Nasopharyngeal recordings of somatosensory evoked potentials document the medullary origin of the N18 far-field

Intraoperative somatosensory evoked potential (SEP) monitoring: an updated position statement by the American Society of Neurophysiological Monitoring

Somatosensory evoked potentials (SEPs) are used to assess the functional status of somatosensory pathways during surgical procedures and can help protect patients' neurological integrity intraoperatively. This is a position statement on intraoperative SEP monitoring from the American Society of Neurophysiological Monitoring (ASNM) and updates prior ASNM position statements on SEPs from the years 2005 and 2010. This position statement is endorsed by ASNM and serves as an educational service to the neurophysiological community on the recommended use of SEPs as a neurophysiological monitoring tool. It presents the rationale for SEP utilization and its clinical applications. It also covers the relevant anatomy, technical methodology for setup and signal acquisition, signal interpretation, anesthesia and physiological considerations, and documentation and credentialing requirements to optimize SEP monitoring to aid in protecting the nervous system during surgery.

Somatosensory evoked potentials recorded from DBS electrodes: the origin of subcortical N18

The different peaks of somatosensory-evoked potentials (SEP) originate from a variety of anatomical sites in the central nervous system. The origin of the median nerve subcortical N18 SEP has been studied under various conditions, but the exact site of its generation is still unclear. While it has been claimed to be located in the thalamic region, other studies indicated its possible origin below the pontomedullary junction. Here, we scrutinized and compared SEP recordings from median nerve stimulation through deep brain stimulation (DBS) electrodes implanted in various subcortical targets. We studied 24 patients with dystonia, Parkinson's disease, and chronic pain who underwent quadripolar electrode implantation for chronic DBS and recorded median nerve SEPs from globus pallidus internus (GPi), subthalamic nucleus (STN), thalamic ventral intermediate nucleus (Vim), and ventral posterolateral nucleus (VPL) and the centromedian-parafascicular complex (CM-Pf). The largest amplitude of the triphasic potential of the N18 complex was recorded in Vim. Bipolar recordings confirmed the origin to be close to Vim electrodes (and VPL/CM-Pf) and less close to STN electrodes. GPi recorded only far-field potentials in unipolar derivation. Recordings from DBS electrodes located in different subcortical areas allow determining the origin of certain subcortical SEP waves more precisely. The subcortical N18 of the median nerve SEP-to its largest extent-is generated ventral to the Vim in the region of the prelemniscal radiation/ zona incerta.

Contribution of different somatosensory afferent input to subcortical somatosensory evoked potentials in humans

OBJECTIVES

To investigate the subcortical somatosensory evoked potentials (SEPs) to electrical stimulation of either muscle or cutaneous afferents.

METHODS

SEPs were recorded in 6 patients suffering from Parkinson's disease (PD) who underwent electrode implantation in the pedunculopontine (PPTg) nucleus area. We compared SEPs recorded from the scalp and from the intracranial electrode contacts to electrical stimuli applied to: 1) median nerve at the wrist, 2) abductor pollicis brevis motor point, and 3) distal phalanx of the thumb. Also the high-frequency oscillations (HFOs) were analysed.

RESULTS

After median nerve and pure cutaneous (distant phalanx of the thumb) stimulation, a P1-N1 complex was recorded by the intracranial lead, while the scalp electrodes recorded the short-latency far-field responses (P14 and N18). On the contrary, motor point stimulation did not evoke any low-frequency component in the PPTg traces, nor the N18 potential on the scalp. HFOs were recorded to stimulation of all modalities by the PPTg electrode contacts.

CONCLUSIONS

Stimulus processing within the cuneate nucleus depends on modality, since only the cutaneous input activates the complex intranuclear network possibly generating the scalp N18 potential.

SIGNIFICANCE

Our results shed light on the subcortical processing of the somatosensory input of different modalities.

Brainstem origins of the n18 component of the somatosensory evoked response

Proposed generator sites for the N18 component of the somatosensory evoked potential (SEP) range in location from the medulla to the thalamus. Additional knowledge regarding the generators of the N18 will be important in interpreting the results of intra-operative monitoring during skull base surgery and providing the surgeon more specific information. The goal of this study was to use both intracranial electrical recording and the effects of acute brainstem ischemia in humans to further define the generators of N18. Monopolar electrodes were used to record SEP (after median nerve stimulation) from the brainstem surface in eight patients undergoing posterior fossa surgical procedures. Recordings were made from various locations, from the cervico-medullary junction to the level of the aqueduct of Sylvius. As the electrode moved rostrally on the brainstem surface, the difference in latencies between the scalp N18 potential and the electrode potential approached zero, suggesting an upper pontine-lower midbrain origin of the N18 potential. These findings were supported by the lack of change in the N18 potentials of ten patients with basilar tip aneurysms who experienced marked changes of their N20/P22 potentials during temporary occlusion of the distal basilar artery.

Efficacy and limitations of intraoperative spinal cord monitoring using nasopharyngeal tube electrodes

Object

Motor evoked potentials are widely used for intraoperative spinal cord monitoring. However, there are problems with anesthetic constraints and high trial-by-trial variability of compound muscle action potential amplitude in muscle motor evoked potential monitoring. It is difficult to determine when to warn the surgeon of an occurrence of spinal cord risk. A method of estimation for motor function in the spinal cord has not been established. To monitor spinal cord function with reliable evoked potentials, including the upper cervical spinal cord and the ventral spinal cord, the authors developed a nasopharyngeal tube electrode that can be placed in front of the upper and ventral cervical spinal cord. The purpose of this study was to investigate the origins and pathways of descending or ascending spinal cord evoked potentials (SCEPs) elicited with this electrode, and the usefulness and limitations of this method.

Methods

A nasopharyngeal tube electrode was inserted into the nostril. A catheter electrode was placed in the epidural or subarachnoid space at the thoracic spine. Ventral SCEP was recorded from the thoracic spinal cord after transpharyngeal stimulation, and dorsal SCEP was recorded with the nasopharyngeal electrode after thoracic spinal cord stimulation. There was no restriction of anesthetic technique in recording. When the amplitude of either of the SCEPs declined to 80% of the baseline, a warning was provided to the surgeon during the observed operative procedure. At the end of surgery, less than 50% or more than 30% of the baseline amplitude was considered a significant change in both SCEPs. The sensitivity and specificity for both SCEPs to detect neurological deterioration were calculated.

Results

The electrode provided noninvasive access to the ventral cervicomedullary junction. The SCEPs showed stable responses. A response change was only observed in situations involving a risky procedure for the spinal cord. Ventral SCEPs showed high sensitivity (73.1%) for identifying patients with new neurological deficits or an exacerbation of preexisting neurological deficits after surgery, but dorsal SCEPs showed lower sensitivity (46.1%) in the total number of cases. Both SCEPs showed high specificities. The sensitivities of ventral SCEP, dorsal SCEP, and either SCEP were 100.0%, 50.0%, and 100.0% for the upper cervical spinal cord, 33.3%, 0%, and 55.6% for the lower cervical spinal cord, and 77.8%, 64.7%, and 88.2% for the thoracic spinal cord.

Conclusions

Combined recording of both SCEPs estimated the ventral and dorsal white matter function in the spinal cord. Measuring the SCEPs with the nasopharyngeal electrode can be another useful approach for upper cervical and thoracic spinal cord monitoring. Ventral SCEP was more reliable for monitoring postoperative spinal cord function than dorsal SCEP. Ventral SCEP does not estimate the gray matter and spinal root functions in the lower cervical spinal cord.

Brain-stem components of high-frequency somatosensory evoked potentials are modulated by arousal changes: nasopharyngeal recordings in healthy humans

OBJECTIVE

Until now, the demonstration that early components of high-frequency oscillations (HFOs) evoked by electrical upper limb stimulation are generated in the brain-stem has been based on the results of scalp recordings. To better define the contribution of brain-stem components to HFOs building, we recorded high-frequency somatosensory evoked potentials (SEPs) in 6 healthy volunteers by means of a nasopharyngeal (NP) electrode. Moreover, since HFOs are highly susceptible to arousal fluctuations, we investigated whether eyes opening can influence HFOs at this level.

METHODS

We recorded right median nerve SEPs from the ventral surface of the medulla by means of a NP electrode as well as from the scalp, in 6 healthy volunteers under two different arousal states (eyes opened versus eyes closed). SEPs have been further analyzed after digital narrow bandpass filtering (400-800 Hz).

RESULTS

NP recordings demonstrated in all subjects a well-defined burst, occurring in the same latency window of the low-frequency P13-P14 complex. Eyes opening induced a significant amplitude increase of the NP-recorded HFOs, whereas scalp-recorded HFOs as well as low-frequency SEPs remained unchanged.

CONCLUSIONS

Our findings demonstrate that slight arousal variations induce significant changes in brain-stem components of HFOs. According to the hypothesis that HFOs reflect the activation of central mechanisms, which modulate sensory inputs depending on variations of arousal state, our data suggest that this modulation is already effective at brain-stem level.

Re-evaluation of short latency somatosensory evoked potentials (P13, P14 and N18) for brainstem function in children who once suffered from deep coma

One of the major clinical features of brain death is deep coma. Therefore, we re-evaluated retrospectively electrophysiological examinations of brainstem function in about 31 children who had once suffered from deep coma in order to reveal its pathophysiological characteristics. The patient age at coma ranged from 1 month to 10 years (mean 2 years 1 month). The electrophysiological examinations were performed, including any of short-latency somatosensory evoked potential (SSEP), brainstem auditory evoked potential (BAEP) and blink reflexes. We first compared results between the fair and poor prognostic groups, and then re-evaluated SSEP results on a few severely impaired patients with persistent vegetative state (PVS). Subsequently, SSEP clarified more specific findings for a deep coma condition than BAEP and blink reflex. A lack of P14, N18 and N20, and an amplitude reduction or vagueness of P13 in SSEP in these children strongly suggested high risk in their future neurological prognosis. In conclusion, electrophysiological examinations, especially SSEP (P13, P14 and N18), might be very useful in obtaining a long-term neurological prognosis after deep coma in children.

Congenital hemiparesis: different functional reorganization of somatosensory and motor pathways

OBJECTIVES

To investigate the reorganization of somatosensory and motor cortex in congenital brain injury.

METHODS

We recorded motor evoked potentials (MEPs) following transcranial magnetic stimulation (TMS) and somatosensory evoked potentials (SEPs) in a 41 year old man with severe congenital right hemiparesis but only mild proprioceptive impairment. Brain magnetic resonance imaging showed a large porencephalic cavitation in the left hemisphere mainly involving the frontal and parietal lobes.

RESULTS

TMS showed fast-conducting projections from the undamaged primary motor cortex to both hands, whereas MEPs were not elicited from the damaged hemisphere. Left median nerve stimulation evoked normal short-latency SEPs in the contralateral undamaged somatosensory cortex. Right median nerve stimulation did not evoke any SEP in the contralateral damaged hemisphere, but a middle-latency SEP (positive-negative-positive, 39-44-48 ms) in the ipsilateral undamaged hemisphere, with a fronto-central scalp distribution.

CONCLUSIONS

Our data show that somatosensory function of the affected arm is preserved, most likely through slow-conducting non-lemniscal connections between the affected arm and ipsilateral non-primary somatosensory cortex. In contrast, motor function was poor despite fast-conducting ipsilateral cortico-motoneuronal output from the primary motor cortex of the undamaged hemisphere to the affected arm. This suggests that different forms of reorganization operate in congenital brain injury and that fast-conducting connections between primary cortex areas and ipsilateral spinal cord are not sufficient for preservation or recovery of function.

Examination of nasopharyngeal and tracheal somatosensory evoked potential recordings in dogs

OBJECTIVE

To investigate the value of nasopharyngeal and tracheal recordings of somatosensory evoked potentials (SSEP) in anesthetized dogs.

ANIMALS

10 healthy mixed-breed dogs (5 males and 5 females).

PROCEDURE

Square-ware electrical stimuli (50 microseconds duration, 4Hz) were delivered through bipolar surface electrodes to the median nerve of the right forelimb with 7 to 12 mA constant current. The SSEP were recorded with soft electrodes placed on the epipharynx and dorsal wall of the trachea, respectively. Traditional scalp and neck recordings of SSEP were also performed, using SC-inserted needle electrodes. The potentials recorded dorsally and ventrally from the neuraxis were compared to assess the application of these signals for intraoperative neurophysiologic monitoring.

RESULTS

Electrical stimulation of the median nerve resulted in multiphasic potentials recorded from all 4 recording sites. Latency and phase shifts were observed between the far-field potentials placed ventrally and dorsally from the neuraxis.

CONCLUSIONS AND CLINICAL RELEVANCE

Potentials recorded with nasopharyngeal and tracheal electrodes are regarded suitable for intraoperative neurophysiologic monitoring in anesthized dogs.

Anatomic origin of P13 and P14 scalp far-field potentials

After median nerve stimulation, noncephalic or earlobe reference montages enable one to record over the scalp a well-defined, positive far-field response, which has been labeled the P14 or P13-P14 complex. It has been ascertained that this wave is generated in the caudal brainstem. Its use is reliable and sometimes mandatory in assessing a number of diseases that affect primarily the brainstem, such as multiple sclerosis or coma. Because of its complex shape as well as discrepant findings in the literature, it is still debated whether this potential is produced by a single or by multiple serial generators. The authors present these different views and summarize the different recording methods, while bearing in mind that some recording techniques are more suitable for routine purposes and others are preferred in selected cases, when more information regarding caudal brainstem function is required.

Anatomic origin and clinical application of the widespread N18 potential in median nerve somatosensory evoked potentials

N18 is a broad negativity, with a duration of approximately 20 msec after positive far-field potentials and is recorded widely over the scalp using a noncephalic reference. Its origin has been controversial but its preservation after pontine or upper medullary lesion while loss after high cervical lesions suggested its medullary origin. Comparison with animal studies and direct recording studies in humans leads the authors to conclude that N18 is most likely generated at the cuneate nucleus by primary afferent depolarization. Namely, dorsal column afferents send collaterals to interneurons within the cuneate nucleus, which in turn synapse on presynaptic terminals of dorsal column fibers and depolarize them as a mechanism of presynaptic inhibition. In this way, an electrical sink is formed on presynaptic terminals, whereas their dorsocaudally situated axons serve as a source. The ventrorostral negative pole of the resultant dipolar potential must correspond to N18. The authors obtained a measure to evaluate medullary function objectively, and therefore N18 may be useful as a diagnostic tool for brain death. Usage of a C2S reference is essential for the accurate estimation of N18. Origins of other somatosensory evoked potential components related to the cuneate nucleus are also discussed.

Neuroanatomic substrates of lower extremity somatosensory evoked potentials

After stimulation of the lower extremity nerve (tibial nerve), N21 and N23 are recorded from L4 and T12 spine respectively. The far-field potentials of P31 and N35 are registered from Fpz-C5s (fifth cervical spine) or CPi (ipsilateral with respect to the side of stimulation)-ear derivation. Additional far-field potentials of P17 and P24 may be recorded from the scalp when a noncephalic (knee) reference is used. The major positive peak, P40, is registered at the vertex and the CPi. Preceding P40, there is a small negative peak, N37, recorded at the contralateral (CPc) hemisphere. Neuroanatomic substrates of these somatosensory evoked potential (SSEP) components are less well clarified compared with those of upper extremity (median nerve) SSEPs, primarily because clinical application of lower extremity SSEPs is more difficult, and all of the aforementioned potentials but one (P40) are not obligatory components. The concept of "paradoxical lateralization" complicates the issue further. Accumulating evidence, however, suggests that the far-field potentials of P17 and P31 arise from the distal portion of the sacral plexus and brainstem respectively. These correspond to P9 and P14 of the median nerve SSEPs respectively. The spinal potential of N23 is equivalent to the N13 cervical potential of the median nerve SSEP. N35 recorded from the ipsilateral hemisphere is analogous to N18 of the median nerve. Paradoxically lateralized P40 has been thought to represent the positive end of a dipole field, reflected by the negativity at the mesial surface of the contralateral hemisphere, and has commonly been considered to be equivalent to the first cortical potentials (N20) of the median nerve SSEP. However, more recent evidence suggests that the primary positivity is at the mesial cortical surface, and it more likely corresponds to P26 of the median nerve SSEP. Thus the first cortical potential corresponding to N20 is probably a small and inconsistent N37 recorded on the contralateral hemisphere. These assumptions need to be verified further by more extensive clinical studies applied to various neurologic disorders.

N18 in median somatosensory evoked potentials: a new indicator of medullary function useful for the diagnosis of brain death

OBJECTIVES

To record N18 in median somatosensory evoked potentials (SEPs) for deeply comatose or brain dead patients and to demonstrate the usefulness of N18 for the diagnosis of brain death in comparison with auditory brain stem responses (ABRs) and P13/14 in median SEPs, which have been conventionally used as complementary tests for the diagnosis of brain death.

METHODS

Subjects were 19 deeply comatose or brain dead patients. Thirteen recordings were performed in deeply comatose but not brain dead conditions, and 12 recordings were performed in brain death. N18 was evaluated in the CPi-C2S lead (or other scalp-C2S leads) to obtain a flat baseline.

RESULTS

N18 was preserved in 12 of 13 non-brain dead comatose recordings whereas it was completely lost for all of the 12 brain death recordings. P13/14 in median SEPs was preserved for all the comatose recordings, whereas apparent P13/14-like potentials, usually of low amplitude, were seen in nine of 12 brain death recordings-that is, frequent false positives. The ABRs already showed features which were characteristic for brain death (loss of components other than wave 1 or small wave 2) for four comatose recordings, in three of which N18 was preserved. The last result not only corresponds with the fact that ABRs can evaluate pontine and midbrain functions and not medullary function, but further supports the medullary origin of N18. In the four patients followed up for the course of progression from coma to brain death, N18s preserved in normal size during the comatose state were completely lost after brain death was established.

CONCLUSIONS

The N18 potential is generated by the cuneate nucleus in the medulla oblongata in the preceding studies. N18 is suggested to be a promising tool for the diagnosis of brain death because there were no false positives and rare false negatives in the present series for detecting the remaining brain stem function.

Median nerve SEP after a high medullary lesion. Preserved N18 and absent P14 components. Case report

Median nerve SEPs recorded from a patient with a high medullary lesion are described. The lesion involved the anteromedial and anterolateral right upper third of the medulla, as documented by MRI. Forty one days after the lesion, left median nerve SEP showed preserved N18 and absent P14 and N20 components; stimulation of the right median nerve evoked normal responses. These findings agree with the proposition that low medullary levels are involved in the generation of the N18 component of the median nerve SEP.

The N18 component of the median nerve SEP is not reduced by vibration

OBJECTIVE

To study the interference of mechanical vibration of the palm of the hand on the median nerve short-latency SEP components.

METHODS

Electrically-elicited short-latency median nerve SEP were obtained before and during mechanical vibration (120 Hz) of the palm in two groups of normal individuals (6 in group I and 9 in group II). The amplitude of the different components was compared between the two conditions through non-parametric statistical tests.

RESULTS

A significant reduction in the amplitude of the N9, P13/14 and N20 components was detected, however no overall significant changes were detected for the N18 component.

CONCLUSIONS

Vibration interference reduced all studied components except the N18, these findings are interpreted as supporting evidence for the proposed association between the N18 component and the inhibitory activities elicited in the dorsal column nuclei.

Generators of short latency human somatosensory-evoked potentials recorded over the spine and scalp

Somatosensory evoked potentials (SEPs) are most commonly obtained after stimulation of the median nerve and the posterior tibial nerve. SEPs reflect conduction of the afferent volley along the peripheral nerve, dorsal columns, and medial lemniscal pathways to the primary somatosensory cortex. Short-latency SEPs are recorded over the spine and scalp. After posterior tibial nerve stimulation, the following waveforms are recorded: N22, W3, the dorsal column volley, N29, P31, N34, and P37. After median nerve stimulation, the brachial plexus volley, dorsal column volley (N11), N13, P14, N18, N20, and P22 potentials are recorded. We discuss the current state of knowledge about the generators of these SEPs. Such information is crucial for proper interpretation of SEP abnormalities.

Somatosensory evoked potentials after posterior tibial nerve stimulation--normative data in children

We report normative data of somatosensory evoked potentials to posterior tibial nerve stimulation from 47 children 4-15 years of age. We recorded near-field potentials from the peripheral nerve, the cauda equina, the lumbar spinal cord and the somatosensory cortex. Far-field potentials were recorded from the scalp electrodes with a reference at Erb's point and on the earlobe. The near-field potentials N8 (peripheral nerve) and P40 (cortex) were present in all children. N20 (near-field from the cauda equina) was recorded in 38 subjects. N22 (near-field from the lumbar spinal cord), P30 and N37 ( both far-field waveforms probably generated in the brainstem) were recorded in 46 subjects each. The latencies and the peripheral conduction time (N8-N22) increased with age, while the central conduction time (N22-P40) and the intracranial conduction time (P30-P40) both decreased with age (up to about 10 years of age). The spinal conduction time (N22-P30) was relatively independent of age. The interpeak latencies allow the assessment of specific portions of this pathway. The subcortical posterior tibial nerve-somatosensory evoked potentials are of particular interest in children when the cortical peaks are influenced by sedation and sleep, or by anaesthesia.

Subcortical somatosensory evoked potentials after median nerve stimulation in children

We report our normative data of subcortical somatosensory evoked potentials (SEPs) after median nerve stimulation from a group of 55 children 4-15 years of age and 18 young adults 18-29 years of age. We recorded near-field potentials from the brachial plexus, the cervical cord and the somatosensory cortex. The far-field potentials P13, P14 and N18 from the brainstem were recorded from the scalp electrodes, when a non-cephalic reference at the contralateral Erb's point or an ear reference was used. The N9 (brachial plexus), N13a (dorsal horn), P13 (caudal medulla oblongata), N18 (medulla oblongata) and N20 (somatosensory cortex) were present in all subjects. The N13b (dorsal column near the foramen magnum or cuneate nucleus) was observed in all children and in 16 adults, P14 (medial lemniscus) in 52 children and 17 adults. The median nerve SEPs provide reliable information about the function of the somatosensory pathway from the upper limb. The subcortical median nerve SEPs should be particularly useful to detect lesions of the upper cervical cord and the cervicomedullary junction. The subcortical SEPs remain unchanged during sleep and facilitate reliable SEP recordings, when sedation is necessary in infants and children.

Preserved widespread N18 and progressive loss of P13/14 of median nerve SEPs in a patient with unilateral medial medullary syndrome

Median nerve somatosensory evoked potentials (SEPs) in a patient with unilateral medial medullary syndrome of recent onset having an MRI-confirmed lesion at upper medulla were investigated. Cortical N20 following stimulation of the affected limb was extremely depressed and delayed, whereas widespread N18, which was best manifested by the CPi-C2S lead (CPi is centroparietal electrode ipsilateral to the stimulation), showed no significant difference regarding amplitude and duration between affected and non-affected sides. The result supported our previous opinion that the principal part of N18, the broad negativity lasting around 20 ms, originates from the cuneate nucleus at the medullary level. Less steep onset of N18 on the affected side suggested that some structures rostral to the cuneate nucleus, possibly the termination of the overall ascending volley, may contribute to the earliest part of N18. P13/14 on the affected side normally preserved at the first examination progressively declined and finally disappeared after 4 months, which suggested that the major part of P13/14 is generated within caudalmost medial lemniscus, as well as the occurrence of retrograde degeneration of lemniscal fibers.

Brain-stem somatosensory dysfunction in a case of long-standing left hemispherectomy with removal of the left thalamus: a nasopharyngeal and scalp SEP study

We have studied median nerve somatosensory evoked potentials (SEPs) in a patient who had undergone early surgical removal of the left cerebral hemisphere and left thalamus. Stimulation of the right side evoked normal latency P9, P11 and P13 potentials at scalp as well as at nasopharyngeal (NP) leads, while P14 and N18 potentials were absent. These SEP abnormalities, that have been described previously in cervico-medullary lesions and in comatose patients with upper brain-stem involvement, suggest that in our patient the removal of the left thalamus has caused retrograde degeneration of the cuneate-thalamic projections. Moreover, this study confirms that P13 and P14 potentials have different generators.

Origin and distribution of P13 and P14 far-field potentials after median nerve stimulation. Scalp, nasopharyngeal and neck recording in healthy subjects and in patients with cervical and cervico-medullary lesions

We studied median nerve SEPs in 10 healthy subjects, by means of simultaneous recording over the scalp, around the neck and near the ventral surface of the medulla using a nasopharyngeal (NP) electrode. This recording technique enabled us to clearly differentiate P13 and P14 potentials. The former was always found in NP records, while the latter was more evident in scalp traces. The same technique was used to study 9 patients with various lesions of the cervical cord or cervico-medullary junction. Patients with high cervical lesions demonstrated abnormalities of both P13 and P14 potentials, while patients with lesions of the cervico-medullary junction demonstrated a clear dissociation between normal P13 in scalp and NP traces, and abnormal scalp P14. Patients with lower cervical lesions, selectively involving the central grey matter, showed normal P13 and P14 potentials, in spite of abnormal N13 cervical responses. Our findings strongly suggest that both scalp and NP P13 have the same generators in higher segments of the cervical cord, and that NP more than scalp records are effective in analyzing the P13 response. We suggest that the selective recording of the P13 potential could be useful in the assessment of focal lesions of the higher cervical cord or of the cervico-medullary junction.

Dissociated changes of frontal and parietal somatosensory evoked potentials in sleep

We studied the changes of frontal and parietal somatosensory evoked potentials (SEPs) in the awake state versus different stages of sleep in 10 normal adult subjects. Frontal and parietal SEP components were affected differentially as sleep stages progressed. In general, the amplitudes of frontal components, notably P22, were increased in sleep, whereas the amplitudes of parietal components were decreased in sleep. A sensitive waveform change from the awake state to sleep was present in the frontal response, where a subtle notched negativity, termed "N40," was present only in the awake state and quickly dissipated in all stages of sleep, including stage 1. The amplitude changes from the awake state to stage 3/4 sleep were neither linear nor parallel among SEP components. The most discordant changes occurred in stage 3/4. The amplitudes for the frontal N18-P22-N30 complex and parietal N20-P26-N32 complex increased from stage 2 to stage 3/4, while those for frontal N30-fP40 and parietal N32-pP40 decreased. In contrast to these divergent amplitude changes, the latencies of all components except P14 and frontal N18 showed progressive prolongation from the awake state to slow-wave sleep. The SEP waveforms and latencies in REM sleep approximated those in the awake state, although amplitudes for frontal peaks still remained slightly higher and amplitudes for parietal peaks slightly lower. We postulate that interactions of excitatory and inhibitory phenomena are responsible for the component-dependent and sleep-stage-dependent amplitude enhancement or depression in sleep.

Effect of manipulation and fractionated finger movements on subcortical sensory activity in man

Previous studies have shown that the somatosensory evoked potentials (SEPs) recorded from the scalp are modified or gated during motor activity in man. Animal studies show corticospinal tract terminals in afferent relays, viz. dorsal horn of spinal cord, dorsal column nuclei and thalamus. Is the attenuation of the SEP during movement the result of gating in subcortical nuclei? This study has investigated the effect of manipulation and fractionated finger movements of the hand on the subcortically generated short latency SEPs in 9 healthy subjects. Left median nerve SEPs were recorded with electrodes optimally placed to record subcortical activity with the least degree of contamination. There was no statistically significant change in amplitude or latency of the P9, N11, N13, P14, N18 and N20 potentials during rest or voluntary movement of the fingers of the left hand or manipulation of objects placed in the hand. The shape of the N13 wave form was not modified during these 3 conditions. It is concluded that in man attenuation of cortical waves during manipulation is not due to an effect of gating in the subcortical sensory relay nuclei.

[Evoked potentials by median nerve stimulation (SSEP): subcortical components]

Este estudo constitui uma revisão de literatura realizada com a finalidade de se relacionar a designação, as características dos campos de potencial e os geradores implicados, para os componentes subcorticais do potencial evocado somatossensorial por estimulação do nervo mediano no punho.

Short latency somatosensory evoked potentials recorded around the human upper brain-stem

We analyzed the intracranial spatiotemporal distributions of the N18 component of short latency median nerve somatosensory evoked potentials (SSEPs) in 3 patients with epilepsy. In these patients, depth electrodes were implanted bilaterally into the frontal and temporal lobes, with targets including the amygdala and hippocampus; the latter two targets are close to the upper pons and midbrain. In this study N18 was divided into the initial negative peak (N18a) and the following prolonged negativity (N18b). Mapping around the upper pons and midbrain showed that: (1) the amplitude of the first negativity, which coincided with scalp N18a, was larger contralateral to the side of stimulation, but showed no polarity change around the upper brain-stem; and (2) the second negativity, which was similar to scalp N18b, did show an amplitude difference or a polarity change. This wave appeared to reflect a positive-negative dipole directed in a dorso-ventral as well as dorso-lateral direction from the midbrain, where positivity arises from the dorsum of the midbrain, contralateral to the side of the stimulation. Recordings from depth electrode derivations oriented in a caudo-rostral direction suggest that N18a and N18b may in part reflect neural activity originating from the upper pons to midbrain region which projects to the rostral subcortical white matter of the frontal lobe as stationary peaks.

Preserved cortical somatosensory evoked potentials in apnoeic coma with loss of brain-stem reflexes: case report

A comatose patient suffering from diffuse cerebellar haemorrhage developed apnoea and brainstem areflexia, i.e. the clinical signs of brain death. However, median nerve somatosensory evoked potential testing 2.5 h and 22 h after the onset of this clinical syndrome showed cortical potentials partly preserved; these were abolished 46 h after the beginning of the clinical signs of brain death. This case report underlines the need for electrophysiological confirmation of brain death in patients with primarily infratentorial lesions.

Abnormalities of somatosensory evoked potentials in the quinolinic acid model of Huntington's disease: evidence that basal ganglia modulate sensory cortical input

Intrastriatal injection of quinolinic acid (QA) in rats provides an animal model that mimics some of the neuropathological and neurochemical alterations observed in the striatum of patients with Huntington's disease (HD). One of the very early neurophysiological signs in HD is a diminution of amplitude of early somatosensory evoked potentials (SEPs) recorded over the parietal cortex. The present study investigated whether the QA model exhibits similar neurophysiological abnormalities. Two weeks after unilateral intrastriatal injection of QA (240 nmol) or of the solvent, early SEPs were recorded with chronically implanted electrodes from the somatosensory cortex or from the ventrobasal nucleus of the thalamus of lightly pentobarbital-anesthetized rats, in response to single-shock electrical stimulation of the contralateral forepaw. Whereas intrastriatal injection of solvent did not influence SEPs, the striatal QA lesion significantly reduced the amplitude of early cortical SEPs by about 40% without affecting the latency. SEPs recorded from the ventrobasal nucleus were unchanged after QA lesion. Histological examination and glial fibrillary acid protein staining after intrastriatal injection of QA revealed no evidence for damage in the somatosensory system. It is concluded that (1) the QA animal model of HD mimics some of the SEP abnormalities of patients, and (2) a striatal lesion modulates somatosensory transmission to the cortex in rats.

Origin of the widespread N18 in median nerve SEP

The widespread N18 potential in median nerve SEP was studied in normal subjects and in patients with high cervical, brain-stem and thalamic lesions who had profound disturbances of deep sensation. N18 was well identified in the HSi-CV2 derivation in every normal subject as a broad elevation from the baseline lasting about 20 msec. The cortical N20 was absent in all patients. N18 was absent in a patient with a dorsal column lesion at C1-2 level. The amplitude and configuration of N18 were normal in all other patients with brain-stem and thalamic lesions, including a patient with a lesion at the ponto-medullary junction. The sagittal distribution of N18 was studied in a patient with a thalamic lesion and an oblique distribution with the maximum region between Cz and nasion was demonstrated. The present results indicate that at least the greater part of N18 is generated at the caudal most brain-stem or through branches from this level. Taking previous animal and intraoperative studies into consideration, we think it most probable that the main part of N18 corresponds to the ventro-rostral negative pole of the dipolar potential generated at the cuneate nucleus by the primary afferent depolarization of presynaptic terminals of dorsal column fibers.

Right or left ear reference changes the voltage of frontal and parietal somatosensory evoked potentials

Short-latency cortical somatosensory evoked potentials (SEPs) to left median nerve stimulation were recorded with either the left or right earlobe as reference. With a right earlobe reference the voltage of the parietal N20 and P27 was reduced while the voltage of the frontal P20 and N30 was enhanced. The effects were consistent, but their size varied with the SEP component considered and also among the subjects. Analysis of SEPs at different scalp sites and at either earlobe suggested that the ear contralateral to the side stimulated picked up transient potential differences, depending a.o. on side asymmetry and geometry of the neural generators as disclosed in topographic mapping. For example, the right ear potential can be shifted negatively by the right N20 field evoked by left median nerve stimulation. The changes involve the absolute potential values, but not the time features or the gradients of potential fields. Scalp current density (SCD) maps are not affected. The results are pertinent for current discussions about which reference to use and document the practical recommendation of recording short-latency cortical SEPs with a reference at the ear ipsilateral (not contralateral) to the side of stimulation.