Dural arteriovenous fistulas with aggressive course: clinical and angiographic correlations in two patients

Cranial dural arteriovenous fistulas (DAVFs) usually present with non-aggressive symptoms. We here report two patients who presented a peculiar clinical picture related to DAVFs, with focal neurological signs and haemorrhagic (case 1) or ischaemic lesions (case 2) respectively. The clinical and angiographic findings and putative pathophysiological mechanisms are discussed.

Repetitive magnetic stimulation A novel therapeutic approach for myofascial pain syndrome

The aim of this study was to evaluate the short, medium and long-term effects of peripheral repetitive magnetic stimulation (rMS) on myofascial pain compared with transcutaneous electrical nerve stimulation (TENS).Fifty-three subjects with myofascial trigger points (TPs) at the level of the superior trapezius muscle were allocated randomly to three groups. The first group (n=17) was treated with rMS, the second (n=18) with TENS and the third (n=18) received a placebo treatment. Each treatment consisted of ten daily 20-minute sessions. Patients were evaluated before and immediately following treatment, and at one and three months after the end of treatment. Outcome measures were: the "neck pain and disability visual analogue scale" (NPDVAS), an algometric evaluation of pain, an evaluation of the TP characteristics, and the range of cervical bending and rotation contralateral to the affected trapezius muscle. At the end of treatment, the rMS group showed a significant improvement in the NPDVAS, algometry, TP characteristics, and cervical contralateral rotation. This improvement also persisted at one and three months post-therapy. After treatment, the TENS group showed significant improvement in the same outcome measures except for algometry. At the one month follow-up visit, this improvement had returned to non significant levels in all outcome measures with the exception of NPDVAS. No significant effect of TENS was seen at the three-month follow-up visit. The placebo group showed no significant improvement in any measure. Our results strongly suggest that at medium and longer term intervals peripheral rMS may be more effective than TENS for the treatment of myofascial pain.

A randomized controlled study on the effect of two different treatments (FREMS AND TENS) in myofascial pain syndrome

AIM

Myofascial pain syndrome (MPS) is a frequent cause of chronic muscoloskeletal pain. Transcutaneous electrical nerve stimulation (TENS) is one of the most frequently employed treatments in MPS. The aim of this study is to compare the short and medium-term effects of frequency modulated neural stimulation (FREMS) to those of TENS in MPS.

METHODS

Forty subjects with upper trapezius MPS were randomly allocated to 1 of 2 groups, treated with either FREMS (n=19) or TENS (n=21). Each treatment consisted in 10 sessions lasting 20 min each. Patients were evaluated before treatment, at 1 week, and at 1 and 3 months after the end of treatment. Clinical evaluation included parameters for measurement of pain levels using the neck pain and disability visual analogue scale (NPDVAS) and algometry, evaluation of myofascial trigger point characteristics and measurement of the range of cervical movement (range of motion, ROM).

RESULTS

The FREMS group showed a significant improvement in the NPDVAS, algometry, in myofascial trigger point characteristics, and in the ROM (homolateral rotation, controlateral rotation, bending and extension) after the end of treatment and at 1 and 3 months follow-up evaluation. The TENS group showed significant improvement in the same outcome measures except for algometry and cervical extension, but these improvements were maintained only at the 1 month follow-up evaluation. However, were not observed statistically significant differences between FREMS of TENS in many of outcome measures.

CONCLUSIONS

Both FREMS and TENS have positive short-term effects on MPS. But, medium-term effects were achieved only with FREMS.

Plastic interactions between hand and face cortical representations in patients with trigeminal neuralgia: a somatosensory-evoked potentials study

Neurophysiological and neuroimaging studies suggest that pain may play a major role in determining cortical somatosensory rearrangements even in the adult brain. The re-organizational power of pain, however, has been tested in models in which massive deafferentation co-existed with pain (e.g. in phantom pain). Moreover, information on whether spinal and brainstem changes contribute to pain-related plasticity in humans is meagre. We used the non-invasive somatosensory evoked potentials technique in patients with right primary trigeminal neuralgia and no clinical signs of large-diameter fibers of trigeminal deafferentation to assess whether pain may induce plastic changes at multiple levels in the somatosensory system. Subcortical and cortical potentials evoked by stimulation of the right median and posterior tibial nerves ipsilateral to the facial pain were compared with those obtained following stimulation of the left median and tibial nerves and with those obtained in a control group tested in comparable conditions. Amplitudes of parietal N20 and P27 and frontal N30 potentials observed following stimulation of the right median nerve ipsilateral to the facial pain were greater than those of the left median nerve and showed a positive correlation with magnitude of pain. This right-left asymmetry was absent following stimulation of the patients' tibial nerves and in control subjects. No changes were found in spinal N13 and brainstem P14. That facial pain is associated with neuroplastic changes within the somatic cortical representation of the hand suggests a pain-related topographic cortical reorganisation.

The role of somatosensory feedback in dystonia: a psychophysical [correction of psycophysical] evaluation

Ten patients with idiopathic dystonia and twelve healthy controls were tested with pairs of non-noxious electrical stimuli separated by different time intervals. Stimuli were delivered (i) to the pad of the index finger (same-point condition), (ii) to the pad and to the base of the index finger (same-finger condition) and (iii) to the pad of the index and ring fingers (different-fingers condition). Subjects were asked to report if they perceived single or double stimuli in the first condition and synchronous or asynchronous stimuli in the second and third conditions. STDTs were significantly higher in dystonic than control subjects in all three conditions. Results extend current knowledge on deficits of somesthetic temporal discrimination in dystonia by showing that temporal deficits are not influenced by spatial variables.

Transient inhibition of the human motor cortex by capsaicin-induced pain. A study with transcranial magnetic stimulation

Motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) of the left motor cortex were recorded from the right first dorsal interosseous (FDI), abductor pollicis brevis (APB), abductor digiti minimi (ADM), flexor carpi radialis (FCR), extensor carpi radialis (ECR) in 17 normal subjects, before and after painful application of capsaicin on the skin overlying the right FDI and FCR muscles. The amplitude of MEPs from the FDI and FCR was significantly reduced from 20 to 30 min after the application of capsaicin over the FDI and FCR muscles, respectively, then progressively returned to the basal values. A similar trend of MEPs inhibition was observed for APB and FCR muscles, but this reduction was not significant. Indices of peripheral nerve (M-wave) and spinal cord excitability (F and H waves) did not change throughout the experiments. Motor cortex inhibition induced by tonic cutaneous pain is maximal to muscles adjacent to the painful area. This inhibition may be due to the activation of the C fibres which mediate 'slow' nociception and might be important to alert subject to possible phasic nociceptive events that may occur close to the painful area.

Somatosensory disinhibition in dystonia

Despite the fact that somatosensory processing is inherently dependent on inhibitory functions, only excitatory aspects of the somatosensory feedback have so far been assessed in dystonic patients. We studied the recovery functions of spinal N13, brainstem P14, parietal N20, P27, and frontal N30 somatosensory evoked potentials (SEPs) after paired median nerve stimulation in 10 patients with dystonia and in 10 normal subjects. The recovery functions were assessed (conditioning stimulus: S1; test stimulus: S2) at interstimuls intervals (ISIs) of 5, 20, and 40 ms. SEPs evoked by S2 were calculated by subtracting the SEPs of the S1 only response from the SEPs of the response to the paired stimuli (S1 + S2), and their amplitudes were compared with those of the control response (S1) at each ISI considered. This ratio, (S2/S1)*100, investigates changes in the excitability of the somatosensory system. No significant difference was found in SEP amplitudes for single stimulus (S1) between dystonic patients and normal subjects. The (S2/S1)*100 ratio at the ISI of 5 ms did not significantly differ between dystonic patients and normal subjects, but at ISIs of 20 and 40 ms, this ratio was significantly higher in patients than in normals for spinal N13 and cortical N20, P27, N30 SEPs. These findings suggest that in dystonia there is an impaired inhibition at spinal and cortical levels of the somatosensory system which would lead to an abnormal sensory assistance to the ongoing motor programs, ultimately resulting in the motor abnormalities present in this disease.

Neuroplastic changes related to pain occur at multiple levels of the human somatosensory system: A somatosensory-evoked potentials study in patients with cervical radicular pain

Studies suggest that pain may play a major role in determining cortical rearrangements in the adult human somatosensory system. Most studies, however, have been performed under conditions whereby pain coexists with massive deafferentation (e.g., amputations). Moreover, no information is available on whether spinal and brainstem changes contribute to pain-related reorganizational processes in humans. Here we assess the relationships between pain and plasticity by recording somatosensory-evoked potentials (SEPs) in patients who complained of pain to the right thumb after a right cervical monoradiculopathy caused by compression of the sixth cervical root, but did not present with clinical or neurophysiological signs of deafferentation. Subcortical and cortical potentials evoked by stimulation of digital nerves of the right thumb and middle finger were compared with those obtained after stimulation of the left thumb and middle finger and with those obtained in a control group tested in comparable conditions. Amplitudes of spinal N13, brainstem P14, parietal N20 and P27, and frontal N30 potentials after stimulation of the painful right thumb were greater than those of the nonpainful left thumb and showed a positive correlation with magnitude of pain. This right-left asymmetry was absent after stimulation of the patients' middle fingers and in control subjects. Results suggest that chronic cervical radicular pain is associated with changes in neural activity at multiple levels of the somatosensory system. The absence of correlation between the amplitude of spinal, brainstem, and cortical components of SEPs suggests that enhancement of cortical activity is not a simple amplification of subcortical enhancement.

Neuropsychological evidence that somatic stimuli are spatially coded according to multiple frames of reference in a stroke patient with tactile extinction

A right-brain damaged patient with pure tactile extinction was asked to report series of single or double light, brief touches delivered to both hands, the thumb or the pinkie of a single hand, the sides of a single index. The stimulated hand was positioned palm up or palm down, in front of or behind the patient, in anatomic or crossed position. In double touch conditions, stimuli coded as left-sided were extinguished not only when delivered to both hands, but also when delivered on a single hand or a single finger. The findings suggest that tactile stimuli may be coded as left or right according to multiple body anchors that are dynamically scaled from the corporeal midline, to axes centered on a single hand or finger.

Abnormal central integration of a dual somatosensory input in dystonia. Evidence for sensory overflow

Several observations suggest impaired central sensory integration in dystonia. We studied median and ulnar nerve somatosensory evoked potentials (SEPs) in 10 patients who had dystonia involving at least one upper limb (six had generalized, two had segmental and two had focal dystonia) and in 10 normal subjects. We compared the amplitude of spinal N13, brainstem P14, parietal N20 and P27 and frontal N30 SEPs obtained by stimulating the median and ulnar nerves simultaneously (MU), the amplitude value being obtained from the arithmetic sum of the SEPs elicited by stimulating the same nerves separately (M + U). Throughout the somatosensory system, the MU : (M + U) ratio indicates the interaction between afferent inputs from the two peripheral nerves. No significant difference was found between SEP amplitudes and latencies for individually stimulated median and ulnar nerves in dystonic patients and normal subjects, but recordings in patients yielded a significantly higher percentage ratio [MU : (M + U)x100] for spinal N13 brainstem P14 and cortical N20, P27 and N30 components. The SEP ratio of central components obtained in response to stimulation of the digital nerves of the third and fifth fingers was also higher in patients than in controls but the difference did not reach a significant level. The possible contribution of subliminal activation was ruled out by recording the ratio of SEPs in six normal subjects during voluntary contraction. This voluntary contraction did not change the ratio of SEP suppression. These findings suggest that the inhibitory integration of afferent inputs, mainly proprioceptive inputs, coming from adjacent body parts is abnormal in dystonia. This inefficient integration, which is probably due to altered surrounding inhibition, could give rise to an abnormal motor output and might therefore contribute to the motor impairment present in dystonia.

Temporal discrimination of somesthetic stimuli is impaired in dystonic patients

Clinical and experimental evidence documents abnormal somatosensory functions in dystonia. Despite the fact that somatosensory processing is inherently temporal, mainly spatial aspects of somatosensory functions have so far been assessed in dystonic patients. Seven patients with idiopathic dystonia and nine healthy controls were given pairs of non-noxious electrical stimuli separated by different time intervals and asked to report if they perceived single or double stimuli. Somesthetic temporal discrimination thresholds (STDT) were obtained by computing the shortest time interval at which stimuli, applied to the left or the right hand, were perceived as separate. STDT were significantly higher in dystonic than in controls thus showing for the first time that temporal and not only spatial somatosensory processing is altered in dystonia.

Evidence for an abnormal cortical sensory processing in dystonia: selective enhancement of lower limb P37-N50 somatosensory evoked potential

We evaluated brain stem P30, contralateral frontal N37, and the vertex-ipsilateral central P37, N50 somatosensory evoked potentials (SEPs) obtained in response to stimulation of the tibial nerve in 10 patients with idiopathic dystonia. Results were compared with those obtained in 10 healthy subjects matched for age and sex. The amplitude of the brain stem P30 potential and of the contralateral frontal N37 response in dystonic patients was not significantly different from that recorded in normal subjects. The vertex- ipsilateral central P37-N50 complex, which is thought to originate in the pre-rolandic cortex, was significantly enhanced in patients compared with the control group. These results suggest the enhancement of the vertex-ipsilateral central P37-N50 complex might reflect an abnormal response to somatosensory inputs of a precentral cortex which is excessively activated because of a disorder of the basal ganglia. Such inefficient sensory processing in motor areas might contribute to motor impairment in dystonia.

Nerve conduction changes during lower limb lengthening. Somatosensory evoked potentials (SEPs) and F-wave results

The effect of lower limb lengthening on nerve conduction was investigated in 5 achondroplastic subjects who underwent callotasis on a "cross-over" basis. Somatosensory evoked potentials (SEPs) and F waves from the posterior tibial nerve (PTN) were studied preoperatively and then after removal of the axial fixators. SEPs at the end of lengthening showed that both the latency of the plexus potential (P9) and, albeit to a lesser extent, the interpeak time between the plexus and the spinal cord (N15) potentials were significantly increased. The central conduction time (N15-P33) and the amplitude of the scalp responses were not modified. The latencies of the F waves were much longer at the end of bone distraction than in basal conditions. The increases in both PTN SEPs and F-wave latencies are consistent with a slowing of conduction The extent of these latency shifts correlated closely with the degree of limb lengthening. We calculated that, on average, each cm of lengthening could produce 0.21 msec and 0.22 msec delays respectively, suggesting a similar effect of the stretching on both sensory and motor fibers. Our findings indicated that the damage could be widely distributed along the whole length of the nerve, affecting both the peripheral (trunk) and proximal (plexus and root) segments. The electrophysiological changes were not associated with any persistent clinical complaint.

Parkinson's disease and lower limb somatosensory evoked potentials: apomorphine-induced relief of the akinetic-rigid syndrome and vertex P37-N50 potentials

We evaluated brainstem P30, vertex-central P37-N50 and contralateral frontal N37 somatosensory evoked potentials (SEPs) from the tibial nerve in 14 patients affected by Parkinson's disease (PD) with akinetic-rigid syndrome. In seven patients SEPs were recorded after administration of apomorphine. The cortical P37-N50 complex was either absent (five patients, eight tested sides) or significantly smaller in patients as compared to the control group (n = 18). There was a relationship between abnormalities of early vertex potentials and degree of motor impairment. Administration of apomorphine was followed by an increase in amplitude of P37-N50 response, which was maximal after 15-30 min and then progressively returned to basal values in parallel with clinical improvement. Amplitude of brainstem P30 and frontal N37 responses was within normal values and did not vary following drug administration. These results suggest that the P37-N50 complex arises from independent cortical generators, probably located in the pre-rolandic cortex, which may be selectively affected by basal ganglia dysfunction. Amplitude decrease of the P37-50 complex may reflect an abnormal processing of somatosensory inputs within the pre-central cortex due to defective modulation exerted by basal ganglia circuitry on cortical excitability. SEP potentiation following apomorphine, besides indicating that this dysfunction is partly reversible, might suggest objective method to measure therapeutic efficacy.

Inhibitory action of forearm flexor muscle afferents on corticospinal outputs to antagonist muscles in humans

1. To find out whether muscle afferents influence the excitability of corticospinal projections to antagonist muscles, we studied sixteen healthy subjects and one patient with a focal brain lesion. 2. Using transcranial magnetic and electrical brain stimulation we tested the excitability of corticomotoneuronal connections to right forearm muscles at rest after conditioning stimulation of the median nerve at the elbow. Somatosensory potentials evoked by median nerve stimulation were also recorded in each subject. 3. Test stimuli delivered at 13-19 ms after median nerve stimulation significantly inhibited EMG responses elicited in forearm extensor muscles by transcranial magnetic stimulation, but did not inhibit responses to electrical stimulation. In contrast, magnetically and electrically elicited responses in forearm flexor muscles were suppressed to the same extent. 4. The higher the intensity of the test shocks, the smaller was the amount of median nerve-elicited inhibition. Inhibition in extensor muscles was also smaller during tonic wrist extension, or if the induced electrical stimulating current in the brain flowed from posterior to anterior over the motor strip rather than vice versa. Test responses evoked by magnetic transcranial stimulation in the first dorsal interosseous and in brachioradialis muscles were not inhibited after median nerve stimulation at the elbow. Stimulation of digital nerves failed to inhibit motor potentials in extensor muscles. 5. Test stimuli delivered at 15 and 17 ms after radial nerve stimulation significantly inhibited EMG responses elicited in forearm flexor muscles by magnetic transcranial stimulation. 6. In the patient with a focal thalamic lesion, who had dystonic postures and an absent N20 component of the somatosensory-evoked potentials but normal strength, median nerve stimulation failed to inhibit magnetically evoked responses in forearm extensor muscles. 7. We propose that activation of median nerve muscle afferents can suppress the excitability of cortical areas controlling the antagonist forearm extensor muscles acting on the hand. The inhibitory effect occurs at short latency and might assist spinal pathways mediating reciprocal inhibition by contrasting the co-activation of antagonistic pools of corticospinal cells.

Neurophysiological evidence of neuroplasticity at multiple levels of the somatosensory system in patients with carpal tunnel syndrome

The human somatosensory cortex (S1) is capable of modification after partial peripheral deafferentation, but it is not known whether spinal and brainstem changes contribute to this process. We recorded spinal, brainstem and cortical somatosensory evoked potentials following ulnar nerve stimulation in patients affected by unilateral carpal tunnel syndrome with EMG evidence of chronic alterations in median nerve sensorimotor conduction at the wrist lasting at least 4 weeks, and compared them with those from the unaffected hand and with those obtained in a control group. Amplitudes of spinal N13 and brainstem P14 potentials following stimulation of the ulnar nerve ipsilateral to the deafferented median nerve were greater than those following stimulation of the contralateral ulnar nerve. Side-to-side amplitude differences in N13 and P14 were greater in patients than in the control group. Parietal N20 and P27 potentials, supposedly generated in S1, were also significantly increased. The present results suggest that a chronic pathological modification of peripheral sensorimotor inputs is associated with changes in neural activity at multiple sites of the somatosensory system. Changes in spinal and brainstem structures could contribute to the mechanisms subserving changes in the S1. Changes in synaptic strength and unmasking inputs secondary to disconnection of the normally dominant inputs to the 'median nerve' cortex may be the mechanisms underlying ulnar nerve SEP changes.

Scalp topography and source analysis of interictal spontaneous spikes and evoked spikes by digital stimulation in benign rolandic epilepsy

OBJECTIVES

We report the analysis of scalp topography and dipole modeling of the rolandic spikes in 6 patients suffering of benign rolandic epilepsy of childhood with extremely high amplitude SEP by tapping stimulation of the finger of the hand.

METHODS

EEG and BESA analysis were performed for both rolandic spontaneous interictal spikes and high amplitude scalp activity evoked by tapping and electrical stimulation of the first finger of the right hand.

RESULTS

The evoked responses showed a morphology characterized by a rapid phase (spike) followed by a slow phase (slow wave). The spike presented an early small positive component followed by a main negative component. Similar morphology, dipole configuration and source localization were observed for both rolandic spikes and evoked high amplitude scalp responses. Dipole localization showed an overlap of spatial coordinates between rolandic and evoked spikes.

CONCLUSIONS

These findings suggest that the extremely high amplitude SEPs could be evoked spikes which probably had the same cortical generators of the spontaneous rolandic spikes.

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

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

Influence of somatosensory input on paroxysmal activity in benign rolandic epilepsy with 'extreme somatosensory evoked potentials'

We studied six patients suffering from benign rolandic epilepsy of childhood with central temporal spikes who presented so-called 'extreme somatosensory evoked potentials (SEPs)' following peripheral somatosensory stimulation. Stimuli were delivered to the fingers of one hand using both a triggered tendon hammer and low-intensity electrical stimulation. The electrical stimulation was delivered in sequences in different conditions (i.e. random order, 1, 3 and 10 Hz). Both tapping and electrical stimulation produced scalp evoked potentials in all subjects, characterized by a spike followed by a slow wave, similar in morphology and scalp distribution to the spontaneously occurring spikes. This paroxysmal activity was sensitive to stimulus rate; the number of evoked spikes was inversely related to the frequency of stimulation, being maximal at 1 Hz and disappearing at high frequencies (10 Hz). Spontaneous spikes disappeared during high-frequency stimulation but were present during low-frequency stimulation. Averaged SEPs at 3-Hz stimulation showed a late high-amplitude component, identical in morphology and distribution to the single evoked spike. We therefore conclude that, in these subjects, the so-called 'extreme SEPs' are evoked spikes and that evoked and spontaneous spikes share common cortical sensorimotor generators. The evidence that these generators can be influenced by afferent input provides important information regarding the functional mechanisms involved in modulating cortical excitability in benign rolandic epilepsy. Moreover, we suggest that peripheral electrical stimulation can be used as an additional activation test in this kind of epilepsy.

Modulation of ipsilateral motor cortex in man during unimanual finger movements of different complexities

To understand the role of the ipsilateral motor cortex in the control of unimanual movements, we evaluated changes in cortical motor evoked potentials (MEP) from the left abductor pollici brevis (APB) to transcranial magnetic stimulation (TMS) of the right hemisphere in nine normal subjects during execution of right finger movements of different complexities. The motor tasks were (a) repetitive opposition movement (thumb tapping the 3rd finger); (b) isolated finger movements in a 'usual' sequence (thumb tapping fingers 2, 3, 4 and 5) and (c) in an 'unusual' sequence (thumb tapping fingers 3, 5, 2 and 4). Subjects were trained before the study up to disappearance of EMG synkinetic activity in the left APB. As compared to the rest condition, MEP amplitude was enhanced in all subjects during paradigm (b) and even more during (c), but remained unchanged during paradigm (a). The MEP increase disappeared in four out of the nine subjects undergoing overtraining. No significant modifications in MEP amplitude were found in the left proximal muscle (biceps, five subjects). The H reflex induced by left median nerve stimulation at the elbow (four subjects) and MEPs from the left APB to transcranial electrical stimulation (three subjects) were not significantly affected by any of the motor paradigms, indicating that the motor cortex was the site of change. These results provide evidence of an increased excitability of cortical motor outputs targeting the unmoving hand muscles during contralateral sequential finger movements which disappears with overtraining. We conclude that during motor learning there is an interhemispheric transfer of information, possibly in order to inhibit the opposite hemisphere from interfering when a fine unimanual movement is required.

Crossed and direct effects of digital nerves stimulation on motor evoked potential: a study with magnetic brain stimulation

We studied the influence of contralateral and ipsilateral cutaneous digital nerve stimulation on motor evoked potentials (MEPs) elicited in hand muscles by transcranial magnetic stimulation (TMS). We tested the effect of different magnetic stimulus intensities on MEPs recorded from the thenar eminence (TE) muscles of the right hand while an electrical conditioning stimulus was delivered to the second finger of the same hand with an intensity four times above the sensory threshold. Amplitude decrement of conditioned MEPs as a function of magnetic stimulus intensity was observed. The lowest TMS stimulus intensity produced the largest decrease in conditioned MEPs. Moreover, we investigated the effects of ipsilateral and contralateral electrical digital stimulation on MEPs elicited in the right TE and biceps muscle using an intensity 10% above the threshold. Marked MEP inhibition in TE muscles following both ipsilateral and contralateral digital stimulation is the main finding of this study. The decrease in conditioned MEP amplitude to ipsilateral stimulation reached a level of 50% of unconditioned MEP amplitude with the circular coil and 30% with the focal coil. The amplitude of conditioned MEPs to contralateral digital stimulation showed a decrease of 60% with the circular coil and more than 50% with the focal coil. The onset of the inhibitory effect of contralateral stimulation using the focal coil occurred at a mean of 15 ms later than that of ipsilateral stimulation. No MEP inhibition was observed when recording from proximal muscles. Ipsilateral and contralateral digital stimulation had no effect on F wave at appropriate interstimulus intervals, where the main MEP suppression was noted. We stress the importance of selecting an appropriate test stimulus intensity to evaluate MEP inhibition by digital nerves stimulation. Spinal and cortical sites of sensorimotor integration are adduced to explain the direct and crossed MEP inhibition following digital nerves stimulation.

Reversible changes of motor cortical outputs following immobilization of the upper limb

We mapped the cortical representations of the abductor pollicis brevis, flexor carpi radialis, biceps and deltoid muscles in six subjects with unilateral wrist fractures, immediately after the removal of the splint. This was repeated 1 month later in three out of the six subjects. Duration of immobilization was 1 month. Muscle maps were obtained by delivering four focal magnetic pulses for each scalp position (1 cm apart with reference to Cz) over the contralateral hemisphere. Motor evoked potentials (MEPs) were averaged off-line and expressed as a percentage of the motor action potential evoked by supramaximal peripheral nerve stimulation. Volume, area and threshold of the motor maps showed no significant hemispheric differences within each muscle in 10 control subjects. In the first recording session the volume of each immobilized muscle was distinctly higher when compared to that of controls in terms of absolute value and side-to-side ratio. This finding disappeared 1 month later. Moreover, MEP amplitude difference recorded from hand muscle could be reversed during a small tonic voluntary contraction. Immobilization had no significant effect on the threshold for activation of the target muscles and on the area of the motor map. The increase in MEP amplitudes occurred without changes in spinal excitability as tested by the F wave. These findings suggest that immobilization of the upper limb induces a reversible enhancement of the excitability of structures along the corticomotoneuronal pathway. Sustained restriction of volitional movements and reduction in somatic sensory inputs might promote this functional modulation of the motor system.

Selective gating of lower limb cortical somatosensory evoked potentials (SEPs) during passive and active foot movements

We evaluated subcortical and cortical somatosensory evoked potentials (SEPs) in response to posterior tibial nerve stimulation in 4 experimental conditions of foot movement and compared them with the baseline condition of full relaxation. The experimental conditions were: (a) active flexion-extension of the stimulated foot; (b) active flexion-extension of the non-stimulated foot; (c) passive flexion-extension of the stimulated foot in complete relaxation; (d) tonic active flexion of the stimulated foot. We analyzed latencies and amplitudes of the subcortical P30 potential, of the contralateral pre-rolandic N37 and P50 responses and of the P37, N50 and P60 potentials recorded over the vertex. Latencies did not vary in any of the paradigms. The amplitude of subcortical P30 potential did not change during any of the paradigms. Among the cortical waves, P37, N50 and P60 amplitudes were significantly attenuated in all conditions except active movement of the non-stimulated foot (b). This attenuation was less during passive (c) than during active movements of the stimulated foot (a and d). The contralateral pre-rolandic waves N37 and P50 showed no significant decrease during any of the paradigms. These results suggest that gating occurs rostrally to the cervico-medullary junction, probably at cortical level. The different behavior of N37, P50 and P37, N50 cortical responses during movement of the stimulated foot provides evidence suggestive of a highly localized gating process occurring at cortical level. These potentials could reflect activation of separate, functionally distinct generators.

Amplitude changes of tibial nerve cortical somatosensory evoked potentials when the ipsilateral or contralateral ear is used as reference

We performed topographical mapping of somatosensory evoked potentials (SEPs) to the posterior tibial nerve using earlobe references both ipsilateral and contralateral to the stimulation side. The voltage of the frontal contralateral N37 and P50 components was enhanced, while the voltage of the parietal ipsilateral P37 and N50 components was reduced when the contralateral earlobe was substituted by the ipsilateral earlobe reference. Maps of the same data documented concomitant changes in negative and positive potential fields, showing an expansion of the pre-Rolandic N37 toward the centrotemporal contralateral regions, and a tendency of the parietal ipsilateral P37, N50 components to be more focally distributed at the vertex. SEPs recorded at each earlobe (Cv6 reference) provided an explanation of these results: The contralateral earlobe detected a negative potential corresponding to the N37 potential recorded over the scalp, followed by a P50 potential that attenuated the contralateral responses and enhanced the ipsilateral ones. The ipsilateral earlobe had no significant effects on scalp SEPs, since it detected only a large N33 negativity. Current source density (CSD) maps were, of course, not influenced by the ear used as reference. Our results suggest that the ipsilateral ear reference is better than the contralateral one for recording "genuine" cortical SEPs. Therefore, it can be recommended in the clinical domain for mapping studies of lower-limb cortical SEPs.

Binocular interaction in normal vision studied by pattern-reversal visual evoked potential (PR-VEPS)

Monocular and binocular visual evoked potentials (VEPs) in response to different check size (15-21-38-84 minutes or arc) were studied in 14 subjects with normal visual acuity and stereopsis. The binocular VEP amplitude is slightly higher than the VEP amplitude on stimulation of the "better eye" and significantly higher than the VEP amplitude on stimulation of the "worse eye"; this effect is observed using small checks and almost exclusively involved N75-P100. Both the N75 and P100 peaks occur earlier after binocular than monocular stimulation. The shortening of the N75 mean latency is significantly greater than that of the P100 mean latency when larger check sizes are used. The mean latency of the N145 potential is not significantly different in monocular and binocular stimulus conditions. The slight summation effect and latency shortening in the binocular VEPs are not consistent with the hypothesis that it is the sum of separate monocular signals originating from the visual cortex that gives rise to the response. The early components of both monocular and binocular VEPs are thought to be of post-synaptic origin (outside layer 4c of area 17), where the inputs become mixed so that most cells receive information from both eyes. The amplitude enhancement of binocular VEPs, which mainly occurs when using small checks, may be related to the increase in the total amount of cortical activity representing the macular region; this may account for binocular superiority in fine spatial resolution. The latency shortening in binocular conditions can be explained by considering that the critical determinant of the latency is the fundamental spatial frequency of the pattern. When coarse patterns are used, their effectiveness in parafoveal stimulation may affect the VEPs, with a significant contribution coming from the more peripheral retina. The enlargement of the visual field when the eyes see simultaneously may therefore further reduce the latency of the response when using the larger checks suitable for eccentric stimulation.

Transient deafferentation in humans induces rapid modulation of primary sensory cortex not associated with subcortical changes: a somatosensory evoked potential study

Human somatosensory cortex (S1) is capable of rapid modification after temporary peripheral deafferentation but it is not known whether subcortical changes contribute to this modulation. We recorded spinal, brainstem and cortical somatosensory evoked potentials (SEPs) to median nerve stimulation following anaesthetic block of the ipsilateral ulnar nerve. Spinal N13 and subcortical P14, N18 potentials remained unchanged during the experiment. N20/P20, P27 and N30 cortical potentials, which are generated in different subareas of the S1 (N20/P20, N30 in area 3b; P27 in area 1), showed different increases in amplitude during the anaesthesia, which were more marked for N20/P20 and N30 than for P27 potentials. These results suggest that changes in S1 neural activity induced by transient deafferentation may be primarily intracortical in origin and appear to be segregated within the different subareas of the somatosensory cortex. Unmasking of pre-existing thalamo-cortical projections from median nerve territories, induced by ipsilateral ulnar nerve deafferentation, may be the mechanism underlying cortical SEP enhancement.

Neural generators of tibial nerve P30 somatosensory evoked potential studied in patients with a focal lesion of the cervicomedullary junction

The tibial nerve P30 potential was studied in 6 patients with focal lesions located in the vicinity of the cervicomedullary junction. P30 potential was unaffected while cortical P39 was abnormal in the patients with a supramedullary lesion affecting the somatosensory pathway just above its decussation. Conversely, P30 was abnormal in the presence of a lesion situated caudally to the cervicomedullary junction affecting the lower limb sensory fibers just below their decussation. Median nerve P14 behaved similarly to the P30 potential in these cases. These clinical observations suggest that P30 potential, as P14 of median nerve somatosensory evoked potentials, is generated in the lower brain stem probably before the decussation of the sensory fibers; nucleus gracilis and medial lemniscus fibers in the lower brain stem are probably the anatomical structures generating P30 potential. This suggests that P30 potential may be used to study intraspinal and intracranial conduction times separately in the afferent somatosensory pathways.

Motor neuron disease with pyramidal tract dysfunction involves the cortical generators of the early somatosensory evoked potential to tibial nerve stimulation

We evaluated somatosensory evoked potentials (SEPs) to tibial nerve stimulation in 39 patients with sporadic motor neuron disease using multiple scalp derivations (earlobe reference). SEPs were altered in 22 of 29 amyotrophic lateral sclerosis (ALS) patients, whereas they were unaffected in 10 progressive muscular atrophy (PMA) patients. The main changes involved the amplitude and the field distribution of the early P40 and N37 cortical potentials with different modalities varying from a selective loss of the P40 potential (33% of tested sides) to absence of all early cortical SEPs (22% of tested sides). The later components following N50 were generally spared. The commonly used Cz-Fz montage was inadequate for detecting these alterations. Central afferent conduction was slightly affected. The selective loss of cortical SEPs and their close correlation with clinicoelectrophysiologic evidence of central motor system involvement strongly support a cortical origin of the SEP alterations in ALS. We suggest that neuronal loss in the somatosensory cortex may selectively affect the generator sites of the cortical SEPs to lower limb stimulation.

'Direct' and 'crossed' modulation of human motor cortex excitability following exercise

Rapid repetitive movements of the thumb (1 min duration) produce a reversible decrease in the activated primary motor cortex (MI) excitability to transcranial magnetic stimulation (TMS) with recovery within 35-40 min. In the present study we investigated (1) the role of peripheral sensory feedback in inducing such decrease and (2) possible effects of exercise on the non-activated MI. Stimulation of peripheral Ia afferent fibres, induced by 1 min vibration of thenar muscles and 2 Hz electrical stimulation of the median nerve at the wrist, have no effect on motor evoked potential (MEP) amplitude to TMS suggesting no role for sensory feedback to produce MI excitability modulation. Exercise produces a significant (P < 0.01) decrease of MEPs for homologous non-exercised muscles with concomitant contraction of corresponding motor cortical output maps, suggesting that changes in MI excitability also occur in the nonactivated hemisphere. This 'crossed' effect might relate to an interhemispheric transfer of information, via homotopic connections of the corpus callosum.

Effect of stimulus rate on the cortical posterior tibial nerve SEPs: a topographic study

We performed topographical mapping of somatosensory evoked potentials (SEPs) in response to posterior tibial nerve stimulation delivered at 2, 5 and 7.5 Hz in 15 healthy subjects. P37 was significantly attenuated at 5 and 7.5 Hz and the N50 component attenuated only at 5 Hz, its amplitude remaining stable for further increases in stimulus frequency. Frontal N37 and P50 potentials showed no significant decrease when the stimulus repetition frequency was changed from 2 to 7.5 Hz. P60 showed an attenuation of the amplitude only at 7.5 Hz. Latency and scalp topographies of all cortical components examined remained unchanged for the 3 stimulus rates tested. The optimal stimulus rate for mapping of tibial nerve SEPs was lower than 5 Hz. The distinct recovery function of the contralateral. N37-P50 and ipsilateral P37-N50 responses suggests that these potentials arise from separate generators.

Subcortical P30 potential following tibial nerve stimulation: detection and normative data

Stimulation of the tibial nerve evokes a P30 far-field potential over the scalp which, like the median nerve P14, probably originates in the vicinity of the cervico-medullary junction. Unlike the P14 potential, P30 recording has not been systematically performed in clinical practice, probably because of doubts about the generator of the potential and the possibility of consistently recording it on the scalp after the unilateral stimulation of the tibial nerve. In this study, we tested the reliability of the tibial nerve scalp far-field P30 potential in 34 normal subjects using different montages, of which the Fpz-Cv6 derivation gave the highest signal to noise ratio, making it possible to obtain a P30 potential peaking at 29.2 +/- 1.6 msec in all normal subjects. This suggests that this component should to be included in the routine recording of tibial nerve SEPs in order to evaluate the spinal and intracranial conduction of the somatosensory pathway separately.

Long-lasting depression of motor-evoked potentials to transcranial magnetic stimulation following exercise

We used transcranial magnetic stimulation to study the modulation of motor cortex excitability after rapid repetitive movements. Eleven healthy subjects aged 24-32 years were evaluated. Serial motor-evoked potential (MEP) recordings were performed from the right thenar eminence every 5 min for a period of 20 min at rest and for a period of 35 min after repetitive abduction-adduction of the thumb at maximal frequency for 1 min. All subjects presented distinct changes in MEP amplitude after exercise with an approximately 55% mean maximal decrease compared with basal conditions and complete recovery 35 min after the end of the exercise. The time course of MEP amplitude changes presented the following trend: (1) a rapid decrease phase within the first 5 min; (2) a maximal depression phase of 10 min duration (from the 5th to the 15th min); and (3) a slow recovery phase. No significant modifications in post-exercise MEP amplitude were found in ipsilateral non-exercised muscles. In order to determine the level where these changes take place, we recorded the M and F waves induced by median nerve stimulation at the wrist (all subjects) and MEPs in response to transcranial electrical stimulation (five subjects) at rest and during the decrease and maximal depression phases. None of these tests were significantly affected by exercise, indicating that the motor cortex was the site of change. Evaluation of maps of cortical outputs to the target muscle, performed in four subjects, showed an approximately 40% spatial reduction in stimulation sites evoking a motor response during the maximal depression phase. These data prove that exercise induces a reversible, long-standing depression of cortical excitability, probably related to intracortical presynaptic modulation, which transitorily reduces the motor representation area.

Dynamic F wave from lower limbs: value and clinical application

We evaluated changes in F wave from the lower limbs after walking in normals. Twenty-two healthy subjects ranging in age from 24 to 74 years (mean 40.2 SD 15.2) were investigated. The peroneal nerve was examined in 16 subjects and the posterior tibial nerve in 22 subjects on the dominant side. F waves were evaluated at rest and 1 minute and 10 minutes after effort testing consisting in prolonged walking for 15 minutes. F wave minimum, maximum and mean latencies, chronodispersion, duration, persistence and amplitude were evaluated. Only chronodispersion showed a significant decrease for both the peroneal and tibial nerve (p < 0.05) after walking. Moreover we evaluated the F wave after effort in five patients affected by neurogenic claudication: three patients presented a further slowing of proximal conduction, while two patients exhibited a transitory conduction block. In conclusion, F wave changes observed in dynamic conditions in normals probably reflect a synchronization of motorneuron firing requiring a certain amount of descending facilitation. On the other hand, the marked F wave changes after effort observed in patients increase the sensitivity of this method in the clinical setting and may provide insights into the pathophysiologic basis of neurogenic claudication.

Assessment of intraspinal and intracranial conduction by P30 and P39 tibial nerve somatosensory evoked potentials in cervical cord, brainstem, and hemispheric lesions

In routine recordings of tibial nerve somatosensory evoked potentials (SEPs), a global central conduction time is evaluated by measuring the interval between the segmental spinal N22 potential, recorded in the lumbar region, and the cortical P39 potential. In this study, we tested the reliability of the scalp far-field P30 potential, which originates in the vicinity of the cervico-medullary junction, in order to evaluate separately intraspinal and intracranial conduction in normal subjects and patients with cervical cord and intracranial lesions. P30 and cortical P39 potentials were studied in 23 healthy subjects and in 70 patients with cervical cord (n = 47), brainstem (n = 11) or hemispheric lesions (n = 12) selected on the basis of neuroimaging--computed tomography (CT) or magnetic resonance (MR)--findings. Median nerve SEPs were also recorded in all patients. Of the several montages tested to obtain the P30 potential, the Fpz-Cv6 derivation gave the highest signal-to-noise ratio; it permitted to obtain a P30 potential that peaked at 29.2 +/- 1.6 ms in all normal subjects. P30 abnormalities were observed only in patients with cervical or cervico-medullary lesions; these were associated with a normal P39 in only two of 33 abnormal recordings. Conversely, P30 was consistently normal in lesions situated above the cervico-medullary junction whether associated with normal, delayed, or reduced P39. P30 abnormalities were subclinical in 42% of abnormal recordings. All patients with normal tibial and median nerve SEPs on both sides had normal touch, joint, and vibration sensation in the four limbs. There was a strong correlation between tibial nerve P30 and median nerve P14 data in the whole series of patients; both potentials behaved similarly in all cases of intracranial supramedullary lesions. Combined abnormalities of P30 and P39 potentials thus indicate that conduction is impaired at the spinal level and proved to be particularly informative for detecting spinal cord dysfunction in patients with neuroimaging evidence of a narrowed cervical canal. Recording of abnormal N13, P14, or P30 potentials provided evidence of a cervical cord dysfunction in 66% of patients who had a suspected spondylotic myelopathy. Recording of tibial nerve P30 potential has proven to give reliable and useful information when a separate assessment of intraspinal and intracranial somatosensory conduction is needed; it merits inclusion, as does the upper limb N13 potential, in the evaluation of patients whose MR image indicates cervical canal narrowing.

Two distinct cervical N13 potentials are evoked by ulnar nerve stimulation

To investigate the dual nature of the posterior neck N13 potential, we attempted to establish the presence of a latency dissociation between caudal (cN13) and rostral (rN13) potentials on stimulating the ulnar nerve, in view of its lower radicular entry compared to the median nerve. SEPs were evaluated in 24 normal subjects after both median and ulnar nerve stimulation. cN13 was prominent in the lower cervical segments, and rN13 was localized mainly in the upper ones using anteroposterior and longitudinal bipolar montage, respectively. The N9-cN13 interpeak latency did not differ significantly from N9-rN13 when stimulating the median nerve. On the other hand, the N9-rN13 interpeak was significantly longer than the N9-cN13 interpeak when the ulnar nerve was stimulated. The rN13 presented the same latency as P13-P14 far-field potentials in 17 out of 24 ulnar nerves tested. Therefore, the ulnar nerve stimulation evokes two distinct posterior neck N13 potentials. It is widely accepted that the caudal N13 is a postsynaptic potential reflecting the activity of the dorsal horn interneurons in the lower cervical cord. We suggest that the rostral N13 is probably generated close to the cuneate nucleus, which partly contributes to the genesis of P13-P14 far-field potentials.

Neurophysiological and urodynamic examinations in the functional assessment of the spinal cord below the injury site

We examined the characteristics of specific components of the evoked electrospinogram (EESG) in response to tibial nerve stimulation in 28 patients with traumatic injury of the dorsal and cervical spinal cord. The data were correlated with clinical, urodynamic and additional neurophysiological findings. In the majority of patients (82%), 11 with complete and 12 with partial spinal cord lesions, the lumbar components of the EESG were normal. In 4 of these patients with complete lesion above T2 the dorsal EESG was absent. In 5 cases (18%), the lumbosacral EESG was altered in the presence of an atypical clinical syndrome characterized by persistent urinary retention associated with lower leg atrophy and reduced tendon jerks. In these cases, MRI provided evidence of an unexpected sacral lesion. Unlike the dorsal EESG reflecting the afferent dorsal column volley, the lumbar components of the EESG are usually unaffected by interruption of the cord pathways; these last events thus originate from segmental neuronal activity, the involvement of which bears witness to additional lumbosacral damage.

Cortical output modulation after rapid repetitive movements

Transcranial magnetic stimulation was used to evaluate changes in motor cortex excitability after rapid repetitive movements in five healthy subjects aged 23-30 years, by considering the amplitude of motor evoked potentials (MEPs) at rest and after one minute of maximal frequency repetitive abduction-adduction movements of the thumb. In addition, M and F waves were evaluated by stimulating the median nerve at the wrist. All of the examined subjects showed a clear modification in post-exercise MEP amplitudes, with a mean maximal reduction of 50-60% in comparison with basal values and complete recovery after a period of about 35 minutes. The time course of this phenomenon showed a triphasic pattern: (I) a rapid decrease phase up to the fifth minute; (II) a maximal depression phase for a period of about ten minutes; (III) a slow return to basal values. No significant changes were observed in post-exercise M and F waves. These results show the existence of a reversible modulation of the excitability of the upper motor neuron after rapid repetitive movements. It is likely that this modulation takes place at the level of the motor cortex and that its anatomo-functional substrate is represented by the activation of inhibitory intracortical circuits.