The patient's hair is prepared. Depending on the thickness, the hair can remain in place and be prepped with the skin or, in patients with thicker hair (particularly with tight or braided hair), a small area can be clipped and shaved followed by the surgical prep. Routinely, the scalp is prepped with Betadine scrub followed by alcohol and Betadine solution, which is left over the area. In patients undergoing DBS placement, it is necessary to prep the burr hole region of the head and the posterior extension down the neck and down over the chest (the region of the pectoral muscle and the clavicle) where the subsequent pulse generator will be placed. The bed is set up such that the patient's face, hands,and body are separated from the surgeon and clearly visible; this allows the patient to interact freely with other staff during the surgery. Because full patient cooperation is necessary and evaluation of the movement disorder is critical to the procedure, no anesthetics or sedatives are used during the procedure except during sedation of scanning and frame placement. The anesthesiologist strictly monitors the patient's blood pressure and vital signs because of concern of an intraoperative hemorrhage if the blood pressure is allowed to elevate (particularly in patients with hypertension). This is not an uncommon occurrence in patients with PD; when a patient is off dopaminergic medication, the blood pressure is frequently elevated as a rebound effect.

At this point, the Leksell stereotactic arc system is assembled and attached to the stereotactic frame with the coordinates set according to values previously determined in the presurgical planning. The frame itself can be utilized to mark the entry point; however, we have found that establishing the system with the appropriate arc angle, ring angle, and X, Y,and Z coordinates provides that entry site. This can be confirmed by targeting the entry site itself as a separate target. After marking the scalp, the scalp is infiltrated with 1% lidocalne with epinephrine solution as utilized earlier, and a small 2-3 cm incision is made. The arc angle, the ring angle, and the X, Y, and Z coordinates have already been specified at the planning stage and are set to provide the exact trajectory and targeting. At this point, the stereotactic system can be used to create a twist drill hole. The advantage of the twist drill hole is that it provides less cerebrospinal fluid (CSF) loss during the procedure and therefore less potential for brain shift. When there is concern of injury to a surface artery or vein, particularly from the surface-rendered views of the braln in the 3D planning, a standard 14-mm burr hole can be created. The technique established for DBS electrode placement utilizes the standard 14-mm burr hole and therefore either technique can be employed by the surgeon based on individual needs.

After creating either a twist drill or burr hole, the dura is opened sharply and coagulated. A brain cannula, which will accommodate the lesioning electrode or the DBS, can be placed and inserted. The cannula will end some distance above the target so that either the lesioning electrode or the DBS electrodes will be open at the distal end to provide the necessary testing. Previously, we utilized microelectrode recording techniques; however, we found that this lengthened the time of the procedure considerably and did not improve the operative results. With the emphasis placed on more sophisticated MRI and localization to a known and anatomically recognized target, microelectrode recordings are no longer used.

In the case of pallidotomy or thalamotomy, lesioning is begun only after testing the safety of the procedure using electrical stimulation. This stimulation is a macrostimulation and is used malnly to ensure safety although frequently it will confirm that the target localization is an effective target. In the case of DBS placement, macrostimulation is performed using the same DBS electrode that will be maintained permanently for the procedure. Indeed, some of its development is based on the fact that macrostimulation frequently gave a similar result as the lesioning itself and, therefore, the concept of stimulation replacing lesioning surgery developed and has reached its final form as a DBS system. The macroelectrode that we use for stimulation and lesioning measures 1.3 mm in diameter and 3 mm in length during the surgical procedure. Stimulation can then be performed to test for safety and effectiveness in this procedure. If the electrode is placed to target stimulation or proceeded 1 mm past the target, the electrode will be within the ansa lenticularis. Stimulation can then be performed using square wave signals, with a pulse width of 1 msec. At this location, stimulation at 2 Hz should produce twitching in the contralateral arm and side of the face at 2-3 mV. If motor contractions occur below 2 V, the electrode is considered to be too close to the internal capsule and will need to be moved approximately 2 mm laterally. If the threshold is less than 1 mV, the target may need to be moved a millimeter or so anteriorly. Likewise, stimulation parameters at 50 Hz at this site will increase the tone of the contralateral upper limb and facial muscles usually between 1.5 and 3 V. The patient is then asked to close their eyes and the room is completely darkened. Stimulation at 2 Hz should not elicit any visual response until the voltage is above 4 mV. The reason for low-frequency versus high-frequency stimulation when testing visual response is that lower frequencies tend to produce bright flashes whereas higher frequencies tend to produce colored responses. The lower frequencies appear to be better identified. If any type of visual phenomena (e.g., flashes, stars, or bright lights) is visualized in the contralateral visual field at voltage less than 4, the electrode is too close to the optic tract and must be elevated at least 1-2 mm superiorly. Final placement of the lesioning electrode at its target site should not elicit these responses; in fact, stimulation at 5 Hz and subsequently followed by 100 Hz may provide some improvement in the patient's neurological complaints from the Parkinson's or other disease, giving a suggestion of the result That will be obtained during lesioning.

Lesioning at the thalamic site is similar to that described at the pallidal site, except that tremor is the desired observed response during this stimulation testing. Close neurological examination of visual, motor, and sensory functions during this time is critical to ensure that the stimulation testing does not produce unwanted complications and, particularly in the case of lesioning, will not produce any permanent neurological deficit.

Deep Brain Stimulation Electrode Placement
Once stimulation has been noted to control the tremor by DBS placement within the Vim nucleus of the thalamus, the electrode is secured in one of several ways to the exiting burr hole site. The DBS system produced by Medtronic contains a burr hole cap that can be used to secure the electrode to the border of the skull at the exit from the burr hole. All effort is maintained to ensure no movement of the electrode once it has been seated in the appropriate position; this is facilitated by a special adapter that attaches to the Leksell frame superior to the electrode placement site followed by securing the electrode to its burr hole exit site. The electrode is tested with variations of the voltage, the amperage, the pulse square duration, and other parameters to determine an ideal stimulation pattern that maintains tremor control. This is most frequently done via a technical assistant standing away from the operating table so that adjustments can be made through a separate extension lead. At this time, the remaining portion of the electrode is placed underneath the patient's scalp and the wound is closed in layers.

The patient is instructed that from this point he/she will be under general anesthetia so that the pulse generator system (Itrel II) can be placed in a subcutaneous pocket. After being placed under anesthesia, the patient is prepped and draped and an area is prepared several finger widths below the clavicle over the pectoralis fascia for placement of the pulse generator system. This can be easily created using blunt finger dissection above the pectoralis fascia; the pulse generator is then placed within the pocket. An extension lead is placed from this pocket to the superior pocket, frequently through an separate incision further posterior in the scalp (at least 5cm or more directly posterior to the frontal incision); the contact between the DBS electrode and the extension lead can then be secured. The wounds can be closed in layers in standard fashion.

Extensive neurological examination of the patient during lesioning is required. Usually we perform a test lesion using 60°C for 15 seconds to observe any neurological change. This period of time creates a very small amount of irreversible damage, which if noted to cause neurological decrement can be immediately stopped and results in no long-lasting clinical effects. During the test lesion, a motor, sensory, speech, and visual examination is performed.

Lesioning
The final pallidal lesion is created using thermal lesions. We routinely produce three thermal lesions 2 mm apart starting at the target site; after completion of the first lesion, lesions are produced at 2 and 4 mm more proximal along the trajectory. This creates a cvlindrical lesion that encompasses part of the ansa lenticularis, the globus pallidus interna, and occasionally the inferior part of the giobus pallidus externa. During that lesioning, the patient will notice immediate improvement of rigidity in the contralateral limbs and often will have complete resolution of contralateral tremor; many will show improvement in bradykinesia. In our center, standard lesioning is at 80oC for 90 seconds starting at the pallidal base. Again, it is important that a neurological examination be done continuously during and after each thermal lesion. We have noted that arm tone usually improves at the deepest lesion site, whereas leg tone improves as one ascends along the trajectory. Rarely, a patient will show increased dyskinesias after the first lesion, but in most instances the dyskinesia disappears completely at that time.

We do not perform bilateral pallidal lesioning procedures in any patient with tremor or PD and so, after the final lesioning, the electrode can be removed with inspection for any bleeding source. We place Gelfoam into a burr hole if a hole is utilized to prevent CSF leakage and to prevent seepage of subgaleal bleeding down through the burr hole, potentially creating a subdural hematoma. The wound is then closed in two layers using two interrupted 3-0 Vicryl stitches followed by a 4-0 nylon closure of the skin.

Closure
In the operating room, the stereotactic system is disassembled and the frame is removed; sterile dressings are placed over the pin-hole sites for approximately 1-2 minutes to ensure pressure control of any bleeding. At that point, Bacitracin ointment can be placed over these sites. A clean Telfa dressing followed by tape for closure is placed over the cranial wound. Postoperatively, the dressing can be removed after two days and subsequent sutures removed at seven days.

Following the procedure, the patient is transferred to a neurosurgical step-down unit for frequent neurological examinations and maintenance of blood pressure within reasonable limits. The greatest concern during that period is the possibility of postoperative hemorrhage, which is why blood pressure monitoring is critical.

The patient's parkinsonian medicines are restarted immediately. In patients undergoing pallidotomy or thalamotomy, this is easily done since the patient has had no anesthetic. In patients undergoing DBS, however, there will be a short period of time prior to restarting their medications when they are not able to take p.o. well. Unless the patient has chronically very high blood pressure, any blood pressure above 160/90 should be treated with antihypertensive medications.