Amplitude abnormalities in the scalp far-field N18 of SSEPs to median nerve stimulation in patients with midbrain-pontine lesion

Median nerve somatosensory evoked potential alarm related to head and neck positioning for carotid surgery

Head positioning in carotid surgery represents an often overlooked but sensitive period in the surgical plan. A 53-year-old male presented a significant decrement in median nerve somatosensory evoked potential (mSEP) following head and neck positioning for carotid pseudoaneurysm repair before skin incision.Neurophysiological monitoring was performed with mSEP and electroencephalography early during the patient's preparation and surgery. Within five minutes after rotation and extension of the head to properly expose the surgical field, the contralateral m-SEP significantly decreased in both cortical (N20/P25) and subcortical (P14/N18) components. Partial neck correction led to m-SEP improvement, allowing to proceed with the carotid repair. We discuss possible underlying pathophysiological mechanisms responsible for these changes and highlight the relevance of an early start on monitoring to avoid neurological deficits.

Sensorimotor integration and motor learning during a novel visuomotor tracing task in young adults with attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder that has noted alterations to motor performance and coordination, potentially affecting learning processes and the acquisition of motor skills. This work will provide insight into the role of altered neural processing and sensorimotor integration (SMI) while learning a novel visuomotor task in young adults with ADHD. This work compared adults with ADHD (n = 12) to neurotypical controls (n = 16), using a novel visuomotor tracing task, where participants used their right-thumb to trace a sinusoidal waveform that varied in both frequency and amplitude. This learning paradigm was completed in pre, acquisition, and post blocks, where participants additionally returned and completed a retention and transfer test 24 h later. Right median nerve short latency somatosensory-evoked potentials (SEPs) were collected pre and post motor acquisition. Performance accuracy and variability improved at post and retention measures for both groups for both normalized (P < 0.001) and absolute (P < 0.001) performance scores. N18 SEP: increased in the ADHD group post motor learning and decreased in controls (P < 0.05). N20 SEP: increased in both groups post motor learning (P < 0.01). P25: increased in both groups post motor learning (P < 0.001). N24: increased for both groups at post measures (P < 0.05). N30: decreased in the ADHD group and increased in controls (P < 0.05). These findings suggest that there may be differences in cortico-cerebellar and prefrontal processing in response to novel visuomotor tasks in those with ADHD.NEW & NOTEWORTHY Alterations to somatosensory-evoked potentials (SEPs) were present in young adults with attention-deficit/hyperactivity disorder (ADHD), when compared with neurotypical controls. The N18 and N30 SEP peak had differential changes between groups, suggesting alterations to olivary-cerebellar-M1 processing and SMI in those with ADHD when acquiring a novel visuomotor tracing task. This suggests that short-latency SEPs may be a useful biomarker in the assessment of differential responses to motor acquisition in those with ADHD.

Differential changes in somatosensory evoked potentials and motor performance: pursuit movement task versus force matching tracking task

Force modulation relies on accurate proprioception, and force-matching tasks alter corticocerebellar connectivity. Corticocerebellar (N24) and corticomotor pathways are impacted following the acquisition of a motor tracing task (MTT), measured using both somatosensory evoked potentials (SEPs) and transcranial magnetic stimulation. This study compared changes in early SEP peak amplitudes and motor performance following a force-matching tracking task (FMTT) to an MTT. Thirty (18 females) right-handed participants, aged 21.4 ± 2.76, were electrically stimulated over the right-median nerve at 2.47 Hz and 4.98 Hz (averaged 1,000 sweeps/rate) to elicit SEPs, recorded via a 64-channel electroencephalography cap, before, and after task acquisition using the right abductor pollicis brevis muscle. Retention was measured 24 h later. Significant time-by-group interactions occurred for the N20 SEP: 6.3% decrease post-FMTT versus 5.5% increase post-MTT (P = 0.013); P25 SEP: 4.0% decrease post-FMTT versus 10.3% increase post-MTT (P = 0.006); and N18 SEP: 113.4% increase post-FMTT versus 4.4% decrease post-MTT (P = 0.006). N18 and N30 showed significant effect of time (both P < 0.001). Motor performance: significant time-by-group interactions-postacquisition: FMTT improved 15.3% versus 24.3% for MTT (P = 0.025), retention: FMTT improved 17.4% and MTT by 30.1% (P = 0.004). Task-dependent differences occurred in SEP peaks associated with cortical somatosensory processing (N20 and P25), and cerebellar input (N18), with similar changes in sensorimotor integration (N30), with differential improvements in motor performance, indicating important differences in cerebellar and sensory processing for tasks reliant on proprioception.NEW & NOTEWORTHY This study demonstrates neurophysiological differences in cerebellar and somatosensory cortex pathways when learning a motor task requiring visuomotor tracking versus a task that requires force-matching modulation, in healthy individuals. The clear neurophysiological differences in early somatosensory evoked potentials associated with cortical somatosensory processing, cerebellar input, and sensorimotor integration between these two tasks demonstrate some of the neural correlates of force modulation and validate the force-matching task for use in future work.

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.

Altered central integration of dual somatosensory input after cervical spine manipulation

OBJECTIVE

The aim of the current study was to investigate changes in the intrinsic inhibitory interactions within the somatosensory system subsequent to a session of spinal manipulation of dysfunctional cervical joints.

METHOD

Dual peripheral nerve stimulation somatosensory evoked potential (SEP) ratio technique was used in 13 subjects with a history of reoccurring neck stiffness and/or neck pain but no acute symptoms at the time of the study. Somatosensory evoked potentials were recorded after median and ulnar nerve stimulation at the wrist (1 millisecond square wave pulse, 2.47 Hz, 1 x motor threshold). The SEP ratios were calculated for the N9, N11, N13, P14-18, N20-P25, and P22-N30 peak complexes from SEP amplitudes obtained from simultaneous median and ulnar (MU) stimulation divided by the arithmetic sum of SEPs obtained from individual stimulation of the median (M) and ulnar (U) nerves.

RESULTS

There was a significant decrease in the MU/M + U ratio for the cortical P22-N30 SEP component after chiropractic manipulation of the cervical spine. The P22-N30 cortical ratio change appears to be due to an increased ability to suppress the dual input as there was also a significant decrease in the amplitude of the MU recordings for the same cortical SEP peak (P22-N30) after the manipulations. No changes were observed after a control intervention.

CONCLUSION

This study suggests that cervical spine manipulation may alter cortical integration of dual somatosensory input. These findings may help to elucidate the mechanisms responsible for the effective relief of pain and restoration of functional ability documented after spinal manipulation treatment.

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.

Changes in median nerve somatosensory transmission and motor output following transient deafferentation of the radial nerve in humans

OBJECTIVE

To determine if transient anaesthetic deafferentation of the radial nerve would lead to alterations in processing of early somatosensory evoked potentials (SEPs) from the median nerve or alter cortico-motor output to the median nerve innervated abductor pollicis brevis (APB) muscle.

METHODS

Spinal, brainstem, and cortical SEPs to median nerve stimulation were recorded before, during and after ipsilateral radial nerve block with local anaesthesia. Motor evoked potentials (MEPs) and motor cortex output maps were recorded from the APB muscle.

RESULTS

There were no significant changes to most early SEP peaks. The N30 peak, however, showed a significant increase in amplitude, which remained elevated throughout the anaesthetic period, returning to baseline once the anaesthetic had completely worn off. MEP amplitude of the median nerve innervated APB muscle was significantly decreased during the radial nerve blockade. There was also a significant alteration in the APB optimal site location, and a small but significant decrease in the silent period during the radial nerve blockade.

CONCLUSIONS

Transient anaesthetic deafferentation of the radial nerve at the elbow leads to a rapid modulation of cortical processing of median nerve input and output. These changes suggest an overall decrease in motor cortex output to a median nerve innervated muscle not affected by the radial nerve block, occurring concomitantly with an increased amplitude of the median nerve generated N30 SEP peak, thought to represent processing in the supplementary motor area (SMA). Independent subcortical connections to the SMA are thought to contribute to the N30 response observed in this study. Unmasking of pre-existing but latent cortico-cortical and/or thalamo-cortical connections may be the mechanism underlying the cortical SEP increases observed following radial nerve deafferentation.

SIGNIFICANCE

Transient deafferentation of the radial nerve, which supplies wrist and hand extensor muscles, has been shown to alter sensory processing from and motor output to the median nerve innervated thenar muscles.

Topographical relationships between the brainstem auditory and somatosensory evoked potentials and the location of lesions in posterior fossa stroke

The topographical relationships between the location of brainstem lesions detected by magnetic resonance imaging and abnormality of brainstem auditory evoked potentials (BAEPs) and short-latency somatosensory evoked potentials (SSEPs) were studied in 57 patients with stroke in the posterior fossa. Abnormal BAEPs or SSEPs were associated with lesions involving the pontine tegmentum, and abnormal BAEPs also with lesions at the cerebellar peduncle. Absence of the V wave in BAEPs and N20 in SSEPs was associated with a localized overlapping area in the pontine tegmentum contralateral to stimulation. The overlapping area associated with loss of N20 coincided with the location of the medial lemniscus. Lesions widely involving the pontine tegmentum caused the disappearance of multiple waves in the BAEPs and SSEPs. Patients who entered prolonged coma or died had total loss of the III, IV, and V waves, bilateral absence to the contralateral response in BAEPs, or loss of N18 in SSEPs. The loss of N18 in SSEPs had a statistically significant correlation with bad outcome, which suggests the superiority of SSEPs for predicting the outcome of stroke and indicates the involvement of some system excluding the medial lemniscus in the generation of N18.

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.

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.

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.

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.

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.

P30 and N33 of posterior tibial nerve SSEPs are analogous to P14 and N18 of median nerve SSEPs

Generator sources of far-field P30 and N33 components produced by posterior tibial nerve stimulation were compared with those of the P14 and N18 components of median nerve stimulated somatosensory evoked potentials. Intracranial spatio-temporal distributions of P30 and N33 were similar to those of the P14 and N18 obtained by median nerve stimulation. In clinical cases, the changes in P30 and N33 were correlated with those in P14 and N18, indicative that P30 and N33 are derived from activities similar to those that produce P14 and N18.

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.