Motor unit behavior during clonus

Properties of the surface electromyogram following traumatic spinal cord injury: a scoping review

Traumatic spinal cord injury (SCI) disrupts spinal and supraspinal pathways, and this process is reflected in changes in surface electromyography (sEMG). sEMG is an informative complement to current clinical testing and can capture the residual motor command in great detail-including in muscles below the level of injury with seemingly absent motor activities. In this comprehensive review, we sought to describe how the sEMG properties are changed after SCI. We conducted a systematic literature search followed by a narrative review focusing on sEMG analysis techniques and signal properties post-SCI. We found that early reports were mostly focused on the qualitative analysis of sEMG patterns and evolved to semi-quantitative scores and a more detailed amplitude-based quantification. Nonetheless, recent studies are still constrained to an amplitude-based analysis of the sEMG, and there are opportunities to more broadly characterize the time- and frequency-domain properties of the signal as well as to take fuller advantage of high-density EMG techniques. We recommend the incorporation of a broader range of signal properties into the neurophysiological assessment post-SCI and the development of a greater understanding of the relation between these sEMG properties and underlying physiology. Enhanced sEMG analysis could contribute to a more complete description of the effects of SCI on upper and lower motor neuron function and their interactions, and also assist in understanding the mechanisms of change following neuromodulation or exercise therapy.

Modulation of soleus stretch reflexes during walking in people with chronic incomplete spinal cord injury

In people with spasticity due to chronic incomplete spinal cord injury (SCI), it has been presumed that the abnormal stretch reflex activity impairs gait. However, locomotor stretch reflexes across all phases of walking have not been investigated in people with SCI. Thus, to understand modulation of stretch reflex excitability during spastic gait, we investigated soleus stretch reflexes across the entire gait cycle in nine neurologically normal participants and nine participants with spasticity due to chronic incomplete SCI (2.5–11 year post-injury). While the participant walked on the treadmill at his/her preferred speed, unexpected ankle dorsiflexion perturbations (6° at 250°/s) were imposed every 4–6 steps. The soleus H-reflex was also examined. In participants without SCI, spinal short-latency “M1”, spinal medium latency “M2”, and long-latency “M3” were clearly modulated throughout the step cycle; the responses were largest in the mid-stance and almost completely suppressed during the stance-swing transition and swing phases. In participants with SCI, M1 and M2 were abnormally large in the mid–late-swing phase, while M3 modulation was similar to that in participants without SCI. The H-reflex was also large in the mid–late-swing phase. Elicitation of H-reflex and stretch reflexes in the late swing often triggered clonus and affected the soleus activity in the following stance. In individuals without SCI, moderate positive correlation was found between H-reflex and stretch reflex sizes across the step cycle, whereas in participants with SCI, such correlation was weak to non-existing, suggesting that H-reflex investigation would not substitute for stretch reflex investigation in individuals after SCI.

Temporal Indices of Ankle Clonus and Relationship to Electrophysiologic and Clinical Measures in Persons With Spinal Cord Injury

BACKGROUND AND PURPOSE

Clonus arising from plantar flexor hyperreflexia is a phenomenon that is commonly observed in persons with spastic hypertonia. We assessed the temporal components of a biomechanical measure to quantify ankle clonus, and validated these in persons with spasticity due to spinal cord injury.

METHODS

In 40 individuals with chronic (>1 year) spinal cord injury, we elicited ankle clonus using a standardized mechanical perturbation (drop test). We examined reliability and construct validity of 2 components of the drop test: clonus duration (timed with a stopwatch) and number of oscillations in the first 10-second interval (measured via optical motion capture). We compared these measures to the Spinal Cord Assessment Tool for Spastic reflexes (SCATS) clonus score and H-reflex/M-wave (H/M) ratio, a clinical and electrophysiologic measure, respectively.

RESULTS

Intra- and interrater reliability of clonus duration measurement was good [intraclass correlation coefficient, ICC (2, 1) = 1.00]; test-retest reliability was good both at 1 hour [ICC (2, 2) = 0.99] and at 1 week [ICC (2, 2) = 0.99]. Clonus duration was moderately correlated with SCATS clonus score (r = 0.58). Number of oscillations had good within-session test-retest reliability [ICC (2, 1) > 0.90] and strong correlations with SCATS clonus score (r = 0.86) and soleus H/M ratio (r = 0.77).

DISCUSSION AND CONCLUSIONS

Clonus duration and number of oscillations as measured with a standardized test are reliable and valid measures of plantar flexor hyperreflexia that are accessible for clinical use. Tools for objective measurement of ankle clonus are valuable for assessing effectiveness of interventions directed at normalizing reflex activity associated with spasticity.Video Abstract available for more insights from the authors (see Supplemental Digital Content 1, http://links.lww.com/JNPT/A179).

Myoclonus in the critically ill: Diagnosis, management, and clinical impact

Myoclonus is the second most common involuntary non-epileptic movement in intensive care units following tremor-like gestures. Although there are several types of myoclonus, they remain underappreciated, and their diagnostic and prognostic associations are largely ignored. This review discusses clinical, electrophysiological, neuroanatomical, and neuroimaging characteristics of different types of myoclonus in critically ill adults along with their prognostic impact and treatment options. Myoclonus is characterized by a sudden, brief, and sometimes repetitive muscle contraction of body parts, or a brief and sudden cessation of tonic muscle innervation followed by a rapid recovery of tonus. Myoclonus can resemble physiologic and other pathologic involuntary movements. Neurologic injuries, anesthetics, and muscle relaxants interfere with the typical appearance of myoclonus. Identifying "real myoclonus" and determining the neuroanatomical origin are important, as treatment responses depend on the involved neuroanatomical structures. The identification of the type of myoclonus, the involved neuroanatomical structures, and the associated illnesses is essential to direct treatment. In conclusion, the combined clinical, electrophysiological, and neuroradiological examination reliably uncovers the neuroanatomical sources and the pathophysiology of myoclonus. Recognizing cortical myoclonus is critical, as it is treatable and may progress to generalized convulsive seizures or status epilepticus.

Locomotor training modifies soleus monosynaptic motoneuron responses in human spinal cord injury

The objective of this study was to assess changes in monosynaptic motoneuron responses to stimulation of Ia afferents after locomotor training in individuals with chronic spinal cord injury (SCI). We hypothesized that locomotor training modifies the amplitude of the soleus monosynaptic motoneuron responses in a body position-dependent manner. Fifteen individuals with chronic clinical motor complete or incomplete SCI received an average of 45 locomotor training sessions. The soleus H-reflex and M-wave recruitment curves were assembled using data collected in both the right and left legs, with subjects seated and standing, before and after training. The soleus H-reflexes and M-waves, measured as peak-to-peak amplitudes, were normalized to the maximal M-wave (M(max)). Stimulation intensities were normalized to 50% M(max) stimulus intensity. A sigmoid function was also fitted to the normalized soleus H-reflexes on the ascending limb of the recruitment curve. After training, soleus H-reflex excitability was increased in both legs in AIS C subjects, and remained unchanged in AIS A-B and AIS D subjects during standing. When subjects were seated, soleus H-reflex excitability was decreased after training in many AIS C and D subjects. Changes in reflex excitability coincided with changes in stimulation intensities at H-threshold, 50% maximal H-reflex, and at maximal H-reflex, while an interaction between leg side and AIS scale for the H-reflex slope was also found. Adaptations of the intrinsic properties of soleus motoneurons and Ia afferents, the excitability profile of the soleus motoneuron pool, oligosynaptic inputs, and corticospinal inputs may all contribute to these changes. The findings of this study demonstrate that locomotor training impacts the amplitude of the monosynaptic motoneuron responses based on the demands of the motor task in people with chronic SCI.

Clinical understanding of spasticity: implications for practice

Spasticity is a poorly understood phenomenon. The aim of this paper is to understand the effect of spasticity on daily life and identify bedside strategies that enhance patient's function and improve comfort. Spasticity and clonus result from an upper motor neuron lesion that disinhibits the tendon stretch reflex; however, they are differentiated in the fact that spasticity results in a velocity dependent tightness of muscle whereas clonus results in uncontrollable jerks of the muscle. Clinical strategies that address function and comfort are paramount. This is a secondary content analysis using a qualitative research design. Adults experiencing spasticity associated with neuromuscular disorder were asked to participate during inpatient acute rehabilitation. They were asked to complete a semistructured interview to explain and describe the nature of their experienced spasticity on daily basis. Spasticity affects activities of daily living, function, and mobility. Undertreated spasticity can lead to pain, immobility, and risk of falls. There were missed opportunities to adequately care for patients with spasticity. Bedside care strategies identified by patients with spasticity are outlined. Uses of alternative therapies in conjunction with medications are needed to better manage spasticity. Patient reports on spasticity are important and should be part of clinical evaluation and practice.

Identification and classification of involuntary leg muscle contractions in electromyographic records from individuals with spinal cord injury

Involuntary muscle contractions (spasms) are common after human spinal cord injury (SCI). Our aim was to compare how well two raters independently identified and classified different types of spasms in the same electromyographic records (EMG) using predefined rules. Muscle spasms were identified by the presence, timing and pattern of EMG recorded from paralyzed leg muscles of four subjects with chronic cervical SCI. Spasms were classified as one of five types: unit, tonic, clonus, myoclonus, mixed. In 48h of data, both raters marked the same spasms most of the time. More variability in the total spasm count arose from differences between muscles (84%; within subjects) than differences between subjects (6.5%) or raters (2.6%). Agreement on spasm classification was high (89%). Differences in spasm count, and classification largely occurred when EMG was marked as a single spasm by one rater but split into multiple spasms by the other rater. EMG provides objective measurements of spasm number and type in contrast to the self-reported spasm counts that are often used to make clinical decisions about spasm management. Data on inter-rater agreement and discrepancies on muscle spasm analysis can both drive the design and evaluation of software to automate spasm identification and classification.

Recovery of neuronal and network excitability after spinal cord injury and implications for spasticity

The state of areflexia and muscle weakness that immediately follows a spinal cord injury (SCI) is gradually replaced by the recovery of neuronal and network excitability, leading to both improvements in residual motor function and the development of spasticity. In this review we summarize recent animal and human studies that describe how motoneurons and their activation by sensory pathways become hyperexcitable to compensate for the reduction of functional activation of the spinal cord and the eventual impact on the muscle. Specifically, decreases in the inhibitory control of sensory transmission and increases in intrinsic motoneuron excitability are described. We present the idea that replacing lost patterned activation of the spinal cord by activating synaptic inputs via assisted movements, pharmacology or electrical stimulation may help to recover lost spinal inhibition. This may lead to a reduction of uncontrolled activation of the spinal cord and thus, improve its controlled activation by synaptic inputs to ultimately normalize circuit function. Increasing the excitation of the spinal cord with spared descending and/or peripheral inputs by facilitating movement, instead of suppressing it pharmacologically, may provide the best avenue to improve residual motor function and manage spasticity after SCI.

Operant conditioning to increase ankle control or decrease reflex excitability improves reflex modulation and walking function in chronic spinal cord injury

Ankle clonus is common after spinal cord injury (SCI) and is attributed to loss of supraspinally mediated inhibition of soleus stretch reflexes and maladaptive reorganization of spinal reflex pathways. The maladaptive reorganization underlying ankle clonus is associated with other abnormalities, such as coactivation and reciprocal facilitation of tibialis anterior (TA) and soleus (SOL), which contribute to impaired walking ability in individuals with motor-incomplete SCI. Operant conditioning can increase muscle activation and decrease stretch reflexes in individuals with SCI. We compared two operant conditioning-based interventions in individuals with ankle clonus and impaired walking ability due to SCI. Training included either voluntary TA activation (TA↑) to enhance supraspinal drive or SOL H-reflex suppression (SOL↓) to modulate reflex pathways at the spinal cord level. We measured clonus duration, plantar flexor reflex threshold angle, timed toe tapping, dorsiflexion (DF) active range of motion, lower extremity motor scores (LEMS), walking foot clearance, speed and distance, SOL H-reflex amplitude modulation as an index of reciprocal inhibition, presynaptic inhibition, low-frequency depression, and SOL-to-TA clonus coactivation ratio. TA↑ decreased plantar flexor reflex threshold angle (-4.33°) and DF active range-of-motion angle (-4.32°) and increased LEMS of DF (+0.8 points), total LEMS of the training leg (+2.2 points), and nontraining leg (+0.8 points), and increased walking foot clearance (+ 4.8 mm) and distance (+12.09 m). SOL↓ decreased SOL-to-TA coactivation ratio (-0.21), increased nontraining leg LEMS (+1.8 points), walking speed (+0.02 m/s), and distance (+6.25 m). In sum, we found increased voluntary control associated with TA↑ outcomes and decreased reflex excitability associated with SOL↓ outcomes.

Automatic analysis of EMG during clonus

Clonus can disrupt daily activities after spinal cord injury. Here an algorithm was developed to automatically detect contractions during clonus in 24h electromyographic (EMG) records. Filters were created by non-linearly scaling a Mother (Morlet) wavelet to envelope the EMG using different frequency bands. The envelope for the intermediate band followed the EMG best (74.8-193.9 Hz). Threshold and time constraints were used to reduce the envelope peaks to one per contraction. Energy in the EMG was measured 50 ms either side of each envelope (contraction) peak. Energy values at 5% and 95% maximal defined EMG start and end time, respectively. The algorithm was as good as a person at identifying contractions during clonus (p=0.946, n=31 spasms, 7 subjects with cervical spinal cord injury), and marking start and end times to determine clonus frequency (intra class correlation coefficient, α: 0.949), contraction intensity using root mean square EMG (α: 0.997) and EMG duration (α: 0.852). On average the algorithm was 574 times faster than manual analysis performed independently by two people (p ≤ 0.001). This algorithm is an important tool for characterization of clonus in long-term EMG records.

Characteristics of lower extremity clonus after human cervical spinal cord injury

Clonus can interfere with self-care and rehabilitation of people with spinal cord injury. Our aim was to characterize clonus and to evaluate factors that influence clonus duration in muscles paralyzed chronically by spinal cord injury. Electromyographic activity was recorded from soleus and 7 other limb muscles (5 ipsilateral, 2 contralateral) during clonus. In 14 subjects, clonus frequency in soleus averaged 5.4±0.9 Hz and was slower when the reflex path was longer. Contraction frequency slowed at the beginning and end of clonus (sometimes by 2 Hz). The magnitude of one cycle changed the timing and magnitude of the next cycle. These data suggest that afferent input influences the frequency and maintenance of clonus. Recording from many muscles revealed that clonus was prolonged (>40 sec) when only ipsilateral triceps surae or triceps surae and tibialis anterior were involved. Therefore, localized inputs to spinal circuits were important to sustain clonus. Clonus was intermediate (median: 21 sec) with activation of three or four ipsilateral muscles and these contractions were associated with greater activation of ipsilateral flexors. Clonus was short (<5 sec) when ipsilateral and contralateral muscles were activated (five or six muscles). Activation of extraneous afferent input, particularly contralateral muscles, may provide a way to shorten clonus after spinal cord injury.

Reduced reciprocal inhibition during assisted stepping in human spinal cord injury

The aim of this study was to establish the modulation pattern of the reciprocal inhibition exerted from tibialis anterior (TA) group I afferents onto soleus motoneurons during body weight support (BWS) assisted stepping in people with spinal cord injury (SCI). During assisted stepping, the soleus H-reflex was conditioned by percutaneous stimulation of the ipsilateral common peroneal nerve at one fold TA M-wave motor threshold with a single pulse delivered at a short conditioning-test interval. To counteract movement of recording and stimulating electrodes, a supramaximal stimulus at 80-100 ms after the test H-reflex was delivered. Stimuli were randomly dispersed across the step cycle which was divided into 16 equal bins. The conditioned soleus H-reflex was significantly facilitated throughout the stance phase, while during swing no significant changes on the conditioned H-reflex were observed when compared to the unconditioned soleus H-reflex recorded during stepping. Spontaneous clonic activity in triceps surae muscle occurred in multiple phases of the step cycle at a mean frequency of 7 Hz for steps with and without stimulation. This suggests that electrical excitation of TA and soleus group Ia afferents did not contribute to manifestation of ankle clonus. Absent reciprocal inhibition is likely responsible for lack of soleus H-reflex depression in swing phase observed in these patients. The pronounced reduced reciprocal inhibition in stance phase may contribute to impaired levels of co-contraction of antagonistic ankle muscles. Based on these findings, we suggest that rehabilitation should selectively target to transform reciprocal facilitation to inhibition through computer controlled reflex conditioning protocols.

Ankle clonus and its relationship with the medium-latency reflex response of the soleus by peroneal nerve stimulation

Ankle clonus and soleus medium-latency reflex are stretch-induced responses. Clonus is traditionally considered to be the result of oscillation in the group Ia mediated spinal stretch reflex but the soleus medium-latency reflex response originates mainly from the activation of group II afferents. The medium latency reflex response (MLR) was recorded in soleus muscle by peroneal nerve stimulation and clonus beats were recorded in soleus muscle using EMG in 19 spastic patients. The dorsiflexion (DF) and plantarflexion (PF) times of clonus and the half-period were calculated based on accelerometric measurements in 11 patients. The MLR of the soleus was 73.63 ± 8.9 ms. The half-period of the clonus was 79.34 ± 12.31 ms. The difference between the MLR and half-period was significant. The PF was 71.75 ± 6.73 ms, and the DF was 88.63 ± 10.83 ms. The difference between the soleus MLR and PF part of the clonus beat was not significant. The PF part of the clonus beat is due to soleus muscle contraction and controlled by the neural part of the oscillation. There may be relationship between the soleus MLR and the PF part of the clonus. Clonus is considered to be the result of oscillations in the group Ia spinal stretch reflex, but there is sufficient time for group II afferents to be involved.

Pre- and post-alpha motoneuronal control of the soleus H-reflex during sinusoidal hip movements in human spinal cord injury

The aim of this study was to establish the contribution of hip-mediated sensory feedback to spinal interneuronal circuits during dynamic conditions in people with incomplete spinal cord injury (SCI). Specifically, we investigated the effects of synergistic and antagonistic group I afferents on the soleus H-reflex during imposed sinusoidal hip movements. The soleus H-reflex was conditioned by stimulating the common peroneal nerve (CPN) at short (2, 3, and 4 ms) and long (80, 100, and 120 ms) conditioning test (C-T) intervals to assess the reciprocal and pre-synaptic inhibition of the soleus H-reflex, respectively. The soleus H-reflex was also conditioned by medial gastrocnemius (MG) nerve stimulation at C-T intervals ranging from 4 to 7 ms to assess changes in autogenic Ib inhibition during hip movement. Sinusoidal hip movements were imposed to the right hip joint at 0.2 Hz by the Biodex system while subjects were supine. The effects of sinusoidal hip movement on five leg muscles along with hip, knee, and ankle joint torques were also established during sensorimotor conditioning of the reflex. Phase-dependent modulation of antagonistic and synergistic muscle afferents was present during hip movement, with the reciprocal, pre-synaptic, and Ib inhibition to be significantly reduced during hip extension and reinforced during hip flexion. Reflexive muscle and joint torque responses--induced by the hip movement--were entrained to specific phases of hip movement. This study provides evidence that hip-mediated input acts as a controlling signal of pre- and post-alpha motoneuronal control of the soleus H-reflex. The expression of these spinal interneuronal circuits during imposed sinusoidal hip movements is discussed with respect to motor recovery in humans after SCI.

Depression of involuntary activity in muscles paralyzed by spinal cord injury

Involuntary muscle contractions are common after spinal cord injury (SCI). Increased sensitivity to Ia muscle afferent input may contribute to the development of these spasms. Since tendon vibration results in a period of postactivation depression of the Ia synapse, we sought to determine whether Achilles tendon vibration (80 HZ for 2 s) altered involuntary contractions evoked by superficial peroneal nerve (SPN) stimulation (5 pulses at 300 HZ) in paralyzed leg muscles of subjects with chronic (>1 year) SCI. Responses to SPN stimulation that were conditioned by vibration were reduced in 66% of trials (by 33+/-12% in tibialis anterior and 40+/-16% in soleus). These reductions in electromyographic activity are unlikely to be mediated by changes at the Ia synapse or motoneuron because vibration did not alter the magnitude of the soleus H reflex. The electromyographic reductions may involve long-lasting neuromodulatory effects on spinal inhibitory interneurons or synapses involved in the flexor reflex pathway. Vibration-evoked depression of electromyographic activity may be clinically useful in controlling involuntary muscle contractions after SCI.