Intermuscular Coherence in the Presence of Electrical Stimulation.

The nervous system uses oscillations to convey information efficiently. Inter-muscular coherence in the 15-35 Hz range is thought to represent common cortical drive to muscles, but is also in the frequency band in which electrical stimulation is applied to restore movement following neurological disease or injury. We wished to determine if, when stimulation is applied at the peak frequency of the coherence spectra it was still possible to determine voluntary effort. Using healthy human subjects we stimulated muscles in the arms and legs, separate experiments, while recording EMG activity from pairs of muscles including the stimulated muscles. Offline coherence analysis was performed. When stimulation is greater than motor threshold, and applied at the peak of the coherence spectra a new peak appears in the spectra, presumably representing a new frequency of oscillation within the nervous system. This does not appear at lower stimulation levels, or with lower frequencies. The nervous system is capable of switching oscillatory frequencies to account for noise in the environment.

Uncrossed corticospinal tracts presenting as transient tumor-related symptomatology.

Ipsilateral corticospinal innervation is rare. No prior cases have described ipsilateral tumor-associated symptoms as the presentation of an uncrossed corticospinal tract. Herein, we describe a case associated with a left frontal tumor, presenting with transient ipsilateral hemiparesis and aphasia. Due to the fluctuating symptomatology, we suspected a cerebrovascular cause and initially performed a workup for stroke. Ipsilateral motor innervation was discovered with intraoperative monitoring during the resection of the tumor, and confirmed with postoperative diffusion tensor imaging (DTI). Neurosurgeons should be aware of uncrossed motor system, and include it in the differential of ipsilateral deficit in patients with intracranial tumors.

Phenomenology of neurophysiologic changes during surgical treatment of carotid stenosis using signal analysis.

OBJECTIVE: To describe the changes in the shape and topology of the somatosensory evoked potential (SSEP) during carotid endarterectomy, with particular reference to the time of clamping. METHODS: Routine intraoperative monitoring was performed on 30 patients undergoing carotid endarterectomy (15) or undergoing stenting (15) using median nerve SSEPs. Post-operatively the first and second derivatives of the potential were examined. Separate analysis of the SSEP using wavelets was also performed. RESULTS: In no instances did changes in the SSEP reach clinical significance. The first derivative showed significant changes that were temporally related to the clamp period. After clamping the 'velocity' was higher than baseline. There were changes in the wavelets related to the clamp period with more marked spectral edges at the conclusion of the procedure than baseline. In all instances the patient had a good clinical outcome. CONCLUSIONS: Wavelet and derivative analysis of evoked potentials show changes that are not apparent with measures of amplitude and latency. The clinical relevance of these changes remains uncertain and await larger studies. SIGNIFICANCE: Increased velocity and spectral edges may be markers of increased cerebral blood flow, at least in the setting of pre-existing carotid stenosis.

Interpretation of surgical neuromonitoring data in Canada: a survey of practising surgeons.

Intraoperative neuromonitoring is a specialized skill set performed in the operating room to reduce the risk of neurologic injury. There appears to be a shortage of qualified personnel and a lack of Canadian guidelines on the performance of the task. We distributed a web-based survey on the attitude of the surgeons to the interpretation of intraoperative neuromonitoring data among surgeons who use the technique. At present, most of the interpretation is performed by either technologists or by the surgeons themselves. Most surgeons would prefer professional oversight from a neurologist or neurophysiologist at the doctoral level. There is a lack of personnel in Canada with the appropriate training and expertise to interpret intraoperative neuromonitoring data.

Utility of neurophysiology in the diagnosis of tethered cord syndrome.

OBJECT: The diagnosis of tethered cord syndrome (TCS) remains difficult, and the decision to operate is even more complex. The objective of this study was to examine how detailed examination of neurophysiological test results can affect the diagnosis for patients undergoing a surgical cord release. METHODS: Patients undergoing tethered spinal cord releases were matched by age and sex with control patients undergoing scoliosis correction in the absence of spinal cord pathology. The latency and width of the P37 peak of the posterior tibial nerve somatosensory evoked potential (SSEP) and the motor evoked potential (MEP) latencies were examined. Immediate changes as a result of the surgical procedure were reported. RESULTS: The width of the P37 response differed significantly between TCS and control patients and changed significantly during the surgical procedure. Nonsignificant trends were seen in SSEP and MEP latencies. CONCLUSIONS: The width of the P37 response may be a useful marker for TCS and may play a role in presurgical decision making.

Simultaneous Measurement of Breathing Kinematics and Surface Electromyography of Chest Wall Muscles during Maximum Performance and Speech Tasks in Children: Methodological Considerations.

OBJECTIVE: To develop a standardized paediatric protocol for acquiring simultaneous chest wall kinematics and surface electromyography (EMG) of chest wall muscles during maximum performance and speech tasks. PATIENTS AND METHODS: Eighteen healthy participants included: (a) a younger age group (n = 6; ages 4.0-6.5 years), (b) an older age group (n = 6; ages 7.0-10.5 years), and (c) an adult group (n = 8; ages 21-33 years). A child (age 10 years) with spastic-type cerebral palsy (CP) served as a 'proof of protocol feasibility'. Chest wall kinematics and surface EMGs (intercostals, rectus abdominus, external oblique, latissimus dorsi, and erector spinae) were acquired during maximum performance and speech tasks. RESULTS: Successful calibration of the EMG signal and reliable detection of muscle activation onset, offset, and amplitude relative to vital capacity and percent maximum voluntary contraction in children were demonstrated. Kinematic and surface EMG measurements were sensitive to non-speech and speech tasks, age, and neurological status (i.e. CP). CONCLUSION: The simultaneous measurement of kinematics and EMG of the chest wall muscle groups provides a more comprehensive description of speech breathing in children. This protocol can be used for the observation and interpretation of clinical outcomes seen in children with motor speech disorders following treatments that focus on increasing overall respiratory and vocal effort.

Preservation of motor evoked potentials under anesthesia in children with spinal muscular atrophy type II undergoing spinal deformity surgery.

Spinal muscular atrophy is a progressive condition in which movement is gradually lost as a result of the loss of spinal motor neurons. Individuals with this condition may require surgical correction of a secondary scoliosis. Motor evoked potentials were recorded using transcranial electrical stimulation in four such individuals undergoing surgery. All the patients were nonambulatory and in wheelchairs. Motor evoked potentials were recordable in both upper and lower limb muscles, with similar stimulation parameters to control subjects undergoing surgery for idiopathic scoliosis. The amplitudes of the motor evoked potentials were similar to those in control subjects, although the latencies were shorter reflective of the smaller stature of the spinal muscular atrophy patients. The relative preservation of the motor evoked potentials despite the patients' poor voluntary motor control suggests that there is a selective preservation of the motor neurons mediating the motor evoked potential in spinal muscular atrophy and a maintenance of the conduction velocities of the corticospinal tract.

Neural activity generated in the neural placode and nerve roots in the neonate with spina bifida.

OBJECT: The authors conducted a study to determine the neurophysiological capacity of the neural placode in spina bifida neonates and to determine if the spinal nerve roots in these neonates had normal stimulation. METHODS: The authors present a case series of 2 neonates born with open neural tube defects who underwent neural tube closure within 24 hours of birth. Neurophysiological monitoring and electrical stimulation of the placode and nerve roots was performed before and after closure of the neural tube. RESULTS: Stimulation of nerve roots resulted in evoked electromyographic responses in distinct muscle groups, indicative of the myotome innervation pattern. Stimulation threshold did not change significantly after closure of the placode. Stimulation within the placode generated an alternating pattern of activity in the left and right legs. CONCLUSIONS: Closure of the neural tube did not affect the stimulation threshold of the nerve roots, which remained easily excitable. The viability of the nerve roots suggests that they may be candidates for neural prostheses in the future. The neural placode contains basic neural elements for generating a locomotor-like pattern in response to tonic neural inputs.

A comparison of a commercially made pedicle stimulating probe with a custom-made device: does the commercial device detect pedicle wall breaches more reliably?

STUDY DESIGN: Clinical trial. OBJECTIVE: To compare the efficacy of a commercially available stimulating pedicle probe with a custom-made probe for the detection of pedicle wall breaches during screw insertion for the surgical correction of scoliosis. SUMMARY OF BACKGROUND DATA: Stimulus triggered electromyography has been used to detect small breaches in the walls of the spinal pedicles during pedicle screw insertion. We routinely use a reusable, custom-made clip that can be attached to the screw, pedicle probe, or other instruments. Commercial systems are available in which the instrument is electrically instrumented to deliver current. METHODS: In five patients (173 pairs of tests), we compared the threshold current required to trigger an electromyographic response during testing of the pedicle. Each track or screw was tested with both the custom-made and the commercial probe and the threshold current recorded. RESULTS: Both systems were able to detect pedicle wall breaches using triggered electromyography. The threshold current recorded was not significantly different between the two systems (P > 0.1, paired t test) nor was the difference (0.16 mA) clinically significant. CONCLUSION: No difference was found between the thresholds detected with either system. There is however, a significant difference in the costs of the two probe systems.

Afferent inputs to mid- and lower-lumbar spinal segments are necessary for stepping in spinal cats.

Afferent inputs are known to modulate the activity of locomotor central pattern generators, but their role in the generation of locomotor patterns remains uncertain. This study sought to investigate the importance of afferent input for producing bilateral, coordinated hindlimb stepping in adult cats. Following complete spinal transection, animals were trained to step on the moving belt of a treadmill until proficient, weight-bearing stepping of the hindlimbs was established. Selective dorsal rhizotomies of roots reaching various segments of the lumbosacral enlargement were then conducted, and hindlimb stepping capacity was reassessed. Depending on the deafferented lumbosacral segments, stepping was either abolished or unaffected. Deafferentation of mid-lumbar (L3/L4) or lower-lumbar (L5-S1) segments abolished locomotion. Locomotor capacity in these animals could not be restored with the administration of serotonergic or adrenergic agonists. Deafferentation of L3, L6, or S1 had mild effects on locomotion. This suggested that critical afferent inputs pertaining to hip position (mid-lumbar) and limb loading (lower-lumbar) play an important role in the generation of locomotor patterns after spinal cord injury.

Changes in locomotor muscle activity after treadmill training in subjects with incomplete spinal cord injury.

Intensive treadmill training after incomplete spinal cord injury can improve functional walking abilities. To determine the changes in muscle activation patterns that are associated with improvements in walking, we measured the electromyography (EMG) of leg muscles in 17 individuals with incomplete spinal cord injury during similar walking conditions both before and after training. Specific differences were observed between subjects that eventually gained functional improvements in overground walking (responders), compared with subjects where treadmill training was ineffective (nonresponders). Although both groups developed a more regular and less clonic EMG pattern on the treadmill, it was only the tibialis anterior and hamstring muscles in the responders that displayed increases in EMG activation. Likewise, only the responders demonstrated decreases in burst duration and cocontraction of proximal (hamstrings and quadriceps) muscle activity. Surprisingly, the proximal muscle activity in the responders, unlike nonresponders, was three- to fourfold greater than that in uninjured control subjects walking at similar speeds and level of body weight support, suggesting that the ability to modify muscle activation patterns after injury may predict the ability of subjects to further compensate in response to motor training. In summary, increases in the amount and decreases in the duration of EMG activity of specific muscles are associated with functional recovery of walking skills after treadmill training in subjects that are able to modify muscle activity patterns following incomplete spinal cord injury.

Changes in sensory-evoked synaptic activation of motoneurons after spinal cord injury in man.

Following spinal cord injury (SCI), prolonged muscle spasms are readily triggered by brief sensory stimuli. Animal and indirect human studies have shown that a substantial portion of the depolarization of motoneurons during a muscle spasm comes from the activation of persistent inward currents (PICs). The brief (single pulse) sensory stimuli that trigger the PICs and muscle spasms in chronically spinalized animals evoke excitatory post-synaptic potentials (EPSPs) that are broadened to more than 500 ms, the duration of depolarization required to activate a PIC in the motoneuron. Thus, in humans, we investigated if post-synaptic potentials (PSPs) evoked from brief (<20 ms) sensory stimulation are changed after SCI and if they are broadened to > or =500 ms to more readily activate motoneuron PICs and muscle spasms. To estimate both the shape and duration of PSPs in human subjects we used peristimulus frequencygrams (PSFs), which are plots of the instantaneous firing frequency of tonically active single motor units that are time-locked to the occurrence of the sensory stimulus. PSFs in response to cutaneomuscular stimulation of the medial arch or toe of the foot, a sensory stimulus that readily triggers muscle spasms, were compared between non-injured control subjects and in spastic subjects with chronic (>1 year), incomplete SCI. In non-injured controls, a single shock or brief (<20 ms) train of cutaneomuscular stimulation produced PSFs consisting of a 300 ms increase in firing rate above baseline with an interposed period of reduced firing. Parallel intracellular experiments in motoneurons of adult rats revealed that a 300 ms EPSP with a fast intervening inhibitory PSP (IPSP) reproduced the PSF recorded in non-injured subjects. In contrast, the same brief sensory stimulation in subjects with chronic SCI produced PSFs of comparatively long duration (1200 ms) with no evidence for IPSP activation, as reflected by a lack of reduced firing rates after the onset of the PSF. Thus, unlike non-injured controls, the motoneurons of subjects with chronic SCI are activated by very long periods of pure depolarization from brief sensory activation. It is likely that these second-long EPSPs securely recruit slowly activating PICs in motoneurons that are known to mediate, in large part, the many seconds-long activation of motoneurons during involuntary muscle spasms.

Persistent mirror movements for over sixty years: the underlying mechanisms in a cerebral palsy patient.

OBJECTIVE: To determine the mechanisms underlying the mirroring of distal movements in both upper and lower limbs present in one individual from birth. METHODS: Transcranial magnetic stimulation (TMS), magnetic resonance imaging (MRI), functional magnetic resonance imaging (fMRI), voluntary and reflexly evoked electromyograms (EMG) and force measurements were used to obtain information about the motor pathways responsible for the mirror movements. RESULTS: MRI showed a significant loss of brain tissue from one hemisphere and fMRI indicated a significant functional reorganization had taken place. An obligatory mirroring of voluntary movement on the sound side occurs on the affected side, but some independent movement can be produced on the affected side, if enabled by weak contractions on the sound side. TMS mapping revealed bilateral projections from one hemisphere and virtually absent projections from the primary motor cortex of the other hemisphere. Spinal reflexes were restricted to the stimulated side. Transcortical reflexes were evoked bilaterally from the sound side, but not from the affected side. CONCLUSIONS: The physiological and imaging data are consistent with a mirroring from the intact motor cortex via the supplementary motor area. SIGNIFICANCE: Mirror movements in this individual represent a major cortical reorganization and a partial solution to the neonatal loss of substantial amounts of brain tissue.