complicated HFS patients.