Electrophysiology of Sensory and Motor Nerve Root Fibers in Selective Dorsal Rhizotomies.

AIMS: Spasticity remains a major impediment in the treatment of cerebral palsy (CP). The single-level selective dorsal rhizotomy (SDR) is a minimally invasive intervention that reduces spasticity in select patients. We provide a descriptive set of normative data that practitioners can utilize to help guide the single-level SDR procedure, including (1) physiological threshold values used to dissociate ventral from dorsal roots; (2) response characteristics of muscles; (3) descriptions of abnormal physiological responses; and (4) percentage of rootlets transected during surgery. METHODS: We examined data from 38 patients with CP who underwent SDR. Dorsal and ventral roots were classified based on the amplitude of electromyographic (EMG) responses, number of muscles activated, and abnormal response characteristics. RESULTS: Ventral roots activated more muscles at significantly lower stimulus thresholds and demonstrated larger EMG responses than did dorsal roots. Of the transections made, 64.72 ± 1.69% of each rootlet was transected. Ventral and dorsal roots can be readily separated based on a few key physiological characteristics including response thresholds and the spread of muscle activation. It was observed that a threshold of approximately 0.4 mA could be used to dissociate ventral and dorsal roots during surgery. CONCLUSIONS: These data illustrate the range of physiological variance observed while performing SDR in patients with spastic CP. Notably, we encountered outlier patients whose roots demonstrated aberrant response characteristics and displayed uncharacteristically low dorsal root thresholds or abnormally high ventral root thresholds. Practitioners should be prepared to individualize their threshold criteria and customize treatment on a patient-by-patient basis.

Dexmedetomidine Does Not Affect Evoked Potentials During Spine Surgery.

BACKGROUND: The effect of dexmedetomidine on evoked potentials (EPs) has not been elucidated. We aimed to investigate the effect of dexmedetomidine on somatosensory, motor, and visual EPs. METHODS: After IRB approval, 40 adult patients scheduled for elective spine surgery using total IV anesthesia with propofol and remifentanil were randomly assigned to receive either dexmedetomidine (n = 20) or placebo (n = 20) in a double-blind, placebo-controlled trial. After obtaining informed consent, positioning, and baseline EPs recording, patients were randomly assigned to either IV dexmedetomidine 0.6 µg/kg infused over 10 minutes, followed by 0.6 µg/kg/h, or a corresponding volume of IV normal saline (placebo). EP measures at 60 ± 30 minutes after initiation of study drug were defined as T1 and at 150 ± 30 minutes were defined as T2. Changes from baseline to T1 (primary end point) and from baseline to T2 (secondary end point) in EP latencies (milliseconds) and amplitudes (microvolts) were compared between groups. Data presented as mean ± SD (95% confidence interval). RESULTS: Data from 40 patients (dexmedetomidine: n = 20; age, 54 ± 3 years; 10 males; placebo: n = 20; age, 52 ± 2 years; 5 males) were analyzed. There was no difference between dexmedetomidine versus placebo groups in primary end points: change of somatosensory EPs at T1, latency: 0.01 ± 1.3 (-0.64, 0.65) vs 0.01 ± 1.3 (-0.64, 0.65), P = 0.43 (-1.24, 0.45); amplitude: 0.03 ± 0.14 (-0.06, 0.02) vs -0.01 ± 0.13 (-0.07, 0.05), P = 0.76 (-0.074, 0.1); motor EPs amplitude at T1: 65.1 ± 194.8 (-35, 165; n = 18) vs 109.2 ± 241.4 (-24, 243; n = 16), P = 0.57 (-113.5, 241.57); visual EPs at T1 (right eye), amplitude: 2.3 ± 3.6 (-0.4, 5.1; n = 11) vs 0.3 ± 6.0 (-3.3, 3.9; n = 16), P = 0.38 (-6.7, 2.6); latency N1: 2.3 ± 3.6 (-0.4, 5.1) vs 0.3 ± 6.0 (-3.3, 3.9), P = 0.38 (-6.7, 2.6); latency P1: -1.6 ± 13.4 (-11.9, 8.7) vs -1.4 ± 8.1 (-6.3, 3.5), P = 0.97 (-9.3, 9.7) or secondary end points. There were no differences between right and left visual EPs either at T1 or at T2. CONCLUSIONS: In clinically relevant doses, dexmedetomidine as an adjunct to total IV anesthesia does not seem to alter EPs and therefore can be safely used during surgeries requiring monitoring of EPs.

A comparison of the effects of desflurane versus propofol on transcranial motor-evoked potentials in pediatric patients.

PURPOSE: The aim was to compare the effects of propofol and desflurane anesthesia on transcranial motor evoked potentials (MEPs) from pediatric patients undergoing surgery for spinal deformities. METHODS: Desflurane and propofol cohorts (25 patients each) were obtained retrospectively and matched for patient characteristics and surgical approach. MEPs from the thenar eminence and abductor hallucis were compared during maintenance anesthesia on desflurane (0.6-0.8 MAC) or propofol infusion (150-300 µg/kg/min). MEP amplitudes and durations were obtained for successive 30-min intervals for 150 min, beginning 60 min after maintenance anesthesia. RESULTS: Mean peak to peak amplitudes of MEPs under desflurane anesthesia from the thenar eminence (419 µV) and abductor hallucis (386 µv) were not significantly different from those under propofol (608 µV, 343 µV, thenar, and abductor hallucis, respectively). Stimulation was greater by 42 V and 136 mA, and trains were slightly longer in the desflurane compared to the propofol group (p < 0.05). Most MEP amplitudes for the desflurane and propofol cohorts remained the same or increased (71 % of cases) when those after 150 min were compared to those in the first 30-min interval. CONCLUSIONS: MEPs with good amplitudes were obtained under desflurane only anesthesia that were comparable to propofol only anesthesia in pediatric patients during surgery for spinal deformities. There was no evidence for anesthetic fade over the time period examined. When used by itself, desflurane can be considered a viable alternative to propofol anesthesia.

Human motor evoked potential responses following spinal cord transection: an in vivo study.

Motor evoked potential (MEP) monitoring has been used increasingly in conjunction with somatosensory evoked potential monitoring to monitor neurological changes during complex spinal operations. No published report has demonstrated the effects of segmental spinal cord transection on MEP monitoring. The authors describe the case of an 11-year-old girl with lumbar myelomeningocele and worsening thoracolumbar scoliosis who underwent a T11-L5 fusion and spinal transection to prevent tethering. Intraoperative MEP and somatosensory evoked potential monitoring were performed, and the spinal cord was transected in 4 quadrants. The MEPs were lost unilaterally as each anterior quadrant was sectioned. This is the first reported case that demonstrates the link between spinal cord transection and MEP signaling characteristics. Furthermore, it demonstrates the relatively minor input of the ipsilateral ventral corticospinal tract in MEP physiology at the thoracolumbar junction. Finally, this study further supports the use of MEPs as a specific intraoperative neuromonitoring tool.

Assessing the effects of subthalamic nucleus stimulation on gait and mobility in people with Parkinson disease.

PURPOSE: To determine the effects of unilateral and bilateral subthalamic nucleus (STN) stimulation on gait and mobility in persons with Parkinson disease (PD). METHOD: We examined eight individuals with advanced PD who underwent staged stimulator implantation surgeries. Gait and mobility were assessed in the medication-on state with a variety of clinical and laboratory measures (Unified Parkinson Disease Rating Scale items, Timed Up and Go Test, gait speed) at three time points: prior to surgery, after the first surgery (unilateral stimulation) and after the second surgery (bilateral stimulation). RESULTS: Despite overall improvements in motor function and reduction of dyskinesia, there were no significant group effects of unilateral or bilateral stimulation on gait and mobility compared to pre-surgical function. However, there were clinically meaningful changes, both improvements and declines, at the individual level. CONCLUSIONS: Because of the consequences of gait deficits and mobility limitations for people with PD, future research should examine the effects of STN stimulation on gait in the medication-on state using sensitive and specific measures such as gait speed. Accurate assessment of gait changes is necessary to improve the evaluation of STN effects and the prediction of individuals in need of rehabilitation services to manage gait and mobility deficits.

Effects of anesthetic regimens and other confounding factors affecting the interpretation of motor evoked potentials during pediatric spine surgery.

Children undergoing corrective spine surgery are at risk of serious neurologic injury. Monitoring transcranial electric motor evoked potentials (TCeMEPs) during these procedures may identify and help prevent injury to motor pathways. The difficulty in obtaining consistent motor evoked potential (MEP) responses during pediatric spine surgery can result in part to the suppression of evoked responses caused by volatile inhalational anesthetics, elevated levels of propofol, and/or physiologic variables. Data obtained from 140 pediatric patients who underwent spine surgery with MEP monitoring were retrospectively analyzed and evaluated for age and anesthetic effects on stimulation variables. MEPs acquired under inhalational anesthetic agents required greater stimulation compared to intravenous propofol anesthesia. Additionally, the responses were more variable when inhalational agents were used. These effects were more prominent in younger age patients. The number of alerts of MEP loss or reduction related to anesthetic levels or blood pressure changes was higher under inhalational agents.

Facial Nerve Branching Patterns Vary With Vascular Anomalies.

OBJECTIVES: At our institution, in vivo facial nerve mapping (FNM) is used during vascular anomaly (VAN) surgeries involving the facial nerve (FN) to create an FN map and prevent injury. During mapping, FN anatomy seemed to vary with VAN type. This study aimed to characterize FN branching patterns compared to published FN anatomy and VAN type. STUDY DESIGN: Retrospective study of surgically relevant facial nerve anatomy. METHODS: VAN patients (n = 67) with FN mapping between 2005 and 2018 were identified. Results included VAN type, FN relationship to VAN, FNM image with branch pattern, and surgical approach. A Fisher exact test compared FN relationships and surgical approach between VAN pathology, and FN branching types to published anatomical studies. MATLAB quantified FN branching with Euclidean distances and angles. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) analyzed quantitative FN patterns amongst VAN types. RESULTS: VANs included were hemangioma, venous malformation, lymphatic malformation, and arteriovenous malformation (n = 17, 13, 25, and 3, respectively). VAN FN patterns differed from described FN anatomy (P < .001). PCA and HCA in MATLAB-quantified FN branching demonstrated no patterns associated with VAN pathology (P = .80 and P = .91, one-way analysis of variance for principle component 1 (PC1) and priniciple component 2 (PC2), respectively). FN branches were usually adherent to hemangioma or venous malformation as compared to coursing through lymphatic malformation (both P = .01, Fisher exact). CONCLUSIONS: FN branching patterns identified through electrical stimulation differ from cadaveric dissection determined FN anatomy. This reflects the high sensitivity of neurophysiologic testing in detecting small distal FN branches. Elongated FN branches traveling through lymphatic malformation may be related to abnormal nerve patterning in these malformations. LEVEL OF EVIDENCE: NA Laryngoscope, 130:2708-2713, 2020.

Preoperative Facial Nerve Mapping to Plan and Guide Pediatric Facial Vascular Anomaly Resection.

Importance: Facial vascular anomalies are surgical challenges due to their vascularity and facial nerve distortion. To assist facial vascular anomaly surgical treatment, presurgical percutaneous facial nerve stimulation and recording of compound motor action potentials can be used to map the facial nerve branches. During surgery, the nerve map and continuous intraoperative motor end plate potential monitoring can be used to reduce nerve injury. Objective: To investigate if preoperative facial nerve mapping (FNM) is associated with intraoperative facial nerve injury risk and safe surgical approach options compared with standard nerve integrity monitoring (NIM). Design, Setting, and Participants: This investigation was a historically controlled study at a tertiary vascular anomaly center in Seattle, Washington. Participants were 92 pediatric patients with facial vascular anomalies undergoing definitive anomaly surgery (from January 1, 1999, through January 1, 2015), with 2 years' follow-up. In retrospective review, a consecutive FNM patient cohort after 2005 (FNM group) was compared with a consecutive historical cohort (1999-2005) (NIM group). Main Outcomes and Measures: Postoperative facial nerve function and selected surgical approach. For NIM and FNM comparisons, statistical analysis calculated odds ratios of nerve injury and operative approach, and time-to-event methods analyzed operative time. Results: The NIM group had 31 patients (median age, 3.3 years [interquartile range, 2.2-11.4 years]; 20 [65%] male), and the FNM group had 61 patients (median age, 4.4 years [interquartile range, 1.5-11.0 years]; 26 [43%] male). In both groups, lymphatic malformation resection was most common (19 of 31 [61%] in the NIM group and 32 of 61 [52%] in the FNM group), and the median anomaly volumes were similar (52.4 mL; interquartile range, 12.8-183.3 mL in the NIM group and 65.4 mL; interquartile range, 18.8-180.2 mL in the FNM group). Weakness in the facial nerve branches at 2 years after surgery was more common in the NIM group (6 of 31 [19%]) compared with the FNM group (1 of 61 [2%]) (percentage difference, 17%; 95% CI, 3%-32%). Anterograde facial nerve dissection was used more in the NIM group (27 of 31 [87%]) compared with the FNM group (28 of 61 [46%]) (percentage difference, 41%; 95% CI, 24%-58%). Treatment with retrograde dissection without identification of the main trunk of the facial nerve was performed in 21 of 61 (34%) in the FNM group compared with 0 of 31 (0%) in the NIM group. Operative time was significantly shorter in the FNM group, and patients in the FNM group were more likely to complete surgery sooner (adjusted hazard ratio, 5.36; 95% CI, 2.00-14.36). Conclusions and Relevance: Facial nerve mapping before facial vascular anomaly surgery was associated with less intraoperative facial nerve injury and shorter operative time. Mapping enabled direct identification of individual intralesional and perilesional nerve branches, reducing the need for traditional anterograde facial nerve dissection, and allowed for safe removal of some lesions after partial nerve dissection through transoral or direct excision.

Spinal accessory neuropathy after deep brain stimulation for Parkinson's disease.

We report a man with Parkinson's disease who developed right spinal accessory neuropathy after right subthalamic nucleus deep brain stimulator and infraclavicular pulse generator implantation. He complained of right shoulder pain and weakness in the post-operative period. He was subsequently diagnosed with a right spinal accessory nerve injury, confirmed by neuromuscular electrodiagnostic studies - electromyography (EMG) and nerve conduction (NC) -, possibly caused by a stretch injury to the nerve at the time of creation of the subcutaneous tunnel for placement of the extension lead of the deep brain stimulator system. However, he had near complete clinical resolution of the spinal accessory neuropathy within nine months after surgery. As a result of this complication, we now map the spinal accessory nerve electrophysiologically during deep brain stimulation surgery.

Intraoperative neurophysiological monitoring technology: recent advances and evolving uses.

Intraoperative neurophysiological monitoring has evolved over the last 25 years to become an important component of many types of orthopedic and neurosurgical procedures. From its foundations in VIII cranial nerve surgeries and scoliosis corrections surgeries, intraoperative neurophysiological monitoring has expanded to incorporate nearly all spine procedures and many involving the brain and brainstem. Fundamental to this growth in the use of intraoperative neurophysiological monitoring has been the development of the technology used to perform the neurophysiological tests. Advancements in electronics and computer technology have resulted in significant improvements in the capacity, ease of use, quality and reliability of the equipment as well as the quality of and control over the acquired data. These technological advancements have resulted in remarkable improvements in not only the quality and availability of intraoperative neurophysiological monitoring, but also, as a consequence, patient care, and have arguably propelled the expansion of the use that intraoperative neurophysiological monitoring has seen over the last 10 years.

Clinical experience with dexmedetomidine for implantation of deep brain stimulators in Parkinson's disease.

The pharmacologic profile of the alpha-2 agonist dexmedetomidine (Dex) suggests that it may be an ideal sedative drug for deep brain stimulator (DBS) implantation. We performed a retrospective chart review of anesthesia records of patients who underwent DBS implantation from 2001 to 2004. In 2003, a clinical protocol with Dex sedation for DBS implantation was initiated. Demographic data, use of antihypertensive medication, and duration of mapping were compared between patients who received Dex (11 patients/13 procedures) and patients who did not receive any sedation (controls: 8 patients/9 procedures). There were no differences in severity of illness between the two groups. Dex provided patient comfort and surgical satisfaction with mapping in all cases, and significantly reduced the use of antihypertensive medication (54% in the Dex group, versus 100% in controls, P = 0.048). In DBS implantation, sedation with Dex did not interfere with electrophysiologic mapping, and provided hemodynamic stability and patient comfort. Routine use of Dex in these procedures may be indicated.

Unilateral subthalamic nucleus deep brain stimulation contralateral to thalamic stimulation in Parkinson disease.

The effects of unilateral subthalamic nucleus (STN) stimulation contralateral to thalamic stimulation in Parkinson disease (PD) have not been previously reported. We are reporting a patient who developed left arm tremor in 1994, at age 62, as her first PD symptom. She underwent right thalamic DBS surgery in 1999 that resulted in complete resolution of left arm tremor. Her PD symptoms progressed and she developed severe motor fluctuations and disabling dyskinesias. In 2003, she underwent left STN electrode implantation. Left STN stimulation improved contralateral motor scores in the medication OFF state, and allowed for reduced medication doses and less dyskinesia. However, there was no significant improvement in activities of daily living (ADL), motor scores in the medication ON state, gait, or postural stability.

Deep brain stimulation in a patient on immunosuppressive therapy after renal transplant.

We performed thalamic deep brain stimulation (DBS) surgery to treat severe essential tremor in a 36 year-old woman who had undergone cadaveric renal transplant four years earlier. She was receiving chronic immunosuppressive therapy. Post-operative healing was normal and there have been no infections of the DBS hardware. There were no peri-operative complications and no rejection of the transplanted kidney. She remains on the same systemic immunosuppressive agents as pre-operatively: prednisone, cyclosporine, and mycophenolate mofetil (CellCept). DBS surgery may be safely performed in carefully selected patients on systemic immunosuppression after renal transplant.

Bilateral subthalamic nucleus stimulation after bilateral pallidotomies in a patient with advanced Parkinson's disease.

A 54-year-old man with advanced Parkinson's disease (PD) presented to our institution in early 2000. He had undergone a right pallidotomy in 1994, a left pallidotomy in 1996, and bilateral subthalamic nucleus (STN) electrode implants in 1999. The patient had cervical myelopathy for which he had undergone neck surgery in 1998. We used the Unified Parkinson's Disease Rating Scale (UPDRS) to evaluate motor performance in four states: combinations of stimulation OFF or ON and medication OFF or ON. There was no significant change in motor UPDRS scores with STN stimulation or with medications. Multiple attempts to optimize stimulation parameters and medication dosages did not result in significant and sustained improvement in activities of daily living or motor performance. To our knowledge, this is the first reported case of bilateral STN stimulation after bilateral pallidotomies. The presence of cervical myelopathy and the limited response to anti-Parkinson medications in this patient underscores the importance of patient selection for functional neurosurgery in PD.

Mechanical failure of the electrode wire in deep brain stimulation.

The feasibility and efficacy of deep brain stimulation (DBS) has offered new possibilities for treatment of movement disorders. Mechanical failure of the DBS system is a potential complication. Here we report five patients who presented with mechanical failure of the DBS system. Radiographs of the skull and cervical spine were analyzed for disruptions. Seven instances of lead breakage near the connection of the DBS electrode with the extension wire were identified. In one patient this was in the paramastoid area over the skull, while in all others were in the supraclavicular location. The patients consisted of three men and two women ranging in age from 24 to 78 (at the time of first operation), one person suffering three breakages. The length of spanned time from implantation to presentation ranged from 8 to 32 months. Palpation of the electrode lead wire in the neck for breakage proved unreliable. Radiography localized the site of breakage in all but one patient who required intraoperative exploration, which revealed that although the lead wire was disrupted, the two ends remained in contact. The fact that all breakages occurred near the connection wire suggests that to-and-fro motion of the DBS electrode with repeated head turning leads to fatigue and eventual disruption.

Electrophysiologic intraoperative monitoring for spine procedures.

The advent of equipment capable of performing SEPs, MEPs, and EMG in a multiplexed manner and in a timely fashion brings a new level of monitoring that far exceeds the previous basic monitoring done with SEPs only. Whether this more comprehensive monitoring will result in greater protection of the nervous system awaits future analysis. In any event, monitoring of the spinal cord with SEPs is an accepted standard of care for cases that place the spinal cord at risk. Likewise, nerve root monitoring with EMG is a widely practiced form of monitoring and shows great benefit. MEPs and reflex monitoring, which address the descending pathways and the interneuronal connections, is efficacious in detecting abnormalities that may be missed by SEPs.

Alternative sites for intraoperative monitoring of cranial nerves X and XII during intracranial surgeries.

During intracranial surgeries, cranial nerve (CN) X is most commonly monitored with electromyographic endotracheal tubes. Electrodes on these endotracheal tubes may be displaced from the vocal folds during positioning, and there is a learning curve for their correct placement. Cranial nerve XII is most commonly monitored with electrodes in the dorsum of the tongue, which are also prone to displacement because of their proximity to the endotracheal tube. A retrospective review was conducted of a consecutive series of 83 skull base surgeries using alternative sites for monitoring CN X and XII. On-going (spontaneous) and evoked electromyography (EMG) were obtained from the cricothyroid muscle for CN X and submental genioglossus for CN XII. Stimulation of CN X or XII evoked specific compound motor action potentials from these muscles, and well-defined on-going EMG was observed during tumor resection in the vicinity of CN X and XII. Volume-conducted responses from the adjacent platysma muscle during CN VII stimulation were identified by concomitant responses from the orbicularis oris and oculi. In conclusion, during skull base surgeries, CN X may be monitored with electrodes in the cricothyroid muscle and CN XII with electrodes in the submental genioglossus. These alternative sites are less prone to displacement of electrodes compared with the more commonly used EMG endotracheal tube and electrodes in the dorsum of the tongue. The cricothyroid muscle should not be used when the recurrent laryngeal nerve is at risk.

Facial nerve mapping and monitoring in lymphatic malformation surgery.

OBJECTIVE: Establish the efficacy of preoperative facial nerve mapping and continuous intraoperative EMG monitoring in protecting the facial nerve during resection of cervicofacial lymphatic malformations. METHODS: Retrospective study in which patients were clinically followed for at least 6 months postoperatively, and long-term outcome was evaluated. Patient demographics, lesion characteristics (i.e., size, stage, location) were recorded. Operative notes revealed surgical techniques, findings, and complications. Preoperative, short-/long-term postoperative facial nerve function was standardized using the House-Brackmann Classification. Mapping was done prior to incision by percutaneously stimulating the facial nerve and its branches and recording the motor responses. Intraoperative monitoring and mapping were accomplished using a four-channel, free-running EMG. Neurophysiologists continuously monitored EMG responses and blindly analyzed intraoperative findings and final EMG interpretations for abnormalities. RESULTS: Seven patients collectively underwent 8 lymphatic malformation surgeries. Median age was 30 months (2-105 months). Lymphatic malformation diagnosis was recorded in 6/8 surgeries. Facial nerve function was House-Brackmann grade I in 8/8 cases preoperatively. Facial nerve was abnormally elongated in 1/8 cases. EMG monitoring recorded abnormal activity in 4/8 cases--two suggesting facial nerve irritation, and two with possible facial nerve damage. Transient or long-term facial nerve paresis occurred in 1/8 cases (House-Brackmann grade II). CONCLUSIONS: Preoperative facial nerve mapping combined with continuous intraoperative EMG and mapping is a successful method of identifying the facial nerve course and protecting it from injury during resection of cervicofacial lymphatic malformations involving the facial nerve.

What's new in MRI of peripheral nerve entrapment?

New developments in clinical peripheral nerve imaging with MRI over the past few years, primarily those related to nerve entrapment syndromes, are reviewed. The basic principles of peripheral nerve imaging are described briefly. Relevant current or forthcoming technical innovations are described, and then recent work describing novel findings, organized by anatomic location (brachial plexus, upper extremity, and lower extremity), is reviewed. The review concludes with a summary and suggestions of areas in which future clinical research would be particularly helpful.