Long-term evaluation of intraoperative neurophysiological monitoring-assisted tethered cord surgery.

PURPOSE: Patients with tethered spinal cord have been investigated for short-term effects after tethered spinal cord surgery in the past. However, little is known about the long-term effects in this patient group. In this retrospective, longitudinal, observational study, a patient sample of a previous report of 65 patients was reassessed to observe the long-term effects of intraoperative neurophysiological monitoring-assisted tethered cord surgery. METHODS: With the use of patient charts and a survey, patients were scored on four domains: (1) neurological deficits, (2) urological deficits, (3) pain symptoms, and (4) orthopedic deficits. Measurements were performed at four moments in time: (1) preoperatively, (2) postoperatively, (3) follow-up 1 (4.6 years), and (4) follow-up 2 (11.2 years). Besides this, a subgroup analysis and a quality of life questionnaire were performed. RESULTS: When observing the symptom domains in the long-term, the pain domain appeared to improve most postoperatively after which it remained stable over time. The neurological and urological domains showed a stable, slightly decreasing trend in the long-term follow-up. The orthopedic domain showed a significant increase of the number of patients with scoliosis during the long-term follow-up. CONCLUSIONS: Lasting effects of stability in the neurological, urological, and pain domains were observed. Close monitoring during follow-up might contribute to early recognition of progressive scoliosis, in spite of detethering, in a risk group defined by females who underwent tethered cord surgery at or under the age of 12 years old with either lipomyelomeningocele, split cord malformation, or myelomeningocele. Detethering does not appear to protect these patients against progressive scoliosis.

Optimum interpulse interval for transcranial electrical train stimulation to elicit motor evoked potentials of maximal amplitude in both upper and lower extremity target muscles.

OBJECTIVE: The aim of this study was to determine the optimum interpulse interval (OIPI) for transcranial electrical train stimulation to elicit muscle motor evoked potentials (TES-MEP) with maximal amplitude in upper and lower extremities during intra-operative spinal cord monitoring. METHODS: Intraoperative spinal cord monitoring with TES-MEP was performed in 26 patients who had (corrective) spine surgery. Optimum interpulse interval (OIPI) were determined for the abductor pollicis brevis muscle (APB) representing the upper extremity and the anterior tibialis muscle (TA) representing the lower extremity. The IPI was varied between 0.5 and 4.0ms, where the OIPI was defined as the IPI with the highest muscle MEP amplitude for each muscle group. Differences between upper and lower extremity OIPIs were analyzed. Furthermore, the MEP amplitudes difference between the upper and lower extremity OIPIs and between the OIPI and IPI 2 ms was determined. RESULTS: The mean OIPI(APB) representing the upper extremity was 1.78 ± 1.09 ms on the left side and 1.82 ± 0.93 ms on the right side. The lower extremity showed a mean OIPI(TA) of 2.26 ± 1.16 ms on the left and 2.73 ± 0.88 ms on the right side. The mean differences between the OIPI(APB) and OIPI(TA) were significant for p=0.019 (Student's T-test). No within patient differences in OIPIs between the left and the right side were found. The mean MEP amplitude reduction, the APB amplitude at OIPI(TA) compared to the APB at OIPI(APB), was 32.5 ± 27.9%. For the TA a mean amplitude reduction of 33.4 ± 27.4% was found. The mean amplitude reduction for the OIPI amplitudes compared to the amplitudes at IPI 2 ms was 53.6 ± 25.5% for the APB and 45.8 ± 28.3% for the TA. CONCLUSION: Large intra- and interindividual differences were found between the mean OIPIs of the TA and APB muscles (range 1.78-2.73 ms) representing the upper and lower extremity. SIGNIFICANCE: Based on the results of this study, it is advisable to perform a set-up procedure for each individual patient undergoing TES-MEP to determine the optimal parameter settings when using supramaximal intensity of TES.

Conditioning stimulation techniques for enhancement of transcranially elicited evoked motor responses.

INTRODUCTION: In spite of the use of multipulse, transcranial electrical stimulation (TES) is still insufficient in a subgroup of patients to elicit motor-evoked potentials during intraoperative neurophysiological monitoring (IONM). Classic facilitation methods used in awake patients are precluded under general anaesthesia. Conditioning techniques can be used in this situation. OBJECTIVE: To present clinical experimental data and models of motor-neuron (MN) excitability for homonymous and heteronymous conditioning and discuss their applications in IONM. MATERIAL AND METHODS: Data were obtained in a prospective study on multipulse TES-conditioning of the monosynaptic H-reflex and double multipulse TES. DISCUSSION: The principle of facilitation by conditioning stimulation is to apply a test stimulus when motor neurons (MNs) have been made maximally excitable by a conditioning stimulus. Both conditioning and test stimuli recruit separate populations of MNs. The overlapping fraction of MNs controls the efficacy of facilitation. Heteronymous conditioning stimulation, which is performed at a different site from the test stimulus, is illustrated by the TES-conditioned H-reflex (HR). Autonomous conditioning stimulation, which is performed at the same stimulation site, is illustrated by double-train TES (dt-TES). The facilitating curves obtained by conditioning stimulation are often 3-modal and show peaks of facilitation at short intertrain intervals (S-ITIs) of 10ms and between 15 and 20ms and at longer intertrain intervals (L-ITI) of over 100ms. The facilitation curves from HR and dt-TES are not always identical since different alphaMN pools are involved. Dt-TES is often successful in neurologically impaired patients whereas facilitation of the HR can be used when conditioned by TES at subthreshold levels allowing continuous IONM without movement in the surgical field. Alternatively, facilitation by conditioning from peripheral-nerve stimulation can be used for selective transmission of subthreshold TES motor responses to peripheral muscles, permitting motor-monitoring by a so-called selective motor-gating technique. CONCLUSIONS: Facilitation techniques offer many possibilities in IONM by enhancing low-amplitude TES-MEP responses. They can also selectively enhance responses in a few muscle groups for the reduction of movement.

Criteria for transcranial electrical motor evoked potential monitoring during spinal deformity surgery A review and discussion of the literature.

Transcranial electrical stimulated motor evoked potential monitoring (TES-MEP) has proven to be a successful and reliable neuromonitoring technique during spinal correction surgery. However, three criteria for TES-MEP monitoring have been described in the literature. This study aims at discussing and comparing the following criteria: (1) the "threshold level criterion" introduced by Calancie et al. (J Neurosurg 88 (1998) 457-70): a more than 100V over more than 1h increase of threshold level to get useful TES-MEP responses indicated neurological impairment; (2) the "amplitude criterion": for TES-MEP monitoring in corrective surgery of the spine, a more than 80% decrease of one or more response amplitudes was considered a valuable criterion for impending neurological deficits by Langeloo et al. (Spine 28 (2003) 1043-50); (3) "the morphology criterion": introduced in 2005 by Quinones et al. (Neurosurgery 56 (2005) 982-93), it is based on the morphology of the MEP-compound muscle action potentials (CMAP). The criterion was applied during TES-MEP monitoring during intramedullary spinal cord tumour resection. Neurological events are defined by a sharp decrease of response duration and/or waveform complexity and an increase in voltage threshold of 100V or greater. Although all methods have been reported to be successful during spinal surgery, the threshold criterion and the morphology change criterion carry several drawbacks. We consider the amplitude reduction method to be most useful during corrective spinal surgery. The sequences of observations and decisions during a TES-MEP monitoring that is based on this criterion are schematized in a flowchart.

Intraoperative neurophysiological assessment of disabling symptoms in DBS surgery.

INTRODUCTION: Neurophysiological assessment can provide quantitative measures for the selected motor signs that have been targeted for surgery and may be helpful in predicting the therapeutic effects of deep brain stimulation (DBS) on pathological tremor, motor performance, and rigidity. OBJECTIVE: To present a survey and demonstrate the contribution of neurophysiological assessment of side effects and effects on disabling motor symptoms at various steps of DBS surgery, and to confirm its role for optimal target localization, as an adjuvant to anatomic imaging. MATERIAL AND METHODS: The data result from 192 nuclei in 118 procedures on patients with Parkinson's disease (84), essential tremor (24), Hallenvorder Spatz dystonia (4), multiple sclerosis (4), and Holmes tremor (2). The intraoperative neurophysiological monitoring (IOM) protocol consists of semimicroelectrode recording (for subthalamic nuclei), whereas accelerotransducers and spectral analysis allow assessment of tremor, finger tapping (FT), diadochokinesis (DDK), and determination of the distance between DBS electrodes and internal capsule (IC). Rigidity is assessed by surface EMG recordings in combination with a goniometer. RESULTS: The determination of the functional distance between the DBS electrode and the IC is based on the activation functions of axons in the IC. We show the high sensitivity of accelerometers for tremor over a large part of the body, the relationship between clinical scores and spectral frequencies of FT and DDK. Parkinsonian rigidity can be assessed from surface EMG (sEMG) by means of a balance coefficient, which can detect negative rigidity, for low unified Parkinson's disease rating scale (UPDRS) scores (0-2) and quantified EMG when negative rigidity is excluded. CONCLUSION: Accelerometer and sEMG recording have shown their value for intraoperative assessment of disabling motor symptoms and side effects during surgery, to optimize the target position electrodes for DBS. The combination with contemporary signal analyzing techniques permit intraoperative monitoring without a significant delay. IONM improves sensitivity and adds objective neurophysiological data.

Chronic stimulation of the subthalamic nucleus increases daily on-time without dyskinesia in advanced Parkinson's disease.

We assessed the efficacy of chronic stimulation of the subthalamic nucleus (STN-DBS) in 20 patients with Parkinson's disease (PD) by means of clinical assessments and patient diaries 12 months after surgery. STN-DBS reduced the UPDRS part III off-medication score by 33%, and successively improved complete daily on-time without dyskinesia at 12 months significantly. In conclusion, our study demonstrates the efficacy of chronic STN-DBS on motor features in a selected population of advanced PD patients. In addition to clinical assessments, patients' diaries serve as an essential tool to evaluate the functional motor status after STN-DBS.

Influence of electrode impedance on threshold voltage for transcranial electrical stimulation in motor evoked potential monitoring.

Motor potentials evoked by transcranial electrical stimulation (TES) are used for monitoring the motor pathways, with emphasis on the spinal cord and brainstem. The stimulus voltage threshold is the voltage below which no motor response can be elicited. It has frequently been used as a monitoring parameter. However, its value can be limited, because it is affected by the impedance of the stimulus electrode. For example, the voltage threshold can change owing to formation of oedema of the scalp. The relationship between the TES voltage threshold and the electrode impedance of different electrode types was studied and discussed in the context of neuromonitoring: 323 impedance and voltage threshold pairs were studied, and TES was performed with disc cup EEG electrodes (six), corkscrew electrodes (type I: seven, type II: eight), multiple EEG needle electrodes (16) and a large needle electrode Cz' (anode) together with a ground strip over the forehead (cathode) (286). The study found the voltage threshold to be strongly dependent on electrode impedance when the impedance was higher than 460 omega (correlation: R2=0.87; p < 0.001). Below 460 omega, which included 91% of the category with the largest electrode surfaces, 25% of the multiple EEG electrodes and 75% of type II corkscrew electrodes, no significant correlation (R2=0.0064; p=0.15) was found. It was concluded that the correlation between the TES voltage threshold and electrode impedance can be markedly reduced by using TES electrodes with large contact surfaces, resulting in limit values for these parameters. This also may improve the reliability of TES motor evoked potential monitoring.

Improved neuromonitoring during spinal surgery using double-train transcranial electrical stimulation.

Motor evoked potentials (MEPs) evoked by transcranial electrical stimulation (TES) have become an important technique for monitoring spinal cord function intra-operatively, but can fail in some patients. A new technique of double-train stimulation is described. A multipulse transcranial electrical stimulus is preceded by a preconditioning pulse train that leads to larger MEP responses. An MEP monitoring system was adapted for double-train transcranial stimulation (DTS). MEP responses from 160 anterior tibial muscles obtained by double-train stimulation were analysed. All patients received propofol/remifentanil/O2/N2O anaesthesia. Fifty-two (83%) out of 63 single-train tibial MEPs with response amplitudes below 100 microV were magnified to over 100 microV, with an inter-train (inter-stimulus) interval ITI = 10-35 ms. These 63 amplitudes were magnified by an overall logarithmic mean factor of 15.5. For 97 MEPs with amplitudes above 100 microV, the logarithmic mean facilitation factor was 2.4. It was concluded that double-train TES stimulation can markedly facilitate responses to a single stimulus train (STS). The facilitation appears to be most effective when the responses to STS would otherwise be small or absent. This preconditioning stimulation technique is therefore useful when an STS leads to responses that are too small for effective monitoring.

A new application of TCE-MEP: spinal cord monitoring in patients with severe neuromuscular weakness undergoing corrective spine surgery.

The importance of spinal cord monitoring during corrective scoliosis surgery is widely acknowledged. However, for patients with preoperative neurologic deficits, its use is technically difficult. In this article, we describe intraoperative transcranial electrical motor evoked potential (TCE-MEP) monitoring in nine patients with neuromuscular weakness who underwent corrective scoliosis surgery. In each patient, eight muscle groups were recorded and stable responses were obtained in all patients. We conclude that TCE-MEP permits monitoring of the spinal cord in patients with disturbed motor function.

Effects of exposure to elemental mercury on the nervous system and the kidneys of workers producing natural gas.

Early signs of alterations in renal and neurological functions were studied in three groups of workers who were exposed to different levels of mercury that were below the current biological exposure index of 35 microg/g creatinine. There were no differences among the three study groups with respect to either motor nerve conduction velocity or tremor frequency spectra of physiological tremors. Also, no significant correlations were found between the results of the neurological tests and any of the present or historical biological monitoring data. In contrast, N-acetyl-beta-D-glucosaminidase was increased significantly in the group with the higher exposure, compared with either the lower-exposure or control groups. N-acetyl-beta-D-glucosaminidase was correlated strongly with mercury concentration in urine and was correlated weakly with historical biological monitoring data; however, there was no correlation with duration of exposure. These results suggest that after exposure to mercury at levels below the biological exposure index, a transient increase in N-acetyl-beta-D-glucosaminidase can be observed, but is not an early indicator of developing renal dysfunction.

Design of a myo-seismic transducer for non-invasive transcutaneous vectorial recording of locally fast muscle-fibre micro-contractions.

Mechanical recording usually concerns the analysis of movements in bio-mechanical research projects. Mechanical recording of locally fast muscle-fibre micro-contractions, however, is a little-developed and rarely-applied myographic technique. In the last decade, acoustic or myophonic measurements came increasingly into the picture when they were also applied to research on general muscle activity, such as in muscle fatigue studies. In this paper, a new micro-seismic recording technique is introduced. The technique registers extremely local activity in the velocity and force vector of skin movement as a function in time. The recording method is sensitive to micro excursions caused by muscle fibres under the skin. The resolution in time is at least 100 us, which is demonstrated in an experiment where a mechanical contraction is provoked by electrical stimulation of the median nerve. This indicates a seismic variant, refered to as seismic-myography (SMG), of surface EMG's, and offers complementary features. The most important features are: 1. Insensitivity to low frequent, large movement artefacts. 2. Sensitivity to fast mechanical micro-excursions and velocities. 3. Fast and precise discrimination of local mechanical events. 4. Vectorial reconstruction of superficial mechanic activity which can be used for the identification and functional behaviour of subcutaneous muscle fibres and, in addition, for the localisation of motor endplate zones. 5. The method is easy to use.(ABSTRACT TRUNCATED AT 250 WORDS)

Design and application of sensor for recording sounds over human eye and nose.

The recording of sounds over the orbit of the eye has been found to be useful in the detection of intracranial aneurysms. A hydrophone for auscultation over the eye has been developed and is tested under controlled conditions. The tests consist of measurement over the eyes in three healthy volunteers at rest, during voluntary breathing, during eyeball movements and during sustained orbicular muscular contractions. Furthermore, measurements are performed at the side of the nose. Major features of the hydrophonic transducer are high sensitivity to physiological sounds and a high degree of insensitivity to environmental sounds propagated through the air. It can be concluded that the hydrophone may be useful for the early detection of intracranial aneurysms and also for apnoea detection.

Mechanical muscle fibre conduction velocity of the biceps as measured by a new seismic technique.

A recently-developed technique, called seismic myography (SMG) has the characteristic of recording fast micro-mechanical response times. These times can be determined with sub-millisecond accuracy. The response times can be compared to response times of EMG recordings. The "muscular electro-seismic response" (MESR) latencies, due to direct electrical stimulation of the biceps muscle, are used for explorative measurements of the mechanical conduction velocity of the muscle fibres. The measurements are performed by means of a general-purpose physiological multimeter which is equiped with the micro-seismic function. Measurements are performed on two healthy subjects, aged 22 years. The MESR-latencies are measured along a medial and a lateral trajectory on their biceps muscles. The MESR-latencies at stimulus-cathodal to seismic transducer distances of 2,0-3,5 cm, are in the range of 2.0-3.8 ms, while at distances in the range of 7.5-8.9 cm the MESR-latencies varied between 3.4 and 4.7 ms. The calculated mechanical muscle fibre conduction velocities (MMFCV) are in the range between 36 and 89 m/s. There is a reproducability error of maximum 20%. The MMFCV's of the lateral and medial trajectory do not differ within the accuracy of the present method. However, the MMFCV's are considerably higher than the electrical muscle fibre conduction velocities of MUAPS ((E)MFCV). Some aspects of the MMFCV and possible consequences to surface EMG recordings are discussed. It is concluded that this seismic method for measuring MMFCV is a new accessible and simple to handle tool for the description of muscle function, and offers an interesting new contribution in experimental muscular research.

ESTRACE--1: a stethoscope for functional checking of electrostimulators and low cost body surface mapping system.

In a project on the mapping of body surface potentials evoked by implanted spinal cord stimulators and transcutaneous electrostimulators, a simple device was developed for use in polyclinical practice for easy checking of the function of these stimulators. The device is an electrical stethoscope and consists of an instrumentation amplifier, bandpass filter, a distortion circuit and a headphone. In vivo measurements demonstrate the generation of an amplitude-dependent tone by the distortion circuit. The apparatus is in many aspects similar to a stethoscope: simple to understand and a practical tool for a fast Go/No-Go test. The device can be optionally connected with a PC and enables body surface mapping documentation within a few minutes.

Stimulation characteristics, complications, and efficacy of spinal cord stimulation systems in patients with refractory angina: a prospective feasibility study.

OBJECTIVES: In a prospective study with a 1-year follow-up we evaluated: (1) the feasibility of a method for the adjustment of spinal cord stimulator (SCS) parameters, (2) complications of SCS, and (3) efficacy of SCS. METHODS: In patients receiving an SCS for severe angina unresponsive to standard therapies, SCS characteristics were evaluated within 1 week and at 4, 14, 26, and 52 weeks after SCS treatment. Step-by-step adjustment of pulse output parameters was performed at the electrode configuration at which paresthesias occurred ("sensory threshold"), covered the anginal area ("adjusted setting"), or provoked pain ("motor threshold"). In addition, the number of anginal attacks and intake of glyceryl trinitrate (GTN) tablets were recorded at regular intervals. RESULTS: Twenty-two patients with either a bipolar (14) or a unipolar (8) system were evaluated. In the 14 patients with bipolar systems, alteration in paresthesias required 26 reprogrammings of the configuration. In the eight patients with bipolar systems who completed the follow-up without lead dislocation, the mean "sensory threshold" was 3.3 V (1.7-5.6), the mean "adjusted stimulation" output was 4.5 V (2.8-7.6), and the mean "motor threshold" was 4.9 V (2.8-7.7) after 4 weeks SCS. The mean stimulation duration per 24 hours was 14.1% (5%-24%), and the mean standardized impedance was 821 omega (748-893) after 4 weeks SCS. The unipolar group demonstrated comparable results. After 1-year follow-up the parameters had not changed significantly. During the 1-year follow-up, 6 of 22 patients experienced lead dislocation that required surgery. In all patients, anginal attacks (P < 0.003) and GTN intake (P < 0.005) were reduced significantly with SCS. The effect lasted during the 1 year. CONCLUSIONS: During a 1-year follow-up, the stimulation parameters did not change significantly in the 16 patients without lead dislocations. Our standardized method appears to be feasible for follow-up of SCS. Moreover, SCS seems to be an effective adjuvant therapy for intractable angina, despite a relatively frequent dislocation of the electrode.

Noisy intracranial tumours.

Transorbital sound recordings were obtained from 21 patients with intracranial tumours, 28 patients with intracranial aneurysms and 20 control patients. The group of patients with tumours consisted of 12 patients with gliomas, of whom 6 had low-grade gliomas and 6 had high-grade gliomas, and 9 patients with meningiomas. All patients with gliomas, including the subgroup of patients with low-grade gliomas, as well as patients with aneurysms, had significantly different sound recordings in comparison to control patients. Recordings from glioma patients did not differ significantly from recordings of aneurysm patients. Radiological evaluation of the tumours was performed in order to establish which tumour characteristics were associated with abnormal sound recordings. It was found that the type of tumour, i.e., histology or malignancy grade, was a significant associated factor, whereas other tumour characteristics such as size, mass effect and amount of oedema were not. In conclusion, patients with specific types of intracranial tumours produced abnormal sounds which could not be distinguished from abnormal sounds recorded in patients with aneurysms. These results may be important for the interpretation of sounds recorded for the detection of intracranial pathology, especially for aneurysm screening.

Ultrasound myography: application in nerve conduction velocity assessment and muscle cooling.

A new application of ultrasound for studying muscle twitch induced by electrical stimulation is described and some preliminary results are presented. The method, called "ultrasound myography" (UMG), uses Doppler ultrasound to measure muscle movement velocity. The Doppler signals were measured simultaneously with the electromyography (EMG) signals from the thenar muscle of a healthy subject. Averaged EMG and full-wave rectified UMG responses to repeated electrical stimuli were measured after cooling of the hand and adaptation to room temperature. Latency times over the wrist of cold hands adapted to a surrounding temperature of 8 degrees C were 4.5 ms and 16.9 ms for the EMG and averaged rectified UMG responses, respectively. Both latency times decreased considerably after 1 h adaptation to a room temperature of 21 degrees C:20% for the EMG response and 35% for the UMG response. The conduction velocities of the median nerve in the forearm determined by both methods yield comparable results. The results of both methods are discussed. It is concluded that UMG possibly offers a new method in clinical practice for the assessment of nerve conduction velocities in the forearm, and basically is a new simple-to-use technique for noninvasive analysis of deep biomechanical processes.

The acoustic detection of intracranial aneurysms: a clinical study.

A new recording method for the acoustical detection of intracranial aneurysms is presented. A study examining the capability of the method to discriminate between patients with an aneurysm and control patients by a simple, objective parameter is reported. Sound signals were recorded over the eyes, and a real-time spectral analysis was performed on these signals. For this study, recordings performed on 26 patients with an aneurysm were compared with recordings on 26 age- and sex-matched control patients without intracerebral abnormalities. As a result of measures taken to reduce artifacts and to improve the signal-to-noise ratio, the measurements were performed reliably, with little inconvenience for the patients; all measurements could be used for analysis. The power spectra measured in the control patients showed a typical, smoothly descending pattern; those measured in the aneurysm patients clearly differed from this pattern, showing peaks of varying width, height, and dominant frequency. For the objective judgment of the power spectra, the power median is introduced. The sensitivity and specificity of this parameter were determined. Possible methods to improve the results will be discussed.