F111. Isolating C8 nerve root technique with focal digital stimulation

Human median nerve somatosensory evoked potentials (SEP) contain a low-amplitude (<500 nV) high-frequency (˜600Hz) burst of repetitive wavelets (HFOs) which are superimposed onto the primary cortical response 'N20'. The following studies in human subjects are aimed to further clarify the cortical and subcortical structures involved in the generation of the HFOs and the (patho)physiological meaning of this somatosensory oscillatory activity. 1) 128-channel SEP recordings were obtained to right median nerve stimulation and ulnar nerve stimulation. Different source evaluation strategies provided converging evidence for a cortical HFO origin, with two different almost orthogonally oriented generators being active in parallel. The cortical HFO source constellation points to a „precortical“ source in terminals of thalamocortical fibers and a second intracortical HFO origin. Additionally, generation of the HFOs was localized in subcortical, near thalamic and subthalamic source sites. 2) We tested the hypothesis that HFOs might be more sensitive to temporal dispersion caused by demyelinating lesions in multiple sclerosis (MS) than the N20. HFOs in median nerve SEPs in 50 patients with definite MS and in 30 healthy controls were recorded. Three patterns of SEP alterations were found: (i) abolished HFOs with either normal (11% of stimulated limbs), or delayed N20 (16%); (ii) an attenuation of N20 amplitude with preserved HFOs (13%); and (iii) a mixture of both patterns (21%). The first pattern -normal N20 with abolished HFOs- indicates that the HFOs are a sensitive marker of slight demyelination. The second pattern is suggestive of a mainly axonal lesion type, while the third pattern points to a combined axonal/demyelinating process or a conduction block. Analysis of HFOs allows identification of slight demyelinating processes in MS patients in whom the N20 SEP component remains unaffected. 3) The behavior of HFOs in a cohort of schizophrenic patients in comparison to an age- and sex-matched group of controls was tested. HFOs in the group of patients showed a delayed latency. In the low frequency part of the SEPs an increase in amplitude was found. These results are interpreted as a lack of somatosensory inhibition in the somatosensory pathway, either on a thalamic or a cortical level. HFOs are an interesting tool to explore functionality and pathophysiology of the lemniscal system, especially of dysfunctions in thalamocortical impulse propagation.

The effects of stimulus rates upon median, ulnar and radial nerve somatosensory evoked potentials

The clinical and EMG findings in 44 patients with syringomyelia who were seen at the Mayo Clinic between 1976 and 1985 are presented. In 10 of the patients, somatosensory evoked potentials (SEPs) of the upper and lower extremities were obtained. All 44 patients had radiographic or surgical evidence of a cervical syrinx. The most common abnormality on nerve conduction studies was a reduced hypothenar compound muscle action potential amplitude (23 patients). Abnormal findings on needle electromyography were present in 33 patients and included sparse fibrillation potentials, reduced motor unit potential (MUP) recruitment, and chronic neurogenic MUP changes in muscles innervated by the C-5, T-1 roots, with the most pronounced changes in small hand muscles. Ulnar and median nerve SEPs were usually normal in the presence of a dissociated sensory loss and were usually abnormal when all sensory modalities were impaired. Abnormalities of tibial nerve SEPs were frequent and were related to impaired proprioceptive sensation in the lower extremities.

Somatosensory evoked potential (SEP) monitoring has been used to intraoperatively evaluate central nervous system (CNS) integrity during selected surgical procedures, such as carotid endoarterectomy, intracranial vascular surgery, thoracic aortic surgery, and spinal cord procedure (Fisher et al., 1995; Nuwer et al., 1995; Cunningham et al., 1987; Gugino and Chabot, 1990). Several intraoperative factors including hypothermia, cooling of the stimulated extremities, hypoxia, hypercapnia, hypotension, and anesthetic agents, can influence the outcome of intraoperative SEP monitoring (Gugino and Chabot, 1990). Although the influence of factors other than anesthetic agents can be largely eliminated by careful control of body temperature and cardio-respiratory status, alteration in SEPs by anesthetic agents is unavoidable in patients undergoing procedures that require general anesthesia. Therefore, effects of anesthetics on SEPs should be considered when interpreting changes in SEPs. Different anesthetics have disparate effects, and different components of evoked potentials have distinct sensitivities to a given anesthetic. In general, early components (subcortical SEPs) are less sensitive to anesthetics than are late components (cortical SEPs). This is probably because anesthetics affect synaptic transmission more than axonal conduction — early components are due more to axonal conduction, whereas late components depend primarily on multisynaptic transmission. For intraoperative cortical SEP monitoring, special attention is required to ensure that the agents and dose levels selected favorably preserve late waves.

Somatosensory evoked potentials (SEPs) were recorded for stimulation of both median nerves and both posterior tibial nerves in control subjects and in subjects with multiple sclerosis (MS) of several years' duration, who were ambulatory and not experiencing exacerbation. Documentation of peripheral nerve conduction revealed no abnormalities in any of the subjects. Centrally, abnormal responses to median and posterior tibial nerve stimulation were found at the spinal level and/or the scalp in nearly all MS patients. Using the latency of the initial negativity of scalp SEPs, posterior tibial SEPs were abnormal significantly more often than median SEPs. Calculations suggested a significant increase in spinal conduction time. The high incidence of abnormal SEPs does not support any substantial physiological recovery in this group of MS patients.

Two cases illustrate an uncommon failure of perioperative somatosensory evoked potential (SEP) monitoring to detect iatrogenic lesions causing temporary quadriparesis during straightforward cervical surgery. In both cases, anterior cervical discectomy...

Global cerebral ischemia (GCI) following cardiac arrest (CA) is often associated with a poor prognosis and longterm disorders of consciousness. Bilateral absence of cortical responses of the median nerve somatosensory evoked potentials (SSEP) is believed to predict poor outcome with almost 100 % specificity [1–4]. We report a case of a patient with good outcome despite repeatedly absent cortical SSEP responses and without hypothermia. A 16-year-old student suffered CA while playing soccer. Cardiopulmonary resuscitation began within 3 min. Initial rhythm analysis showed ventricular fibrillation (VF, Fig. 1a). Both pupils were dilated and unresponsive to light. After 25 min of resuscitation, stable cardiac output was reestablished and he was admitted to an intensive care unit. Despite the lack of sedative drugs, he was comatose. The GCS motor score was three and pupils were reactive to light. Because of aspiration pneumonia, the patient was ventilated and sedation was started with propofol, midazolam, and fentanyl for 48 h. Hypothermia was not induced, because this was only optional for children according to the 2005 European resuscitation council guidelines [5]. Neurological assessment after 72 h (with no sedation for the last 24) revealed a GCS of 5 (E1V1M3). At this time, median nerve SSEP were conducted in a specialized neurophysiology lab ([3000 recordings/year). N20 cortical responses were bilaterally absent (Fig. 1b). The EEG showed severe encephalopathy. Serum markers for neuronal damage were not examined. A brain CT scan after 96 h showed slight brain edema and obscuration of the caudate nucleus head (Fig. 1c). Based on the lack of arousal and the unfavourable SSEP results, the parents were informed about the diagnosis of severe GCI and the nearly inevitable poor prognosis. Because of a myoclonic status of the diaphragm, he was treated with repeated single doses of barbiturates and muscle relaxants on days 4–7. Sedation was discontinued on day 7, but again the patient failed to recover consciousness. On day 9, SSEP were repeated and again showed no signs of cortical N20 responses. He was extubated on day 12. On day 27, he was transferred to a specialized neurorehabilitation center in the minimally conscious state. He remained in inpatient rehabilitation for 7 months with constant improvement of neurological function. He was able to leave the rehabilitation facility on foot together with his mother (Fig. 1d). 3 years after cardiac arrest, the patient now lives at home with his parents (Barthel index 90/100 points). He rates his quality of life as 65 % (EuroQol, ranging from 0 to 100 %). Dysarthria and fine motor control problems are his biggest limitations on participation in life. A mutation in the SCN5A-gene was A. Bender (&) K. Howell M. Frey Department of Neurology, Therapiezentrum Burgau, Dr.-Friedl-Str. 1, 89331 Burgau, Germany e-mail: andreas.bender@med.uni-muenchen.de

N30 and the effect of explorative finger movements: a model of the contribution of the motor cortex to early somatosensory potentials

Central and peripheral SEP defects in neurologically symptomatic and asymptomatic subjects with low vitamin B12 levels

The experience with cortical somatosensory evoked potential (SEP) recording during 13 cases of spinal neurological surgery is described. Good quality cortical SEPs were obtained in eight patients with a variety of intradural and extradural spinal disorders. The short latency components of these waveforms were stable during anaesthesia with nitrous oxide, 0.5% halothane and fentanyl. Fluctuations in signal amplitude were, however, common. In the one patient in whom the cortical SEP waveform was distorted intra-operatively, there was an increased neurological deficit. In one normal and four patients with impairment of dorsal column function, no intra-operative cortical SEP was recorded. In these five patients spinal SEPs were recorded rostral to the level of spinal disease. Monitoring spinal cord function using cortical SEPs can provide useful neurophysiological information, however, there are limitations to its utility. These relate to difficulties in signal acquisition, the low signal amplitude, attenuation of the signal during intramedullary surgery and uncertainties in signal interpretation. All these problems are exacerbated if the patient has a pre-operative clinical somatosensory deficit. Although most of these problems can be overcome using spinal SEP monitoring, intra-operative SEP recordings are not an infallible guide to spinal cord integrity since they reflect the functional status of only the dorsal column-medial lemniscus pathway.

Parallels exist between the coma associated with cerebral malaria and general anaesthesia. They both produce reversible loss of consciousness. In the case of cerebral malaria and in the absence of other complications, patients recover without sequelae. General anaesthetics are so designed that patients recover from their anaesthetics very quickly and show no 'after effects'. This study compares brain function in these two clinical conditions by examining auditory- (AEPs) and median nerve somatosensory-evoked potentials (SEPs). The AEPs studied (waves Pa and Nb) are thought to arise from the primary auditory cortex and the median nerve SEPs (waves P15, N20, P25, N35, P45) from the pons, thalamus and primary somatosensory cortices. Six comatosed patients with malaria (three males, three females) aged between 19 and 38 yr were studied in Zimbabwe. Their Glasgow Coma Scores on admission were 4, 3, 6, 7, 7 and 11. Their AEPs and median nerve SEPs were recorded daily over 4 days. The data were compared with those previously collected in the UK on patients and volunteers anaesthetized with desflurane, isoflurane, sevoflurane and propofol. In general, patients with cerebral malaria showed AEPs and SEPs similar to those of light to moderate anaesthesia i.e. 0.5-1.25 measure of anaesthetic potency (MAC), where 1 MAC is the minimum alveolar concentration necessary to prevent movement to surgical incision in 50% of patients. The appearance of the AEPs and SEPs bore no relationship to the degree of coma. The auditory brainstem-evoked response was retained in all degrees of coma, as would be expected. Otherwise, it would not be possible to interpret the waveform. In most instances, the early cortical complex Pa/Nb/Pb of the AER was present. When comatose patients emerged from malarial coma or were stimulated by talking loudly to them, they showed changes in the Pa/Nb/Pb complex similar to those seen on awakening from anaesthesia. The somatosensory-evoked response showed clear P15, N20 and P25 peaks at the expected latencies, and in some instances the waveforms of cerebral malaria and lightly anaesthetized volunteers were very similar. The sensory-evoked responses of the cerebral malaria patients recorded in this study were not markedly different from those seen in light-to-moderately anaesthetized patients and volunteers. The profound depression of the AEPs and SEPs associated with deeper levels of anaesthesia were not seen, with the exception of one patient several hours before death.

Riche-Cannieu anastomosis as an inherited trait

Upper-limb somatosensory evoked potential monitoring in lumbosacral spine surgery: a prognostic marker for position-related ulnar nerve injury

Giant somatosensory evoked potentials are uncommon in patients with post-anoxic coma and paroxysmal electroencephalographic activity

BackgroundPrevious literature has shown that the frontal N30 is increased during movement of the hand contralateral to median nerve stimulation. This finding was a result of non-dominant left hand movement in right-handed participants. It is unclear however if the effect depends upon non-dominant hand movement or if this is a generalized phenomenon across the upper-limbs. This study tests the effect of dominant and non-dominant hand movement upon contralateral frontal and parietal somatosensory evoked potentials (SEPs) and further tests if this relationship persists in left hand dominant participants. Median nerve SEPs were elicited from the wrist contralateral to movement in both right hand and left hand dominant participants alternating the movement hand in separate blocks. Participants were required to volitionally squeeze (~ 20% of a maximal voluntary contraction) a pressure-sensitive bulb every ~3 seconds with the hand contralateral to median nerve stimulation. SEPs were continuously collected during the task and individual traces were grouped into time bins relative to movement according to the timing of components of the Bereitschaftspotential. SEPs were then averaged and quantified from both FCZ and CP3/4 scalp electrode sites during both the squeeze task and at rest.ResultsThe N30 is facilitated during non-dominant hand movement in both right and left hand dominant individuals. There was no effect for dominant hand movement in either group.ConclusionsN30 amplitude increase may be a result of altered sensory gating from motor areas known to be specifically active during non-dominant hand movement.