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Figure 5 Final position of electrode in the target on axial computed tomographic scan.

If conventional cordotomy is performed using X-ray guidance, use of an image intensifier is recommended. Visualization of the dentate ligament in the lateral X-ray is mandatory. Because water-soluble dyes do not demonstrate the dentate ligament [19,20], oil-based contrast material must be used; however, the risk of arachnoiditis with such dyes remains a problem. Theoretically the anterior part of the spinal cord could be visualized by air myelogram, and the posterior part by oil-based contrast material. Antero-posterior imaging is usually used to demonstrate the position of the needle just in the lateral margin of the odontoid process.

In the anterior approach, the skin, subcutaneous tissues, and paraver-tebral fascia are infiltrated with local anesthetic. An 18-gauge, thin-walled spinal needle is inserted opposite the cordotomy site, medial to the carotid sheath and lateral to the trachea and esophagus at the C4-C5, C5-C6 or C6-C7 level. With the help of X-ray imaging or CT, the needle is observed as it passes through the disc space and is placed in the anterior lateral part of the spinal cord [23-25]. After reaching the subarachnoid space, only the anterior part of the spinal cord can be visualized indirectly on air myelogram [23,24].

To confirm whether the electrode is in the cerebrospinal fluid (CSF) or the spinal cord, the surgeon obtains impedance values and determines the neurophysiologic response of the compartment where the electrode is located. Impedance measurements are an important indication of passage into a new medium along the path of the electrode. Impedance values are approximately 400 Ohms in the CSF; an increase of approximately 200 Ohms is observed when there is contact between the electrode tip and the pia. The value is almost always greater than 1000 Ohms after insertion into the spinal cord.

Real neurophysiology confirmation of the target is obtained by stimulation, necessitating that the patient be alert to cooperate. As a rule of functional neurosurgery, stimulation must be initiated at minimum voltage values: 2-5 Hz stimulation with 0.4 to 1.5 volts causes ipsilateral trapezius muscle contraction, indicating that the electrode is within or near the anterior gray matter of the LST. Ipsilateral motor responses in the arm or leg indicate that the electrode is in the corticospinal tract; 100 Hz stimulation with 0.2 to 1.5 volts causes pain, paresthesia, or warmth in the LST. Use of a curved electrode allows the surgeon to rotate the needle 0.5 mm anteriorly or posteriorly to place the electrode in a specific part of the tract in the lateral-to-medial plane. In CT-guided cordotomy, the position of the electrode must be confirmed by new CT images. The author believes that if stimulation is confirmed by the CT image, the effectiveness, safety, and selectivity of PC are gained in this golden state. This is currently only obtained with CT imaging and stimulation. The final step of the procedure is to make con trolled radiofrequency lesions. A test lesion should be made at 55° to 60° C for 60 seconds before making the final lesions. The author then recommends making two to three lesions at a temperature of 70° to 80° C for 60 seconds, checking the patient's neurological function, particularly analgesia level and motor function, after making each lesion.

Bilateral PC is usually performed with a 1-week interval. Bilateral procedures may present technical difficulties, but we recommend using bilateral selective cordotomy for intractable pain in the lower Th10 dermatome. If the pain is located in the upper segment, C1-C2 percutaneous lateral cordotomy is performed on one side and percutaneous anterior cordotomy is performed on the other [23-25]. The author prefers to use a small-caliber electrode for CT-guided bilateral cordotomy only in cases with somatic lower body pain. The pain-dominant side is selected for the first denervation. After the test lesion is made, one or two lesions are made at a temperature of 70° to 80° C.

The patient is kept in the supine position with the head elevated for 1 hour after the procedure. After an observation period of 6 hours, unilateral cordotomy patients may go home if conditions permit, but bilateral cordo-tomy patients must be observed in the intensive care unit. Blood pressure must be monitored carefully because of the risk of hypotension, especially on the day of the procedure. Other important problems are related to lesions of the reticulospinal tract, which controls the rhythm and depth of ventilation. Blood gases must be evaluated and sleep patterns monitored. Patients with respiratory complications are kept in the hospital for 2 to 3 weeks. The surgeon who performs cordotomy and controls pain successfully must remember not to stop morphine therapy suddenly. Most patients reduce their dosages progressively and discontinue morphine use over time.

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