Visual evoked potentials in multiple sclerosis

A study was carried out of 32 patients suffering from multiple sclerosis the diagnois of which was quite definite according to Mac Alpine's criteria and which had developed over more than two years. Comparison was made with patient suffering from optic neuritis of a different aetiology and with normal subjects. The method of investigation was dynamic electroretinoencephalography : the visual evoked potentials were investigated after stimulation by white and coloured light before and after adapatation to darkness. The specificity of the results was tested by, firstly, electro-retinogram recording for control of the visual system, and secondly, by the recording of auditory evoked potentials for control cerebral function. Abnormalities in visual evoked potentials were found in 77 percent of the cases. In particular, there was delayed latency, that is, a slowing of retino-cortical conduction. The significance of these abnormalities is discussed, by analyzing the electroclinical correlationships according to whether there was obvious or subclinical optic neuritis, and the dissemination, activity or duration of the disease. The incidence of visual evoked potential abnormalities was considerably greater than that in optic neuritis (35 percent). The links between abnormalities in nervous conduction and demyelinisation are emphasized. The role and specificity of electrophysiological investigation in diagnosis of multiple sclerosis are discussed.     

Median nerve somatosensory evoked potentials during isoflurane anaesthesia

PURPOSE: The effect of isoflurane on the subcortical P14 component of the median nerve somatosensory evoked potential (SEP) is poorly known. We studied whether the P14 wave from the upper brainstem, recorded with a nasopharyngeal electrode, was attenuated at the isoflurane-induced EEG burst-suppression level. We also compared the effect of isoflurane on the P14, cervical N13 and cortical N20, N35 and N6, components. METHODS: Seventeen elective patients were anaesthetized with isoflurane. Somatosensory evoked potentials were recorded prior to anaesthesia, at 0.5 MAC and 1 MAC end-tidal isoflurane as well as at the level when EEG was in burst-suppression (mean 1.9 vol% end-tidal isoflurane). RESULTS: Isoflurane had varying effects on the subcortical components of median SEP. The amplitude of nasopharyngeal P14 was stable, but the mean latency increased from 14.4 +/- 1.2 msec at 0.5 MAC to 15.2 +/- 1.1 msec at burst-suppression level (P < 0.05). In contrast, the N13 neck response amplitude was attenuated from 3.3 +/- 0.6 microV to 2.6 +/- 0.5 microV (P < 0.005) without latency changes. The latency of the cortical N20 wave was increased from 19.7 +/- 1.1 msec at awake to 24.4 +/- 1.6 msec at burst-suppression level (P < 0.0001) and amplitude was reduced from 3.3 +/- 1.1 microV to 1.3 +/- 0.6 microV (P < 0.0001). The later cortical components were attenuated even during 0.5 MAC isoflurane and were not recordable during EEG burst-suppression. CONCLUSION: We conclude that P14 can reliably be recorded with nasopharyngeal electrodes during isoflurane anaesthesia, even during EEG burst-suppression, when the N20 wave is attenuated. In contrast, the middle-latency SEP components are sensitive to isoflurane anaesthesia.     

Usefulness of trigeminal somatosensory evoked potentials to detect subclinical trigeminal impairment in multiple sclerosis patients

Trigeminal somatosensory evoked potentials (TSEPs) by surface electric pulse stimulation were recorded in 30 normal subjects and in 70 multiple sclerosis (MS) patients, 13 of whom presenting clinical trigeminal impairment. We observed significant prolongation of all TSEPs parameters in MS group. TSEPs were abnormal in 45 patients (64.3%). Clinical and neurophysiological data agreed in 36 patients (51%) on 84 sides (60%). TSEPs were able to detect clinically silent lesions 54 times. TSEPs recording proves to be an additional useful test in MS multimodal evoked potential protocols.     

Median nerve middle-long latency somatosensory evoked potentials in children with tactile evoked spikes

It is known that hand tapping during electroencephalography (EEG) recording evokes centrotemporal spikes in some 4- to 12-year-old children. This phenomenon occurs in both healthy and epileptic children. The same children were reported to show giant middle-long latency evoked potentials (MLSEPs). To study the relationship between tactile evoked spikes (TES) and MLSEPs, we performed a neurophysiologic investigation in 25 children with TES and in 25 children without any EEG abnormality. Such an investigation included a MLSEP study after both electrical and tactile stimulation with simultaneous recording of digital EEG. MLSEPs consistently showed three negative (NI, NII, NIII) and two positive (PI, PII) peaks. Children with TES had MLSEPs of enlarged amplitude. The analysis of MLSEPs and TES characteristics led us to the conclusion that they are the same neurophysiologic event, with NII being the evoked spike and NIII the following slow-wave.     

The cortical representation of somatosensory evoked potentials of the phrenic nerve

Parasitic third-stage larvae of the sheep abomasal nematode Haemonchus contortus develop and molt in vitro to the fourth stage (L4) in 48-72 hr, at which time they begin feeding. Coincident with the third molt, larvae begin to secrete significant amounts of protein into culture fluids, including a zinc metalloproteinase. This culture-derived zinc metalloproteinase differs from a previously described metalloproteinase from infective third-stage larvae (L3[2M]), which mediates the ecdysis process. These differences include time of expression, molecular mass, and substrate specificity. The purified proteinase, from cultures of L4, has a molecular weight of approximately 46 kDa, functions as an endopeptidase, and digests several native proteins of host origin including fibrinogen and fibronectin.     

MRI of optic nerve and postchiasmal visual pathways and visual evoked potentials in secondary progressive multiple sclerosis

We studied the relationship between abnormalities shown by MRI and functional disturbances in the visual pathway as assessed by the visual evoked potential (VEP) in 25 patients with established multiple sclerosis (MS); only 4 of whom had a history of acute optic neuritis. Optic nerve MRI was abnormal in 19 (76%) and is thus useful in detecting subclinical disease. Optic nerve total lesion length and area on the STIR sequence was found to correlate significantly with prolongation of the VEP latency. This may reflect a predominantly demyelinating rather than inflammatory origin for the signal change in the optic nerve.     

The origins of the subcortical component of the median nerve somatosensory evoked potentials

Short latency median nerve somatosensory evoked potentials (SEPs) were studied in two patients presenting with well localized unilateral pontine lesions producing hemisensory deficits. Both the early subcortical (N11-N13) and the later cortical component (N19-P22) of the SEP were abnormal in the patient whose lesion involved the medial lemniscus, while they remained unaffected in a second case of lateral inferior pontine infarction in which the medial lemniscus was spread. These results suggest that the N11-N13 component of the scalp recorded SEP emanates from the brain stem medial lemniscal pathway.     

Brain-stem trigeminal and auditory evoked potentials in multiple sclerosis: physiological insights

Although suramin has long been used to treat human trypanosomiasis, recent clinical trials have tested its efficacy against the acquired immunodeficiency syndrome (AIDS) and various malignancies. Thromobocytopenia was observed in early trials with suramin in AIDS, but has been uncommon in patients treated for solid tumors. Here we describe 5 patients out of a total of 67 (7%) who developed severe thrombocytopenia while receiving suramin as part of a phase II clinical trial for metastatic prostate carcinoma refractory to hormonal therapy. IgG purified from one patient's plasma caused suramin-dependent platelet aggregation. There was also evidence of crossreactivity between suramin and heparin in this system. An immune mechanism, however, could not be documented in the other cases, suggesting that multiple mechanisms may be responsible for severe thrombocytopenia in this patient population.     

Dermatomal and mixed nerve somatosensory evoked potentials in the diagnosis of neurogenic thoracic outlet syndrome

To evaluate the diagnostic utility of dermatomal and mixed nerve somatosensory evoked potentials (SEPs) in patients with thoracic outlet syndrome (TOS) and to compare their value with routine electrodiagnostic methods, we studied a group of 44 patients with neurogenic TOS and 30 healthy controls. In addition to bilateral median and ulnar SEPs, evoked potentials were recorded after stimulation of C6 and C8 dermatomes from the first and fifth digits, respectively. The patients were classified into 3 groups according to the nature of their clinical condition. The abnormality rate for both ulnar and C8 dermatomal SEPs was 100% in a small group of patients with severe neurological signs like atrophy. In groups of patients with lesser degrees of neurogenic damage, abnormality rates for ulnar and C8 dermatomal SEPs on affected limb(s) were 67 and 50%, respectively. Same abnormality rates were 25 and 18% in patients with only subjective symptoms. In patients with objective neurological signs, the major increase in sensitivity was with electromyography (EMG). Abnormalities of routine nerve conduction studies and F-wave latency were observed in patients with severe neurogenic damage. We concluded that the most useful tests in the diagnosis of neurogenic TOS are needle EMG and ulnar SEPs.     

One-week test-retest reliability of spinal and cortical somatosensory evoked potentials by tibial nerve stimulation

Spinal (Th12) and cortical somatosensory evoked potentials by right and left posterior tibial nerve stimulation at the ankle were performed in 20 healthy volunteers (10 females and 10 males) aged 23-50 years. The procedure was repeated after one week to assess the reliability of the parameters and to establish upper normal variability limits. Reliability was measured by the intraclass correlation coefficient and was excellent for all absolute latencies and at least good for amplitudes and for the spinal-cortical conduction time. Upper variability limits were calculated using a method based on the within-subject mean square, which can be also applied in the case of more than two repetitions.     

Correlations of evoked potentials with MR imaging and MR spectroscopy in multiple sclerosis

The current state-of-the-art imaging technique in multiple sclerosis (MS) is magnetic resonance (MR) imaging. With improved imaging technology, MR spectroscopy offers the capacity to identify those chemical changes associated with MS and promises to enhance our ability to understand this disease. Physiologic function in the central nervous system can be measured using evoked potentials. This article analyzes the correlation between these two techniques.     

Development of cerebellar somatosensory evoked potentials in the rat

The effect of radio and microwave radiation on dietary atherosclerosis of rabbits was tested. 16 New Zealand white rabbits were exposed to microwave (2.45 GHz) radiation at a power density of 20--30 mW/cm2 for 4 h a day, 5 days a week for 8 to 10 weeks. Irradiated animals had serum cholesterol concentrations, aortic wall cholesterol concentrations and percentage of intimal surface involved in atherosclerotic lesions which were not different from age and weight matched controls. Continuous radio frequency irradiation (1 MHz) for 8 to 11 weeks with a field strength of 30 V/cm also failed to change these indices of atherogenesis. We conclude that under the conditions of these experiments radio and microwave irradiation do not affect the course of diet induced atherogenesis.     

High frequency oscillations in early cortical somatosensory evoked potentials

OBJECTIVE: To evaluate the characteristics of high frequency (HF) components of the early cortical somatosensory evoked potentials (SEPs). METHODS: We recorded 8-channel SEPs from the frontal and left centro-parietal scalp after right median nerve stimulation with a wide band-pass (0.5-2000 Hz) and digitized at 40 kHz sampling rate in 12 healthy subjects. HF components were analyzed after digital band-pass filtering (300-1000 Hz). The power spectrum was obtained by a maximum entropy method. RESULTS: HF oscillations (maximum power at 600-800 Hz) consisting of 5 to 8 peaks were discriminated from the preceding P14 far-field in all cases and their phases were reversed between the frontal and contralateral parietal regions. In addition, in subjects with a high amplitude central P22 potential in original wide-band recordings, a single HF oscillation with a maximum at the central region was present. Furthermore, this component showed no phase reversal over the centro-parietal area. CONCLUSION: We therefore conclude that HF oscillations are superimposed not only on the tangential N20-P20 but on the radial P22 potential, and are generated from both tangential (area 3b) and radial (area 1) current sources.     

Effect of simultaneous bilateral tibial nerve stimulation on somatosensory evoked potentials (SEP) in dogs

Since somatosensory evoked potentials (SEP) by hindlimb nerve stimulation are known to ascend bilaterally in the spinal cord, it was investigated whether or not simultaneous bilateral stimulation causes facilitation or inhibition of the stimuli. In an experiment using 36 adult Beagle dogs, the difference between simultaneous bilateral stimulation and unilateral stimulation was studied as to latencies and amplitudes. No significant difference was noted. However, the detection of far-field potentials by bilateral stimulation was not effective in which case far-field potentials did not recorded by unilateral stimulation. It was therefore confirmed that simultaneous bilateral stimulation does not cause facilitation or inhibition in the relay pathways, and does not affect the latency.     

Effects of voluntary contraction on tibial nerve somatosensory evoked potentials: gating of specific cortical responses

We evaluated vertex-parietal P37, N50, and contralateral N37 somatosensory evoked potentials (SEPs) to posterior tibial nerve stimulation during weak (20 to 30%) and strong (80 to 90%) ipsilateral gastrocnemius-soleus contraction. The results were compared with data obtained during full relaxation. P37 and N50 were attenuated significantly during weak contraction and then abolished during strong contraction, whereas the contralateral N37 was not. The N37 potential spreads over the vertex and over the ipsilateral parietal region during strong contraction. The Cz'-F3 montage was not appropriate for detecting these SEP patterns. These findings suggest that thalamic or cortical gating mechanisms affect specific cortical responses. P37 and N50 could reflect the arrival of the afferent volley into the motor areas from thalamic and cortical (subareas 1 and 2 of S1) projections. N37 could be generated in subarea 3b. Differential analysis of N37 and P37 is required in clinical practice, mainly in those conditions that involve the motor system and in those conditions in which tonic muscular activity is increased.     

Facilitation of somatosensory evoked potentials by exploratory finger movements

Modification of somatosensory processing depending on the behavioral setting was studied. Active alternating movements of the fingers, passive tactile stimuli to the hand, and active exploration of objects were performed during recording of somatosensory evoked potentials (SEPs). SEPs were elicited by compound electrical median nerve stimulation and electrical stimulation at detection threshold of cutaneous median nerve fascicles identified by microneurography. Electrical stimulation was not time-locked to the studied condition. In comparison with SEPs at rest there was attenuation of early cortical potentials up to 25 ms post-trigger in all nonresting conditions. In stimulation of the compound median nerve as well as of isolated cutaneous fascicles of a hand actively exploring an object there was an additional increased negativity, peaking at 28 ms. This facilitory effect was independent of attentional focusing and was absent during exploration using the ipsilateral, non-electrically stimulated hand. In patients with parietal lesions the facilitatory effect was diminished on the affected side. Spline interpolated brain maps at this latency based on 32-channel recordings in healthy volunteers showed a shift of local contralateral positive maximum from frontal to parietal during exploration, indicating enhancement of a tangential dipole. It is suggested that in conditions involving close sensorimotor interaction such as exploratory hand movements there is preactivation of a cortical area which is located in the central sulcus and receives cutaneous somatosensory inputs.     

Contribution of thalamic somatosensory evoked potentials to stereotaxic thalamometry

INTRODUCTION: In order to use short latency somatosensory thalamic evoked potentials (PES) to locate therapeutic targets in functional surgery, thalamic PES were recorded during stereotactic thalamotomy in 25 patients with Parkinson's disease, using a concentric bipolar semi-micro-electrode, 4 mm in diameter. In the 72 trajectories planned. 628 registers were made, obtaining 314 PES in 55 trajectories. These recordings were divided into 5 groups, according to the electrical variables evaluated in each case (absolute latency, inter-peak latency, absolute amplitude and number of phases). MATERIAL AND METHODS: The electrophysiological characteristics of the PES groups obtained, and the spatial representation of these in a tridimensional system of coordinates, is shown. We analyze the sequence of the groups of potentials in each of the trajectories followed. CONCLUSIONS: We consider that the limit between adjacent nuclear edges, ventral intermediate (Vim)-ventro-caudal (Vc), may be represented by the transition of potentials in group 1 to potentials in group 4 and/or potentials of group 3 to those of group 2. This study shows that thalamic PES are useful for locating targets during stereotactic thalamotomy.     

Effects of anesthetics on somatosensory evoked potentials by median nerve stimulation in human--analyses after single administration in volunteers

In the present study, effects of midazolam, thiopental sodium, propofol, and nitrous oxide upon SEP in a clinically used dose were investigated on 24 male volunteers. In addition, antagonistic actions of flumazenil and naloxone against effects of midazolam and nitrous oxide, respectively, on SEP were studied. Midazolam had no effect on latencies of N 20 and P 25, but increased latency of P 45 and attenuated P 100 amplitude. Flumazenil reversed these effects of midazolam of P 45 latency and P 100 amplitude to their control values. While thiopental sodium and propofol suppressed P 100 amplitude, they had no effect on N 20, P 25, P 45 latencies. Nitrous oxide elongated latencies of N 20, P 25, P 45 and decreased P 100 amplitude. Naloxone reversed the effects of nitrous oxide on N 20 and P 25 latencies without affecting increased P 45 latency and attenuated P 100 amplitude. These results suggest that midazolam might have an analgesic action of suppressing cortical sensory neurons, whereas thiopental sodium and propofol have no effect on neurons in the primary sensory cortex. The finding that naloxone antagonized the increased latencies of N 20 and P 25 by nitrous oxide could be explained by the analgesic action of nitrous oxide that could be mediated by opioid receptors. The results also indicate that electrical activities of the cortical neurons in the associated area are more susceptible to psychotropic agents than those in the primary sensory cortex. The effects of anesthetics on SEP appear to reflect their characteristics of functioning mechanisms on cortical neurons. Analysis of SEP is, therefore, useful for the assessment of the mechanism and the acting site of anesthetics in the sensory cortex.     

Multireference adaptive noise cancellation applied to somatosensory evoked potentials

Somatosensory Evoked Potentials (SEP's) contain information that is useful in diagnosing various physiological disorders. However, surface measurements of these potentials suffer from very poor Signal-to-Noise ratio (SNR) resulting in imperceptible SEP waveforms. This factor motivates the employment of dedicated signal processing techniques to improve the quality of the waveform. The objective of this research work is to improve the SNR of SEP by eliminating the predominant myoelectric interference. The strategy followed to achieve this goal is to process the SEP signal by MultiReference Adaptive Noise Cancellation (MRANC). A theoretical model for the MRANC is presented and its performance under the influence of various factors is investigated and compared with other signal processing techniques. The performance of the MRANC is then evaluated by processing simulated and in vivo SEP data. It is found that the MRANC gives a significant improvement in the SNR of the SEP.