Technique Of Percutaneous Rfl For Trigeminal Neuralgia

Patients can undergo radiofrequency rhizotomy in the radiology department, except for patients with pulmonary or cardiac issues that require precise control of anesthesia parameters. In most cases, neither is general anesthesia required nor must an anesthesiologist be present. The equipment list typically includes X-ray fluoroscopey, sterile drapes, sponges and skin prep, ruler and marking pen to delineate landmarks, sterile spinal tray with syringe for metho-hexital anesthetic (Brevital 500 mg 10:1 dilution), intravenous tubing with side injection port to alternate between Brevital and 5% dextrose, atropine 0.4 mg, pins and cotton for sensory testing, and Nipride. Additionally, the Radionics TIC and Tew kits are used. Various electrodes (e.g., straight 7-mm temperature monitoring electrode, curved temperature monitoring Tew electrode, angled 3-mm cordotomy electrode) (16) and generator systems are available; our experience in 1500 cases was with Radionics equipment. The Radionics RFG-3C lesion generator system provides the necessary power output of 15 W or higher and a radiofrequency exceeding 250 kH/s. The electrode tip should include features to monitor voltage, current, radiofrequency, and temperature. A built-in timer and stimulator are essential. Another useful feature of the Radionics RFG-3C model includes the capability for continuous impedance monitoring during

Fig. 1. Components of a Type TIC Kit. (A) Top to bottom: TIC-TM temperature monitoring electrode; TIC-SS solid stylet; TIC-CIO 10-mm tip cannula; TIC-7 7-mm-tip cannula; TIC-C5 5-mm-tip cannula; TIC-C2 2-mm-tip cannula; TIC-IE 19-gage TW indifferent electrode; and TIC-FA flushing adaptor. (B) Kit includes a stainless steel storage case and a sterilizing metal tube containing the TM electrode. (Courtesy of the May-field Clinic.)

Fig. 1. Components of a Type TIC Kit. (A) Top to bottom: TIC-TM temperature monitoring electrode; TIC-SS solid stylet; TIC-CIO 10-mm tip cannula; TIC-7 7-mm-tip cannula; TIC-C5 5-mm-tip cannula; TIC-C2 2-mm-tip cannula; TIC-IE 19-gage TW indifferent electrode; and TIC-FA flushing adaptor. (B) Kit includes a stainless steel storage case and a sterilizing metal tube containing the TM electrode. (Courtesy of the May-field Clinic.)

electrocoagulation. The TIC kit includes a 19-gage Teflon-insulated cannula with 2-, 5-, 7-, and 10-mm tips to create a variety of lesion sizes (Fig. 1).

Patient Preparation

After admission as an outpatient the morning of the procedure, the patient undergoes preoperative laboratory tests, electrocardiogram, and chest X-ray; no food or drink is permitted for 5 h before the procedure. With the patient positioned supine, the surgeon is on the right side regardless of pain location. The fluoroscopy is placed for convenient viewing by the surgeon. Lateral fluoro-scopic images should show overlapping orbits and clinoids and should include the sella turcica, clivus, and petrous apex. The radiofrequency generator is placed for easy access by the surgeon. An adhesive grounding pad or stainless steel plate completes the electric circuit; without a grounding pad, the electrode can monitor temperature but cannot deliver a stimulus or generate a lesion.

Standing opposite the surgeon on the patient's left, an assistant can manage the intravenous, anesthesia, vital, and lesion parameters. The skin is prepared with Betadine and the field is draped with sterile towels. Anatomic landmarks are mapped as described by Hartel (17). Three points are marked on the skin for guidance. The first reference point is 3 cm anterior to the external auditory mea-tus. The second point is beneath the medial aspect of the pupil on the lower lid

Fig. 2. Anatomic landmarks described by Hartel shown in two views. (A) The target foramen is at the intersection of three planes. (B) Surgeon's finger guides placement to the lateral pterygoid wing. 1, beneath the medial aspect of the pupil on the lower end; 2, 3 cm anterior to the external auditory canal; and 3, 2.5 cm lateral to the oral commissure. (Courtesy of the Mayfield Clinic.)

Fig. 2. Anatomic landmarks described by Hartel shown in two views. (A) The target foramen is at the intersection of three planes. (B) Surgeon's finger guides placement to the lateral pterygoid wing. 1, beneath the medial aspect of the pupil on the lower end; 2, 3 cm anterior to the external auditory canal; and 3, 2.5 cm lateral to the oral commissure. (Courtesy of the Mayfield Clinic.)

when the eye is in neutral position. The third point is 2.5 cm lateral to the oral commissure (Fig. 2A).

Insertion of the Electrode

The needle is placed in the retrogasserian ganglion portion of the trigeminal nerve by free-hand manipulation based on the Hartel reference points. After the patient is anesthetized with an intravenous injection of 30-50 mg of methohex-ital (Brevital), a 100-mm-long, 20-gage cannula with a stylet penetrates the skin 2.5 cm lateral to the oral commissure. An oral airway may be placed to prevent closure of the jaw during the procedure. Atropine (0.4 mg) may be given to reduce oral secretions. The surgeon's index finger, placed in the patient's mouth inferior to the lateral pterygoid wing, guides insertion without penetrating mucosa. The needle is aimed toward the intersection of the coronal plane 3 cm anterior to the auditory meatus and the sagittal plane medial to midpupillary line (Fig. 2B). The cannula is advanced based on cine or serial true lateral fluoroscopic images; it then rests 5-10 mm below the sella floor (Fig. 3). Penetration of the foramen ovale by the cannula is signaled by a wince and contraction of the ipsilateral masseter muscle because of contact irritation to the sensory and motor fibers.

Successful access to the retrogasserian ganglion usually produces a free flow of cerebrospinal fluid when the stylet is removed. Spinal fluid may also be obtained from the distal nerve if the dural sleeve extends extracranially along the nerve, or from the infratemporal subarachnoid if the electrode is distal or proximal to the target. Malposition of the cannula poses a risk of carotid artery

Fig. 3. Electrode trajectory. (A) Lateral radiograph. (B) Illustration shows ideal trajectory (5-10 mm below the floor of the sella turcica). (Courtesy of the Mayfield Clinic.)

Fig. 4. Relationship of the electrode tip and surgical anatomy. Lateral radiograph (A) and composite illustration (B) shows the (1) relationship of the electrode tip and its trajectory to the mandibular nerve; (2) carotid artery in Meckel's cave; and (3) cavernous carotid artery. (Courtesy of the Mayfield Clinic.)

Fig. 4. Relationship of the electrode tip and surgical anatomy. Lateral radiograph (A) and composite illustration (B) shows the (1) relationship of the electrode tip and its trajectory to the mandibular nerve; (2) carotid artery in Meckel's cave; and (3) cavernous carotid artery. (Courtesy of the Mayfield Clinic.)

injury. The herald of carotid artery puncture is pulsatile blood from the cannula when the stylet is removed or when rhythmic fluctuation of temperature occurs during monitoring (18). The carotid artery is vulnerable to injury at three sites: the foramen lacerum posteromedially, posterolaterally in Meckel's cave, and the cephalad cavernous sinus (18) (Fig. 4). In the foramen lacerum, the cannula may penetrate the cartilaginous floor; however, Meckel's cave below V3 may lack a barrier of petrous bone. In the event of carotid artery violation, pressure should be applied manually, and the procedure should be postponed for 48 h. Complications of carotid artery injury include hemiparesis from ischemia and carotid cavernous fistula (19). Additional risks include damage to other anatomic structures of the skull base, such as the superior orbital fissure anteriorly and the jugular foramen posteriorly or aberrant foramen near the foramen ovale. Complications of electrode advancement beyond 5 mm proximal to the clivus may include ocular nerve damage or abducens palsy. Cavernous sinus violation may result in trochlear or oculomotor palsy.

Localization

Confirmation of electrode localization is based on physiological and radiographic evidence. Physiological confirmation is the free flow of cerebrospinal fluid from the cannula and the lack of pulsatile blood flow. The patient should experience facial pain and masseter contraction when the electrode enters the foramen ovale. Facial paroxysms of pain should occur with electrical stimulation between 0.3 and 0.5 V. Impedance measurements of the electrode tip also serve as a localizing data point. Impedance of 150-350 Q is typical of cerebro-spinal fluid bathing the retrogasserian nerve roots, whereas impedance exceeding 1000 Q is characteristic of solid tissue.

Radiographic confirmation includes the anterior/posterior projection of a target site 9 mm medial to the lateral wall of the internal auditory meatus, which coincides with the medial dip in the petrous ridge (20). A true lateral projection is useful when one is advancing the electrode until the needle is 5-10 mm below the intersection of a line drawn from the floor of the sella turcica to the clival line. The final target is the intersection of the petrous ridge with the clival line (18). Selective targeting of isolated divisions of the trigeminal nerve root is further defined by radiographic positioning. Division 1 is isolated with a curved electrode directed anteriorly and 5 mm above the clival petrous junction. Division 2 is best isolated with a straight electrode placed at the clival petrous junction. Division 3 may be isolated by placing a posteriorly curved electrode 5 mm below the clival line (Fig. 5).

Stimulation Procedures

Before connection of the cables for stimulation, the generator is either turned off or inactive. A reference dispersive electrode of 150 cm2 or more is recommended. Needle electrodes for reference or ground purposes are avoided because of the possibility of tissue burns at the insertion site. When cables are connected and generators are unplugged, patients occasionally feel a benign stimulus, which is caused by small galvanic charges between electrodes. When one is ready to initiate stimulation, the voltage range is set to 0.0 with a pulse rate of 50 pulses/s and a 1-ms duration. During stimulation, voltage output is slowly increased. Unless a patient has undergone previous treatment with alcohol or compression therapy, typical values of 0.2-0.5 V generate paresthesias in the trigeminal division of the trigger zone. If paroxysmal pain at low threshold stimulation is not produced, the electrode is repositioned. During stimulation, contraction of the masseter muscle or eye movement also indicates the need to reposition the electrode. Eye movement may indicate that the electrode is near or in the cavernous sinus; such placement may result in damage to cranial

Fig. 5. Composite illustration shows the relationship of the trigeminal rootlets to the clivus. When the electrode tip is -5 mm beneath the clivus, the third division is stimulated: at 0, the second division is stimulated; and at +5 mm above the clivus, the first division fibers are stimulated. (Courtesy of the Mayfield Clinic.)

Fig. 5. Composite illustration shows the relationship of the trigeminal rootlets to the clivus. When the electrode tip is -5 mm beneath the clivus, the third division is stimulated: at 0, the second division is stimulated; and at +5 mm above the clivus, the first division fibers are stimulated. (Courtesy of the Mayfield Clinic.)

nerves III, IV, and VI. During repositioning, the curved electrode is retracted to prevent mechanical damage to nerves. After its retraction, it may be rotated to a new position that is confirmed by fluoroscopy.

Radiofrequency Lesion Procedure

After confirmation of electrode physiologic and radiographic localization, the creation of the lesion begins. Anesthesia, administered in 30-50 mg boluses with the short-acting barbiturate methohexital (Brevital), induces somnolence and amnesia without compromise of the airway. The patient's blood pressure is closely monitored from the time of electrode insertion. Nipride is available in case of a hypertensive induced by nerve manipulation, stimulation, or ablation. An initial lesion temperature of 60°C for 90 s is sufficient for patients who have not undergone treatment by other means. If stimulation thresholds are high, initial temperature requirements may be elevated.

After the effects of methohexital resolve, within 2 to 3 min, sensory testing is performed. The goal of sensory testing, which includes pinprick and light touch, is preservation of corneal reflex and touch while inducing analgesia in the trigger zone. Additionally, corneal reflexes and extraocular muscle, ptery-goid, and masseter strength may be checked between lesioning intervals. Every precaution is taken to preserve corneal sensation. If the first lesion results in sensory deficit but not analgesia, further lesion production performed in awakened patients provides the most reliable sensory evaluations. Another lesion-localizing sign is erythema in the trigeminal distribution of coagulation; this occurs in fewer than 50% of patients. Although this physiologic mechanism is not understood, it is a valuable sign that the division is being destroyed.

Constant-temperature RFL depends on the heat-conducting properties of the surrounding milieu. The electrode tip may be placed in the retrogasserian ganglion, rootlets, or cerebrospinal fluid. As impedance and stimulation intensities vary, so do fluctuations of current, voltage, and heat to maintain the appropriate temperature. Cerebrospinal fluid is a heat sink, so that as heat disperses, increases in radiofrequency current are needed. Close monitoring of current and voltage aids in resolution of this issue and in the identification of tissue boiling characterized by a rapid rise in volts and sudden drop in current.

Postoperative care includes the gradual discontinuation of neurogenic pain medication except for long-term use. Medications are tapered slowly to prevent withdrawal symptoms. Postoperative pain can be alleviated by a mild narcotic and by periodic application of an ice pack to the jaw. The patient is observed in recovery and typically discharged that same day. If oral sensation is diminished, a soft diet is temporarily recommended. Postoperative instructions include meticulous eye care and mandatory use of artificial tears (methylcellulose) if corneal sensation is compromised.

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