We report a 75-year-old male with a 10-year history of progressive OT and unsuccessful medical treatment. He used a portable stool or cane when he had to stand in place. Upon standing, he would have the immediate onset of fine tremors in both legs. He could only stand in place for 20 seconds at most before needing to sit, lean, or hold on to something. The patient had been initially trialed on acetazolamide and primidone over the course of 5 years prior to his referral to our institution. He tolerated the medications, but they were discontinued because of ineffectiveness. He was then started on topiramate, up to 50 mg twice per day, but discontinued because of side effects of anorexia and significant weight loss. He was subsequently begun on clonazepam, up to 0.5 mg twice per day, but was discontinued because of sedation. Gabapentin and then valproic acid were trialed without any improvement in his symptoms. The severity of the side effects of the clonazepam and topiramate and the ineffectiveness of the other medications resulted in referral for consideration of DBS surgery.
Pre-operative surface EMG showed 13 Hz tremors of the lower extremities upon standing, which spread upwards to his paraspinal, truncal, and upper limb muscles. The patient underwent bilateral thalamic DBS. The target coordinates were based upon the midcomissural point and 11.5 mm lateral to the wall of the ipsilateral third ventricle. Intraoperative electrophysiology was performed to locate and confirm the leg portion of the Vim nucleus. Microelectrode recordings were followed by microstimulation looking for sensory side effects in the contralateral leg. In the right brain, leg side effects were noted at 13 mm lateral to the ipsilateral wall of the third ventricle, whereas in the left brain, final laterality was 11.5 mm. Macrostimulation was carried out to further refine final electrode placement based upon intraoperative improvement of contralateral leg tremor. This was assisted by surface EMG recordings of the legs, asking the patient to push down on each leg against resistance or on a footboard (Figure 1). Final electrode placement per side was determined based on stimulation side effects (sensory symptoms in the contralateral leg) and improvement of the contralateral leg tremor on surface EMG with macrostimulation (Figure 2).
One month following surgery, initial programming was done. The left Implantable Programmable Generator (IPG) settings were: contacts 0 and 1(-); contact 2(+); amplitude 4 volts; pulse width 60 milliseconds; and frequency 185 Hz. The right IPG settings were: contact 4(-); contact 5(+); amplitude 2.5 volts; pulse width 90 milliseconds; and frequency 185 Hz.
He reported an 80% subjective improvement of his OT in the left leg and 50% improvement in the right leg. He was able to stand in place for 7 minutes before needing to sit. He no longer required his portable stool/cane. A repeat surface EMG study was performed 3 months post-DBS. On standing from a seated position, there was immediate onset of tremor in the lower extremities on surface EMG, although the tremor bursts were less continuous, less rhythmic, and often showing bursts of slower frequency than the previous 13 Hz tremor seen pre-operatively (Figures 3-4). At 11 months post-DBS, the patient developed an infection and skin erosion around the IPG, which required removal of the pulse generator. Interestingly, it took 1 month before he returned to baseline severity of his OT. A new pulse generator was placed 2 months later, resulting in prompt improvement of his OT. The patient has been followed at our institution by the treating movement disorders neurologist and implanting neurosurgeon. Since surgery, the patient has had 19 visits to the DBS clinic demonstrating continued improvement. At last follow-up 30 months following DBS, he continued to experience a good response to stimulation alone (with no anti-tremor medications). He was able to stand in place at least 7 minutes or more before needing to sit or lean on something. The left IPG settings on last follow-up were: contacts 4 and 5(-); contact 6(+); amplitude 2.2 volts; pulse width 90 milliseconds; and frequency 185 Hz. The right IPG settings were: contact 0(-); contact 1(+); amplitude 2.7 volts; pulse width 90 milliseconds; and frequency 185 Hz.