Epilepsy, Functional Neurosurgery, and Pain.

Most neurosurgical procedures can be categorized into 4 divisions: cerebro-vascular, neuro-oncology, spine, and stereotactic and functional neurosurgery. Stereotactic and functional neurosurgery can be further subcategorized into epilepsy, movement disorders, and pain. The major categories of neurosurgery overlap extensively, as do the subcategories of stereotactic and functional neurosurgery. Recent advances in stereotactic and functional neurosurgery have been validated through rigorous clinical trials over the past decade, deepening our understanding of these disorders. The benefits of surgical treatment of epilepsy and ablative procedures for movement disorders and chronic pain are well documented. Neuromodulation is a treatment strategy that aims to regulate neural circuits by electrical stimulation or chemical agents. There is a well-defined role for neuromodulation in the management of epilepsy, movement disorders, and pain treatment, and this role appears to be expanding. Deep brain stimulation (DBS) and spinal cord stimulation are common neuromodulation therapies. DBS is approved by the US Food and Drug Administration to treat Parkinson disease, essential tremor, and epilepsy. It is also approved for treatment of primary dystonia and obsessive compulsive disorder in selected patients via a humanitarian device exemption. Responsive neurostimulation and vagal nerve stimulation are also approved neuromodulation therapies for epilepsy. Spinal cord stimulation is used for chronic pain syndromes affecting the back or extremities and is considered for patients in whom lumbar spine surgery has failed. The aim of functional neurosurgery is to restore function in patients. This section will outline the techniques and principles behind the methods that are used to accomplish this goal. All patients in the case presentations gave written informed consent for diagnostic imaging and treatment. Institutional review board approval was not sought and was unnecessary. CHAPTER 1: TEMPORAL LOBECTOMY FOR EPILEPSY Case Presentation A 42-yr-old right-handed woman presented with epilepsy. She had her first convulsive seizure when she was 31 yr of age, although she reported having had staring spells for several years before that first seizure. At the time of her first seizure, she underwent electroencephalography (EEG) and was started on single-agent medical management. She was seizure-free for 8 yr before having a recurrence of complex partial seizures several times a month that affected the left side of her body. Despite changing antiepileptic medication and starting a second agent, she continued to have seizures. She described an aura, including strange smells, that preceded the events. She underwent prolonged video EEG in the epilepsy monitoring unit, which confirmed a right temporal lobe seizure origin. Magnetic resonance imaging (MRI) demonstrated atrophy of the hippocampus on the right. Questions Where is the “safe zone” for temporal lobectomy on the left (dominant) side? 4 to 5 cm from the anterior tip 6 to 7 cm from the anterior tip 8 to 9 cm from the anterior tip There is no safe zone on the dominant hemisphere What is the most common cause of intractable temporal lobe epilepsy? Brain tumor Herpes simplex virus Arteriovenous malformation Mesial temporal sclerosis Which of the following is not a potentially curative surgical treatment option for epilepsy? Temporal lobectomy Selective amygdalohippocampectomy (SAH) Vagus nerve stimulation Laser thermal ablation Which of the following is a consequence of resecting too much of the temporal lobe? Aphasia Contralateral quadrant homonymous hemianopsia Contralateral hemiplegia All of the above Decision Making: Treatment of Epilepsy Epilepsy is a disease of recurrent, often unprovoked, seizures. If left untreated, recurrent seizures can result in injury from falls and trauma, cognitive and memory decline, or even sudden death. The first-line treatment of epilepsy is antiepileptic drugs (AEDs). However, approximately one-third of patients with epilepsy will continue to have uncontrolled seizures despite trials of multiple AEDs, often in combination. If trials of 2 well-tolerated, appropriately chosen AEDs, either as separate monotherapies or in combination, have failed to control the seizures, the patient is considered to have medically refractory epilepsy and is unlikely to achieve seizure freedom with medical management alone. Patients with medically refractory epilepsy should be referred to an epilepsy center for evaluation to determine whether they are candidates for epilepsy surgery that may either cure their epilepsy or decrease their seizure frequency. Preoperative Workup for Epilepsy Surgery Deciding whether an epilepsy patient is a candidate for a surgical treatment often involves answering the following question: Can the seizure onset be localized to 1 area? The answer to this question may be related to a lesion, such as a tumor or cortical dysplasia that is visible on imaging, or to a location of onset that may not be a lesion. Seizure onset is localized by having patients undergo an EEG with video monitoring, often for a prolonged time, in an EEG monitoring unit. AEDs are withdrawn during this period to capture typical seizures. Patients also undergo MRI to allow the clinician to look for anatomic abnormalities, such as mesial temporal sclerosis (the most common cause of temporal lobe epilepsy), that may predispose the patient to seizures. Seizure semiology (the type and presentation of the seizures) and detailed neuropsychological testing also help localize seizure onset. An array of other tests may further help localize seizure onset (eg, magnetoencephalography, interictal positron emission tomography/computed tomography, and ictal single-photon emission computed tomography) or may help localize eloquent areas of the brain (eg, functional MRI or intra-arterial sodium amobarbital Wada testing). If an obvious lesion with consistent localization is found on EEG, by semiology, or with another test, the patient may proceed directly to surgery. If questions remain, then the next step is usually invasive monitoring. Invasive Monitoring Placing electrodes directly on, or in, the brain for invasive intracranial monitoring (sometimes called phase-2 monitoring) provides a much more detailed and accurate localization of seizure onset than with the testing noted above. After a craniotomy, intracranial monitoring can be performed by using subdural recording grids placed directly on the brain surface (electrocorticography) and by using depth recording electrodes placed into the brain to evaluate deep structures (eg, hippocampus). Alternatively, multiple-depth electrodes can be placed stereotactically through small holes drilled in the skull in a procedure called stereoelectroencephalography. The patient is admitted to an epilepsy monitoring unit, where the electrodes are kept in place for a week or longer to provide enough time to capture multiple seizures and perhaps localize 1 focus of seizure onset. If the seizures can be localized, the patient will usually undergo surgery to remove the seizure focus. If the patient has seizure onset from more than 1 area in the brain, or if no definitive focus can be identified, then the patient may still qualify for palliative neuromodulation treatments, such as vagus nerve stimulation, to decrease the frequency of seizures. Other applications for intracranial electrodes include stimulation mapping to localize speech and motor function. Surgical Treatment for Temporal Lobe Epilepsy Temporal Lobectomy Most patients with temporal lobe epilepsy have seizures arising from the mesial structures, namely, the amygdala and hippocampus. Mesial temporal sclerosis is the most common underlying pathology for these patients. For appropriately selected patients with medically refractory, temporal lobe epilepsy, temporal lobectomy has been demonstrated to be superior to medical management in a randomized controlled trial, with approximately two-thirds of surgical patients achieving complete seizure freedom. This finding is particularly impressive in light of the failure of previous multiple medical regimens aimed simply at seizure reduction in these patients. Temporal lobectomy involves removal of the anterior temporal cortex, the amygdala, and the hippocampus (and the adjacent parahippocampal gyrus) posteriorly to the level of the quadrigeminal plate (Figure 1). The extent of anterior temporal cortex removal, as measured from the temporal pole along the middle temporal gyrus, depends on the side. In the dominant (usually left) hemisphere, resection is usually limited to 4 to 5 cm, whereas in the nondominant hemisphere, resection may be extended to 6 to 7 cm. Overly aggressive temporal lobe resection can injure the optic radiations of Meyer's loop, which course over the roof of the temporal horn of the lateral ventricle and can cause contralateral homonymous superior quadrantanopia. Resections carried too far posteriorly on the dominant temporal lobe can injure areas vital to speech, resulting in aphasia. Injury to branches of the anterior choroidal artery, which run in the choroid plexus of the lateral ventricle adjacent to the hippocampus, can result in contralateral hemiplegia. Despite the effectiveness of temporal lobectomy for seizure control, the surgery is significantly underutilized.FIGURE 1.: Illustration demonstrating an inferior view of the cerebrum in the cut-away skull, showing 2 approaches to the mesial temporal lobe. A temporal lobectomy or SAH is performed via a temporal craniotomy. The subtemporal approach is used for an SAH. Used with permission of Barrow Neurological Institute, Phoenix, Arizona.Selective Amygdalohippocampectomy Because seizures caused by temporal lobe epilepsy usually originate in the mesial temporal lobe structures—the amygdala and the hippocampus—some surgeons will spare the temporal cortex and perform only an SAH. The goal is to preserve as much normal cortex as possible to minimize any neuropsychological adverse effects of surgery. No direct randomized controlled comparison of temporal lobectomy and SAH has been performed, but a meta-analysis suggests that the seizure freedom rate achieved with SAH is slightly lower than that achieved with temporal lobectomy. For patients undergoing SAH who continue to have seizures, another option is the completion of the temporal lobectomy, with resection of the anterior temporal cortex. Three main approaches allow surgeons to reach the mesial temporal structures to perform an SAH: (1) a transsylvian approach; (2) a transcortical approach, in which a corticectomy is made in the middle temporal gyrus to reach the temporal horn of the ventricle; and (3) a subtemporal approach (Video 1), in which the corticectomy is made in the parahippocampal gyrus. {"href":"Single Video Player","role":"media-player-id","content-type":"play-in-place","position":"float","orientation":"portrait","label":"VIDEO 1.","caption":"Right-sided SAH for mesial temporal sclerosis.ITG, inferior temporal gyrus; MTG, middle temporal gyrus; STG, superior temporal gyrus. Used with permission from Barrow Neurological Institute, Phoenix, Arizona. This video can be accessed in the HTML version of the article. Please visit www.operativeneurosurgery-online.com to view this article in HTML and play the video.","object-id":[{"pub-id-type":"doi","id":""},{"pub-id-type":"other","content-type":"media-stream-id","id":"1_6v98x73h"},{"pub-id-type":"other","content-type":"media-source","id":"Kaltura"}]} Laser Ablation Laser thermal ablation, also known as laser interstitial thermal therapy, is a newer method for treating a seizure focus. Laser ablation is a minimal access approach that requires only a small incision and a bur hole drilled in the skull instead of a traditional craniotomy. An infrared, cooled laser wire is placed stereotactically to the desired point in the brain, and the laser energy heats and ablates the surrounding brain tissue. The temperature achieved is monitored by intraoperative magnetic resonance thermography to provide controlled lesion ablation. Laser ablation has been used as an alternative method to perform SAH, by placing the laser trajectory along the axis of the amygdala and hippocampus. Although larger studies are currently underway, early experience suggests that the seizure freedom rate is less than that for temporal lobectomy or SAH. Neuromodulation for Epilepsy Patients with generalized onset seizures or multiple seizure foci are not usually candidates for seizure focus resection. For example, patients with seizures arising independently from both hippocampi cannot undergo bilateral hippocampal resection, as it would result in severe anterograde and temporally graded retrograde amnesia. However, these patients may be candidates for palliative stimulation therapies that decrease the frequency of seizures. Vagus nerve stimulation involves implanting an electrode around the left vagus nerve and connecting it to a subcutaneous pulse generator under the clavicle. On average, vagus nerve stimulation decreases seizure frequency by 30% at 5 yr. Another stimulation approach, called responsive neurostimulation with the RNS System (NeuroPace Inc, Mountain View, CA) uses intracranial electrodes (surface grids and depth wires) connected to a pulse generator that senses seizure activity and attempts to disrupt it with feedback stimulation. In a study, at 2 yr post-treatment, approximately 50% of patients with the responsive neurostimulation system had a greater than 50% reduction in seizure frequency. The goal of this neuromodulation system is seizure reduction, as the system rarely provides seizure freedom. Discussion of Case The patient underwent right-sided subtemporal SAH to treat right temporal onset partial complex seizures. Because the MRI, EEG, and seizure semiology findings were consistent with right-sided seizure onset, second-stage monitoring with intracranial electrodes was not deemed necessary. Pathologic findings confirmed hippocampal sclerosis. The patient was free of seizures after surgery. After 1 yr without seizures, the patient was slowly weaned off AEDs. Answers to Questions A. The safe zone for resection of the dominant temporal lobe is 4 to 5 cm. Resecting beyond that point increases the risk of speech and cognitive side deficits. D. Mesial temporal sclerosis is a common and treatable cause of intractable temporal lobe epilepsy. C. On average, vagus nerve stimulation decreases seizure frequency by 30% at 5 yr. Patients rarely become seizure free and therefore remain ineligible for activities such as driving. D. The safe zone for temporal lobectomy is intended as a guide to avoid complications such as aphasia, contralateral quadrant homonymous hemianopsia, and contralateral hemiplegia. Pearls ✓ Epilepsy patients who are not responsive to 2 or more AEDs are considered to have medication-refractory epilepsy and should be evaluated in an epilepsy center to determine whether they are surgical candidates. ✓ Level I evidence supports the use of temporal lobectomy in temporal lobe epilepsy. Nevertheless, surgical treatment of epilepsy is underutilized. ✓ Temporal lobectomy is effective in well-selected patients. SAH is an alternative treatment with potentially fewer postoperative cognitive deficits; however, efficacy in seizure reduction may be less than with a temporal lobectomy. ✓ Neuromodulation of epilepsy seldom succeeds in rendering a patient seizure free. CHAPTER 2: HEMISPHERECTOMY FOR EPILEPSY Case Presentation A 29-yr-old woman in status epilepticus was refractory to all medical therapy, including barbiturate-induced coma. Withdrawal of care was considered. As an alternative, the patient's family elected to proceed with subdural grid mapping after surface electrodes vaguely localized the seizure focus to the left hemisphere. Multiple subpial transections were subsequently performed, and the patient's status epilepticus resolved. Questions What is an indication for corpus callosotomy? Drop attacks (atonic seizures) Infantile hemiplegia syndrome Cavernous malformation causing seizure Mesial temporal sclerosis A and B C and D How much of the corpus callosum can be resected safely in a callosotomy? Anterior one-third Middle one-third Posterior one-third Anterior two-thirds Posterior two-thirds Which of the following is not a surgical disconnection operation? Callosotomy Hemispherectomy Multiple subpial transections SAH Which of the following is true about disconnection syndrome? It is a common adverse effect of vagus nerve stimulation. Symptoms include decreased spontaneity of speech and incontinence. Complete resolution of symptoms is expected 3 mo after surgery. On examination, left homonymous hemianopsia with macular sparing is seen in >85% of cases. Decision-Making: Surgical Treatment for Severe Epilepsy Three strategies can be used to treat epilepsy surgically: resection, stimulation, and disconnection. In a hemispherectomy, one-half of the brain is resected or disconnected from the other half. This procedure is reserved for patients with severe forms of epilepsy when the seizure source covers a broad area of the brain. It is used only when seizures cannot be controlled by medication and when more targeted surgery is not an option. Rasmussen encephalitis or Sturge-Weber syndrome are 2 conditions that may cause such seizures. These conditions are typically observed in young children who have an underlying disease that has caused extensive damage to a brain hemisphere. The seizure-control rate with hemispherectomy is approximately 80%. Patient Selection Hemispherectomy is an inherently high-risk procedure. The operation is performed predominantly in children because younger brains have more potential for neuroplasticity. Neural circuits in the remaining hemisphere are able to remodel to accommodate the tasks previously performed by the resected hemisphere. The high potential for permanent functional deficits also requires a careful risk-benefit analysis and discussion with the family of the patient before proceeding with surgery. A hemispherectomy is typically considered for patients who are severely disabled and incapacitated by the seizures. Conversely, patients with good neurological function are usually not offered this treatment because of its high risk. When the procedure is being considered for an adult, an intracarotid amobarbital test (Wada test) is performed to determine which hemisphere is dominant for functions such as speech and memory. This test is performed in an awake patient with endovascular administration of amobarbital. The medication is selectively delivered to 1 hemisphere at a time, and it suppresses neural activity in that hemisphere. Neuropsychological testing is performed to assess the different components of speech and memory functions that are affected. Disconnection Procedure Options Hemispherectomy A hemispherectomy is considered when the source of seizure is unilateral, with widespread cortical involvement, and when preexisting neurological deficits correspond to the involved areas of the brain. Any cortex that is not resected is disconnected from the underlying white matter. An anatomical hemispherectomy is the resection of an entire half of the brain. This aggressive resection removes widespread epileptic foci, thus reducing the likelihood of seizure recurrence. A functional hemispherectomy is a more selective procedure, involving resection of only the epileptic regions, with disconnection of the remaining brain tissue in the hemisphere. This more limited form of hemispherectomy results in a similar seizure control rate with fewer complications than with an anatomical hemispherectomy. In a peri-insular hemispherectomy, the hemisphere is disconnected but not removed. The basal ganglia are left intact in all these procedures. Corpus Callosotomy The corpus callosum is a broad band of nerve fibers that connects the 2 hemispheres of the brain. A corpus callosotomy is a surgical procedure to disconnect the 2 hemispheres from each other. This operation is used for generalized seizures, such as “drop attacks” (ie, atonic seizures). In a drop attack, loss of posture or tone can result in falls and subsequent injury. The corpus callosotomy reduces drop attacks by approximately 70%. A full corpus callosotomy may result in disconnection syndrome, which may be avoided by limiting the callosotomy to the anterior two-thirds of the corpus callosum. Multiple Subpial Transections When EEG or depth electrode monitoring localizes the seizure focus to an eloquent area of the brain, multiple subpial transections are an option (Figure 2; Video 2). The goal is to interrupt the cortical spread of the seizure while maintaining the functional connections of the affected cortex to the deeper structures. The multiple subpial transections technique has also been used to treat status epilepticus after localization of seizure focus to eloquent cortex (Video 2).FIGURE 2.: Illustration of the multiple subpial transections technique. Used with permission from Barrow Neurological Institute, Phoenix, Arizona. {"href":"Single Video Player","role":"media-player-id","content-type":"play-in-place","position":"float","orientation":"portrait","label":"VIDEO 2.","caption":"Video of multiple subpial transections technique. Used with permission from Barrow Neurological Institute, Phoenix, Arizona. This video can be accessed in the HTML version of the article. Please visit www.operativeneurosurgery-online.com to view this article in HTML and play the video.","object-id":[{"pub-id-type":"doi","id":""},{"pub-id-type":"other","content-type":"media-stream-id","id":"1_f4ifd8cn"},{"pub-id-type":"other","content-type":"media-source","id":"Kaltura"}]} Discussion of Case Postoperatively, the patient was weaned from barbiturates and started following commands 4 d after surgery. Six weeks later, she was ambulatory and without focal weakness. Over the long term, she continued to take 3 antiepileptic medications. Answers to Questions E. Corpus callosotomy is infrequently performed. Drop attacks and infantile hemiplegia syndrome are both conditions for which this technique is used. D. Resecting the anterior two-thirds of the corpus callosum is believed to be safe; resecting more can result in significant postoperative deficits such as disconnection syndrome. D. SAH is a resective procedure. B. Disconnection syndrome can also lead to other symptoms such as aphasia, left-sided apraxia, tactile aphasia, and incontinence. Pearls ✓ The 3 basic approaches for epilepsy surgery are resection, stimulation, and disconnection. ✓ A corpus callosotomy can be an effective treatment for atonic seizures or drop attacks. ✓ In children undergoing a hemispherectomy, neuroplasticity may allow the remaining hemisphere to take over tasks previously performed by the hemisphere that was resected. ✓ Hemispherectomy carries higher risks in adults than children, and it is performed infrequently. It is reserved for patients for whom the risk for significant neurological impairment is acceptable, or in whom preexisting profound neurological deficits render the resection of the hemisphere functionally inconsequential. CHAPTER 3: DBS FOR PARKINSON DISEASE Case Presentation A 61-yr-old man developed right-hand resting tremor 7 yr before presentation to our clinic. He was diagnosed with Parkinson disease 1 yr after the development of tremors and was started on levodopa-carbidopa. For the first 4 yr, he responded well to levodopa-carbidopa. During that time, he developed symptoms on his left side, as well as bradykinesia, rigidity, and painful toe dystonia. Over the previous 2 yr, he had noticed more fluctuations of his symptoms during the day and less predictability of when his symptoms would be controlled with levodopa. This inconsistency affected his confidence in going out with friends. At the time of presentation, he was taking Parkinson medications 7 times a day. He complained that about an hour after taking a dose, he developed involuntary movements. Questions Which statement accurately describes DBS for Parkinson disease? DBS is recommended as a salvage treatment when all other therapies have been exhausted. DBS helps treat later-stage symptoms of Parkinson disease, such as cognitive impairment and dopa-unresponsive freezing of gait. DBS reduces the amount of time patients spend in the “off” state. DBS remains an experimental therapy. Which DBS target in the brain allows for the most noticeable reduction in requirements for Parkinson disease medication? Subthalamic nucleus Globus pallidus interna Ventral intermediate nucleus Pedunculopontine nucleus Which of the following modalities can be used to confirm appropriate placement of the DBS lead? Single-unit microelectrode recording Test stimulation to assess response of symptoms and side effects MRI or computed tomography imaging All the above What results in adverse effects of stimulation affecting the internal capsule and causing muscle contractions? A subthalamic nucleus lead that is too far anterolateral A ventral intermediate nucleus lead that is too far medial A globus pallidus interna lead that is too far anterolateral Muscle contractions are never an adverse effect of DBS DBS for Parkinson Disease Background Levodopa therapy was introduced in the late 1960s, and it remains the cornerstone of medical management of patients with Parkinson disease. It is an effective treatment for the cardinal features of the disease, including tremor, bradykinesia, and rigidity. These features of the disease are thought to arise as a consequence of the pathological activity within the cortico-basal ganglia-thalamo-cortical loop of the brain (Figure 3A). Over time, as the severity of the disease progresses, patients can develop motor fluctuations such that they spend an increasing amount of time during the day in the so-called “off” state, when their symptoms are uncontrolled. They can also develop dyskinesias, which are abnormal involuntary movements that are side effects of medication. Motor fluctuations and dyskinesias are 2 types of complications of medical therapy for Parkinson disease for which a surgical solution can be considered.FIGURE 3.: A, Illustration demonstrates the corticospinal tract, starting at the motor cortex and extending to the spinal cord. The cortico-basal ganglia-thalamo-cortical loop, which is a circuit that coordinates motor function, is affected in Parkinson disease. B, DBS leads are placed through bur holes made in the frontal bone. The leads are connected via extension wires to an internalized pulse generator in the chest. C, An MRI of the brain, T2-weighted fast-spin echo sequence, at the Z = –4 plane, demonstrates the subthalamic nucleus. D, An MRI of the brain, proton density sequence, at the Z = 0 plane, demonstrates the globus pallidus interna (GPi). E, A stereotactic frame is used to place the DBS leads. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.DBS Surgery DBS is a surgical procedure in which electrodes are placed within nodes of the motor circuit of the brain through bur holes in the skull (Video 3). The electrodes are connected to a pulse generator in the chest (Figure 3B). Much like how a cardiac pacemaker is used to correct abnormal heart rhythms, DBS can be thought of as a mechanism for correcting abnormal brain rhythms, and it functions, in effect, as a pacemaker for the brain. {"href":"Single Video Player","role":"media-player-id","content-type":"play-in-place","position":"float","orientation":"portrait","label":"VIDEO 3.","caption":"This video shows the use of the stereotactic frame to place DBS electrodes. Intraoperative computed tomography is used to verify lead position. After lead placement, the internal pulse generator is placed in the right chest and connected to the DBS leads. Used with permission from Barrow Neurological Institute, Phoenix, Arizona. This video can be accessed in the HTML version of the article. Please visit www.operativeneurosurgery-online.com to view this article in HTML and play the video.","object-id":[{"pub-id-type":"doi","id":""},{"pub-id-type":"other","content-type":"media-stream-id","id":"1_81pow3n9"},{"pub-id-type":"other","content-type":"media-source","id":"Kaltura"}]} Typical DBS targets include the subthalamic nucleus (Figure 3C) and the globus pallidus interna (Figure 3D). These are both components of the cortico-basal ganglia-thalamo-cortical loop. The ventral intermediate nucleus is another target that has been used in patients with Parkinson disease, but targeting this region appears to be effective only for Parkinson tremor, and it provides no benefit for treating rigidity or bradykinesia. The ventral intermediate nucleus continues to be used for the treatment of essential tremor. The surgical procedure consists of using a stereotactic apparatus to place the leads accurately to a target 6 to 8 cm deep in the brain through a dime-sized hole in the skull (Figure 3E). The deep brain targets can be visualized on specific MRI sequences, which are used for planning purposes. The operation is typically performed with the patient awake. Microelectrode recording of individual neurons can be performed to localize the electrophysiological target. Test stimulation of the DBS lead can be used to verify the response of symptoms to stimulation and to identify adverse effects caused by stimulation. Benefits of DBS Numerous randomized clinical trials have evaluated DBS therapy, and the safety and efficacy of DBS is now well established (See Weaver et al reference in reading list below). DBS has a levodopa-like effect on motor symptoms, and the most accurate predictor of long-term improvement after DBS is the magnitude of the patient's levodopa