Intraoperative microrecordings of the subthalamic nucleus in Parkinson's disease

THE POSTERIOR SUBTHALAMIC AREA IN THE TREATMENT OF MOVEMENT DISORDERS

THE INTRODUCTION OF thalamotomy in 1954 led naturally to exploration of the underlying subthalamic area, with the development of such procedures as campotomy and subthalamotomy in the posterior subthalamic area. The most popular of these procedures was the subthalamotomy, which was performed in thousands of patients for various movement disorders. Today, in the deep brain stimulation (DBS) era, subthalamic nucleus DBS is the treatment of choice for Parkinson's disease, whereas thalamic and pallidal DBS are mainly used for nonparkinsonian tremor and dystonia, respectively. The interest in DBS in the posterior subthalamic area has been quite limited, however, with a total of 95 patients presented in 14 articles. During recent years, interest has increased, and promising results have been published concerning both Parkinson's disease and nonparkinsonian tremor. We reviewed the literature to investigate the development of surgery in the posterior subthalamic area from the lesional era to the present.

Automatic noise-level detection for extra-cellular micro-electrode recordings

Extra-cellular neuro-recording signals used for functional mapping in deep brain stimulation (DBS) surgery and invasive brain computer interfaces, may suffer from poor signal to noise ratio. Therefore, a reliable automatic noise estimate is essential to extract spikes from recordings. We show that current methods are biased toward overestimation of noise-levels with increasing neuronal activity or artifacts. An improved and novel method is proposed that is based on an estimate of the mode of the distribution of the signal envelope. Our method makes use of the inherent characteristics of the noise distribution. For band-limited Gaussian noise the envelope of the signal is known to follow the Rayleigh distribution. The location of the peak of this distribution provides a reliable noise-level estimate. It is demonstrated that this new 'envelope' method gives superior performance both on simulated data, and on actual micro-electrode recordings made during the implantation surgery of DBS electrodes for the treatment of Parkinson's disease.

Typical variations of subthalamic electrode location do not predict limb motor function improvement in Parkinson's disease

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an effective treatment for patients with medically refractory Parkinson's disease (PD). The degree to which the anatomic location of the DBS electrode tip determines the improvement of contralateral limb movement function has not been defined. This retrospective study was performed to address this issue. Forty-two DBS electrode tips in 21 bilaterally implanted patients were localized on postoperative MRI. The postoperative and preoperative planning MRIs were merged with the Stealth FrameLink 4.0 stereotactic planning workstation (Medtronic Inc., Minneapolis, MN, USA) to determine the DBS tip coordinates. Stimulation settings were postoperatively optimized for maximal clinical effect. Patients were videotaped 1 year postoperatively and assessed by a movement disorder neurologist blinded to electrode tip locations. The nine limb-related components of the Unified PD Rating Scale Part III were tabulated to obtain a limb score, and the electrode tip locations associated with the 15 least and 15 greatest limb scores were evaluated. Two-tailed t-tests revealed no significant difference in electrode tip location between the two groups in three-dimensional distance (p=0.759), lateral-medial (x) axis (p=0.983), anterior-posterior (y) axis (p=0.949) or superior-inferior (z) axis (p=0.894) from the intended anatomical target. The range of difference in tip location and limb scores was extensive. Our results suggest that anatomic targeting alone may provide the same clinical efficacy as is achieved by "fine-tuning" DBS placement with microelectrode recording to a specific target.

Mean-field modeling of the basal ganglia-thalamocortical system. I Firing rates in healthy and parkinsonian states

Parkinsonism leads to various electrophysiological changes in the basal ganglia-thalamocortical system (BGTCS), often including elevated discharge rates of the subthalamic nucleus (STN) and the output nuclei, and reduced activity of the globus pallidus external (GPe) segment. These rate changes have been explained qualitatively in terms of the direct/indirect pathway model, involving projections of distinct striatal populations to the output nuclei and GPe. Although these populations partly overlap, evidence suggests dopamine depletion differentially affects cortico-striato-pallidal connection strengths to the two pallidal segments. Dopamine loss may also decrease the striatal signal-to-noise ratio, reducing both corticostriatal coupling and striatal firing thresholds. Additionally, nigrostriatal degeneration may cause secondary changes including weakened lateral inhibition in the GPe, and mesocortical dopamine loss may decrease intracortical excitation and especially inhibition. Here a mean-field model of the BGTCS is presented with structure and parameter estimates closely based on physiology and anatomy. Changes in model rates due to the possible effects of dopamine loss listed above are compared with experiment. Our results suggest that a stronger indirect pathway, possibly combined with a weakened direct pathway, is compatible with empirical evidence. However, altered corticostriatal connection strengths are probably not solely responsible for substantially increased STN activity often found. A lower STN firing threshold, weaker intracortical inhibition, and stronger striato-GPe inhibition help explain the relatively large increase in STN rate. Reduced GPe-GPe inhibition and a lower GPe firing threshold can account for the comparatively small decrease in GPe rate frequently observed. Changes in cortex, GPe, and STN help normalize the cortical rate, also in accord with experiments. The model integrates the basal ganglia into a unified framework along with an existing thalamocortical model that already accounts for a wide range of electrophysiological phenomena. A companion paper discusses the dynamics and oscillations of this combined system.

ELECTRODE POSITION DETERMINED BY FUSED IMAGES OF PREOPERATIVE AND POSTOPERATIVE MAGNETIC RESONANCE IMAGING AND SURGICAL OUTCOME AFTER SUBTHALAMIC NUCLEUS DEEP BRAIN STIMULATION

OBJECTIVE

The electrode position is important to the surgical outcome after subthalamic nucleus (STN) deep brain stimulation (DBS). The aim of this study was to compare the surgical outcome of bilateral STN DBS with the electrode position estimated using fused magnetic resonance imaging.

METHODS

Bilateral STN DBS was performed in 60 patients with advanced Parkinson's disease. Patients were evaluated with the Unified Parkinson's Disease Rating Scale, Hoehn and Yahr staging, Schwab and England Activities of Daily Living, L-dopa equivalent dose, and Short Form-36 Health Survey before and at 3 and 6 months after surgery. Brain magnetic resonance imaging (1.5-T) was performed in 53 patients at 6 months after STN DBS. The electrode position was estimated in the fused pre- and postoperative magnetic resonance images and correlated with the surgical results.

RESULTS

As a group, the Unified Parkinson's Disease Rating Scale, Hoehn and Yahr staging, Schwab and England Activities of Daily Living, and Short Form-36 Health Survey scores improved at 3 and 6 months after STN DBS. The L-dopa equivalent dose decreased by 60% at 3 and 6 months after STN DBS. The electrode position was divided into 6 types according to its relationship to the STN and the red nucleus. Most off-medication Unified Parkinson's Disease Rating Scale motor subscale scores improved regardless of the type of electrode position. The off-medication speech subscale score improved only in the patients whose electrodes were correctly positioned in the STN bilaterally.

CONCLUSION

The electrodes accurately positioned in the STN led to improved speech after bilateral STN DBS. An effort should be made in each patient to document the electrode position to monitor surgical performance and to improve the surgical outcome after STN DBS.

Tremor entrainment by patterned low-frequency stimulation

High-frequency test stimulation for tremor suppression is a standard procedure for functional target localization during deep brain stimulation. This method does not work in cases where tremor vanishes intraoperatively, for example, due to general anaesthesia or due to an insertional effect. To overcome this difficulty, we developed a stimulation technique that effectively evokes tremor in a well-defined and quantifiable manner. For this, we used patterned low-frequency stimulation (PLFS), i.e. brief high-frequency pulse trains administered at pulse rates similar to neurons' preferred burst frequency. Unlike periodic single-pulse stimulation, PLFS enables one to convey effective and considerably greater integral charge densities without violation of safety requirements. In a computational investigation of an oscillatory neuronal network temporarily rendered inactive, we found that PLFS evokes synchronized activity, phase locked to the stimulus. While a stronger increase in the amount of synchrony in the neuronal population requires higher stimulus intensities, the portion of synchronously active neurons nevertheless becomes strongly phase locked to PLFS already at weak stimulus intensities. The phase entrainment effect of PLFS turned out to be robust against variations in the stimulation frequency, whereas enhancement of synchrony required precisely tuned stimulation frequencies. We applied PLFS to a patient with spinocerebellar ataxia type 2 (SCA2) with pronounced tremor that disappeared intraoperatively under general anaesthesia. In accordance with our computational results, PLFS evoked tremor, phase locked to the stimulus. In particular, weak PLFS caused low-amplitude, but strongly phase-locked tremor. PLFS test stimulations provided the only functional information about target localization. Optimal target point selection was confirmed by excellent post-operative tremor suppression.

Subthalamic deep brain stimulation after anesthetic inhalation in Parkinson disease: a preliminary study

OBJECT

The authors of this preliminary study investigated the outcome and feasibility of intraoperative microelectrode recording (MER) in patients with Parkinson disease (PD) undergoing deep brain stimulation of the subthalamic nucleus (STN) after anesthetic inhalation.

METHODS

The authors conducted a retrospective analysis of 10 patients with PD who received a desflurane anesthetic during bilateral STN electrode implantation. The MERs were obtained as an intraoperative guide for final electrode implantation and the data were analyzed offline. The functional target coordinates of the electrodes were compared preoperatively with estimated target coordinates.

RESULTS

Outcomes were evaluated using the Unified Parkinson's Disease Rating Scale 6 months after surgery. The mean improvement in total and motor Unified Parkinson's Disease Rating Scale scores was 54.27 +/- 17.96% and 48.85 +/- 16.97%, respectively. The mean STN neuronal firing rate was 29.7 +/- 14.6 Hz. Typical neuronal firing patterns of the STN and substantia pars nigra reticulata were observed in each patient during surgery. Comparing the functional target coordinates, the z axis coordinates were noted to be significantly different between the pre- and postoperative coordinates.

CONCLUSIONS

The authors found that MER can be adequately performed while the patient receives a desflurane anesthetic, and the results can serve as a guide for STN electrode implantation. This may be a good alternative surgical method in patients with PD who are unable to tolerate deep brain stimulation surgery with local anesthesia.

STN DBS of Advanced Parkinson's Disease Experienced in a Specialized Monitoring Unit with a Prospective Protocol

OBJECTIVE

In the evaluation of patients with Parkinson's disease (PD), most neurologists only see their patients during a limited period of their fluctuating 24-hour-a-day lives. This study aimed to assess the short-term outcome of STN stimulation for patients with advanced PD evaluated in a 24-hour monitoring unit for movement disorder (MUMD) using a prospective protocol.

METHODS

Forty-two patients with advanced PD consecutively treated with bilateral STN stimulation using multi-channel microelectrode recording were included in this study. All patients were evaluated using a 24-hour MUMD with a video recording/editing system and were evaluated with a prospective protocol of the Unified Parkinson's Disease Rating Scale, Hoehn and Yahr Staging, Schwab and England Activities of Daily Living, levodopa equivalent daily dose (LEDD), Short Form-36 Health Survey, and neuropsychological tests. Magnetic resonance (MR) images of the brain were performed prior to and six months after surgery.

RESULTS

All patients were evaluated at three and six months after surgery. There was a rapid and significant improvement of the motor symptoms, especially in tremor and rigidity, after STN stimulation with low morbidity. Dyskinesia was markedly decreased with much lowered LEDD values by 50% after STN stimulation. 1.5T MR images were safely taken according to the manufacturer's guidelines at six months after surgery without any adverse effects in 41 patients treated with STN stimulations.

CONCLUSION

Evaluations in a 24-hour monitoring unit could reduce the dose of medication efficiently to an optimal level with patients'comfort and improve the clinical symptoms in harmony with STN stimulation.

Neuronal correlates of obsessions in the caudate nucleus

BACKGROUND

Metabolic overactivity of corticosubcortical loops including the caudate nucleus (CN) has been reported in obsessive-compulsive disorder (OCD) using functional imaging techniques. However, direct proof of a modification of neuronal activity within the CN of OCD patients is still lacking. We tested the hypothesis that obsessions or compulsions might be associated with particular features of neuronal activity in the CN of OCD patients.

METHODS

Single unit recordings were performed peroperatively in the CN of three patients with severe forms of obsessive-compulsive disorder (OCD) who were candidates for deep brain stimulation of the CN. Severity of obsessions was assessed preoperatively with the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) and peroperatively with a subjective obsession score based on a visual analog scale (VAS).

RESULTS

Frequency of CN discharge and variability of interspike intervals were found to be abnormally high in two patients with a high VAS score during surgery but not in one with a low VAS score. Lateralization and depth of recording influenced neuronal activity variably among patients.

CONCLUSIONS

Because the three patients had high Y-BOCS scores before surgery, these findings suggest that caudate hyperactivity in OCD is concomitant with the occurrence of the obsession process.

Direct localization of subthalamic nucleus supplemented by single-track electrophysiological guidance in deep brain stimulation lead implantation: techniques and clinical results

BACKGROUND/AIMS

In subthalamic nucleus (STN) deep brain stimulation (DBS) lead implantation, it is still controversial whether it is more appropriate to employ indirect or direct methods in magnetic resonance imaging (MRI)-based tentative targeting and to select single- or multiple-track recording in electrophysiological definitive targeting. The efficacy of single-track electrophysiological recording through direct targeting was compared with the conventional indirect targeting methods in light of the identified STN thickness and clinical results.

METHODS

The identified mean STN thickness, pre- and 6-month postoperative Unified Parkinson's Disease Rating Scale (UPDRS), dose change of L-dopa and dopaminergic agonists were compared in indirect (midcommissural point-based, 44 procedures) and direct (image-based, 44 procedures) targeting methods.

RESULTS

The identified mean STN thickness was significantly greater in the group employing direct methods. For evaluation of the UPDRS, a significant scale improvement was noted in part 2 OFF for both groups. Significant scale improvements occurred in parts 3 and 4 in the group employing the direct method. Both groups revealed significant L-dopa dose reduction with the tendency towards a greater reduction in the group employing the direct method.

CONCLUSION

MRI-based direct targeting supplemented by single-track recording could be justified as a standard for DBS lead implantation to achieve better clinical results.

Single electrode and multiple electrode guided electrical stimulation of the subthalamic nucleus in advanced Parkinson's disease

OBJECTIVE

It is still debated to what extent intraoperative electrophysiological techniques contribute to the outcome of subthalamic nucleus (STN) deep brain stimulation (DBS). Intraoperative electrophysiological recordings for identification of the STN can be made with one electrode or with multiple, simultaneously implanted electrodes. The latter provide more detailed information about the electrophysiological boundaries of the STN; however, implantation of several electrodes at one time might increase the risk of bleeding. Here we report the results of a study of patients with advanced Parkinson's disease, in which one group of patients underwent bilateral STN DBS with electrophysiological recordings from a single electrode, and the other group received STN DBS with multiple (five or fewer) simultaneously implanted electrodes.

PATIENTS AND METHODS

Fifty-five patients suffering from advanced Parkinson's disease who underwent bilateral STN stimulation were included in this study. Thirty-two patients underwent STN DBS guided by a single semi-microelectrode, and 23 patients underwent STN DBS guided with simultaneously implanted multiple microelectrodes. All patients were examined preoperatively and 3 and 12 months postoperatively with regard to activities of daily living, motor functions, and neuropsychological functions.

RESULTS

We found that the simultaneous implantation of multiple electrodes does not increase the risk of bleeding or any other major intracranial complication. The use of multiple electrodes resulted in better motor results when compared with patients who underwent STN DBS guided with a single recording electrode. There were significantly more improvements in patients' tremor and rigidity, and as a consequence, a better total Unified Parkinson Disease Rating Scale, Part III score was identified during the medication-off phase. Despite better motor effects, patients treated with multiple electrodes showed subtle deterioration in neuropsychological functions, particularly in memory function.

CONCLUSION

STN DBS performed with multiple electrophysiological recording electrodes resulted in better motor outcome but induced specific mild declines in neuropsychological functions.

The role of glutamate transporters in neurodegenerative diseases and potential opportunities for intervention

Extracellular concentrations of the predominant excitatory neurotransmitter, glutamate, and related excitatory amino acids are maintained at relatively low levels to ensure an appropriate signal-to-noise ratio and to prevent excessive activation of glutamate receptors that can result in cell death. The latter phenomenon is known as 'excitotoxicity' and has been associated with a wide range of acute and chronic neurodegenerative disorders, as well as disorders that result in the loss of non-neural cells such as oligodendroglia in multiple sclerosis. Unfortunately clinical trials with glutamate receptor antagonists that would logically seem to prevent the effects of excessive receptor activation have been associated with untoward side effects or little clinical benefit. In the mammalian CNS, the extracellular concentrations of glutamate are controlled by two types of transporters; these include a family of Na(+)-dependent transporters and a cystine-glutamate exchange process, referred to as system X(c)(-). In this review, we will focus primarily on the Na(+)-dependent transporters. A brief introduction to glutamate as a neurotransmitter will be followed by an overview of the properties of these transporters, including a summary of the presumed physiologic mechanisms that regulate these transporters. Many studies have provided compelling evidence that impairing the function of these transporters can increase the sensitivity of tissue to deleterious effects of aberrant activation of glutamate receptors. Over the last decade, it has become clear that many neurodegenerative disorders are associated with a change in localization and/or expression of some of the subtypes of these transporters. This would suggest that therapies directed toward enhancing transporter expression might be beneficial. However, there is also evidence that glutamate transporters might increase the susceptibility of tissue to the consequences of insults that result in a collapse of the electrochemical gradients required for normal function such as stroke. In spite of the potential adverse effects of upregulation of glutamate transporters, there is recent evidence that upregulation of one of the glutamate transporters, GLT-1 (also called EAAT2), with beta-lactam antibiotics attenuates the damage observed in models of both acute and chronic neurodegenerative disorders. While it seems somewhat unlikely that antibiotics specifically target GLT-1 expression, these studies identify a potential strategy to limit excitotoxicity. If successful, this type of approach could have widespread utility given the large number of neurodegenerative diseases associated with decreases in transporter expression and excitotoxicity. However, given the massive effort directed at developing glutamate receptor agents during the 1990s and the relatively modest advances to date, one wonders if we will maintain the patience needed to carefully understand the glutamatergic system so that it will be successfully targeted in the future.

Direct validation of atlas-based red nucleus identification for functional radiosurgery

Treatment targets in functional neurosurgery usually consist of selected structures within the thalamus and basal ganglia, which can be stimulated in order to affect specific brain pathways. Chronic electrical stimulation of these structures is a widely used approach for selected patients with advanced movement disorders. An alternative therapeutic solution consists of producing a lesion in the target nucleus, for example by means of radiosurgery, a noninvasive procedure, and this prevents the use of intraoperative microelectrode recording as a method for accurate target definition. The need to have accurate noninvasive localization of the target motivated our previous work on atlas-based identification; the aim of this present work is to provide additional validation of this approach based on the identification of the red nuclei (RN), which are located near the subthalamic nucleus (STN). Coordinates of RN were obtained from the Talairach and Tournoux (TT) atlas and transformed into the coordinates of the Montreal Neurological Institute (MNI) atlas, creating a mask representation of RN. The MNI atlas volume was nonrigidly registered onto the patient magnetic resonance imaging (MRI). This deformation field was then applied to the RN mask, providing its location on the patient MRI. Because RN are easily identifiable on 1.5 T T2-MRI images, they were manually delineated; the coordinates of the centers of mass of the manually and automatically identified structures were compared. Additionally, volumetric overlapping indices were calculated. Ten patients were examined by this technique. All indices indicated a high level of agreement between manually and automatically identified structures. These results not only confirm the accuracy of the method but also allow fine tuning of the automatic identification method to be performed.

Surgical and hardware complications in subthalamic nucleus deep brain stimulation

OBJECTIVE

To assess the surgical and hardware complications in 26 consecutive patients with movement disorders undergoing subthalamic deep brain stimulation (STN-DBS) in early practice at our institute.

METHODS

The 26 patients in our institute were analyzed retrospectively. Group A included the first eight patients treated while we had no facility for microelectrode recording (MER), 16 intracranial procedures were performed and 8 batteries were implanted. Group B (with MER) included 18 patients, 35 intracranial procedures were performed and 18 batteries were implanted.

RESULTS

The intracranial morbidity was 18.75% in group A and 5.71% in group B. The extracranial morbidity was 37.5% in group A and 16.67% in group B. There was no hardware-related infection in our study. The overall mortality rate was 7.69%, and deaths were not surgical related.

CONCLUSIONS

The associated morbidity is significant in STN-DBS. The use of MER may improve the clinical outcome while decreasing the morbidity.

Impact of chronic subthalamic high-frequency stimulation on metabolic basal ganglia activity: a 2-deoxyglucose uptake and cytochrome oxidase mRNA study in a macaque model of Parkinson's disease

The mechanisms of action of high-frequency stimulation (HFS) of the subthalamic nucleus (STN) remain only partially understood. Hitherto, experimental studies have suggested that STN-HFS reduces the activity of STN neurons. However, some recent reports have challenged this view, showing that STN-HFS might also increase the activity of globus pallidus internalis (GPi) neurons that are under strong excitatory drive of the STN. In addition, most results emanate from studies applying acute STN-HFS, while parkinsonian patients receive chronic stimulation. Thus, the present study was designed to assess the effect of chronic (10 days) STN-HFS in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated nonhuman primate. For this purpose, 2-deoxyglucose (2-DG) uptake, a measure of global synaptic activity, was assessed in the basal ganglia and the motor thalamus after chronic unilateral STN-HFS. Cytochrome oxidase subunit 1 (COI) mRNA expression, a marker of efferent metabolic activity, was additionally assessed in the globus pallidus. Chronic STN-HFS (i) reversed abnormally decreased 2-DG uptake in the STN of parkinsonian nonhuman primates, (ii) reversed abnormally increased 2-DG accumulation in the GPi while COI mRNA expression was increased, suggesting global activation of GPi neurons, and (iii) reversed abnormally increased 2-DG uptake in the ventrolateral motor thalamus nucleus. The simultaneous decrease in 2-DG uptake and increase in COI mRNA expression are difficult to reconcile with the current model of basal ganglia function and suggest that the mechanisms by which STN-HFS exerts its clinical benefits are more complex than a simple reversal of abnormal activity in the STN and its targets.

Stereotactic surgery for subthalamic nucleus stimulation under general anesthesia: A retrospective evaluation of Japanese patients with Parkinson's disease

We compared retrospectively the outcome of bilateral subthalamic nucleus (STN) stimulation in 15 patients with Parkinson's disease who underwent the procedure under general anesthesia (GA) with that achieved in 10 patients under local anesthesia (LA). At 3 months postoperatively, all cardinal parkinsonian motor symptoms, evaluated on Unified Parkinson's Disease Rating Scale were significantly improved compared to preoperative baselines in both groups. The administration of GA did not adversely affect postoperative improvements in motor and daily activity scores, except for off-medication bradykinesia. Our results suggest that GA compares favorably with LA in surgical procedure for bilateral STN stimulation.

Detection of the subthalamic nucleus in microelectrographic recordings in Parkinson disease using the high-frequency (> 500 Hz) neuronal background

✓Accurate and fast localization of the subthalamic nucleus (STN) during intraoperative electrophysiological monitoring can improve the outcome of deep brain stimulation surgery. The authors show a simple method of detecting the STN that is based on an analysis of the high-frequency (> 500 Hz) background (HFB) activity of neurons. The HFB reflects multiunit spiking activity close to the recording electrode, and its characteristic profile, which is higher in the STN than in neighboring structures, and facilitates delineation of both the dorsal and ventral STN borders.

Neuronal activity of the zona incerta in Parkinson's disease patients

The objective of this study was to describe the firing characteristics of the zona incerta (ZI) in Parkinson's disease patients. The ZI constitutes a band of gray matter lying dorsal to the subthalamic nucleus, whose firing properties have not been well defined in humans yet. ZI proved to become hyperactive in 6-OHDA-lesioned rats as compared to normal rats, and regarding these noticeable changes in the discharge patterns it was suggested that ZI could be a putative target for the surgical treatment of Parkinson's disease. Twelve patients who underwent microrecording-guided subthalamic surgery consented to the study. Neurons from different tracts were classified as belonging to the ZI according to their firing features, background extracellular activity, anatomical mapping of trajectories, and atlas confirmation. Fifty-nine neurons were classified as belonging to ZI. The mean firing rate proved to be 29.5 Hz, with a broad dispersion band, even covering subthalamic nucleus (STN) frequency ranges. Pattern analysis showed heterogeneous neuronal signals ranging from tonic to burst and paused neurons. A decrease in extracellular background activity in the defined ZI was also observed. Five of the recorded neurons showed rhythmical spike trains with oscillations of 8 to 14 Hz, and two units were found to discharge trains at 4 Hz. None of the recorded ZI neurons responded to proprioceptive maneuvers. ZI presented firing activities with a broad spectrum in terms of frequency and tonicity. It is differentiated from STN recordings in Parkinson's disease patients mainly because of absent proprioceptive-related units and diminished extracellular background activity.

Subthalamic nucleus neurones in slices from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned mice show irregular, dopamine-reversible firing pattern changes, but without synchronous activity

The loss of dopamine in idiopathic or animal models of Parkinson's disease induces synchronized low-frequency oscillatory burst-firing in subthalamic nucleus neurones. We sought to establish whether these firing patterns observed in vivo were preserved in slices taken from dopamine-depleted animals, thus establishing a role for the isolated subthalamic-globus pallidus complex in generating the pathological activity. Mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) showed significant reductions of over 90% in levels of dopamine as measured in striatum by high pressure liquid chromatography. Likewise, significant reductions in tyrosine hydroxylase immunostaining within the striatum (>90%) and tyrosine hydroxylase positive cell numbers (65%) in substantia nigra were observed. Compared with slices from intact mice, neurones in slices from MPTP-lesioned mice fired significantly more slowly (mean rate of 4.2 Hz, cf. 7.2 Hz in control) and more irregularly (mean coefficient of variation of inter-spike interval of 94.4%, cf. 37.9% in control). Application of ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2-amino-5-phosphonopentanoic acid (AP5) and the GABA(A) receptor antagonist picrotoxin caused no change in firing pattern. Bath application of dopamine significantly increased cell firing rate and regularized the pattern of activity in cells from slices from both MPTP-treated and control animals. Although the absolute change was more modest in control slices, the maximum dopamine effect in the two groups was comparable. Indeed, when taking into account the basal firing rate, no differences in the sensitivity to dopamine were observed between these two cohorts. Furthermore, pairs of subthalamic nucleus cells showed no correlated activity in slices from either control (21 pairs) or MPTP-treated animals (20 pairs). These results indicate that the isolated but interconnected subthalamic-globus pallidus network is not itself sufficient to generate the aberrant firing patterns in dopamine-depleted animals. More likely, inputs from other regions, such as the cortex, are needed to generate pathological oscillatory activity.

Fear extinction in rats: Implications for human brain imaging and anxiety disorders

Fear extinction is the decrease in conditioned fear responses that normally occurs when a conditioned stimulus (CS) is repeatedly presented in the absence of the aversive unconditioned stimulus (US). Extinction does not erase the initial CS-US association, but is thought to form a new memory. After extinction training, extinction memory competes with conditioning memory for control of fear expression. Deficits in fear extinction are thought to contribute to post-traumatic stress disorder (PTSD). Herein, we review studies performed in rats showing that the medial prefrontal cortex plays a critical role in the retention and expression of extinction memory. We also review human studies indicating that prefrontal areas homologous to those critical for extinction in rats are structurally and functionally deficient in patients with PTSD. We then discuss how findings from rat studies may allow us to: (1) develop new fear extinction paradigms in humans, (2) make specific predictions as to the location of extinction-related areas in humans, and (3) improve current extinction-based behavioral therapies for anxiety disorders.

Surgery Insight: deep brain stimulation for movement disorders

Over the past two decades, deep brain stimulation (DBS) has supplanted lesioning techniques for the treatment of movement disorders, and has been shown to be safe and efficacious. The primary therapeutic indications for DBS are essential tremor, dystonia and Parkinson's disease. In the case of Parkinson's disease, DBS is effective for treating the primary symptoms--tremor, bradykinesia and rigidity--as well as the motor complications of drug treatment. Progress has been made in understanding the effects of stimulation at the neuronal level, and this knowledge should eventually improve the effectiveness of this therapy. Preliminary studies also indicate that DBS might be used to treat Tourette's syndrome, obsessive-compulsive disorder, depression and epilepsy. As we will discuss in this review, the success of DBS depends on an appropriate rationale for the procedure, and on collaborations between neurologists and neurosurgeons in defining outcomes.

A technique for minimally altering anatomically based subthalamic electrode targeting by microelectrode recording

OBJECT

Implantation of a subthalamic nucleus (STN) deep brain stimulation (DBS) electrode is increasingly recognized as an effective treatment for advanced Parkinson disease (PD). Despite widespread use of microelectrode recording (MER) to delineate the boundaries of the STN prior to stimulator implantation, it remains unclear to what extent MER improves the clinical efficacy of this procedure. In this report, the authors analyze a series of patients who were treated at one surgical center to determine to what degree final electrode placement was altered, based on readings obtained with MER, from the calculated anatomical target.

METHODS

Subthalamic DBS devices were placed bilaterally in nine patients with advanced PD. Frame-based volumetric magnetic resonance images were acquired and then transferred to a stereotactic workstation to determine the anterior and posterior commissure coordinates and plane. The initial anatomical target was 4 mm anterior, 4 mm deep, and 12 mm lateral to the midcommissural point. The MERs defined the STN boundaries along one or more parallel tracks, refining the final electrode placement by comparison of results with illustrations in a stereotactic atlas. In eight of 18 electrodes, the MER results did not prompt an alteration in the anatomically derived target. In another eight placements, MER altered the target by less than 1 mm and two of 18 electrode positions differed by less than 2 mm. The anterior-posterior difference was 0.53 +/- 0.65 mm, whereas the medial-lateral direction differed by 0.25 +/- 0.43 mm. The ventral boundary of the STN defined by MER was 2 +/- 0.72 mm below the calculated target (all values are the means +/- standard deviation). All patients attained clinical improvement, similar to previous reports.

CONCLUSIONS

In this series of patients, microelectrode mapping of the STN altered the anatomically based target only slightly. Because it is not clear whether such minor adjustments improve clinical efficacy, a prospective clinical comparison of MER-refined and anatomical targeting may be warranted.

Clinical and economic results of bilateral subthalamic nucleus stimulation in Parkinson's disease

BACKGROUND

High frequency stimulation of the subthalamic nucleus (STN) is an alternative but expensive neurosurgical treatment for parkinsonian patients with levodopa induced motor complications.

OBJECTIVE

To assess the safety, clinical effects, quality of life, and economic cost of STN stimulation.

METHODS

We conducted a prospective multicentre study in 95 consecutive Parkinson's disease (PD) patients receiving bilateral STN stimulation and assessed its effects over 12 months. A double blind randomised motor evaluation was carried out at 3 month follow up, and quality of life, self care ability, and predictive factors of outcome following surgery were assessed. The cost of PD was estimated over 6 months before and after surgery.

RESULTS

The Unified Parkinson's Disease Rating Scale (UPDRS) motor score improved by 57% (p<0.0001) and activities of daily living improved by 48% (p<0.0001) at 12 month follow up. Double blind motor scoring improved by 51% at 3 month follow up (p<0.0001). The total PD Quality of Life Questionnaire (PDQL-37) score improved by 28% (p<0.001). The better the preoperative motor score after a levodopa challenge, the better the outcome after STN stimulation. Five patients developed an intracerebral haematoma during electrode implantation with permanent after effects in two. The 6 month costs of PD decreased from 10,087 euros before surgery to 1673 euros after surgery (p<0.0001) mainly because of the decrease in medication. These savings allowed a return on the procedure investment, estimated at 36,904 euros over 2.2 years.

CONCLUSIONS

STN stimulation has good outcomes with relatively low risk and little cost burden in PD patients with levodopa induced motor complications.

Pretargeting For The Implantation Of Stimulation Electrodes Into The Subthalamic Nucleus:A Comparative Study Of Magnetic Resonance Imaging And Ventriculography

OBJECTIVE:

The optimal imaging modality for preoperative targeting of the subthalamic nucleus (STN) for high-frequency stimulation is controversially discussed. Commonly used methods were stereotactic magnetic resonance imaging (MRI), stereotactic ventriculography, and fusion between MRI and stereotactic computer tomography. All of these techniques not only have their own advantages but also specific limitations and drawbacks. The purpose of this study was to evaluate the accuracy of the preoperative MRI targeting as compared with ventriculography in terms of both the STN target as well as the internal landmarks.

METHODS:

Thirty patients with Parkinson's disease who underwent bilateral surgery for STN-high-frequency stimulation received both stereotactic ventriculography and stereotactic MRI. The theoretical target was determined by each of these two imaging modalities. The final electrode placement was performed after extensive electrophysiological evaluation using microrecording and microstimulation. The real target was assumed to be given by the electrode contact with the best clinical result assessed by the United Parkinson's Disease Rating Scale in the postoperative follow-up. In addition, the coordinates of the two landmarks, anterior commissure and posterior commissure, were determined using both imaging methods.

RESULTS:

The mean targeting error was 4.1 ± 1.7 mm (mean ± standard deviation) for MRI and 2.4 ± 1.1 mm for ventriculography (P &lt; 0.0001). The mean target mismatch between the two imaging methods was 2.9 ± 1.2 mm. The length of the anterior commissure-posterior commissure distance differed significantly (P &lt; 0.0001) between MRI (27.6 ± 1.6 mm) and ventriculography (25.0 ± 1.3 mm). The mismatch was mainly induced by an anterior diplacement of the anterior commissure by 1.9 ± 2.2 mm (P &lt; 0.0001) in MRI determination, as compared with ventriculography.

CONCLUSION:

Our findings show that the indirect targeting of the STN using coordinates based on radiological landmarks is more accurate than the direct targeting using anatomic visualization of the target structure. Regardless of the imaging procedure, electrophysiological mapping is required for optimal electrode placement, although in 20% of cases, the target determined by MRI falls out of the radius explored by electrophysiology.

Deep brain stimulation for Parkinson's disease: Surgical issues

Numerous factors need to be taken into account when implanting deep brain stimulation (DBS) systems into patients with Parkinson's disease. The surgical procedure itself can be divided into immediate preoperative, intraoperative, and immediate postoperative phases. Preoperative considerations include medication withdrawal issues, stereotactic equipment choices, imaging modalities, and targeting strategy. Intraoperative considerations focus on methods for physiological confirmation of a given target for DBS electrode deployment. Terms such as microelectrode recording, microstimulation, and macrostimulation will be defined to clarify inconsistencies in the literature. Advantages and disadvantages of each technique will be addressed. Furthermore, operative decisions such as staging, choice of electrode and implantable pulse generator, and methods of device fixation will be outlined. Postoperative issues include imaging considerations, including magnetic resonance safety, device-device interactions, and immediate surgical complications pertaining to the DBS procedure. This report outlines answers to a series of questions developed to address all aspects of the DBS surgical procedure and decision-making with a systematic overview of the literature (until mid-2004) and by the expert opinion of the authors. This is a report from the Consensus on Deep Brain Stimulation for Parkinson's Disease, a project commissioned by the Congress of Neurological Surgeons and the Movement Disorder Society. It outlines answers to a series of questions developed to address all surgical aspects of deep brain stimulation.

Electrophysiological mapping for the implantation of deep brain stimulators for Parkinson's disease and tremor

The vast majority of centers use electrophysiological mapping techniques to finalize target selection during the implantation of deep brain stimulation (DBS) leads for the treatment of Parkinson's disease and tremor. This review discusses the techniques used for physiological mapping and addresses the questions of how various mapping strategies modify target selection and outcome following subthalamic nucleus (STN), globus pallidus internus (GPi), and ventralis intermedius (Vim) deep brain stimulation. Mapping strategies vary greatly across centers, but can be broadly categorized into those that use microelectrode or semimicroelectrode techniques to optimize position prior to implantation and macrostimulation through a macroelectrode or the DBS lead, and those that rely solely on macrostimulation and its threshold for clinical effects (benefits and side effects). Microelectrode criteria for implantation into the STN or GPi include length of the nucleus recorded, presence of movement-responsive neurons, and/or distance from the borders with adjacent structures. However, the threshold for the production of clinical benefits relative to side effects is, in most centers, the final, and sometimes only, determinant of DBS electrode position. Macrostimulation techniques for mapping, the utility of microelectrode mapping is reflected in its modification of electrode position in 17% to 87% of patients undergoing STN DBS, with average target adjustments of 1 to 4 mm. Nevertheless, with the absence of class I data, and in consideration of the large number of variables that impact clinical outcome, it is not possible to conclude that one technique is superior to the other in so far as motor Unified Parkinson's Disease Rating Scale outcome is concerned. Moreover, mapping technique is only one out of many variables that determine the outcome. The increase in surgical risk of intracranial hemorrhage correlated to the number of microelectrode trajectories must be considered against the risk of suboptimal benefits related to omission of this technique.

Real-time refinement of subthalamic nucleus targeting using Bayesian decision-making on the root mean square measure

The subthalamic nucleus (STN) is a major target for treatment of advanced Parkinson's disease patients undergoing deep brain stimulation surgery. Microelectrode recording (MER) is used in many cases to identify the target nucleus. A real-time procedure for identifying the entry and exit points of the STN would improve the outcome of this targeting procedure. We used the normalized root mean square (NRMS) of a short (5 seconds) MER sampled signal and the estimated anatomical distance to target (EDT) as the basis for this procedure. Electrode tip location was defined intraoperatively by an expert neurophysiologist to be before, within, or after the STN. Data from 46 trajectories of 27 patients were used to calculate the Bayesian posterior probability of being in each of these locations, given RMS-EDT pair values. We tested our predictions on each trajectory using a bootstrapping technique, with the rest of the trajectories serving as a training set and found the error in predicting the STN entry to be (mean +/- SD) 0.18 +/- 0.84, and 0.50 +/- 0.59 mm for STN exit point, which yields a 0.30 +/- 0.28 mm deviation from the expert's target center. The simplicity and computational ease of RMS calculation, its spike sorting-independent nature and tolerance to electrode parameters of this Bayesian predictor, can lead directly to the development of a fully automated intraoperative physiological procedure for the refinement of imaging estimates of STN borders.

Anatomical alterations of the subthalamic nucleus in relation to age: a postmortem study

The subthalamic nucleus (STN) is currently the preferred target for chronic electrical high-frequency stimulation in Parkinson's disease. Anatomical determination of the exact position of the STN in the individual patient, using magnetic resonance imaging, remains cumbersome, whereas calculation of the target using a stereotactic atlas bypasses patient interindividual variations in the exact delineation of the STN. The aim of this study was to demonstrate variations in shape and position of the STN during life. In this anatomopathological study, a method was applied to localize the STN in reference to the anterior commissure-posterior commissure line (AC-PC line) in 12 postmortem brains of patients who died of non-neurological diseases. Their age varied from 29 to 84 years. Centers and borders of the STN were macroscopically measured in three spatial orthogonal planes in relation to the AC-PC line, and verified by light microscopy. The AC-PC distance remains almost constant during life (24.4 mm; SD 3.58). With increasing age, the center of the STN tends to move 3.9 mm cranially, 2.6 mm laterally, and 0.2 mm anteriorly. This last result also differs from the position mentioned in the stereotactic brain atlases. The form of the STN also changes. During life, the STN becomes wider in the mediolateral direction and smaller in the superior-inferior and anterior-posterior direction. The shape and spatial position of the STN also change during life. These changes should be taken into account during target determination in deep brain stimulation procedures in Parkinson's disease.

Subthalamic stimulation for Parkinson disease: determination of electrode location necessary for clinical efficacy

Object

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) performed using intraoperative microelectrode recording (MER) to adjust electrode placement has become a widely used treatment for patients with advanced Parkinson disease (PD). Few studies have been conducted to examine the location of implanted electrodes relative to the intended target, and even fewer have been undertaken to investigate the degree to which variations in the location of these electrodes impacts their clinical efficacy. This study was performed to examine these issues.

Methods

The authors located 52 bilaterally implanted DBS electrode tips on postoperative magnetic resonance (MR) images obtained in 26 consecutive patients. Postoperative and preoperative planning MR images were merged to determine the DBS electrode tip coordinates relative to the midcommissural point. Surgical records listed the intended target coordinates for each DBS electrode tip. Clinical outcome assessment included the Unified PD Rating Scale (UPDRS) motor score at 1 year, standardized questionnaires, and routine follow-up visits.

The mean difference between electrode tip location and intended target for all 52 electrodes was less than 2 mm in all axes. Only one electrode was farther than 3 mm from the intended target, and this was the only electrode that had to be replaced due to lack of clinical efficacy (lack of tremor suppression); its reimplantation 4 mm more medially provided excellent tremor control. High correlation coefficients indicate that the MR imaging analysis accurately determined the anatomical location of the electrode tips. Blinded videotape reviews of UPDRS motor scores comparing effects of stimulation in patients who were “on” and “off” medication identified subgroups in whom there was minimal and maximal stimulation response. Patients in these subgroups had no differences between the MR imaging–determined actual electrode tip location and its intended location. Similarly, improvements of dyskinesias and severity of symptoms encountered during the wearing-off period for the drug did not correlate with variations of electrode tip location.

Conclusions

The findings in this study lead the authors to suggest that a DBS electrode placed anywhere within a 6-mm-diameter cylinder centered at the presumed middle of the STN (based on stereotactic atlas coordinates) provides similar clinical efficacy. Future studies may be warranted to evaluate prospectively the degree to which MER modification of the anatomically and/or image-determined target improves clinical efficacy of DBS electrodes.

Usefulness of an Intraoperative Electrophysiological Navigator System for Subthalamic Nucleus Surgery in Parkinson’s Disease

OBJECTS

The characteristics and benefits are presented of an intraoperative neurophysiological navigator computerized system developed in our center (Columbus-Track 1.0) that helps the surgical team in neuronal identification and in strategy processes in subthalamic nucleus surgery for Parkinson's disease (PD).

METHODS

The navigator consists of three assembled parts: (1) neuronal identification, based on wavelet processing, filtering and gaussian characteristics of the signal; (2) track identification, based on anatomical coincidence, somatomotor response and microstimulation quotient, and (3) strategy, coordinating correction for the next track. A retrospective comparative study was performed with 15 consecutive PD patients (30 targets) operated without the system and the next 15 consecutive patients operated with the aid of the system. With the aid of the computerized navigation system, a significant reduction in the number of tracks was observed (t = -2.503, p = 0.0015), with a mean difference of 1.2 tracks per hemisphere. A non-significant reduction in the total intervention time was also observed, with a mean difference of 20 min per hemisphere (t = -1.418, p = 0.161).

CONCLUSIONS

The intraoperative computerized navigation system can aid the surgical team in better identifying the neuronal signal and in defining the optimal track to achieve the target.

Subthalamic high frequency stimulation resets subthalamic firing and reduces abnormal oscillations

High frequency stimulation (HFS) of the subthalamic nucleus (STN) is a well-established therapeutic approach for the treatment of late-stage Parkinson's disease. Although the underlying cause of this illness remains a mystery, changes in firing rate and synchronized activity in different basal ganglia nuclei have been related to its symptoms. Here we investigated the impact of STN-HFS on firing rate as well as correlated and oscillatory activity in the STN network in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned non-human primates by using simultaneous extracellular single-unit recordings. STN-HFS reduced (i) the firing rate of STN neurons, (ii) the oscillatory activity at an individual STN neuron level as well as (iii) the correlated and oscillatory activity between pairs of STN neurons, while contralateral rigidity was improved. A detailed analysis showed that the decrease of mean firing rate resulted from the resetting of firing probability to virtually zero by the stimulus pulse. Subsequently, STN neurons resumed their activity after a mean duration of 2.9 +/- 0.1 ms and their firing probability returned to baseline values approximately 7 ms after the onset of the stimulus pulse, the recovery of the firing probability being represented by a sigmoid function. Thus, the overall decrease of the mean firing rate resulted from the repetition of this dynamical process with a frequency of 130 Hz (interstimulus interval approximately 7.7 ms), allowing the neuron to fire with its baseline firing rate only for a very short period. Although the mechanisms underlying the desynchronization of neuronal activity in the STN network remain unclear, the resetting of STN neuron firing probability by the electrical stimulus would rather be expected to increase oscillatory activity at an individual neuron level as well as correlated and oscillatory activity between pairs of STN neurons. However, assuming the resetting of firing rate to be the consequence of a transient GABAergic inhibition through excitation of presynaptic GABAergic axon terminals, different recovery periods of STN neurons might delay the appearance of synchronized oscillations, particularly if they are not generated locally. In conclusion, our study provides new evidence that STN-HFS decreases oscillatory activity in the STN network. Although the exact relation between oscillatory activity and Parkinson's disease symptoms remains to be determined, the present results suggest that STN-HFS might at least partially exert its beneficial effects through the reduction of oscillatory activity in the STN network and consequently in the entire cortex-basal ganglia-cortex network.

Invasive recordings from the human brain: clinical insights and beyond

Although non-invasive methods such as functional magnetic resonance imaging, electroencephalograms and magnetoencephalograms provide most of the current data about the human brain, their resolution is insufficient to show physiological processes at the cellular level. Clinical approaches sometimes allow invasive recordings to be taken from the human brain, mainly in patients with epilepsy or with movement disorders, and such recordings can sample neural activity at spatial scales ranging from single cells to distributed cell assemblies. In addition to their clinical relevance, these recordings can provide unique insights into brain functions such as movement control, perception, memory, language and even consciousness.

Electrophysiological alterations in subthalamic neurons after unilateral dopamine depletion in the rat

The present study examined the effects of unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNc) on electrophysiological properties of subthalamic neurons (STN) in adult rats. Most neurons displayed regular spontaneous tonic firing patterns in both control and lesioned animals; however, the percentage of neurons with spontaneous burst firing at hyperpolarized membrane potentials was increased significantly in lesioned animals compared with controls (45% vs. 14% respectively). In the presence of bicuculline, a gamma-aminobutyric acid type A (GABAA) receptor antagonist, electrical stimulation of the internal capsule produced monosynaptic excitatory postsynaptic potentials (EPSPs) in almost all recorded neurons. DA (50 microM) increased the amplitude and/or duration of the EPSPs in neurons from both groups, whereas the DA D1 receptor agonist SKF 81297 (10 microM) produced a significant increase in amplitude and/or duration of EPSPs in neurons from the lesioned group only. This latter increase was blocked by pretreatment with the DA D1 antagonist SCH 23390 (10 microM). These data suggest that unilateral degeneration of DA neurons in the SNc changes firing properties and enhances electrophysiological responsiveness of STN neurons to activation of DA D1 receptors.

Surgical treatment for Parkinson's disease

Since the early 1930s, physicians have developed and refined various surgical therapies for the treatment of Parkinson's disease. In this review we examine some of the problems associated with early surgical therapies, the development of new techniques and targets, and the results of clinical trials examining the safety and efficacy of these techniques. Ablative techniques include pallidotomy, thalamotomy, and, more recently, subthalamotomy. Because of concern over the high incidence of side-effects associated with bilateral ablative procedures, alternative approaches were explored. Deep brain stimulation (DBS) was subsequently developed and successfully applied in the internal globus pallidus, subthalamic nucleus, and thalamus for the treatment of Parkinson's disease. Recent approaches include biological neurorestorative techniques--surgical therapies with transplantation, gene therapy, and growth factors are all being studied. Although a great deal of work remains to be done, advances in surgical therapies for the treatment of Parkinson's disease are moving forward at an unprecedented pace.

Most effective stimulation site in subthalamic deep brain stimulation for Parkinson's disease

The optimal stimulation site in subthalamic deep brain stimulation (STN-DBS) was evaluated by correlation of the stereotactic position of the stimulation electrode with the electrophysiologically specified dorsal STN border. In a series of 25 electrodes, best clinical results with least energy consumption were found in contacts located in the dorsolateral border zone, whereas contacts within the subthalamic white matter, e.g., zona incerta, were significantly less effective. We suggest that the dorsolateral STN border should be covered by STN-DBS.

Deep brain stimulation

During the last decade deep brain stimulation (DBS) has become a routine method for the treatment of advanced Parkinson's disease (PD), leading to striking improvements in motor function and quality of life of PD patients. It is associated with minimal morbidity. The rationale of targeting specific structures within basal ganglia such as the subthalamic nucleus (STN) or the internal segment of the globus pallidus (GPi) is strongly supported by the current knowledge of the basal ganglia pathophysiology, which is derived from extensive experimental work and which provides the theoretical basis for surgical therapy in PD. In particular, the STN has advanced to the worldwide most used target for DBS in the treatment of PD, due to the marked improvement of all cardinal symptoms of the disease. Moreover on-period dyskinesias are reduced in parallel with a marked reduction of the equivalent daily levodopa dose following STN-DBS. The success of the therapy largely depends on the selection of the appropriate candidate patients and on the precise implantation of the stimulation electrode, which necessitates careful imaging-based pre-targeting and extensive electrophysiological exploration of the target area. Despite the clinical success of the therapy, the fundamental mechanisms of high-frequency stimulation are still not fully elucidated. There is a large amount of evidence from experimental and clinical data that stimulation frequency represents a key factor with respect to clinical effect of DBS. Interestingly, high-frequency stimulation mimics the functional effects of ablation in various brain structures. The main hypotheses for the mechanism of high-frequency stimulation are: (1) depolarization blocking of neuronal transmission through inactivation of voltage dependent ion-channels, (2) jamming of information by imposing an efferent stimulation-driven high-frequency pattern, (3) synaptic inhibition by stimulation of inhibitory afferents to the target nucleus, (4) synaptic failure by stimulation-induced neurotransmitter depletion. As the hyperactivity of the STN is considered a functional hallmark of PD and as there is experimental evidence for STN-mediated glutamatergic excitotoxicity on neurons of the substantia nigra pars compacta (SNc), STN-DBS might reduce glutamatergic drive, leading to neuroprotection. Further studies will be needed to elucidate if STN-DBS indeed provides a slow-down of disease progression.

Consciousness and Neurosurgery

The neuronal basis of consciousness is the greatest challenge to the scientific worldview. Much relevant empirical work is carried out on the minimal neuronal mechanisms underlying any one specific conscious percept. Two broad approaches are popular among brain scientists: electrophysiological recordings from individual neurons in the cortex of behaving monkeys or behavior combined with functional brain imaging in humans. However, many aspects of consciousness are problematic or remain off-limits to the former approach, while the latter one lacks sufficient spatial and temporal resolution to monitor individual neurons that are key to perception, thought, memory, and action. It is here that neurosurgeons, probing the living human brain on a daily basis, can play a decisive role. This article explores the contributions of neurosurgeons to this quest and outlines some of the results that have already been achieved.

La stimulation cérébrale profonde dans la maladie de Parkinson

The present renewal of the surgical treatment of Parkinson's disease, almost abandoned for twenty Years, arises from two main reasons. The first is the better understanding of the functional organization of the basal ganglia. It was demonstrated in animal models of Parkinson's disease that the loss of dopaminergic neurons within the substantia nigra, at the origin of the striatal dopaminergic defect, induces an overactivity of the excitatory glutamatergic subthalamo-internal pallidum pathway. The decrease in this hyperactivity might lead to an improvement in the pakinsonian symptoms. The second reason is the improvement in stereotactic neurosurgery in relation with the progress in neuroimaging techniques and with intraoperative electrophysiological microrecordings and stimulations, which help determine the location of the deep brain targets. In the 1970s chronic deep brain stimulation in humans was applied to the sensory nucleus of the thalamus for the treatment of intractable pain. In 1987, Benabid and colleagues suggested high frequency stimulation of the ventral intermediate nucleus of the thalamus in order to treat drug-resistant tremors and to avoid the adverse effects of thalamotomies. How deep brain stimulation works is not well known but it has been hypothetized that it could change the neuronal activities and thus avoid disease-related abnormal neuronal discharges. Potential candidates for deep brain stimulation are selected according to exclusion and inclusion criteria. Surgery can be applied to patients in good general and mental health, neither depressive nor demented and who are severely disabled despite all available drug therapies but still responsive to levodopa. The first session of surgery consists in the location of the target by ventriculography and/or brain MRI. The electrodes are implanted during the second session. The last session consists in the implantation of the neurostimulator. The ventral intermediate nucleus of the thalamus was the first target in which chronic deep brain stimulation electrodes were implanted in order to alleviate tremor. This technique can be applied bilaterally without the adverse effects of bilateral thalamotomies. Like pallidotomy, internal globus pallidum stimulation has a dramatic beneficial effect on levodopa-induced dyskinesia but its effects on the parkinsonian triad are less constant and opposite motor effects are sometimes observed in relation with the stimulated contact. The inconstant results, perhaps related to the complexity of the structure led to the development of subthalamic nucleus stimulation. The alleviation of motor fluctuations and the improvement in all motor symptoms allows a significant decrease in levodopa daily dose and in levodopa-induced dyskinesia. Presently, deep brain stimulation is a fashionable neurosurgical technique to treat Parkinson's disease. Subthalamic nucleus stimulation seems to be the most suitable target to control the parkinsonian triad and the motor fluctuations. Because of the possible adverse effects it must be reserved for disabled parkinsonian patients. No large randomized study comparing different targets and different neurosurgical techniques has been performed yet. Such studies, including cost benefit studies would be useful to assess the respective value of these different techniques.

Microelectrode recording revealing a somatotopic body map in the subthalamic nucleus in humans with Parkinson disease

Object. The subthalamic nucleus (STN) is a key structure for motor control through the basal ganglia. The aim of this study was to show that the STN in patients with Parkinson disease (PD) has a somatotopic organization similar to that in nonhuman primates.

Methods. A functional map of the STN was obtained using electrophysiological microrecording during placement of deep brain stimulation (DBS) electrodes in patients with PD. Magnetic resonance imaging was combined with ventriculography and intraoperative x-ray film to assess the position of the electrodes and the STN units, which were activated by limb movements to map the sensorimotor region of the STN. Each activated cell was located relative to the anterior commissure—posterior commissure line. Three-dimensional coordinates of the cells were analyzed statistically to determine whether those cells activated by movements of the arm and leg were segregated spatially.

Three hundred seventy-nine microelectrode tracks were created during placement of 71 DBS electrodes in 44 consecutive patients. Somatosensory driving was found in 288 tracks. The authors identified and localized 1213 movement-related cells and recorded responses from 29 orofacial cells, 480 arm-related cells, 558 leg-related cells, and 146 cells responsive to both arm and leg movements. Leg-related cells were localized in medial (p < 0.0001) and ventral (p < 0.0004) positions and tended to be situated anteriorly (p = 0.063) relative to arm-related cells.

Conclusions. Evidence of somatotopic organization in the STN in patients with PD supports the current theory of highly segregated loops integrating cortex—basal ganglia connections. These loops are preserved in chronic degenerative diseases such as PD, but may subserve a distorted body map. This finding also supports the relevance of microelectrode mapping in the optimal placement of DBS electrodes along the subthalamic homunculus.

High-frequency stimulation of both zona incerta and subthalamic nucleus induces a similar normalization of basal ganglia metabolic activity in experimental parkinsonism

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) alleviates dramatically motor symptoms in Parkinson's disease, and recently it has been suggested that zona incerta (ZI) stimulation might be as beneficial to patients. We used in situ cytochrome oxidase (CoI) mRNA hybridization to investigate and compare the effects of HFS of the STN and the ZI on metabolic activity of the STN, globus pallidus (GP), and substantia nigra reticulata (SNr) in normal rats as well as in rats with 6-hydroxydopamine (6-OHDA) lesion, an animal model of Parkinson's disease. In normal rats, HFS of the STN, as well as of the ZI, induced a significant decrease in CoI mRNA expression within the STN and SNr but an increase within the GP. In 6-OHDA rats, HFS of the STN reversed dopamine denervation-induced changes in the expression of CoI mRNA in the STN, SNr, and GP. Similar results were obtained with HFS of the ZI except for the STN, which showed only a trend toward normalization. These data suggest that the ZI, as well as the STN, are implicated in the functional mechanism of HFS supporting the involvement of GABA transmission for the reduction of neuronal activity in the basal ganglia output structures.