Step 1: Cannulation of the Foramen Ovale

One hour before the procedure, 0.4 mg of atropine is administered intramuscularly to reduce oral secretions and prevent bradycardia. The patient is positioned supine on the fluoroscopic table with the head neutral. Next, Hartel’s three anatomic landmarks are plotted on the face: a point beneath the medial aspect of the pupil, a point 3 cm anterior to the external auditory meatus, and a point 2.5 cm lateral to the oral commissure (Fig. 124-1A and B).¹⁰ The first two points provide the rostral/caudal and medial/lateral trajectories for the penetration of the foramen ovale, and the third point is where the needle penetrates the skin of the jaw. The affected cheek is prepared with Betadine (Purdue-Frederick, Norwalk, CT). A 21-gauge spinal needle placed in the deltoid subcutaneous tissue acts as a reference and grounding electrode, or alternatively, a bovie pad can be used.

FIGURE 124-1 Hartel’s anatomic landmarks for access to the foramen of ovale. A, Surgeon marks three points on the patient’s face: a point 3 cm anterior to the external auditory meatus along a line from the external auditory meatus to the tip of the nose (1), a point beneath the medial aspect of the pupil on the lower eyelid (2), and a point 2.5 cm lateral to the oral commissure (3). B, The target is the foramen ovale, at the intersection of a vertical line (in the sagittal plane) extending through the point beneath the pupil (1) and a horizontal line (in the axial plane) through the point anterior to the external auditory meatus (2).

(Printed with permission from the Mayfield Clinic.)

Before cannulization of the foramen, understanding the skull base anatomy and its anatomic variants is essential (Fig. 124-2). Aberrant placement of the cannula can result in unintended neurovascular injuries. Use of lateral fluoroscopic imaging will aid in avoiding the following: cannulization of the inferior orbital fissure (IOF) anterosuperiorly or the jugular foramen posteroinferiorly, or intracranial placement of the cannula through aberrant foramina (e.g., foramen of Vesalius, which lies anteromedial to the foramen ovale, or innominate canal of Arnold, which lies posterior to the foramen ovale). Pulsatile blood flow through the cannula indicates penetration of the internal carotid artery (ICA). Puncture of the ICA can occur in three locations: at the proximal C2 segment at the carotid canal, the C3 segment with the electrode passing through the cartilage of the foramen lacerum, and the cavernous (C4) segment (Fig. 124-3). This third type of penetration was described by Rish,¹¹ who noted that an anteromedial electrode passing through the foramen ovale can penetrate the C4 segment of the ICA. If ICA penetration occurs, the cannula is withdrawn promptly, manual pressure is applied over the posterior pharyngeal space, the procedure is discontinued, and the patient is allowed 24 to 48 hours to convalesce. Ischemic complications, such as hemiparesis, and carotid-cavernous fistula have resulted from puncture of the ICA.¹¹ Generally, arterial punctures are minimized by refraining from broad angular readjustments of the electrode and diligent attempts to avoid trajectories that are either posterolateral or posteromedial to the foramen of ovale; such trajectories jeopardize the carotid canal and foramen lacerum, respectively.

FIGURE 124-2 Skull base foraminae can be punctured with a cannula when targeting the foramen ovale. Inferior orbital fissure (IOF) can be hit by a cannula aimed too anterior and superior to the foramen ovale (FO). Jugular foramen (JF) can be hit by a cannula aimed too posterior and inferior. Internal carotid artery can be hit as it enters the carotid canal (CC), or at the foramen lacerum (FL).

(Printed with permission from the Mayfield Clinic.)

FIGURE 124-3 The ICA can be punctured with a cannula at three points when targeting the foramen ovale. A trajectory angle either too posterior (1) can puncture the C2 portion of the carotid as it enters the petrous bone, a trajectory angle too medial and posterior (2) can puncture the C3 portion of the carotid where it lies within the foramen lacerum covered by a thin shell of bone or after penetration of the foramen ovale, a trajectory too medial and anterior (3) can puncture the C4 segment in the cavernous sinus.

(Printed with permission from the Mayfield Clinic.)

The primary goal of the cannulization step is penetration of the medial portion of the foramen ovale and placement of the electrode’s tip in the retrogasserian rootlets (Fig. 124-4). Using Hartel’s landmarks, we advocate either the direct penetration technique or the sequential palpation in which the surgeon sequentially walks the cannula down the smooth surface of the infratemporal fossa toward the superior-medial aspect of the foramen (Fig. 124-5). If the cannula enters the posterolateral aspect of the foramen, it may elude the trigeminal cistern and not contact the trigeminal ganglion within its dural investment. Additionally, as the electrode is advanced, it may not reach the maxillary or ophthalmic divisions of the rootlet.

FIGURE 124-4 Optimal cannula placement through the foramen ovale into the trigeminal cistern located retrogasserian. The resting point of the tip should be approximated by the clival line.

(Printed with permission from the Mayfield Clinic.)

FIGURE 124-5 Cannula trajectory toward the superior-medial aspect of the foramen ovale. A, The surgeon’s index finger rests alongside the molars, against the lateral pterygoid, to guide the cannula toward the foramen ovale and prevent penetration of the oral mucosa. The cannula is aimed at the foramen ovale via Hartel’s landmarks. B, If direct penetration of the foramen is not achieved, the surgeon can sequentially walk down the infratemporal fossa (arrow) to the superomedial aspect of the foramen ovale.

(Printed with permission from the Mayfield Clinic.)

To begin, a 9-mm airway turned sideways or a bite block is inserted into the patient’s mouth between the molars on the opposite side to prevent the patient from involuntarily biting the surgeon’s finger. The patient is anesthetized with a rapid intravenous injection of 30 to 50 mg of methohexital (Brevital) followed by a 10-ml saline flush. Once the patient is fully asleep, the surgeon inserts the indexed finger of gloved hand inside the patient’s mouth, sliding along the inferior and medial wall of the lateral pterygoid to a point where it hooks around the lateral pterygoid, to guide the cannula toward the foramen ovale. A standard 100-mm, 20-gauge cannula, with its stylet in place, is inserted into the cheek 2.5 cm lateral to the oral commissure. The cannula is aimed toward the intersection of an axial plane, passing 3 cm anterior to the external auditory meatus, and a sagittal plane, passing through the medial aspect of the pupil. Although placement is by free-hand manipulation, fluoroscopic visualization is important to assist localization. Use of the image intensifier in the lateral plane effectively localizes the needle.⁴ Entrance of the cannula into the foramen ovale is signaled by a wince and a brief contraction of the masseter muscle, indicating contact with the mandibular sensory and motor fibers. Before the cannula is advanced any further, a lateral fluoroscopic image is obtained to confirm proper placement in the foramen ovale (Fig. 124-6). Fluoroscopy allows targeting a point along the lateral projection of the clivus, which is 5 to 10 mm below the floor of the sella. If difficulty penetrating the foramen ovale ensues, one should pause and return to a fundamental principle—namely, the safest approach to the foramen ovale is a trajectory that begins anteromedially to the foramen. In this manner, the surgeon can sequentially palpate, using the cannula along the smooth surface of the infratemporal fossa, and enter the superior medial aspect of the foramen. Use of this technique ensures a cannula trajectory in which the electrode will enter the trigeminal cistern at an angle and then sequentially contact each of the three divisions of the trigeminal root. Furthermore, targeting the anteromedial portion of the foramen ovale reduces the risks associated with probing alternative portions of the skull base, lowers the incidence of hematoma due to venous or arterial hemorrhage, and increases the probability of entering the trigeminal cistern.

FIGURE 124-6 Cannula trajectory. A, Lateral fluoroscopy. B, Shows ideal trajectory 5 to 10 mm below the floor of the sella turcica at the clival line.

(Printed with permission from the Mayfield Clinic.)

Proper positioning of the cannula within the trigeminal cistern allows free flow of cerebrospinal fluid through it in most patients, except for the few who have undergone previous surgical procedures or chemical injection. However, egress of cerebrospinal fluid does not ensure that the cannula lies in the proper position (retrogasserian). Cerebrospinal fluid (CSF) can also be obtained either from the infratemporal subarachnoid space if the needle is too deep or from the region distal to the gasserian ganglion if the dural subarachnoid sleeve extends beyond the rootlets. Fluoroscopy must ascertain that the trajectory does not either project anterior to the sella where the cannula may penetrate the IOF or lie too low on the clivus (greater than 15 mm) where entry into the jugular foramen may occur.

If satisfied with the position of the cannula and the flow of CSF, a small clamp at skin level holds the cannula in place, preventing its further advancement into the trigeminal ganglion. The surgeon removes the bite block and introduces the electrode into the cannula. The cable, connecting the electrode to the stimulator, should be connected to the electrode before its insertion. If connected after it is in the cannula, the electrode can vibrate within the trigeminal nerve root and elicit a trigeminal attack.

Step 2: Stimulation of the Trigeminal Nerve

Subtle adjustments in the position of the electrode tip facilitate a precise stimulation of the division of the trigeminal root. When making an adjustment or generating stimuli, the surgeon aims to achieve two important objectives. The first objective is to precisely reproduce the pattern of the patient’s trigeminal pain. An appropriate stimulus will effectively replicate the pain and lessen the likelihood of lesions in adjacent divisions. Precise stimulation is principally related to electrode tip placement (as described below). The second objective is to identify the intensity threshold required to reproduce the patient’s typical pain, so that the neurosurgeon can judge the duration and temperature needed to generate an effective partial sensory lesion as described later.

Targeted stimulation begins with the manipulation of the electrode’s tip in two dimensions as viewed on lateral fluoroscopy: the relationship of the electrode tip to the profile of the clivus and the curvature of the electrode’s tip. When the tip rests at the clival level, a stimulating pulse typically elicits paresthesias in the maxillary division rootlets. Advancement beyond the clival profile moves the electrode into the territory of the ophthalmic division rootlets, while withdrawal of the needle from the level of the clivus targets the mandibular division rootlets (Fig. 124-7A). The electrode’s tip should not be advanced more than 10 mm deep to the profile of the clivus because the tip can contact the trochlear or abducens nerve in this region. Sometimes the needle must be redirected more anteromedially to bring the tip closer to the posterior clinoid process for closer contact with the ophthalmic division. If the globe moves during stimulation, the cannula is too near the cranial nerves (CNs) in the cavernous sinus or perhaps near the brain stem. Stimulus-evoked facial contractions indicate that the electrode is either too deep, inclined too low on the clivus, or the stimulation level is too high. A lesion should not be made if there is any indication of motor nerve III, IV, VI, or VII stimulation or arterial bleeding.

FIGURE 124-7 A, Composite illustration shows the relationship of the trigeminal rootlets to the clival line and electrode position. A curved electrode can be manipulated to target different regions of the trigeminal ganglion. Depth is varied to target V1 (5 mm beyond the clivus), V2 (at the level of the clivus), or V3 (5 mm before the clivus). B, AP and lateral view of the dermatome distribution of the three divisions of the trigeminal nerve: ophthalmic division (V1, green), maxillary division (V2, blue), and mandibular division (V3, orange). Sensory testing by pinprick should begin in autonomous areas (slightly darker-colored areas) where less innervation overlap occurs between divisions.

(Printed with permission from the Mayfield Clinic.)

After this initial placement and understanding of clival relationships, the axial rotation of the curved electrode permits a secondary form of targeting. Precise anatomic localization within the sensory root is aided by this maneuverability. The curved electrode tip is a coil spring that carries a thermocouple, stimulator, and lesion-generating probe. When the electrode is fully inserted into the cannula, the curved tip extends 5 mm beyond the end of the cannula and projects 3 mm perpendicular to the long axis of the electrode. Insulation of the cannula with polytetrafluoroethylene allows only the extruded portion of the electrode (0 to 5 mm) to be conductive. Rotation of the electrode can occur through a 360-degree axis for stimulation and lesion production (Fig. 124-7A