The repeat reliability of somatosensory evoked potentials

The test-immediate-retest reliability of latency and amplitude values of cervical and cortical somatosensory evoked potentials (SEP) to median nerve stimulation was assessed in 86 normal subjects aged 15 to 71 years. In addition to the stability of data between repeat trials within one test session the standard errors of measurement and the interpretable differences for SEP measures were calculated according to measurement theory. The study revealed retest correlations rtt greater than 0.80 for all latency measures of the cervical and cortical SEPs and all cortical amplitude parameters. The highest stability was found for the latency measures of the cervical components P10, N11, N13, the cortical components P16 and N20 and for the amplitude N20/P25.     

Incidence of identification and configuration variants of the NSEP signal in relation to the recording site and measuring moment

The component evaluation of the spinal and cortical recorded SEP-signals of thirty healthy volunteers after stimulation of the median nerve at the wrist gave information about variation of waveforms and frequency of peak identification with respect to the position of the active electrode and the repetition of the SEP recording. Basically one can distinguish three main groups of NSEP waveforms (Fig. 1). Firstly the "classical form" (i.e. N11 in the ascending part, N13 at the highest peak of the NSEP main component and N14 in the descending part of the curve). Secondly the "plateau-form" and thirdly the "polypeak form" (i.e. many peaks of the same amplitude). The latter two configurations were seen in more than a third of the evaluated NSEP maincomponents. Neither the position of the active electrode (C2 and C7) nor repetition of the SEP recording were found to have significant influence on the waveform. The position of the active electrode influenced the number of identificable peaks: the early components (N9 and N11) were more often recognisable when the recording was made over the lower neckregion (C7), N14 when the recording was made over the upper neckregion (C2). N13 was always identificable in both electrode positions. The repetition of the SEP recordings immediately after the first run is helpful if there was difficulty in identifying the peaks. The normal values of the SEP latency of the spinal and cortical recorded signals were evaluated taking above mentioned criteria into account.     

Somatosensory evoked potentials in moderate hyperthermia]

The effects of moderate whole-body hyperthermia on the cervical and cortical somatosensory evoked potentials (SEP) were studied in healthy male subjects, aged 22-32 years. They were immersed in hot water and heated to a median rectal temperature of 39.0 degrees C. Serial SEPs to median nerve stimulation were recorded during cooling at intervals of 0.1 degrees C. The general wave form and the amplitudes did not systematically change. For a 1 degrees C drop there was a median latency increase of 2.6-3.7% in cervical and 1.5-7.4% in cortical SEP components. In individual cases significant latency delays of cervical N13 and cortical N20 could already be observed at differences of 0.2 degrees and 0.5 degrees respectively. All other components showed significant latency changes at temperature intervals of 0.6 to 0.8 degrees C.     

Intraoperative somatosensory evoked potentials as a prognostic factor of neurologic state after carotid endarterectomy

Somatosensory evoked cervical and cortical potentials (SEP) were analyzed under general anesthesia in 106 patients undergoing carotid endarterectomy. Cortical electrical silence occurred in 5 patients without an inlying shunt; all developed a new neurologic deficit postoperatively. Analysis of the SEP in these patients revealed progredient cerebral ischemia as indicated by an increase in central conduction time (CCT) and a decrease in amplitude of the primary cortical response N20P25 resulting in a complete loss of cortical SEP later on during the clamping period. In 6 patients the insertion of a shunt restored the deteriorated SEP, these patients and those with unchanged SEP after carotid clamping showed an uneventful postoperative recovery. Taking the presence or absence of N20P25 as the sole parameter, the sensitivity of this technique was 83%, specificity 99% and predictability 83%. A normal range for CCT and amplitude of N20P25 during anesthesia and criteria for shunt insertion were developed. The presented monitoring regimen appears to be rational and is based on current concepts of cerebrovascular physiology and pathophysiology.     

Determination of neurophysiological parameters of the aging CNS. I. Evoked potentials (author's transl)]

Somatosensory (SEP) and visual (VEP) evoked potentials were studied in 65 healthy subjects who ranged in age from 62-91 years and in 48 normal persons aged 18-38 years. The amplitudes and latencies of the different components of the evoked potentials were measured. The transmission of evoked potentials from the specific response area into different regions of the ipsi- and contralateral hemisphere were studied. The findings can be summarized as follows: The latencies of the SEP and VEP of aged subjects are significantly prolonged. The increase in latency is relatively slight for the first components, progressively higher for the following components. The amplitudes of the different components of the SEP and VEP are significantly increased in aged subjects. The modifications of the SEP-latencies and of the VEP-amplitudes are more important over the dominant hemisphere. In young subjects the SE are confirmed to the centro-parietal region. In aged persons the SEP spreads over the whole hemisphere. The hypothesis that the aging process involves the synaptic transmitter mechanisms at the cortical level is discussed in relation to the involvement of the latencies. The increase in amplitude of the evoked potentials could be explained by assuming that the inhibition - an essential property of the cortex - decreases during the aging process.     

Value of somatosensory evoked potentials in thoraco- brachial outlet syndrome

INTRODUCTION: Brachial plexus involvement in symptoms of thoracic outlet syndrome (TOS) is often difficult to assess from clinical data. Conventional EMG and nerve conduction studies (NCS) do not seem reliable to all authors. For this reason, our investigations of this syndrome were complemented by study of somatosensory evoked potentials (SEP) in order to compare the results of these different techniques. PATIENTS AND METHODS: Ten patients were studied, all of whom had prominent vascular symptoms which led to their consulting a vascular surgeon. Only one had hand wasting without hypoesthesia. None had cervical rib or cervical spine anomaly. In all cases, diagnosis was confirmed by arteriography or phlebography. Operations were decided on clinical data and results of vascular investigations. Patients were tested with conventional motor and sensitive NCS F-wave studies. Needle EMG was performed in abductor pollicis brevis, first dorsal interosseus or abductor digitiminimi. Their SEP were performed as for controls. Ten controls were studied whose SEP were obtained at Erb's point (N9) and C2 cervical spine level (N13) after percutaneous stimulation of median and ulnar nerves at the wrist on both sides. The criterion of abnormality was the mean of controls + 2.5 SD for latencies. Amplitude was considered as low when it was less than 50% of the contralateral one. RESULTS: For 2 patients EMG, NCS and SEP were abnormal. One had hand wasting and denervation in hand muscles as well as slowed median and ulnar sensory conduction with low amplitude responses. SEP at Erb's point were slightly delayed after ulnar stimulation. No cervical response was obtained after ulnar stimulation. The second one had normal responses at Erb's point but delayed responses at the cervical level. In addition, N13 amplitude after ulnar stimulation was low. Four patients had normal EMG, NCS and SEP. Two patients had normal EMG and NCS, but their SEP was questionable since latencies were normal, even though amplitude was low after median and ulnar stimulation. This was not considered this to be abnormal since it was bilateral. For the remaining 2 patients (F.1), EMG and NCS and Erb's point SEP were normal, but C2 median and ulnar responses were delayed in one case and C2 ulnar response amplitude was very low on one side only in the other. In conclusion, SEP were abnormal for 4 patients out of 10 but gave more information than conventional EMG and NCS for only 2 patients. SEP abnormalities prevailed after ulnar stimulation.     

Spinal and subcortical somatosensory evoked potentials: a comparison with the localization of spinal, medullary and pontine lesions and in brain death

The spinal and subcortical median nerve SEPs were recorded in 65 patients with lesions of the cervical cord, medulla oblongata, pons and in brain death. A recording technique including cephalic, non-cephalic and anterior neck referenced leads was used. The location of the lesions corresponded to different types of SEP alterations: Cervical extramedullary lesions compressing the spinal cord corresponded to a prolonged P9-P14 interpeak latency. Cervical intramedullary lesions corresponded to the loss of N13, normally generated in the spinal interneurons. Vascular lesions of the medulla oblongata (Wallenberg's syndrome) showed normal SEPs. Space occupying medulla oblongata lesions corresponded to reduction in amplitude or loss of P14. Pontine lesions showed normal spinal and subcortical SEPs. In brain death P14 showed a graduate decrease in amplitude or alternatively vanished abruptly. The spinal and subcortical SEPs provide a good tool for testing the function of the lemniscal pathways and the spinal interneurons.     

The median nerve evoked potential in normal children and youths: normal values]

The derivation of cortical somatosensory evoked potentials permits a good local and functional diagnosis of supratentorial structures and the localisation of damage in this area. The special diagnostical relevance of early cortical somatosensory evoked potentials as a sage, non-invasive examination method in pediatric neurology is given both by these characteristics and by the fact that they cannot, in principle, be influenced by sleep or by drugs affecting the central nervous system. Somatosensory evoked cortical potentials of N. medianus were investigated for 35 children (17 boys, 18 girls) between the ages of 6 and 17, for whom there were no anamnestic or clinical indications of neurological or otological diseases. Investigations demonstrate that the SEPs of the N. medianus from the P15 up to and including the N55, is in every case clearly measurable in children from preschool age up to the end of adolescence. In the evaluation of the cortical SEPs, the P15, N20 and P25 were considered to be the components most stable and those most valuable for clinical practice. These could also be presented with the smallest standard deviations and side differences. The latencies of the P15 and N20 of the N. medianus show significant dependencies on the height of the children. The later potential components, side differences, interpeak intervals and amplitudes show no correlations with height. The SEPs of N. medianus show no dependencies on gender or on physical parameters such as weight or head circumference of the experiments. In summary it can be established that, using the procedure mentioned above, the cortical somatosensory evoked potentials in children can be described well and reproducibly and within practical narrow standard limits.     

Topography of early cortical median nerve somatosensory evoked potentials: results for routine use of the method]

The results from a topographic analysis of the early median nerve SEPs allowed to design a method for recording and analysing the SEPs in the routine laboratory. The detailed analysis of the topography of 50 normal subjects revealed: 1. An inter- and intraindividual variability of the location of the maximal amplitudes, 2. A significantly longer latency of the cortical potentials after left side stimulation, 3. A significantly higher amplitude of N20 after left side stimulation and 4. A significantly higher amplitude of the later potentials P25 and N30 after right side stimulation. It was shown that a 4-channel recording from the neck at C7 with a Fz-reference as well as from stimulus contralateral F3, CP3 and P3 or F4, CP4 and P4 with an stimulus contralateral earlobe reference provides all necessary parameters in comparison to an 20-channel recording. The definition of normal values has to take into account these results. Absolute maximum values were taken instead of the standard deviation because all amplitude values were proven to be not distributed normally. In 7 out of 30 MCA-stroke patients pathological SEP amplitudes were obtained using the 4-channel montage, whereas the 1-channel recording from CP3 and CP4 with a Fz-reference revealed normal amplitudes.     

Evoked potentials following cerebral ischemia in the rat: the effect of the stimulus frequency]

EEG and somatosensory evoked potentials were recorded in anesthetized rats before and up to 24 h after 30 min of global forebrain ischemia. Before ischemia, SEP (sweep time 1000 ms) included primary (N25P50) components and late potentials of low amplitude. During ischemia, SEP and EEG were isoelectric. During postischemic recirculation SEP evoked by stimulation at 1.0 Hz remained severely suppressed for 24 h, although long-latency potentials (> 100 ms) recovered. Using a very low stimulation frequency of 0.1 Hz, late components strongly increased in amplitude and the overall evoked activity in the averaged post-stimulus-EEG reached 90% of control. These results demonstrate that the SEP evoked at very low stimulation frequency may serve as an early indicator of functional brain recovery after prolonged cerebro-circulatory arrest.     

Age changes in early somatosensory evoked potentials

There are characteristic age-related changes in the cervical and early cortical somatosensory potentials evoked by electrical stimulation of the median nerve. At an age of 40 to 50 years the latencies of the potential components and the transit times start increasing progressively. Moreover, there is an attenuation of the cervical and an enhancement of the cortical components with age. Considering the presumed neuronal basis of the bioelectric phenomena the changes are discussed in connection with aging processes of the spinal ganglion cells, cortical pyramidal cells and the locus coeruleus.     

SEP monitoring during clonidine therapy of alcohol delirium]

In 12 patients with developing alcohol withdrawal syndrome after abdomino-thoracic surgical procedures who were treated with Clonidine, short latency somatosensory evoked potentials were recorded. Clonidine leads to a good sedation, anxiolysis and moderated the psychotic symptoms. However, there was no significant correlation between the Clonidine medication and the SEP results over the time of drug administration concerning the primary cortical response N20 and the following N20/P25 wave. Therefore, the short-latency SEP-recording cannot provide a reliable monitoring of the good sedative effect of Clonidine in alcoholic withdrawal. The SEP responses after Clonidine are like those of morphine and are thought to have a similar central mode of action.     

The effects of age on auditory event-related potentials.

The effects of age on event-related potentials (ERPs) elicited during a two-tone discrimination ("oddball") task were examined in 97 normal subjects aged from 17-80 years. Strong relationships were found between age and the latencies of the later ERP components N200 and P300. Furthermore the correlation between age and N200 latency at Pz was marginally higher than that of age and P300 latency. For the entire sample, the increase in P300 latency as a function of age was best described at Cz and Pz by linear regression equations. However, a segmented line model better described the P300/age relationship at Fz--the increase in P300 latency with age in subjects over 61 was five times that of subjects younger than 61 years. In this study the task required button-press identification of the targets--the significance of increased age and a delay in N200 latency is discussed with reference to the possibility of N200 latency indexing the speed of cognitive processing.     

Somatosensory evoked cortical potentials (SSEP) in regenerating nerves following suture (author's transl)]

In 37 patients with sutured median and ulnar nerves 106 SSEP were evaluated. Stimulation of the finger nerves was performed at a rate of 4-6/sec. In the early stage of reinnervation, just when the electrical stimulus was perceived, the second positive component (P2) of the SSEP could be recorded, but the primary part of the SEP (N1/P1) was highly reduced or absent. At this time no peripheral nerve action potential or muscle action potential could be obtained. In a follow-up study the sensory regenerating process expressed in a clinical sensibility index (S1-S4+), corresponded well with latency and amplitude of the primary potential of the SSEP: N1-latency decrease and N1/P1 and especially P2/N3-amplitude increase corresponded with improving sensibility. The abnormally high P2-wave is assumed to be an expression of central adaptation and reintegration. The method seems to be of diagnostic value as a test of peripheral nerve function especially in early and late stages of reinnervation, in verification of collateral reinnervation and in objectifying sensory defects in forensic cases.     

The effect of a reference electrode on the configuration of somatosensory evoked potentials

Early somatosensory evoked potentials to median nerve stimulation were recorded in 6 normal adults from the scalp and the neck. Simultaneous derivations consisted of a recording with a cephalic reference, recordings of both electrodes with a noncephalic reference and an arithmetically subtraction of these potentials. The electrodes were placed over the scalp (P4'), the lower and the upper neck and the reference electrodes over the midfrontal region (Fz), both ears (M1/2) and the contralateral hand. P15/P16 in the P4'-Fz derivation had no identity to the far field potentials (FFP) P13/P14 recorded from the scalp with hand reference. FFP P9 and P11 from the ear lobes was abolished in the P4'-M1/2 derivation and the residual positive waves corresponded to P13 and P14. None of the electrodes picked up a wave equivalent to P15 regardless of the reference electrode. In the neck (CV 7) - scalp derivation NSEP9, 13 and 14 corresponded to the FFP P9, 13 and 14. NSEP11 differed in latency from P11 and N11. There was no latency shift from the lower (CV 7) to the upper (CV 2) neck. NSEP9 and 11 was enhanced in amplitude from CV 7, which resulted from different polarities (N11/P11) over CV 7 and CV 2. These results indicate the necessarity of potential analysis with respect to the activity of the recording electrodes.     

Analysis of the generators of early cortical somatosensory evoked potentials (N. medianus) using dipole source analysis: initial results

There is still much controversy about the contribution of the brainstem, the thalamus and the somatosensory and motor areas of the various scalp recorded peaks of the somatosensory evoked potentials (SEP) after median nerve stimulation. This study addressed the generator problem of the scalp recorded potentials using brain electric source analysis. In 11 normal subjects median nerve SEPs were recorded from 32 locations. The brain-electric-source-analysis revealed a minimum of 5 sources with overlapping activities in the interval of 12-35 ms post stimulus. The initial deflections were in the time range of the scalp peaks (P14, P18, N20, P22, N30), but there was no single source to fully explain a scalp peak except for the brainstem source of P14. The other sources appeared to reflect activities of the thalamo-cortical-pathway (P18), of the somatosensory areas 3b (N20) and 1 (P22) and of a fifth source (contribution maximal around 30 ms) with no consistent location. The close location of multiple sources makes the precise separation and localisation of the various sources quite difficult in individual data sets.     

Clinical utility of somatosensory evoked potentials in diabetes mellitus

The posterior tibial nerve and median nerve somatosensory evoked potentials (PTN-SEPs and MN-SEPs) were investigated in 34 patients with diabetes mellitus (DM). We measured the latency of the first positive cortical potential (the cortical P37) of PTN-SEPs and that of the first negative cortical potential (the cortical N18) and Erb's potential of MN-SEPs. In 18 patients (52.9%), the cortical P37 latency was more than 3 SD longer than normal in the tibial nerve. There were positive correlations between the latency of cortical P37 and the duration of DM and the motor nerve conduction velocity of the posterior tibial nerve. Sensory action potentials of the posterior tibial nerve were not detectable in 21 patients, though cortical P37 potential was unambiguously recorded by stimulating the posterior tibial nerve even in those subjects. Diabetic retinopathy and nephropathy also tended to rise with increasing latency of cortical P37. The latency of cortical P37 is an important parameter in assessing diabetic neuropathy.     

Superiority of biphasic shocks in the defibrillation of dogs by epicardial patches and catheter electrodes

Currently available internal cardiac defibrillators use a uniphasic, truncated exponential waveform morphology of about 6 msec in duration at an energy level of 23 to 33 joules. To determine if improved defibrillation could be achieved with a different waveform morphology, we implanted 4.5 cm2 titanium patches to the left and right ventricle of 28 dogs. After ventricular fibrillation was induced, defibrillation was attempted using 7, 12, 13, or 17 joules. A 5 msec rectangular uniphasic waveform morphology was compared with a 10 msec rectangular biphasic waveform with the lagging 5 msec pulse of half the amplitude of the leading 5 msec. In an additional seven dogs, a transvenous bipolar catheter was placed with the distal electrode in the right ventricular apex and the proximal electrode in the superior vena cava. Biphasic and uniphasic shocks were compared at 14 joules. In the patch-patch system, the biphasic waveform was superior to the uniphasic waveform at 7 joules (67% versus 35%, p less than 0.001) and at 12 joules (93% versus 78%, p less than 0.001). No statistically significant differences were achieved at 13 joules or 17 joules. In the catheter electrode system with a delivered energy of 14 joules, the biphasic waveform was more effective than the uniphasic waveform (87% versus 27%, p less than 0.001). Manufacturers of automatic implantable defibrillators should consider this information in the design of future automatic implantable defibrillators.     

Motor pathway function in normoalbuminuric IDDM patients

Summary Central motor pathways were studied in 17 normoalbuminuric insulin-dependent diabetic (IDDM) patients who had been diabetic for more than 20 years, and compared with findings in 17 age-, sex-, and height-matched control subjects. The central motor conduction time was calculated from recordings of the compound muscle action potentials of the abductor pollicis brevis muscle after single transcranial and spinal root magnetic stimulation. The central motor conduction time from motor cortex to cervical spinal roots was 9.8±1.65 ms in diabetic patients and 10.1±1.48 ms in control subjects. In diabetic patients with neuropathy the central motor conduction time was 9.5±1.76 ms vs 10.1±1.56 ms in patients without neuropathy. The excitability of the motor pathways was studied by paired transcranial magnetic stimulation at interstimulation intervals of 30–1000 ms. In normal control subjects, an early facilitation of the amplitude of the compound muscle action potential at an interstimulation interval of 30 ms was found, while no facilitation was present in diabetic patients. In addition the compound muscle action potential latencies were prolonged at interstimulation intervals of 30–50 ms in diabetic patients. The changes of excitability did not correlate with the presence of peripheral neuropathy, metabolic control or diabetes duration. It is concluded that long-term normoalbuminuric IDDM patients have impaired excitability but normal central conduction time of the motor pathways.Abbreviations CMCT Central motor conduction time - CMAP compound muscle action potential - ISI interstimulation interval - MNCV motor nerve conduction velocity - SNCV sensory nerve conduction velocity - APB abductor pollicis brevis muscle - IDDM insulin-dependent diabetes mellitus - NIDDM non-insulin-dependent diabetes mellitus