The deep brain stimulation of the pedunculopontine tegmental nucleus: towards a new stereotactic neurosurgery

Locomotor pattern generation and descending control: a historical perspective

The ability to generate and control locomotor movements depends on complex interactions between many areas of the nervous system, the musculoskeletal system, and the environment. How the nervous system manages to accomplish this task has been the subject of investigation for more than a century. In vertebrates, locomotion is generated by neural networks located in the spinal cord referred to as central pattern generators. Descending inputs from the brain stem initiate, maintain, and stop locomotion as well as control speed and direction. Sensory inputs adapt locomotor programs to the environmental conditions. This review presents a comparative and historical overview of some of the neural mechanisms underlying the control of locomotion in vertebrates. We have put an emphasis on spinal mechanisms and descending control.

Developments in the mechanistic understanding and clinical application of deep brain stimulation for Parkinson's disease

INTRODUCTION. Deep brain stimulation (DBS) is a life-changing treatment for patients with Parkinson's disease (PD) and gives the unique opportunity to directly explore how basal ganglia work. Despite the rapid technological innovation of the last years, the untapped potential of DBS is still high.

AREAS COVERED. This review summarizes the developments in the mechanistic understanding of DBS and the potential clinical applications of cutting-edge technological advances. Rather than a univocal local mechanism, DBS exerts its therapeutic effects through several multimodal mechanisms and involving both local and network-wide structures, although crucial questions remain unexplained. Nonetheless, new insights in mechanistic understanding of DBS in PD have provided solid bases for advances in preoperative selection phase, prediction of motor and non-motor outcomes, leads placement and postoperative stimulation programming.

EXPERT OPINION. DBS has not only strong evidence of clinical effectiveness in PD treatment, but technological advancements are revamping its role of neuromodulation of brain circuits and key to better understanding PD pathophysiology. In the next few years, the worldwide use of new technologies in clinical practice will provide large data to elucidate their role and to expand their applications for PD patients, providing useful insights to personalize DBS treatment and follow-up.

Contribution of different somatosensory afferent input to subcortical somatosensory evoked potentials in humans

OBJECTIVES

To investigate the subcortical somatosensory evoked potentials (SEPs) to electrical stimulation of either muscle or cutaneous afferents.

METHODS

SEPs were recorded in 6 patients suffering from Parkinson's disease (PD) who underwent electrode implantation in the pedunculopontine (PPTg) nucleus area. We compared SEPs recorded from the scalp and from the intracranial electrode contacts to electrical stimuli applied to: 1) median nerve at the wrist, 2) abductor pollicis brevis motor point, and 3) distal phalanx of the thumb. Also the high-frequency oscillations (HFOs) were analysed.

RESULTS

After median nerve and pure cutaneous (distant phalanx of the thumb) stimulation, a P1-N1 complex was recorded by the intracranial lead, while the scalp electrodes recorded the short-latency far-field responses (P14 and N18). On the contrary, motor point stimulation did not evoke any low-frequency component in the PPTg traces, nor the N18 potential on the scalp. HFOs were recorded to stimulation of all modalities by the PPTg electrode contacts.

CONCLUSIONS

Stimulus processing within the cuneate nucleus depends on modality, since only the cutaneous input activates the complex intranuclear network possibly generating the scalp N18 potential.

SIGNIFICANCE

Our results shed light on the subcortical processing of the somatosensory input of different modalities.

Deep Brain Stimulation for Parkinson's Disease: Currents Status and Emerging Concepts

The clinical application of DBS has become manifold and there has been a tremendous growth in DBS technology in the last few decades making it safer and user friendly. The earlier concept of its delayed application in motor fluctuations of Parkinson's disease has been replaced by Class-I evidence of EARLY-STIM trial in 2013, leading to its FDA approval to be used in early-stage despite criticism. Various studies have provided evidence of beneficial effects of bilateral STN-DBS on both motor as well as nonmotor symptoms and different new targets such as the pedunculopontine nucleus, posterior subthalamic area or caudal zona incerta, centromedian-parafascicular complex, and substantia nigra pars reticulata have now become a new area of interest in addition to the subthalamic nucleus and globus pallidus internus for the alleviation of both motor and nonmotor symptoms of Parkinson's disease. New data has confirmed that the DBS is clinically as effective and safe in elderly patients as it is in younger ones. Technological advances like current steering, directional leads, and closed-loop DBS are directed towards reducing the stimulation-induced adverse effects and preservation of the battery life for a longer period. Results of the long-term efficacy of DBS on Parkinson's disease are now available. These have shown that as the motor benefit continues, the clinical progression of Parkinson's disease also continues. We plan to discuss all these in this paper.

Tractography patterns of pedunculopontine nucleus deep brain stimulation

Deep brain stimulation of the pedunculopontine nucleus is a promising surgical procedure for the treatment of Parkinsonian gait and balance dysfunction. It has, however, produced mixed clinical results that are poorly understood. We used tractography with the aim to rationalise this heterogeneity. A cohort of eight patients with postural instability and gait disturbance (Parkinson’s disease subtype) underwent pre-operative structural and diffusion MRI, then progressed to deep brain stimulation targeting the pedunculopontine nucleus. Pre-operative and follow-up assessments were carried out using the Gait and Falls Questionnaire, and Freezing of Gait Questionnaire. Probabilistic diffusion tensor tractography was carried out between the stimulating electrodes and both cortical and cerebellar regions of a priori interest. Cortical surface reconstructions were carried out to measure cortical thickness in relevant areas. Structural connectivity between stimulating electrode and precentral gyrus (r = 0.81, p = 0.01), Brodmann areas 1 (r = 0.78, p = 0.02) and 2 (r = 0.76, p = 0.03) were correlated with clinical improvement. A negative correlation was also observed for the superior cerebellar peduncle (r = −0.76, p = 0.03). Lower cortical thickness of the left parietal lobe and bilateral premotor cortices were associated with greater pre-operative severity of symptoms. Both motor and sensory structural connectivity of the stimulated surgical target characterises the clinical benefit, or lack thereof, from surgery. In what is a challenging region of brainstem to effectively target, these results provide insights into how this can be better achieved. The mechanisms of action are likely to have both motor and sensory components, commensurate with the probable nature of the underlying dysfunction.

Frameless Stereotaxis for Subthalamic Nucleus Deep Brain Stimulation: An Innovative Method for the Direct Visualization of Electrode Implantation by Intraoperative X-ray Control

The recent introduction of frameless devices has enabled stereotactic neurosurgery to reach a level of accuracy that is comparable to traditional frame-based methodologies. Among frameless devices, the Nexframe appears to be very useful in implanting electrodes into the subthalamic nucleus or other structures for deep brain stimulation in Parkinson’s disease. However, frameless devices, including the Nexframe, limit the possibility of intraoperative visual control of the placement of electrodes in the brain. Utilizing intraoperative O-arm Computed tomography (CT) scan or high-field Magnetic Resonance Imaging (MRI) could overcome this limitation, but their high cost restricts their use. Thus, in this paper we propose an innovation in Nexframe surgical planning that allows the intraoperative use of a C-arm X-ray apparatus to establish: (1) the progression of the electrode guide tube and the electrode in the brain; (2) the accuracy of the electrode trajectory; and (3) the correct attainment of the target. The proposed frameless technique using the Nexframe has been developed and successfully applied in our practice. It was shown to be helpful in overcoming the major issues that are usually encountered when electrodes are placed in the brain with frameless neurosurgery and reduced the risk of having to re-operate on patients to reposition the electrodes.

Deep Brain Stimulation in Parkinson's Disease: New and Emerging Targets for Refractory Motor and Nonmotor Symptoms

Parkinson's disease (PD) is a progressive neurodegenerative condition characterized by bradykinesia, tremor, rigidity, and postural instability (PI), in addition to numerous nonmotor manifestations. Many pharmacological therapies now exist to successfully treat PD motor symptoms; however, as the disease progresses, it often becomes challenging to treat with medications alone. Deep brain stimulation (DBS) has become a crucial player in PD treatment, particularly for patients who have disabling motor complications from medical treatment. Well-established DBS targets include the subthalamic nucleus (STN), the globus pallidus pars interna (GPi), and to a lesser degree the ventral intermediate nucleus (VIM) of the thalamus. Studies of alternative DBS targets for PD are ongoing, the majority of which have shown some clinical benefit; however, more carefully designed and controlled studies are needed. In the present review, we discuss the role of these new and emerging DBS targets in treating refractory axial motor symptoms and other motor and nonmotor symptoms (NMS).

Efficacy and Safety of Pedunculopontine Nuclei (PPN) Deep Brain Stimulation in the Treatment of Gait Disorders: A Meta-Analysis of Clinical Studies

BACKGROUND

Pedunculopontine nucleus (PPN) has complex reciprocal connections with basal ganglia, especially with internal globus pallidus and substantia nigra, and it has been postulated that PPN stimulation may improve gait instability and freezing of gait. In this meta-analysis, we will assess the evidence for PPN deep brain stimulation in treatment of gait and motor abnormalities especially focusing on Parkinson disease patients.

METHODS

PubMed and Scopus electronic databases were searched for related studies published before February 2014. Medline (1966-2014), Embase (1974-2010), CINAHL, Web of Science, Scopus bibliographic, and Google Scholar databases (1960-2014) were also searched for studies investigating effect of PPN deep brain stimulation in treatment of postural and postural instability and total of ten studies met the inclusion criteria for this analysis.

RESULTS

Our findings showed a significant improvement in postural instability (p<0.001) and motor symptoms of Parkinson disease on and off medications (p<0.05), but failed to show improvement in freezing of gait.

CONCLUSIONS

Despite significant improvement in postural instability observed in included studies, evidence from current literature is not sufficient to generalize these findings to the majority of patients.

Pedunculopontine Nucleus Region Deep Brain Stimulation in Parkinson Disease: Surgical Techniques, Side Effects, and Postoperative Imaging

The pedunculopontine nucleus (PPN) region has received considerable attention in clinical studies as a target for deep brain stimulation (DBS) in Parkinson disease. These studies have yielded variable results with an overall impression of improvement in falls and freezing in many but not all patients treated. We evaluated the available data on the surgical anatomy and terminology of the PPN region in a companion paper. Here we focus on issues concerning surgical technique, imaging, and early side effects of surgery. The aim of this paper was to gain more insight into the reasoning for choosing specific techniques and to discuss shortcomings of available studies. Our data demonstrate the wide range in almost all fields which were investigated. There are a number of important challenges to be resolved, such as identification of the optimal target, the choice of the surgical approach to optimize electrode placement, the impact on the outcome of specific surgical techniques, the reliability of intraoperative confirmation of the target, and methodological differences in postoperative validation of the electrode position. There is considerable variability both within and across groups, the overall experience with PPN DBS is still limited, and there is a lack of controlled trials. Despite these challenges, the procedure seems to provide benefit to selected patients and appears to be relatively safe. One important limitation in comparing studies from different centers and analyzing outcomes is the great variability in targeting and surgical techniques, as shown in our paper. The challenges we identified will be of relevance when designing future studies to better address several controversial issues. We hope that the data we accumulated may facilitate the development of surgical protocols for PPN DBS.

Auditory and Lower Limb Tactile Prepulse Inhibition in Primary Restless Legs Syndrome: Clues to Its Pathophysiology

The resting sensory discomfort transiently relieved upon movement of the affected area in restless legs syndrome suggests that sensorimotor integration mechanisms, specifically gating, may be altered in the disease. The authors sought to determine the effects of prepulse auditory and tactile stimulation applied to lower limbs on the blink reflex of patients with restless legs syndrome and healthy subjects. Seventeen patients with restless legs syndrome and 17 age- and sex-matched healthy controls were investigated. Auditory stimuli and tactile lower limb stimulation were applied as prepulses. The R2 response of the blink reflex induced by electrical stimulation applied to the right supraorbital nerve was selected as the test stimulus. Time intervals between prepulses and response-eliciting stimuli were 40, 70, 90, 110, and 200 milliseconds. There were no differences in either the auditory or tactile prepulse conditions between patients and controls and no differences between these measures within subject groups. We concluded that the tactile lower limb and the auditory prepulse effects on the brainstem interneurons mediating the blink reflex share common neural pathways. Because forebrain interneurons mediate these prepulse effects, they are likely not involved in the disordered sensorimotor interaction of restless legs syndrome.

Merging DBS with viral vector or stem cell implantation: "hybrid" stereotactic surgery as an evolution in the surgical treatment of Parkinson's disease

Parkinson’s disease (PD) is a complex neurodegenerative disorder that is currently managed using a broad array of symptom-based strategies. However, targeting its molecular origins represents the potential to discover disease-modifying therapies. Deep brain stimulation (DBS), a highly successful treatment modality for PD symptoms, addresses errant electrophysiological signaling pathways in the basal ganglia. In contrast, ongoing clinical trials testing gene and cell replacement therapies propose to protect or restore neuronal-based physiologic dopamine transmission in the striatum. Given promising new platforms to enhance target localization—such as interventional MRI-guided stereotaxy—the opportunity now exists to create hybrid therapies that combine DBS with gene therapy and/or cell implantation. In this mini-review, we discuss approaches used for central nervous system biologic delivery in PD patients in previous trials and propose a new set of strategies based on novel molecular targets. A multifaceted approach, if successful, may not only contribute to our understanding of PD pathology but could introduce a new era of disease modification.

Low-frequency stimulation of the pedunculopontine nucleus affects gait and the neurotransmitter level in the ventrolateral thalamic nucleus in 6-OHDA Parkinsonian rats

The pedunculopontine nucleus (PPN) is connected to spinal, cerebellar and cerebral motor control structures and can be activated with external electrodes. Intrinsic cholinergic neuronal degeneration in the PPN is associated with postural instabilities and gait disturbances (PIGD) in advanced Parkinson's disease (PD). Clinical studies have demonstrated that PPN stimulation may improve PIGD. We investigated this claim and the underlying mechanisms using the 6-hydroxydopamine (6-OHDA) hemilesion model of PD. In this study, gait-related parameters, including the base of support (BOS), stride length, and maximum contact area, were analyzed via CatWalk gait analysis following PPN-low frequency stimulation (LFS) of rats with unilateral 6-OHDA lesions. Additionally, neurotransmitter concentrations in the ventrolateral thalamic nucleus (VL) were measured by microdialysis and liquid chromatography-mass spectrometry (LC-MS). Our data revealed that unilateral 6-OHDA lesions of the medial forebrain bundle (MFB) induced significant gait deficits. PPN-LFS significantly improved the BOS (hindlimb) and maximum contact area (impaired forelimb) scores, whereas no other gait parameters were significantly affected. Unilateral 6-OHDA MFB lesions significantly decreased acetylcholine (ACh) and moderately decreased noradrenaline (NA) concentrations in the VL. PPN-LFS mildly reversed the ACh loss in the VL in the lesioned rats but did not alter the NA levels. Taken together, our data indicate that PPN-LFS is useful for treating gait deficits of PD and that these effects are probably mediated by a rebalancing of ACh levels in the PPN-VL pathway. Thus, our findings provide possible insight into the mechanisms underlying PIGD in PD.

Axial disability and deep brain stimulation in patients with Parkinson disease

Axial motor signs-including gait impairment, postural instability and postural abnormalities-are common and debilitating symptoms in patients with advanced Parkinson disease. Dopamine replacement therapy and physiotherapy provide, at best, partial relief from axial motor symptoms. In carefully selected candidates, deep brain stimulation (DBS) of the subthalamic nucleus or globus pallidus internus is an established treatment for 'appendicular' motor signs (limb tremor, bradykinesia and rigidity). However, the effects of DBS on axial signs are much less clear, presumably because motor control of axial and appendicular functions is mediated by different anatomical-functional pathways. Here, we discuss the successes and failures of DBS in managing axial motor signs. We systematically address a series of common clinical questions associated with the preoperative phase, during which patients presenting with prominent axial signs are considered for DBS implantation surgery, and the postoperative phase, in particular, the management of axial motor signs that newly develop as postoperative complications, either acutely or with a delay. We also address the possible merits of new targets-including the pedunculopontine nucleus area, zona incerta and substantia nigra pars reticulata-to specifically alleviate axial symptoms. Supported by a rapidly growing body of evidence, this practically oriented Review aims to support decision-making in the management of axial symptoms.

Axial disability and deep brain stimulation in patients with Parkinson disease

Axial motor signs-including gait impairment, postural instability and postural abnormalities-are common and debilitating symptoms in patients with advanced Parkinson disease. Dopamine replacement therapy and physiotherapy provide, at best, partial relief from axial motor symptoms. In carefully selected candidates, deep brain stimulation (DBS) of the subthalamic nucleus or globus pallidus internus is an established treatment for 'appendicular' motor signs (limb tremor, bradykinesia and rigidity). However, the effects of DBS on axial signs are much less clear, presumably because motor control of axial and appendicular functions is mediated by different anatomical-functional pathways. Here, we discuss the successes and failures of DBS in managing axial motor signs. We systematically address a series of common clinical questions associated with the preoperative phase, during which patients presenting with prominent axial signs are considered for DBS implantation surgery, and the postoperative phase, in particular, the management of axial motor signs that newly develop as postoperative complications, either acutely or with a delay. We also address the possible merits of new targets-including the pedunculopontine nucleus area, zona incerta and substantia nigra pars reticulata-to specifically alleviate axial symptoms. Supported by a rapidly growing body of evidence, this practically oriented Review aims to support decision-making in the management of axial symptoms.

Long-term follow-up of deep brain stimulation of peduncolopontine nucleus in progressive supranuclear palsy: Report of three cases

Background:

Progressive supranuclear palsy (PSP) is a neurodegenerative disease due to mitochondrial dysfunction. The PSP syndrome presents generally with gait disorder, Parkinsonism, ophthalmoparesis and cognitive alteration. Few reports exist on deep brain stimulation (DBS) in patients with atypical Parkinsonism. The aim of our study was to evaluate further the potential role of DBS in PSP.

Case Description:

We report three patients with PSP with long-term follow up undergoing DBS. Two patients had right peripedunculopontine nucleus (PPN) stimulation and one patient had simultaneous right PPN and bilateral globus pallidus internus DBS. DBS of the PPN alone or combined with globus pallidus internus (GPi) determined an improvement in gait and a reduction in falls sustained over time. Combined target stimulation (GPi-PPN) was correlated with better clinical outcome than single target (PPN) DBS for PSP.

Conclusions:

Although few data on DBS for PSP exist, reported clinical results are encouraging. DBS might be considered as an alternative therapeutic option for patients with PSP presenting with relevant gait imbalance and frequent falls, who fail to respond to pharmacological treatment. Larger cohorts with longer follow-ups are needed to evaluate more exhaustively the efficacy of DBS in PSP.

Clinical implications of local field potentials for understanding and treating movement disorders

BACKGROUND

Deep brain stimulation (DBS) for the treatment of movement disorders has provided researchers with an opportunity to record electrical oscillatory activity from electrodes implanted in deep brain structures. Extracellular activity recorded from a population of neurons, termed local field potentials (LFPs), has shed light on the pathophysiology of movement disorders and holds the potential to lead to refinement in existing treatments.

OBJECTIVE

This paper reviews the clinical significance of LFPs recorded from macroelectrodes implanted in basal ganglia and thalamic targets for the treatment of Parkinson's disease, essential tremor and dystonia.

METHODS

Neural population dynamics and subthreshold events, which are undetectable by single-unit recordings, can be examined with frequency band analysis of LFPs (frequency range: 1-250 Hz).

RESULTS

Of clinical relevance, reliable correlations between motor symptoms and components of the LFP power spectrum suggest that LFPs may serve as a biomarker for movement disorders. In particular, Parkinson's rigidity has been shown to correlate with the power of beta oscillations (13-30 Hz), and essential tremor coheres with oscillations of 8-27 Hz. Furthermore, evidence indicates that the optimal contacts for DBS programming can be predicted from the anatomic location of beta and gamma bands (48-200 Hz).

CONCLUSION

LFP analysis has implications for improved electrode targeting and the development of a real-time, individualized, 'closed-loop' stimulation system.

Future of brain stimulation: new targets, new indications, new technology

In the last quarter of a century, DBS has become an established neurosurgical treatment for Parkinson's disease (PD), dystonia, and tremors. Improved understanding of brain circuitries and their involvement in various neurological and psychiatric illnesses, coupled with the safety of DBS and its exquisite role as a tool for ethical study of the human brain, have unlocked new opportunities for this technology, both for future therapies and in research. Serendipitous discoveries and advances in structural and functional imaging are providing abundant "new" brain targets for an ever-increasing number of pathologies, leading to investigations of DBS in diverse neurological, psychiatric, behavioral, and cognitive conditions. Trials and "proof of concept" studies of DBS are underway in pain, epilepsy, tinnitus, OCD, depression, and Gilles de la Tourette syndrome, as well as in eating disorders, addiction, cognitive decline, consciousness, and autonomic states. In parallel, ongoing technological development will provide pulse generators with longer battery longevity, segmental electrode designs allowing a current steering, and the possibility to deliver "on-demand" stimulation based on closed-loop concepts. The future of brain stimulation is certainly promising, especially for movement disorders-that will remain the main indication for DBS for the foreseeable future-and probably for some psychiatric disorders. However, brain stimulation as a technique may be at risk of gliding down a slippery slope: Some reports indicate a disturbing trend with suggestions that future DBS may be proposed for enhancement of memory in healthy people, or as a tool for "treatment" of "antisocial behavior" and for improving "morality."

The Clinical Effects of Deep Brain Stimulation of the Pedunculopontine Tegmental Nucleus in Movement Disorders May Not Be Related to the Anatomical Target, Leads Location, and Setup of Electrical Stimulation

BACKGROUND:

The pedunculopontine tegmental nucleus (PPTg) is a novel target for deep brain stimulation (DBS) in movement disorders.

OBJECTIVE:

To clarify the relationships between the individual anatomic variations of the brainstem, the site in which the PPTg DBS is applied, and the clinical outcome in a relatively large number of patients affected by Parkinson disease or progressive supranuclear palsy.

METHODS:

Magnetic resonance images have been used to evaluate brainstem anatomy and the relationships between lead position and specific brainstem landmarks. All data were matched on atlas representations of the PPTg and were correlated with Unified Parkinson Disease Rating Scale III (UPDRS III), subitems 27 to 30 of UPDRS III and the Hoehn and Yahr evaluations.

RESULTS:

A high variance of brainstem parameters was evident, affecting the relationships between the position of the nucleus and lead contacts. According to the contacts giving the best clinical outcome, patients could be distinguished between those who required the use of 2 adjacent contacts and those who required stimulation through 2 nonadjacent contacts. Furthermore, in the former group the target coordinates were more lateral and deeper compared with the latter group.

CONCLUSION:

Individual PPTg-DBS planning is required to overcome the inconsistencies linked to the high variability in the brainstem anatomy of patients. The lack of correlations between lead position, contact setup, and clinical outcome indicate that the benefits of PPTg DBS may not be strictly linked to the site of stimulation within the PPTg area, and may not depend upon the neurons still surviving in this region in Parkinson disease or progressive supranuclear palsy.

[Deep brain stimulation for movement disorders]

In the last 25 years deep brain stimulation (DBS) has increased the therapeutic options as well as the pathophysiological understanding of movement disorders (MDS) to an unforeseen extent. This paper covers the state of the art of DBS treatment of Parkinson's disease, tremors, dystonia and other rare forms of MDS and gives an short overview of the mechanisms of action of DBS.

Targeting the Pedunculopontine Nucleus

BACKGROUND:

Pedunculopontine tegmental nucleus (PPTg) deep brain stimulation (DBS) has been used in patients with Parkinson disease.

OBJECTIVE:

To verify the position of the DBS lead within the pons during PPTg targeting.

METHODS:

In 10 Parkinson disease patients undergoing electrode implantation in the PPTg, somatosensory evoked potentials were recorded after median nerve stimulation from the 4 DBS electrode contacts and from 2 scalp leads placed in the frontal and parietal regions.

RESULTS:

The DBS electrode recorded a P16 potential (latency at contact 0, 16.33 ± 0.76 ms). There was a P16 latency shift of 0.18 ± 0.07 ms from contact 0 (lower) to contact 3 (upper). The scalp electrodes recorded the P14 far-field response (latency, 15.44 ± 0.63 ms) and the cortical N20 potential (latency, 21.58 ± 1.42 ms). The P16 potentials recorded by the intracranial electrode contacts are generated by the volley traveling along the medial lemniscus, whereas the scalp P14 potential represents a far-field response generated at the Obex level. Considering that the distance between the electrode contacts 0 and 3 is 6 mm, the distance of the electrode contact 0 from the Obex (ΔObex) was calculated by the equation: ΔObex = 6 × Δlatency P14- PPTg0/Δlatency PPTg0-PPTg3. The Obex-to-brainstem electrode distance obtained by the neurophysiological method confirmed that the electrode was located within the pons in all patients. Moreover, this distance was very similar to that issued from the individual brain magnetic resonance imaging.

CONCLUSION:

Somatosensory evoked potentials may be a helpful tool for calculating the macroelectrode position within the pons during PPTg targeting.