We perform pallidotomy using CRW Stereotactic apparatus and macrostimulation. The stereotactic frame is fixed to the patient’s head with the help of four pins. The area of fixation is numbed with the help of local anaesthetic. The stereotactic frame is placed in a plane parallel to the orbitomeatal line. Following this the patient is taken to the CT scan department where an axial CT scan is performed. The scanner gantry is angled in a plane to include the anterior commissure (AC) and posterior commissure (PC) in one plane. These are fixed landmarks in the brain to which the target can be related. For high degree of accuracy the CT slices are 2mm thick and contiguous. The length of the AC-PC is measured. The pallidal target is 2mm in front of the midpoint of AC-PC line at a laterality of 21-22mm and a depth of 4-6 mm.
An inversion recovery, coronal, MRI scan is performed perpendicular to the AC-PC plane. The pallidal target is on a slice that passes through the mammillary body. The correct laterality and depth of the pallidal target is confirmed on this MRI and the CT target refined accordingly. Once the target is defined the patient is taken back to the operation theatre and made to comfortably lie down on the operation table. A small opening (burr hole) is made in the skull after infiltrating local anaesthetic at the operative site. The target is reached with the help of stereotactic arc system.
The physiological exploration is performed using an electrode with an exposed tip of 2 x 2 mm. This is introduced through a precoronal burr hole. The exploration starts 6 mm above the target and the electrode is advanced in increment of 2 mm using micro drive. At each level stimulation is performed using 5 Hz. and 100 Hz. frequencies. Impedance, which is a measure of resistance of various tissues, is also noted at each level to discriminate between nuclei and fibre tracts. Motor evaluation to check for weakness, dysarthria and fasciculation’s in tongue is performed at 5 Hz. Sensory evaluations is performed at 100Hz. frequency.
During sensory stimulation there are some dyskinetic movements and decrease in rigidity. Patient is also asked to report any flashes of light or visual disturbances, indicating close proximity to the optic tract. If there are any motor or sensory side effects the electrode position is adjusted. If there are no side effects than a test lesion of 42 C for 60 seconds is made at this point, and if there is no deficits than a final lesion of 70C for 60 seconds is made. Similar procedure is repeated at a level 4mm, 2mm and at 0 target level.
Postoperative MRI a: Axial and b: Coronal, following pallidotomy. Note the relationship of the lesion to internal capsule and the optic tract Pallidal stimulation is performed using similar technique. We mainly restrict the use of pallidal stimulation for the treatment of dystonia. We feel that STN stimulation is better than pallidal stimulation in the treatment of advanced Parkinson’s disease. Following the surgery the patient is observed in intensive care unit for 24 hours. In case of thalamic stimulation the IPG (Implantable pulse generator) is implanted on the next day and the programming of the electrode is commenced the day after. The usual hospital stay for thalamotomy is four days, whereas that for the thalamic stimulation is 10 days.
Untoward side effects resulting from a physiologically guided selective Vim thalamotomy are minimal. The most common complications of thalamotomy are pyramidal weakness, dysesthesia, cognitive and speech deficits. The cognitive and speech deficits are more commonly seen in left sided and bilateral thalamotomy. Due to increased incidence of morbidity, bilateral thalamotomy is not performed any more. The risk of intracerebral hemorrhages and infection is less than one per cent.