Key points

Introduction

Microvascular decompression (MVD) is a well-established and effective treatment of trigeminal neuralgia, hemifacial spasm, glossopharyngeal neuralgia, and other associated surgically treated craniofacial pain syndromes. Traditionally, this surgery is performed under microscopic guidance through a retrosigmoid craniotomy. Treatment failures do occur and can be associated with inadequate visualization of the surgical anatomy—in particular, the point of contact of the offending vessel to the root entry zone. This observation reflects an underlying problem with visualization provided by the microscope. Namely, the visualization of the anatomy of interest requires a direct line of sight with the light source when using the microscope. The incorporation of the endoscope into the armamentarium of the neurosurgeon has revolutionized anterior and ventral skull-base neurosurgery. The endoscope was first introduced as an alternative to the microscope in transsphenoidal pituitary surgery. Its use was quickly expanded to approach lesions of the anterior skull base, tuberculum sella, and the clivus. More recently, endoscopic approaches have been developed for use in lateral skull-base surgery, and in particular, in the surgical treatment of trigeminal neuralgia. In this article, the fully endoscopic microvascular decompression (E-MVD) for patients with trigeminal neuralgia is discussed.

Introduction

Microvascular decompression (MVD) is a well-established and effective treatment of trigeminal neuralgia, hemifacial spasm, glossopharyngeal neuralgia, and other associated surgically treated craniofacial pain syndromes. Traditionally, this surgery is performed under microscopic guidance through a retrosigmoid craniotomy. Treatment failures do occur and can be associated with inadequate visualization of the surgical anatomy—in particular, the point of contact of the offending vessel to the root entry zone. This observation reflects an underlying problem with visualization provided by the microscope. Namely, the visualization of the anatomy of interest requires a direct line of sight with the light source when using the microscope. The incorporation of the endoscope into the armamentarium of the neurosurgeon has revolutionized anterior and ventral skull-base neurosurgery. The endoscope was first introduced as an alternative to the microscope in transsphenoidal pituitary surgery. Its use was quickly expanded to approach lesions of the anterior skull base, tuberculum sella, and the clivus. More recently, endoscopic approaches have been developed for use in lateral skull-base surgery, and in particular, in the surgical treatment of trigeminal neuralgia. In this article, the fully endoscopic microvascular decompression (E-MVD) for patients with trigeminal neuralgia is discussed.

Technical and anatomic considerations of cerebellopontine angle endoscopic surgery

There are several principal advantages of the endoscope over the microscope in cerebellopontine angle (CPA) base surgery. First, the endoscope allows for a panoramic and uninhibited view of surrounding anatomy. Unlike the microscope, which requires a direct line of site from the lens to the point of interest for optimal visualization, the endoscope has a field of view of approximately 90° from the tip of the endoscope (45° on each side from the axis of entry). This field of view allows the surgeon to see beyond the typical field of view of the dural opening, which in the case of the endoscopic MVD is quite small. This enhanced visualization can allow a complete understanding of the course of vessels and nerves from the point of origin to where they terminate or exit through the skull base. Second, the endoscope provides unparalleled illumination of the operative field. Finally, smaller exposures and less brain retraction are possible with the endoscope because of these latter 2 qualities, reducing potential morbidity for the patient.

Several investigators have attempted to rigorously evaluate the visualization benefits of the endoscope over the microscope for the retrosigmoid approach. Tang and colleagues quantitatively assessed the visual field of the microscope versus the endoscope in a cadaver and found that the 30° endoscope offered a significantly larger field of view (nearly 2-fold) compared with the microscope. In another cadaveric study, Tang and colleagues developed a numerical grading score for assessing maneuverability and demonstrated that endoscopic-assisted microsurgical MVD had superior scores compared with the microscope both with and without sacrificing the superior petrosal vein. Moreover, the endoscopy scores did not improve with taking the vein, in contrast to microscopy, suggesting an endoscopic approach may decrease the need to compromise the superior petrosal vein. In a detailed anatomic study, Takemura and colleagues demonstrated that the endoscope had superior visualization of the skull-base meati and root entry zones into the brainstem of the cranial nerves and their relationship to surrounding vessels when compared with the microscope.

There is also ample evidence within the clinical literature regarding the benefits afforded by endoscopic visualization. Jarrahy and colleagues performed MVD on 21 patients initially with the microscope; the endoscope was used to confirm the presence of offending vessels initially identified with the microscope and assess the adequacy of the decompression. Interestingly, 14 of the 51 offending vessels were only identified after the endoscope was introduced, and in 5 patients, further decompression was needed after it was deemed inadequate using the endoscope. In another study of patients undergoing MVD initially with the microscope alone, the endoscope was able to better delineate arterial anatomy in 25% of patients and detect vessels completely missed with microscopy in 8% of patients. Rak and colleagues found that the adjunctive use of the endoscope resulted in visualization and subsequent sacrifice of the trigeminal vein compressing the trigeminal nerve at the entrance of Meckel’s cave in several patients within their series; in these cases, bone obscured the full view of the trigeminal nerve from the microscope. In their large case series of endoscopic assisted MVD (EA-MVD), Chen and colleagues discovered vessel-nerve conflicts in 14.7% of their 167 patients only after using the endoscope. Indeed, the senior surgeon (J.Y.K.L.) has found the endoscope invaluable at detecting nerve-vessel contacts that would not otherwise be directly within the line of site of traditional means of visualization ( Figs. 1 and 2 ).

Endoscope identifies additional point of compression from lateral vein along Meckel cave in a case of MVD for trigeminal neuralgia. ( A ) Typical view of dissection of the superior cerebellar artery (SCA) off of the trigeminal nerve. ( B ) Once the Teflon pad is placed between the SCA and the trigeminal nerve, the scope was advanced to reveal a compressive vein ( arrow ) lateral along Meckel cave. ( C ) View of trigeminal complex after decompression complete. Note that the endoscope afforded a complete view of the trigeminal nerve and allowed identification multiple points of vessel-nerve contact.

Endoscope reveals vascular compression from vertebral artery posterior to the CN IX/X complex. ( A ) View of CN IX/X complex and offending anterior inferior cerebellar artery (AICA) after arachnoid dissection. ( B , C ) Dissection of AICA opens a window through which the endoscope is advanced, revealing vascular compression from vertebral artery ( dotted arrow ) and potentially posterior inferior cerebellar artery (PICA; solid arrow ) of the nerve complex ( asterisk ). ( D ) Further dissection on the contralateral side of the nerve further delineates the vertebral artery ( asterisk ) and develops a plane through which Teflon is advanced ( E ). ( F ) Final view after Teflon is positioned posterior to the nerve complex but anterior to the compressive vertebral artery and PICA.

Despite these advantages, the endoscope has not been widely adopted for lateral skull-base surgery for several reasons. The anatomy of the CPA requires delicate manipulation. Unlike endonasal approaches, where the structures encountered largely consist of mucosa, the sinuses, and bone, lateral skull-base surgery requires that the surgeon navigate neural tissues and blood vessels without causing injury; this is further complicated by the fact that the planes of access are largely potential spaces in contrast to the well-aerated spaces within the nasosinal cavities encountered in endonasal surgery. Although visualization of operative anatomy may be superior with the endoscope, some studies have suggested that maneuverability may be limited when compared with the microscope. Furthermore, because the endoscope only captures images anterior to its tip and is frequently deep within the surgical field, the absence of visualization behind the field of view makes inserting or removing instruments from the operative field dangerous, and extreme care must be taken to avoid injuring vessels or neural tissue. Finally, completely endoscopic surgery lacks the depth perception afforded by the binocular vision of the microscope. However, the adept endoscopic surgeon can use visual cues from gentle manipulation of surrounding structures to create 3-dimensional representation of the anatomy. Furthermore, advances in stereoscopic high-definition optics are improving, and once optimized for use in CPA surgery, will likely become widely adopted and enhance surgeon dexterity. Nevertheless, in skilled hands, CPA surgery can be facilitated using the endoscope either as an adjunct to the microscope or as the sole source of illumination and visualization.

Patient selection and preoperative workup

In general, the indications for E-MVD are similar to those for microscopic MVD. The most common clinically entity requiring MVD in the United States is trigeminal neuralgia, although similar approaches are used for glossopharyngeal neuralgia, geniculate neuralgia, hemi-facial spasm, and vestibular nerve sectioning. Although the diagnosis and initial management of trigeminal neuralgia and associated peripheral neuropathies are discussed elsewhere in this text, they are briefly reviewed in Table 1 .

Once the diagnosis of trigeminal neuralgia is made clinically, the patient should undergo imaging with MRI to rule out structural causes of trigeminal pain, although the low negative predictive value obviates the clinical utility of imaging for operative decision-making. Interestingly, imaging may prove useful for the surgeon in deciding whether to use the endoscope. Sandell and colleagues examined the utility of preoperative MRI in predicting the need for endoscopic assistance during a microscopic MVD to visualize the offending vessel. The investigators found an association with the fraction of microscopically visible nerve on preoperative MRI and the need for the endoscope, quoting an optimal cutoff of less than 35% of visible nerve. In addition to imaging, patients should undergo the typical preoperative laboratory assessment and, as this is an elective procedure, should be medically cleared for surgery. Before undergoing surgery, the patient’s level of pain should be graded in a rigorous manner using a pain scale, such as the Brief Pain Inventory–Facial, to accurately assess outcomes postoperatively.