The value of intraoperative neurophysiological monitoring in tethered cord surgery

The value of intraoperative neurophysiological monitoring (IONM) with surgical detethering in dysraphic patients has been questioned. A retrospective analysis of our series of 65 patients is presented with special focus on technical set-up and outcome. All patients were diagnosed with a tethered cord (TC) due to spinal dysraphism. A high-risk group (HRG) was determined consisting of 40 patients with a lipomyelomeningocele and/or a split cord malformation sometimes in combination with a tight filum terminale. The surgical procedure was a detethering operation in all cases performed by a single surgeon during a 9-year period (1999-2008). A standard set-up of IONM was used in all patients consisting of motor-evoked potentials (MEP) evoked by transcranial electrical stimulation (TES) and electrical nerve root stimulation. In young patients, conditioning stimulation was applied in order to improve absent or weak MEPs. IONM responses could be obtained in all patients. Postoperative deterioration of symptoms was found in two patients of whom one patient belonged to the HRG. Mean maximal follow-up of all 65 patients was 4.6 years (median 4.1 years). Long-term deterioration of symptoms was found in 6 of 65 patients with a mean follow-up of 5 years (median 5.3 years). The use of IONM is feasible in all TC patients. The identification of functional nervous structures and continuous guarding of the integrity of sacral motor roots by IONM may contribute to the safety of surgical detethering

Source analysis of beta-synchronisation and cortico-muscular coherence after movement termination based on high resolution electroencephalography

We hypothesized that post-movement beta synchronization (PMBS) and cortico-muscular coherence (CMC) during movement termination relate to each other and have similar role in sensorimotor integration. We calculated the parameters and estimated the sources of these phenomena.We measured 64-channel EEG simultaneously with surface EMG of the right first dorsal interosseus muscle in 11 healthy volunteers. In Task1, subjects kept a medium-strength contraction continuously; in Task2, superimposed on this movement, they performed repetitive self-paced short contractions. In Task3 short contractions were executed alone. Time-frequency analysis of the EEG and CMC was performed with respect to the offset of brisk movements and averaged in each subject. Sources of PMBS and CMC were also calculated.High beta power in Task1, PMBS in Task2-3, and CMC in Task1-2 could be observed in the same individual frequency bands. While beta synchronization in Task1 and PMBS in Task2-3 appeared bilateral with contralateral predominance, CMC in Task1-2 was strictly a unilateral phenomenon; their main sources did not differ contralateral to the movement in the primary sensorimotor cortex in 7 of 11 subjects in Task1, and in 6 of 9 subjects in Task2. In Task2, CMC and PMBS had the same latency but their amplitudes did not correlate with each other. In Task2, weaker PMBS source was found bilaterally within the secondary sensory cortex, while the second source of CMC was detected in the premotor cortex, contralateral to the movement. In Task3, weaker sources of PMBS could be estimated in bilateral supplementary motor cortex and in the thalamus. PMBS and CMC appear simultaneously at the end of a phasic movement possibly suggesting similar antikinetic effects, but they may be separate processes with different active functions. Whereas PMBS seems to reset the supraspinal sensorimotor network, cortico-muscular coherence may represent the recalibration of cortico-motoneuronal and spinal systems

DEMODULATION OF AMPLITUDE MODULATED NOISE - A MATHEMATICAL EVALUATION OF A DEMODULATOR FOR PATHOLOGICAL TREMOR EMGS

A mathematical description is given of amplitude modulated noise, simulating EMG signals of pathological tremors, and its demodulation into a tremor signal by means of a full wave rectifier. In this description these signals are considered in the frequency domain in the form of power spectra. The mathematically derived formulas are compared with the results of an experimental setup using a demodulator designed for use in clinical practice. The purpose of the experiments was to determine the signal-to-noise ratio of the tremor signal with respect to the demodulation noise, the latter being an inevitable consequence of demodulating the amplitude modulated carrying noise. The theoretically derived data are in acceptable agreement with the experimental results obtained in this way. As a result of the theoretical approach to the amplitude modulated noise, the signal-to-noise ratio of the demodulated signal is proportional to the bandwidth of the carrying noise of the amplitude modulation, and, for small modulation depths, is proportional to the square of the modulation depth. Copyright © 1983 by The Institute of Electrical and Electronics Engineers, Inc

AUTOMATIC METHOD TO DETECT AND DETERMINE THE PREFERENTIAL FREQUENCY OF EMG SIGNALS FROM PATIENTS WITH PATHOLOGICAL TREMORS

The paper presents a special purpose automatic-detection method de veloped to determine whether a tremour EMG contains a tremour and to calculate its preferential frequency if a tremour is present. This method is specially designed and applied in a project where only pathological tremours with preferential frequencies within the frequency band 2-10 Hz are recorded. The preferential frequency is calculated approximately as the mean frequency in a 3Hz broad area surrounding the main peak in the power spectrum The detection method assumes that the tremour signal contains only one single preferential frequency The advantages of the detection method are data reduction and objectivity. The level of detection of a tremour is approximately the same as can be derived from inspection of a power spectrum © 1984 IFMBE

DEMODULATION OF EMGS OF PATHOLOGICAL TREMORS - DEVELOPMENT AND TESTING OF A DEMODULATOR FOR CLINICAL USE

In clinical tremour recordings, e.m. g. signals allow highly localised recordings to be made. This is in contrast to mechanical recordings which have been used extensively in the past. E.m.g. recordings make it possible to distinguish between tremours in agonist and antagonist muscle groups. E.m.g. signals, however, have poor signal-to-noise characteristics. In this study the e.m.g. of pathological tremours is considered as amplitude-modulated noise. The design of a demodulator and some tests of its performance are described. The visual detectability of an undemodulated tremour e.m.g., when written on a polygraphic recorder, is expressed as a modulation depth, and is about 60%. This improves after demodulation to a level of 20%. The placement of the electrodes and filtering of the e.m.g. signal will influence the shape of the power spectrum of the e.m.g. itself. However, these factors do not critically influence the demodulated tremour e.m.g. For this reason also a demodulator is a useful instrument for recording clinical tremours. © 1983 IFMBE

DEMODULATION OF EMGS OF PATHOLOGICAL TREMORS - DEVELOPMENT AND TESTING OF A DEMODULATOR FOR CLINICAL USE

In clinical tremour recordings, e.m. g. signals allow highly localised recordings to be made. This is in contrast to mechanical recordings which have been used extensively in the past. E.m.g. recordings make it possible to distinguish between tremours in agonist and antagonist muscle groups. E.m.g. signals, however, have poor signal-to-noise characteristics. In this study the e.m.g. of pathological tremours is considered as amplitude-modulated noise. The design of a demodulator and some tests of its performance are described. The visual detectability of an undemodulated tremour e.m.g., when written on a polygraphic recorder, is expressed as a modulation depth, and is about 60%. This improves after demodulation to a level of 20%. The placement of the electrodes and filtering of the e.m.g. signal will influence the shape of the power spectrum of the e.m.g. itself. However, these factors do not critically influence the demodulated tremour e.m.g. For this reason also a demodulator is a useful instrument for recording clinical tremours. © 1983 IFMBE