Relating clinical and neurophysiological assessment of spasticity by machine learning

Spasticity following spinal cord injury (SCI) is most often assessed clinically using a five-point Ashworth score (AS). A more objective assessment of altered motor control may be achieved by using a comprehensive protocol based on a surface electromyographic (sEMG) activity recorded from thigh and leg muscles. However, the relationship between the clinical and neurophysiological assessments is still unknown. In this paper we employ three different classification methods to investigate this relationship. The experimental results indicate that, if the appropriate set of sEMG features is used, the neurophysiological assessment is related to clinical findings and can be used to predict the AS. A comprehensive sEMG assessment may be proven useful as an objective method of evaluating the effectiveness of various interventions and for follow-up of SCI patients.

Intracortical inhibition of lower limb motor-evoked potentials after paired transcranial magnetic stimulation

The aim of the present study was to determine the characteristics of intracortical inhibition in the motor cortex areas representing lower limb muscles using paired transcranial magnetic (TMS) and transcranial electrical stimulation (TES) in healthy subjects. In the first paradigm (n=8), paired magnetic stimuli were delivered through a double cone coil and motor evoked potentials (MEPs) were recorded from quadriceps (Q) and tibialis anterior (TA) muscles during relaxation. The conditioning stimulus strength was 5% of the maximum stimulator output below the threshold MEP evoked during weak voluntary contraction of TA (33+/-5%). The test stimulus (67+/-2%) was 10% of the stimulator output above the MEP threshold in the relaxed TA. Interstimulus intervals (ISIs) from 1-15 ms were examined. Conditioned TA MEPs were significantly suppressed (P<0.01) at ISIs of less than 5 ms (relative amplitude from 20-50% of the control). TA MEPs tended to be only slightly facilitated at 9-ms and 10-ms ISIs. The degree of MEP suppression was not different between right and left TA muscles despite the significant difference in size of the control responses (P<0.001). Also, conditioned MEPs were not significantly different between Q and TA. The time course of TA MEP suppression, using electrical test stimuli, was similar to that found using TMS. In the second paradigm (n=2), the suppression of TA MEPs at 2, 3, and 4 ms ISIs was examined at three conditioning intensities with the test stimulation kept constant. For the pooled 2- to 4-ms ISI data, relative amplitudes were 34+/-6%, 61+/-5%, and 98+/-9% for conditioning intensities of 0.95, 0.90, and 0.85x active threshold, respectively (P<0.01). In conclusion, the suppression of lower limb MEPs following paired TMS showed similar characteristics to the intracortical inhibition previously described for the hand motor area.

Generators for human P300 elicited by somatosensory stimuli using multiple dipole source analysis

Cognitive event-related potentials, such as P300, are sensitive to manipulations of psychological variables and may provide evidence to support theories of brain mechanisms involved in cognition. However, the relationship between event-related potentials and the active neural structures is not yet understood. Electrical stimulation of the index and little fingers of the left hand in the context of a somatosensory target discrimination task, performed by healthy human subjects, elicited the middle-latency component of somatosensory evoked potentials, N60, the long-latency component, N140, and the P300 component. Identification of the generators for both the earlier components and P300, using equivalent electrical dipole modeling, was performed. Individual spatiotemporal seven-dipole models were developed in order to suggest locations of the sources generating each subject's scalp-recorded wave forms. Three dipoles with fairly weak moments, located in the primary and secondary sensory areas, explained the middle- and long-latency somatosensory evoked potential components, and the remaining four dipoles (4-7), with stronger dipole moments, were active during P300. There was a clear temporal separation of dipole activity between the somatosensory evoked potential components and the P300 component. Dipoles 4 and 5 were found quite symmetrically in the parahippocampal areas of the two hemispheres, while dipoles 6 and 7 were slightly asymmetrical. Dipole 7 was found in the left hippocampal area. Dipole 6 appeared in the right insular cortex. The locations of the four dipoles implicated in the generation of the somatosensory P300 were compared with the locations of four dipoles accounting for the auditory evoked P300 described in our previous paper [Tarkka et al. (1995) Electroenceph. clin. Neurophysiol. 96, 538-545]. No substantial difference in source locations of the P300 was found between auditory and somatosensory modality other than an asymmetrical activity in the somatosensory modality contralateral to the stimulated hand.

Spinal motoneuron excitability after acute spinal cord injury in humans

BACKGROUND

Few studies in humans have assessed the ability of Ia afferent and antidromic motor volleys to activate motoneurons during spinal shock. Hence, little is known about the excitability state of the spinal motoneuron pool after acute spinal cord injury (SCI) in humans.

METHODS

In 14 patients with acute SCI involving anatomic levels T10 and above, we performed clinical and electrophysiologic studies early after injury (within 24 hours in seven subjects) and on day 10, 20, and 30 postinjury. Maximal H:M ratios, F-wave persistence, and tendon tap T-reflexes were recorded. Sixteen normal subjects and eight chronic SCI patients served as control subjects.

RESULTS

Ten of 14 patients had spinal shock (complete paralysis, loss of sensation, absent reflexes, and muscle hypotonia below the injury) at the time of initial evaluation. F-waves were absent in patients with spinal shock, reduced in persistence in patients with acute SCI without spinal shock, and normal in persistence in patients with chronic SCI. H-reflexes were absent or markedly suppressed in patients with spinal shock within 24 hours of injury but recovered to normal amplitudes within several days postinjury. This recovery occurred despite absence of F-waves that persisted for several weeks postinjury. Deep tendon reflexes were proportionally more depressed in spinal shock than were H-reflexes. All patients had elicitable H-reflexes for days or weeks before the development of clinical reflexes.

CONCLUSIONS

Rostral cord injury causes postsynaptic changes (hyperpolarization) in caudal motoneurons. This hyperpolarization is a major physiologic derangement in spinal shock. The rise in H-reflex amplitude despite evidence of persistent hyperpolarization is due to enhanced transmission at Ia fiber-motoneuron connections below the SCI. Finally, the observation that the stretch reflex is proportionally more depressed than the H-reflex is consistent with fusimotor drive also being depressed after SCI.

Dissociation of cortical areas responsible for evoking excitatory and inhibitory responses in the small hand muscles

Noninvasive transcranial magnetic stimulation (TMS) of the brain using a focal eight-shaped coil with 100% stimulation output was performed in eleven healthy subjects to find out if excitatory and inhibitory responses in the small hand muscles could be dissociated. Motor evoked potentials (MEP) as well as silent periods (SP) were recorded from the right abductor pollicis brevis (APB), and first dorsal interosseus (FDI) muscles at rest and during weak voluntary contraction. Mapping of the cortical representation area was performed over different scalp locations on the left hemisphere. The cortical representation maps for ABP and FDI recorded during contraction covered much larger area and were more elongated in the anterior-posterior than in the medial-lateral direction compared to maps obtained during relaxation. The distribution maps for SPs covered larger scalp areas compared to the maps of MEPs obtained during voluntary contraction. Also during voluntary contraction the locations for evoking the longest SPs were not identical to locations for evoking the peak MEP amplitudes; the longest SPs were observed during stimulation of more medial and frontal locations compared to peak MEPs. Interestingly, stimulation of some locations resulted in the appearance of an isolated MEP without the following SP and in other locations an isolated SP was recorded. The areas for evoking isolated MEPs were in the center, whereas the areas for isolated SPs were located in the periphery of the map. Features such as exclusive locations for MEPs and SPs, and different locations for peak MEP amplitudes and longest SPs, suggest dissociation of the excitatory and inhibitory cortical processes evoked by transcranial magnetic stimulation during voluntary contraction.

Modification of motor control of wrist extension by mesh-glove electrical afferent stimulation in stroke patients

OBJECTIVE

To study the effect of mesh-glove afferent stimulation on motor control of voluntary wrist movement in stroke patients who have chronic neurological deficits.

DESIGN

Case series. Motor control was evaluated by surface EMG of the arm muscles and kinematics of voluntary wrist movements on 3 occasions: before and immediately after the initial session of mesh-glove stimulation, and then after a daily mesh-glove stimulation program conducted over several months.

SETTING

Tertiary care center.

PATIENTS

The inclusion criteria were: a history of stroke lasting longer than 6 months; completion of a rehabilitation program during early recovery; and preserved cognitive and communicative ability. Fourteen referred patients (age 63 +/- 9yr; time since stroke 31 +/- 22mo) fulfilled the criteria and completed the daily stimulation program.

INTERVENTION

A single initial and then daily mesh-glove electrical afferent stimulation was applied to the hand of the involved upper limb for 20 to 30min.

MAIN OUTCOME MEASURES

Surface EMGs from the affected biceps brachii and wrist extensor muscles and amplitudes of wrist movements were analyzed.

RESULTS

The single, initial mesh-glove application had no effect on outcome measures. Following a daily mesh-glove stimulation program, however, both the amplitude of wrist extension movement and wrist extensor integrated EMG were significantly increased while coactivation of biceps brachii decreased. These findings were most prominent in subjects with partially preserved voluntary wrist movements.

CONCLUSION

We conclude that daily mesh-glove stimulation can modify altered motor control and improve voluntary wrist extension movement in stroke subjects with chronic neurological deficits.

Spinal motor neuron excitability during the cutaneous silent period

The physiologic mechanisms generating the cutaneous silent period (CSP) remain uncertain. It is not known whether the CSP occurs because of inexcitability of the spinal motor neuron. We therefore, assessed excitability of the motor neuron during the CSP using F-wave responses. H-reflexes were also elicited during the CSP. Electrical stimulation to the fifth digit produced the CSP in the voluntarily contracting abductor pollicis brevis muscle (APB). Median nerve stimulation at the wrist elicited control F or H responses during isometric APB contraction (condition 1) and in resting muscle (condition 2). Control amplitudes were compared to those elicited in the midst of the CSP. In Condition 1, F-wave amplitudes and frequency during the CSP were unchanged compared with controls. However, F-waves were increased in amplitude and frequency during the CSP (P < 0.001) relative to responses elicited in resting muscle (condition 2). H-reflexes during the CSP were suppressed (P < 0.001) compared with controls elicited during contraction (condition 1), but facilitated relative to the resting state (condition 2) in which no H-reflexes were elicitable. We conclude that spinal motor neurons remain excitable to antidromic volleys at the same time that the corticospinal volley is inhibited to produce the CSP. Moreover, motor neuron excitability appears to be increased during the CSP compared to the relaxed state.

Electric source localization of the auditory P300 agrees with magnetic source localization

The event-related cortical potential elicited in the context of auditory target detection tasks includes the N1, P2 and P3 components. The aim of the present study was to identify the sources of these scalp-recorded components using an electrical multiple dipole model. Nine healthy adults volunteered for the study. An auditory oddball paradigm was used. Stimuli (18% target and 82% non-target tones) were delivered through ear-phones and subjects were required to silently count the targets. Event-related potentials (ERPs) to these stimuli were recorded by 30 electrodes placed on the scalp. The identification of the sources of the ERP was attempted using the brain electric source analysis (BESA) program. The instantaneous source locations of N1, P2 and P3 reported in magnetoencephalographic (MEG) literature were used as initial starting locations for the spatio-temporal multiple dipole modeling of the EEG data. First the auditory long latency responses were modeled separately. Bilateral superior temporal plane sources with almost vertical orientations explained the first 250 msec window of the non-target tone recording including N1/P2 complex. This agrees with MEG source localization of N1m/P2m. Two slightly deeper dipoles in superior temporal gyri and bilateral dipoles in hippocampi or parahippocampal areas explained P3 (analysis window 250-600 msec). The final model explained the complete epoch of 600 msec with 6 dipoles and the residual variances of individual models ranged from 3.83% to 7.77%. The concordance between MEG and BESA source localization results supports the notion of generators in temporal lobes for the N1/P2 complex and generators in temporal and hippocampal areas for the P3 component.

Focal depression of cortical excitability induced by fatiguing muscle contraction: a transcranial magnetic stimulation study

Motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (TES) of the motor cortex were recorded in separate sessions to assess changes in motor cortex excitability after a fatiguing isometric maximal voluntary contraction (MVC) of the right ankle dorsal flexor muscles. Five healthy male subjects, aged 37.4 +/- 4.2 years (mean +/- SE), were seated in a chair equipped with a load cell to measure dorsiflexion force. TMS or TES was delivered over the scalp vertex before and after a fatiguing MVC, which was maintained until force decreased by 50%. MEPs were recorded by surface electrodes placed over quadriceps, hamstrings, tibialis anterior (TA), and soleus muscles bilaterally. M-waves were elicited from the exercised TA by supramaximal electrical stimulation of the peroneal nerve. H-reflex and MVC recovery after fatiguing, sustained MVC were also studied independently in additional sessions. TMS-induced MEPs were significantly reduced for 20 min following MVC, but only in the exercised TA muscle. Comparing TMS and TES mean MEP amplitudes, we found that, over the first 5 min following the fatiguing MVC, they were decreased by about 55% for each. M-wave responses were unchanged. H-reflex amplitude and MVC force recovered within the 1st min following the fatiguing MVC. When neuromuscular fatigue was induced by tetanic motor point stimulation of the TA, TMS-induced MEP amplitudes remained unchanged. These findings suggest that the observed decrease in MEP amplitude represents a focal reduction of cortical excitability following a fatiguing motor task and may be caused by intracortical and/or subcortical inhibitory mechanisms.

N-acetylcysteine inhibits muscle fatigue in humans

N-acetylcysteine (NAC) is a nonspecific antioxidant that selectively inhibits acute fatigue of rodent skeletal muscle stimulated at low (but not high) tetanic frequencies and that decreases contractile function of unfatigued muscle in a dose-dependent manner. The present experiments test the hypothesis that NAC pretreatment can inhibit acute muscular fatigue in humans. Healthy volunteers were studied on two occasions each. Subjects were pretreated with NAC 150 mg/kg or 5% dextrose in water by intravenous infusion. The subject then sat in a chair with surface electrodes positioned over the motor point of tibialis anterior, an ankle dorsiflexor of mixed-fiber composition. The muscle was stimulated to contract electrically (40-55 mA, 0.2-ms pulses) and force production was measured. Function of the unfatigued muscle was assessed by measuring the forces produced during maximal voluntary contractions (MVC) of ankle dorsiflexor muscle groups and during electrical stimulation of tibialis anterior at 1, 10, 20, 40, 80, and 120 Hz (protocol 1). Fatigue was produced using repetitive tetanic stimulations at 10 Hz (protocol 1) or 40 Hz (protocol 2); intermittent stimulations subsequently were used to monitor recovery from fatigue. The contralateral leg then was studied using the same protocol. Pretreatment with NAC did not alter the function of unfatigued muscle; MVC performance and the force-frequency relationship of tibialis anterior were unchanged. During fatiguing contractions stimulated at 10 Hz, NAC increased force output by approximately 15% (P < 0.0001), an effect that was evident after 3 min of repetitive contraction (P < 0.0125) and persisted throughout the 30-min protocol. NAC had no effect on fatigue induced using 40 Hz stimuli or on recovery from fatigue. N-acetylcysteine pretreatment can improve performance of human limb muscle during fatiguing exercise, suggesting that oxidative stress plays a causal role in the fatigue process and identifying antioxidant therapy as a novel intervention that may be useful clinically.

Characteristics of the silent period after transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) of human cortex during voluntary muscle contraction produces a transient period of inhibition (i.e., silent period, SP) in the electromyographic (EMG) activity. The duration of the SP in relation to the level of muscle force (10%, 50% and 100% of maximum voluntary contraction) as well as possible cumulative effects of sequential TMS on the SP were studied. Methodologic problems were encountered in defining the SP and thus the duration of both an absolute (complete EMG silence) and relative (return of uninterrupted EMG activity) SP was measured. In all subjects, shortening of the SP duration occurred in relation to an increase in force when the criterion for absolute SP was used. Conversely, the relative SP duration suggested a trend toward prolongation with increasing force of contraction. No cumulative effects of TMS were observed on the absolute SP duration, whereas two subjects showed a cumulative effect of TMS on the relative SP. We conclude that the effect of muscle force and sequential TMS on the SP duration is dependent on the methods used to measure the SP. It is therefore essential to agree on methodology before SP measurements are clinically useful.