Fig. 9.1
A panoramic representation of anatomical relationships seen through a left retrosigmoid approach. The medulla is associated with vertebral and posterior-inferior cerebellar arteries, while the anterior-inferior cerebellar artery courses over the pons and in relation to the vestibulocochlear (VIII) and facial nerves (VII). The hypoglossal nerve (XII) arises from the anteriolateral medulla and courses over the posterior-inferior cerebellar artery origin. The glossopharyngeal (IX), vagal (X), and accessory (XI) nerves pass over the jugular tubercle and enter the jugular foramen. The flocculus (Fl) and choroid plexus (not shown) overlie the lateral portion of the pontomedullary sulcus. Circles: Surgical views gained by variations of approach trajectory and retraction
9.4.1 Cranial Nerves
The glossopharyngeal nerve is predominately a sensory nerve, but also carries secretomotor fibers. The functional components include (1) general somatic afferents from small areas of the postauricular skin and posterior fossa meninges terminating in the spinal tract and spinal nucleus of the trigeminal nerve; (2) general visceral afferents of touch, pain, and temperature sensation from the mucous membranes of the posterior third of the tongue, tonsil, posterior wall of the upper pharynx, and eustachian tube; (3) special visceral afferents from the carotid sinus and body as well as taste from the posterior third of the tongue (the visceral afferents terminate in the solitary tract nucleus); (4) general visceral efferents arising from the inferior salivary nucleus provide parasympathetic input to the otic ganglion and parotid gland; and (5) special visceral efferents arising from the nucleus ambiguus innervate the stylopharyngeus muscle (Carpenter 1985).
Rootlets of the glossopharyngeal nerve arising and terminating at the intramedullary nuclei emerge from the brainstem at the upper third of the postolivary sulcus of the medulla and converge into one or two bundles. These arise immediately inferior to the root exit point of the facial nerve that emerges from the pontomedullary sulcus. The proximal glossopharyngeal nerve is closely associated with the flocculus and choroid plexus that protrudes from the foramen of Luschka. It then traverses the subarachnoid space, crosses over the jugular tubercle, and enters at the jugular foramen. The glossopharyngeal nerve may traverse the subarachnoid space adjacent to the vagus nerve fibers or be separated by several millimeters. At the jugular foramen, the glossopharyngeal nerve enters a dural meatus, separated from the more inferiorly situated vagal meatus by a dural septum that measures between 0.5 and 4.9 mm in width (Katsuta et al. 1997). Other components of the jugular foramen are not visible from the retrosigmoid approach, but include the inferior petrosal sinus situated superomedially and the sigmoid sinus situated inferior laterally from the neural elements. Injury to the glossopharyngeal nerve alone causes minimal deficits of reduced oropharyngeal sensation and diminished gag reflex.
The vagus nerve is closely related to the glossopharyngeal nerve sharing common nuclei and also containing a variety of both afferent and efferent fiber types. The functional components include (1) general somatic afferents from the cutaneous areas, back of the ear, and external auditory meatus terminating in the spinal tract and spinal nucleus of the trigeminal nerve; (2) general visceral afferents from the pharynx, larynx, trachea, esophagus, and thoracoabdominal viscera terminating in the solitary tract nucleus; (3) special visceral afferents from taste buds in the region of the epiglottis terminating in the solitary tract nucleus; (4) general visceral efferents from the dorsal vagal nucleus distributed to parasympathetic ganglia of the thoracic and abdominal viscera; and (5) special visceral efferents arising from the nucleus ambiguus that innervate the striated muscles of the pharynx and larynx.
The 8–10 vagal rootlets emerge from the postolivary sulcus, in a straight vertical line that is in continuity with the glossopharyngeal rootlets above and accessory nerve rootlets below. These enter the vagal meatus of the jugular foramen together with the accessory nerve. Dysfunction of the vagus nerve unilaterally will lead to uvular deviation to the normal side, hoarseness, dysphagia, and dyspnea related to a paralysis of the ipsilateral soft palate, pharynx, and larynx. An ipsilateral loss of cough reflex is related to anesthesia of the pharynx and larynx, while loss of visceral motor fibers of the vagus nerve will decrease the carotid sinus reflex.
The accessory nerve has two distinct parts. The cranial portion is a special visceral efferent arising from the nucleus ambiguus and innervates intrinsic muscles of the pharynx and larynx. These fibers emerge from the postolivary sulcus of the medulla, join the vagus nerve rootlets entering the jugular foramen, and continue as an “accessory” to the vagus nerve. The spinal portion of the accessory nerve is a general somatic efferent that arises from a column of anterior horn cells of the cervical spinal cord and innervate the sternocleidomastoid and upper portion of the trapezius muscles. The fibers emerge from the lateral aspect of the upper cervical cord segments and come together as a common trunk posterior to the dentate ligament. The nerve enters the posterior fossa through the foramen magnum and continues superiorly along the dural surface to the jugular foramen. Unilateral dysfunction of the accessory nerve produces weakness in turning the head against resistance and sagging of the shoulder with downward and outward rotation of the scapula.
The hypoglossal nerve is a general somatic efferent that provides motor innervation to the tongue. Fibers arise from the hypoglossal nucleus beneath the floor of the fourth ventricle and emerge as a series of rootlets from a groove between the lower two-thirds of the olive and medullary pyramid, the ventrolateral medullary sulcus. These fibers arise in the same vertical line as the ventral spinal roots. The rootlets course anterolaterally through the subarachnoid space and enter the hypoglossal canal. Dysfunction of the hypoglossal nerve produces a lower motor neuron paralysis of the ipsilateral tongue.
There is a close anatomical relationship between the lower cranial nerves arising from the medulla and the vestibulocochlear and facial nerves that arise from the pontomedullary sulcus. The vestibulocochlear nerve is most laterally situated and tethered to the anterior cerebellum and flocculus with fine arachnoid. Retraction of the cerebellum may therefore exert mechanical stress on the vestibulocochlear nerve that is particularly sensitive to even minor degrees of manipulation and stretching (Sekiya et al. 1986). Its axons are invested with the central oligodendrocyte-derived myelin throughout the cisternal course. Furthermore, the cochlear nerve separates into multiple tiny filaments that traverse the lamina cribrosa at the fundus of the internal auditory canal, where they may be torn with minor degrees of traction (Sekiya and Moller 1987; Sekiya and Moller 1988).
The facial nerve root exit point also emerges from the pontomedullary sulcus, immediately medial to the vestibulocochlear nerve and directly superior to the glossopharyngeal nerve. It then courses superiorly, adherent to the bulging pontine surface before detaching from the pons and continuing across the prepontine cistern toward the internal auditory meatus. Its Obersteiner-Redlich zone is situated 1–2 mm distal to the root detachment point, such that the entire attached segment has myelin derived from oligodendrocytes. This long root exit zone of the facial nerve, from root exit point to the transition zone, is an important consideration during microvascular decompression for hemifacial spasm (Campos-Benitez and Kaufmann 2008; Kaufmann and Wilkinson 2005) but also highlights the relative sensitivity of the nerve to mechanical injury during surgery for glossopharyngeal neuralgia.
9.4.2 Arteries
The arteries of the posterior fossa have usual anatomical relationships with the cranial nerves, although there exist many variations and asymmetries. The length of these vessels, also, may vary considerably and predispose to vascular loops impinging on the cranial nerve roots as culprit neurovascular compression (Rhoton 2003). A thorough examination of the preoperative diagnostic imaging should allow the neurosurgeon to anticipate the course of these vessels (Fig. 9.2).
Fig. 9.2
Magnetic resonance imaging of a patient with left-sided glossopharyngeal neuralgia. Left panel: An axial CISS imaging sequence through the medulla. The glossopharyngeal nerve (vertical arrow) courses from the postolivary sulcus, across the jugular tubercle, and enters the jugular foramen. Culprit neurovascular compression is seen at the root entry zone (horizontal arrow). Right panel: A magnetic resonance angiography reconstruction with the elongated culprit posterior-inferior cerebellar artery loop (asterisk)
The distal cervical portion of the vertebral artery penetrates the dura lateral to the cervicomedullary junction and continues superiorly and anteromedially before merging with its contralateral mate at the level of the pontomedullary sulcus, becoming the basilar artery. The vertebral artery typically courses medial to the hypoglossal nerve and vertebrobasilar junction is between the abducens nerves. Branches of the vertebral artery include the anterior and posterior spinal arteries, meningeal arteries, medullary perforating arteries, and the posterior-inferior cerebellar artery origin.
The posterior-inferior cerebellar artery arises from the posterior lateral surface of the vertebral artery and usually courses laterally around the inferior end of the olive. This anterior medullary segment is inferior to most of the hypoglossal nerve rootlets but sometimes may go above or between them. The lateral segment then courses posteriorly between rootlets of the cranial nerves emerging from the postolivary sulcus or rarely rostral to the glossopharyngeal nerve. The vessel then continues posteriorly and inferiorly in the tonsillomedullary fissure, forms a caudal loop, and then ascends along the medial surface of the tonsil. It then forms a cranial loop before dividing over the inferior cerebellar surface.
The course of the posterior-inferior cerebellar artery varies considerably, as the length of each segment is commonly greater than required. Similarly, the vertebral arteries are often also asymmetrical and elongated. A common finding is a lateral and superior displacement of the vertebral artery with the posterior-inferior cerebellar artery origin situated well superior to the olive. In this configuration, fibers of the hypoglossal nerves may be stretched superiorly and cross the vertebral artery at the posterior-inferior cerebellar artery origin before looping inferiorly again toward the hypoglossal foramen. Elongated loops of the posterior-inferior cerebellar artery are the typical culprit in glossopharyngeal neuralgia, usually impinging on the medial aspect of the root entry zone but occasionally on the lateral side.
The anterior-inferior cerebellar artery is a rare cause of culprit neurovascular compression in glossopharyngeal neuralgia. The vessel arises from the basilar artery, usually between its inferior and middle thirds. The vessel loops around the inferior pons and through the cerebellopontine angle in close association with the facial and vestibulocochlear nerves. A loop toward the internal auditory meatus gives rise to the internal auditory artery. The distal vessel bifurcates with a superior and inferior division supplying the anterior cerebellum. An anterior-inferior cerebellar artery loop may rarely come into contact with the glossopharyngeal nerve root and its root entry zone, particularly when associated with an aberrant or hypoplastic posterior-inferior cerebellar artery.
The brainstem perforating arteries require special attention during microvascular decompression surgery. Both direct and circumflex vessels arise from the vertebral artery, anterior spinal artery, first three segments of the posterior-inferior cerebellar artery, as well as anterior-inferior cerebellar artery. In the setting of an elongated parent vessel, the perforating arteries will also be elongated between their origin and perforating point on the brainstem surface.
9.5 Surgical Technique
9.5.1 Opening
The patient is positioned lateral decubitus with the knees slightly bent and positioned such that the lumbar region is perpendicular to the floor. Rigid cranial fixation is employed with the head drawn posteriorly, while the chin is flexed, preserving a space of two fingerbreadths between the chin and thyroid cartilage. Additional adjustments include a 10–15° drop of the vertex and contralateral rotation to bring the retrosigmoid bone toward a horizontal plane.
The incision is planned to provide maximal access to the most anterior-inferior aspect of the retrosigmoid space, at the junction of the posterior fossa floor and sigmoid sinus. Landmarks include the mastoid tip that extends a couple of millimeters caudal to the posterior fossa floor, and the posterior edge of the mastoid process parallels the underlying sigmoid sinus. A straight line drawn between the lateral canthus, root of zygoma, and inion approximates the inferior edge of the distal transverse sinus behind the ear. A linear incision is marked 1 cm behind the mastoid process, superiorly to just above the transverse sinus line and inferiorly across from the mastoid tip (Fig. 9.3).
Fig. 9.3
Left retromastoid craniectomy. Left panel: The course of the transverse and sigmoid sinuses (blue) is determined form surface landmarks (see text). A linear skin incision (orange) provides the required exposure for a bony opening that extends to the sigmoid sinus laterally and posterior fossa floor inferiorly. Right panel: Soft tissue is held with a self-retaining retractor and bony opening completed. The planned dural flap (dotted line) provides maximal anterior and inferior access
The incision is carried down through successive layers and a single self-retaining retractor placed to maintain the exposure. The occipital artery coursing between the splenius capitis and underlying longus capitis muscles may cross the field. Superiorly, the suboccipital muscle origin at the nuchal line is elevated in the subperiosteal plane. Inferiorly, the exposure should extend to the inferior edge of the occipital squamous bone. The posterior aspect of the mastoid process and digastric groove is cleared. An emissary vein usually coursing anterolaterally through the mastoid foramen toward the mid-sigmoid sinus is controlled with wax.
A low retrosigmoid craniectomy measuring up to 3 × 2 cm is fashioned with a high-speed pneumatic burr and 3–5-mm angulated rongeurs, exposing the posterior edge of the sigmoid sinus along its length. The occipital squamous bone is removed inferiorly until the floor is seen end-on (Fig. 9.4). Maximizing the anterolateral and inferior extent of this exposure provides direct access to the deep arachnoid cisterns and a clear surgical trajectory to the lower cranial nerve roots and medulla (Ferroli et al. 2009; Gaul et al. 2011; Olds et al. 1995; Patel et al. 2002; Resnick et al. 1995; Sindou and Mertens 1993). Some surgeons have advocated for additional anterior-inferior exposure through a transcondylar approach (Hitotsumatsu et al. 2003; Kawashima et al. 2010; Matsushima et al. 2000; Sampson et al. 2004), although we have used this only in cases with more complex pathologies such as aneurysms and cranial base tumors.
Fig. 9.4
Three-dimensional computerized tomographic reconstructions demonstrating a right retromastoid craniectomy, with cranioplasty closure (asterisk). The bony opening extends to the sigmoid sinus laterally and posterior fossa floor inferiorly
An L-shaped dural opening is reflected anteriorly overtop a saline-soaked Gelfoam. The inferior dural edge is retracted with a stitch or can be incised inferiorly if required to visualize the posterior fossa floor. Cerebrospinal fluid may spontaneously drain from the subdural space, or gentle compression of the cerebellum may facilitate its release, resulting in relaxation of the cerebellum. If the cerebellum remains full, the microscope is brought into use, and the cerebellum is elevated from the posterior fossa floor to access and open the deep arachnoid cisterns (Fig. 9.5). Access to cerebrospinal fluid can also be achieved by intraoperative lumbar puncture performed by the anesthesiologist using a 25-gauge spinal needle, while the surgeon remains attentive to the operative field. This is rarely required but helpful particularly for repeat microvascular decompression surgeries if early access to CSF is impaired by scarring of dura to the cerebellum.
Fig. 9.5
Approach through a left retromastoid craniectomy. Top left: Mastoid bone edges have been sealed with wax and the sigmoid sinus edge is visible laterally. Top right: The dura has been opened in a curved fashion with an anterior flap reflected over moistened Gelfoam; a retraction stitch placed inferiorly. Bottom left: The cerebellum is gently depressed with a folded “slider,” fashioned with a sheet of latex beneath a 1/2 × 3-inch cottonoid. Bottom right: The slider has been unfolded and advanced beneath the cerebellum parallel to the posterior fossa floor. This corridor will provide access to the deep arachnoid cistern
9.5.2 Approach
The anterior-inferior aspect of the cerebellum is elevated a couple of millimeters, and retraction is applied by suction and bayonet overtop a 1/2 × 3-inch cottonoid and latex sheet “slider” (Fig. 9.6). Bridging veins are identified as retraction is advanced. These can be released for the cerebellum with arachnoid dissection and preserved or divided after coagulation. The cervical portion of the accessory nerve courses on the dura from the foramen magnum toward the jugular foramen and should be identified to mechanical or thermal injury, particularly if coagulating a bridging vein.
Fig. 9.6
Microsurgical materials. Left panel: A “slider” fashioned from a sheet of latex beneath a 1/2 × 3-inch cottonoid protects the brain. Various sizes of shredded Teflon felt implants are prepared (see text). Left panel: Microsurgical instruments include a tapered 3 mm self-retaining retractor blade, 5F smooth tipped suction, microdissectors and fine forceps, as well as microscissors and bipolar (not shown)
The arachnoid exposed inferior to the jugular foramen provides the most direct access to the deep cistern, and when opened the cerebrospinal fluid release will further “deflate” the cerebellum. Further sharp dissection opens the arachnoid between the lower cranial nerves and cerebellum and is continued superiorly across the cerebellopontine angle. Separation of the cerebellum from the vestibulocochlear nerve can be achieved with careful dissection of fine arachnoid tethering these structures. This will facilitate brainstem exposure with minimal cerebellar retraction. Finally, exposure of the facial, glossopharyngeal, and upper vagal root exit/entry zones is achieved with elevation of the flocculus and closely associated choroid plexus (Fig. 9.7).
Fig. 9.7
Paired operative photographs and illustrations of microvascular decompression for left-sided glossopharyngeal neuralgia. Top left: Arachnoid has been opened over the lower cranial nerves entering the jugular foramen. A bridging vein is seen to cross over the cervical portion of accessory nerve, and the choroid plexus overlies the superior vagal and glossopharyngeal nerve roots. Top middle: The posterior-inferior cerebellar artery lateral segment courses tightly against the medulla, between the lower vagal rootlets and associated with a bridging vein. Top right: Superiorly in the surgical field, the arachnoid covers the cerebellum and flocculus. This will be widely opened and the flocculus further separated from the vestibulocochlear and glossopharyngeal nerves. Lower left: The flocculus is retracted superiorly to expose the glossopharyngeal and vagal rootlets emerging from the medulla. Immediately superior to this, the attached segment of the facial nerve emerges from the pontomedullary sulcus. A loop of the proximal posterior-inferior cerebellar artery is seen to impinge and distort the vagal and glossopharyngeal nerve root entry zone. Bottom middle: Mobilization is initiated away from the point of maximum compression, in this case elevating the lateral segment of the distal posterior-inferior cerebellum artery anteriorly off the medulla and maintaining this new position with a shredded Teflon felt implant placed between the nerve rootlets. This results in a shift of the more proximal culprit vascular loop distally along the lower cranial nerve rootlets, alleviating compression at the root entry zone. Bottom right: The culprit vascular loop is further mobilized distally toward the jugular tubercle and additional shredded Teflon felt implants placed beneath this vessel to maintain its new position
Throughout surgery, the degree of retraction should always be minimized as much as possible in order to avoid cerebellar compression and cranial nerve traction injuries. Each repositioning of retraction is gauged by observation of any resultant distortion or traction on neurovascular elements as well as intraoperative monitoring feedback, as further discussed below. Focused retraction is directed to provide clear exposure of one anatomical region at a time, rather than an all encompassing single panoramic view. On rare occasions we have utilized an angulated mirror to view behind the cranial nerves, as can be achieved also with an angulated endoscope (Ferroli et al. 2009). More commonly, all neural and vascular elements can be visualized by varying the microscope trajectory and adjusting retraction (Fig. 9.1).
9.5.3 Decompression
The most common culprit vessel in glossopharyngeal neuralgia is the posterior-inferior cerebellar artery, with the apex of a loop impinging against the glossopharyngeal and vagal nerve root entry zone. Displacement of the proximal nerve rootlets or indention of the medulla is often apparent, although even gentle pulsating compression is a sufficient cause. Conversely, vessels in simple contact with the cisternal portion of the nerve roots are incidental. The vertebral artery, anterior-inferior cerebellar artery, or a large vein may also contribute to neurovascular compression or rarely may be the sole culprit.
The technical objective of microvascular decompression surgery is to reorient the axis of the offending artery loop away from the target root entry zone, rather than simply cushioning the neurovascular conflict. Beginning proximally and distally, away from the point of maximum compression, the vessel is lifted off the brainstem. Working toward the apex of the loop in a stepwise fashion, the entire vessel is shifted distally along the nerves, alleviating compression on the root entry zone. Care is taken to avoid tension on perforating vessels, and all arachnoid tethers are sharply divided. Implanted material is used to maintain the vessel in its new orientation, positioned between the vessels and brainstem, without pressure on the root entry zone or proximal rootlets. The tail of larger implants may also extend over the cerebellum to be further leveraged when retraction is eased (Fig. 9.8). When the offending artery forms a long loop, it is often possible to entirely transpose the vessel to a new position relative to the lower cranial nerves (Fig. 9.9). There is potential to kink particularly long vessel loops during transposition, and this can be recognized by visual inspection and checked with a Doppler flow probe. Rarely culprit veins may be encountered and considered for coagulation, although mechanical or thermal injury to brainstem rootlets and perforating vessels must be avoided.
Fig. 9.8
A series of illustrations from four microvascular decompression surgeries for glossopharyngeal neuralgia showing culprit vessel position before and after mobilization away from the root entry zone. Top left: An elongated loop of the culprit posterior-inferior cerebellar artery compresses and distorts the glossopharyngeal and vagal root entry zone. This vessel is mobilized anteriorly from the brainstem and distally along the nerve roots, with shredded Teflon felt implants (speckled) maintaining the new vessel position. Complete pain relief was achieved. Top right: A prominent anterior-inferior cerebellar artery courses in close proximity to the glossopharyngeal root entry zone, as seen after elevation of the flocculus and choroid plexus. This vessel is mobilized further superiorly and shredded Teflon felt implants positioned. A small vein tracking around the vagal root entry zone is coagulated and divided, while the hypoplastic posterior-inferior cerebellar artery appears coincidental. This patient had nonclassical symptoms of glossopharyngeal neuralgia that did not improve after surgery. Bottom left: The posterior-inferior cerebellar artery courses between vagal nerve rootlets, causing a distortion at the root entry zone. Mobilization is limited by a small direct perforating branch. This patient did not improve after surgery and underwent subsequent sectioning of the glossopharyngeal nerve root and upper vagal rootlets at a second operation with complete pain relief but moderate laryngeal and pharyngeal weakness as a consequent to the ablative procedure. Bottom right: This patient had undergone a prior surgery elsewhere with sponge implants placed between the elongated vertebral artery and underlying medulla (clear ovals). At reoperation, the posterior-inferior cerebellar artery was seen to deeply impinge into the medulla, immediately lateral to the vagal root entry zone. This vessel loop was mobilized away from the brainstem and placed over the anterior cerebellum, with additional shredded Teflon felt implants placed to elevate the vertebral artery more distally along the nerve root and maintain separation of vessels from the vagal and glossopharyngeal root entry zone. Complete pain relief was achieved
