Evidence for fusimotor drive in stroke patients based on muscle spindle thixotropy

Evaluation of spasticity: IFCN Handbook Chapter

There is no generally accepted definition of spasticity, but hyperexcitable stretch reflexes, exaggerated tendon jerks, clonus, spasms, cramps, increased resistance to passive joint movement, sustained involuntary muscle activity and aberrant muscle activation, including co-contraction of antagonist muscles are all signs and symptoms which are usually associated clinically to the term spasticity. This review describes how biomechanical and electrophysiological techniques may be used to provide quantitative and objective measures of each of these signs and symptoms. The review further describes how neurophysiological techniques may be used to evaluate pathophysiological changes in spinal motor control mechanisms. It is emphasized that understanding the pathophysiology and distinguishing the specific signs and symptoms associated with spasticity, using objective, valid, and reproducible measurements, is essential for providing optimal therapy.

Spastic dystonia: Still a valid term

Hypertonia in childhood may arise because of a variable combination of neuronal and non-neuronal factors. Involuntary muscle contraction may be due to spasticity or dystonia, which represent disorders of the spinal reflex arch and of central motor output respectively. Whilst consensus definitions for dystonia have been established, definitions of spasticity vary, highlighting the lack of a single unifying nomenclature in the field of clinical movement science. The term spastic dystonia refers to involuntary tonic muscle contraction in the context of an upper motor neuron (UMN) lesion. This review considers the utility of the term spastic dystonia, exploring our understanding of the pathophysiology of dystonia and the UMN syndrome. An argument is advanced that spastic dystonia is a valid construct that warrants further exploration. WHAT THIS PAPER ADDS: There is no single universally accepted definitions for terms commonly used to describe motor disorders. Spasticity and dystonia are phenomenologically and pathophysiologically distinct entities. Spastic dystonia represents a subset of dystonia, but with pathophysiological mechanisms more in common with spasticity.

Neurointerface with oscillator motifs for inhibitory effect over antagonist muscles

The effect of inhibitory management is usually underestimated in artificial control systems, using biological analogy. According to our hypothesis, the muscle hypertonus could be effectively compensated via stimulation by bio-plausible patterns. We proposed an approach for the compensatory stimulation device as implementation of previously presented architecture of the neurointerface, where (1) the neuroport is implemented as a DAC and stimulator, (2) neuroterminal is used for neurosimulation of a set of oscillator motifs on one-board computer. In the set of experiments with five volunteers, we measured the efficacy of motor neuron inhibition via the antagonist muscle or nerve stimulation registering muscle force with and without antagonist stimulation. For the agonist activation, we used both voluntary activity and electrical stimulation. In the case of stimulation of both the agonist and the antagonist muscles and nerves, we experimented with delays between muscle stimulation in the range of 0-20 ms. We registered the subjective discomfort rate. We did not identify any significant difference between the antagonist muscle and nerve stimulation in both voluntary activity and electrical stimulation of cases showing agonist activity. We determined the most effective delay between the stimulation of the agonist and the antagonist muscles and nerves as 10-20 ms.

[Spasticity: from pathophysiology to treatment]

The article presents modern views on the pathophysiology of spasticity, which is a frequent disabling consequence to the upper motor neuron (UMN) damage. Morphological and functional system of motion organization and the changes after the UMN damage is considered. The authors analyze existing definitions of spasticity. Stages of spasticity development are described in the context of neuroplasticity as well as in the framework of pathogenesis and sanogenesis. Existing ideas of its pathogenesis are compared with the typical clinical symptoms. The occurring pathological processes in muscles, tendons and joints that can aggravate the development of spasticity and complicate the diagnosis are considered. In addition, the main pathological spasticity patterns are described and the current development of diagnostic techniques is estimated. A review of main methods of spasticity treatment is presented. Special attention is paid to the botulinum neurotoxin type A (BoNT) preparations and central action muscle relaxants. The pathophysiological basement for complex treatment of spasticity as a part of the general rehabilitation process is given, so that the BoNT can be considered as the obligatory element of standard rehabilitation programs.

Emerging Therapies for Spastic Movement Disorders

Spasticity develops as a result of central nervous system (CNS) injury; however, secondary changes within the muscles and connective tissue also contribute to muscle stiffness. The hyaluronan hypothesis postulates that the accumulation of hyaluronan promotes the development of muscle stiffness. Intramuscular injections of the enzyme hyaluronidase, which hydrolyzes long-chained hyaluronan polymers to smaller polymers, was shown to reduce muscle stiffness and increase passive and active range of motion in patients with spasticity. These results provide preliminary evidence of the hyaluronan hypothesis and suggest an emerging therapy to reduce muscle stiffness using the enzyme hyaluronidase.

Different Effects of Cold Stimulation on Reflex and Non-Reflex Components of Poststroke Spastic Hypertonia

OBJECTIVE

To use an established biomechanical approach to quantify reflex and non-reflex responses from spastic-paretic elbow flexors in response to controlled cold and heat stimulation.

METHODS

Thirteen spastic-hemiplegic stroke subjects were tested in the experiment. The spastic elbow joint was stretched into extension for 50° at two speeds (5°/s and 100°/s) in a customized apparatus. Thermal stimulation (HEAT at heat pain threshold, COLD at 0°C, or BASELINE at room temperature) was applied to the thenar eminence of the contralateral hand immediately prior to stretching for at least 30 s.

RESULTS

Total torque was greater at 100°/s than at 5°/s. Total torque was significantly increased after COLD, but not HEAT as compared to BASELINE. When normalized to total torque at baseline, HEAT decreased total torque by 6.3%, while COLD increased total torque by 11.0%. There was no significant difference in the reflex torque among three thermal conditions.

CONCLUSION

The findings demonstrate differentiated effects of cold stimulation on the total resistance from spastic muscles. They provide objective evidence for anecdotal clinical observations of increased muscle spasticity by cold exposure.

Contributions of voluntary activation deficits to hand weakness after stroke

BACKGROUND

Hemiparetic stroke survivors often exhibit profound weakness in the digits of the paretic hand, but the relative contribution of potential biomechanical and neurological impairment mechanisms is not known. Establishing sources of impairment would help in guiding treatment.

OBJECTIVE

The present study sought to quantify the role of diminished capacity to voluntarily active finger flexor and extensor muscles as one possible neurological mechanism.

METHODS

Two groups of stroke survivors with "severe" (N = 9) or "moderate" (N = 9) hand impairment and one group of neurologically intact individuals (N = 9) participated. Subjects were asked to create isometric flexion force and extension force, respectively, with the tip of the middle finger. The maximum voluntary force (MVF) and the maximum stimulated force (MSF) produced by an applied train of electrical current pulses (MSF) were recorded for flexion and extension. Percent voluntary activation (PVA) was computed from MVF and MSF.

RESULTS

Significant deficits in both MVF and PVA were observed for stroke subjects compared to control subjects. For example, activation deficits were >80% for extensor digitorum communis (EDC) for the "severe" group. Maximum voluntary force and PVA deficits were greater for EDC than for flexor digitorum superficialis (FDS) for stroke subjects with severe impairment. Maximum voluntary force and PVA correlated significantly for stroke subjects but not for control subjects.

CONCLUSIONS

Although extrinsic finger muscles could be successfully recruited electrically, voluntary excitation of these muscles was substantially limited in stroke survivors. Thus, finger weakness after stroke results predominantly from the inability to fully activate the muscle voluntarily.

Pathophysiology of spasticity: implications for neurorehabilitation

Spasticity is the velocity-dependent increase in muscle tone due to the exaggeration of stretch reflex. It is only one of the several components of the upper motor neuron syndrome (UMNS). The central lesion causing the UMNS disrupts the balance of supraspinal inhibitory and excitatory inputs directed to the spinal cord, leading to a state of disinhibition of the stretch reflex. However, the delay between the acute neurological insult (trauma or stroke) and the appearance of spasticity argues against it simply being a release phenomenon and suggests some sort of plastic changes, occurring in the spinal cord and also in the brain. An important plastic change in the spinal cord could be the progressive reduction of postactivation depression due to limb immobilization. As well as hyperexcitable stretch reflexes, secondary soft tissue changes in the paretic limbs enhance muscle resistance to passive displacements. Therefore, in patients with UMNS, hypertonia can be divided into two components: hypertonia mediated by the stretch reflex, which corresponds to spasticity, and hypertonia due to soft tissue changes, which is often referred as nonreflex hypertonia or intrinsic hypertonia. Compelling evidences state that limb mobilisation in patients with UMNS is essential to prevent and treat both spasticity and intrinsic hypertonia.

Position-dependent, hyperexcitable patellar reflex dynamics in chronic stroke

OBJECTIVES

To quantify tendon tap response (TTR) properties and their position dependence using multiple neuromechanical parameters, and to analyze correlations among neuromechanical and clinical measures.

DESIGN

Hyperexcitable dynamics of TTR were investigated in a case-control manner. An instrumented hammer was used to induce the patellar deep tendon reflex (DTR), with reflex-mediated electromyography and torque responses measured across a range of knee flexion.

SETTING

Research laboratory in a rehabilitation hospital.

PARTICIPANTS

Chronic hemiplegic stroke survivors (n=9) and healthy subjects (n=13).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Neuromechanical measures (system gain, contraction rate, half-relaxation rate, reflex loop delay, peak reflex torque, peak reflex electromyography, and reflex threshold in tapping force) were measured to characterize neuromuscular properties of patellar TTR. Clinical measurements were taken using the DTR scale and the Modified Ashworth Scale.

RESULTS

The system gain, contraction rate, half-relaxation rate, and peak reflex-mediated torque in the stroke group were generally higher, whereas the reflex threshold in the stroke group was significantly lower than their counterparts in the control group across 45° to 90° of knee flexion (P<.05). The 4 parameters were significantly higher at 60° and 75° of flexion than at 15°, 30°, 45°, and 90°, and their correlations with the 2 clinical scales at 60°, 75°, and 90° of flexion were also significantly higher than those at 15°, 30°, and 45° (P<.05).

CONCLUSIONS

The results showed hyperexcitability of TTR in stroke, quantified using a number of neuromechanical measures. Those measures peak around 60° to 75° of knee flexion and were correlated with clinical scales.

The spinal pathophysiology of spasticity--from a basic science point of view

Spasticity is a term, which was introduced to describe the velocity-sensitive increased resistance of a limb to manipulation in subjects with lesions of descending motor pathways. This distinguishes spasticity from the changes in passive muscle properties, which are often seen in these patients, but are not velocity-sensitive. Increased excitability of the stretch reflex is thus a central component of the definition of spasticity. This review describes changes in cellular properties and transmission in a number of spinal reflex pathways, which may explain the increased stretch reflex excitability. The review focuses mainly on results derived from the use of non-invasive electrophysiological techniques, which have been developed during the past 20-30 years to investigate spinal neuronal networks in human subjects, but work from animal models is also considered. The reflex hyperexcitability develops over several months following the primary lesion and involves adaptation in the spinal neuronal circuitries caudal to the lesion. In animal models, changes in cellular properties (such as 'plateau potentials') have been reported, but the relevance of these changes to human spasticity has not been clarified. In humans, numerous studies have suggested that reduction of spinal inhibitory mechanisms (in particular that of disynaptic reciprocal inhibition) is involved. The inter-subject variability of these mechanisms and the lack of objective quantitative measures of spasticity have impeded disclosure of a clear causal relationship between the alterations in the inhibitory mechanisms and the stretch reflex hyperexcitability. Techniques which make such a quantitative measure possible as well as longitudinal studies where development of reflex excitability and changes in the inhibitory mechanisms are followed over time are in great demand.

Temporal facilitation of spastic stretch reflexes following human spinal cord injury

Recent evidence suggests that alterations in ionic conductances in spinal motoneurones, specifically the manifestation of persistent inward currents, may be partly responsible for the appearance of hyperexcitable reflexes following spinal cord injury (SCI). We hypothesized that such alterations would manifest as temporal facilitation of stretch reflexes in human SCI. Controlled, triangular wave, ankle joint rotations applied at variable velocities (30-120 deg s(-1)) and intervals between stretches (0.25-5.0 s) were performed on 14 SCI subjects with velocity-dependent, hyperexcitable plantarflexors. Repeated stretch elicited significant increases in plantarflexion torques and electromyographic (EMG) activity from the soleus (SOL) and medial gastrocnemius (MG). At higher velocities (> or = 90 deg s(-1)), reflex torques declined initially, but subsequently increased to levels exceeding the initial response, while mean EMG responses increased throughout the joint perturbations. At lower velocities (< or = 60 deg s(-1)), both joint torques and EMGs increased gradually. Throughout a range of angular velocities, reflex responses increased significantly only at intervals < or = 1 s between stretches and following at least four rotations. Ramp-and-hold perturbations used to elicit tonic stretch reflexes revealed significantly prolonged EMG responses following one or two triangular stretches, as compared to single ramp-and-hold excursions. Post hoc analyses revealed reduced reflex facilitation in subjects using baclofen to control spastic behaviours. Evidence of stretch reflex facilitation post-SCI may reflect changes in underlying neuronal properties and provide insight into the mechanisms underlying spastic reflexes.

A neuromusculoskeletal model to simulate the constant angular velocity elbow extension test of spasticity

We developed a neuromusculoskeletal model to simulate the stretch reflex torque induced during a constant angular velocity elbow extension by tuning a set of physiologically-based parameters. Our model extended past modeling efforts in the investigation of elbow spasticity by incorporating explicit musculotendon, muscle spindle, and motoneuron pool models in each prime elbow flexor. We analyzed the effects of changes in motoneuron pool and muscle spindle properties as well as muscle mechanical properties on the biomechanical behavior of the elbow joint observed during a constant angular velocity elbow extension. Results indicated that both motoneuron pool thresholds and gains could be substantially different among muscles. In addition, sensitivity analysis revealed that spindle static gain and motoneuron pool threshold were the most sensitive parameters that could affect the stretch reflex responses of the elbow flexors during a constant angular velocity elbow extension, followed by motoneuron pool gain, and spindle dynamic gain. It is hoped that the model will contribute to the understanding of the underlying mechanisms of spasticity after validation by more elaborate experiments, and will facilitate the future development of more specific treatment of spasticity.

Pathophysiology of spastic paresis. II: Emergence of muscle overactivity

In the subacute and chronic stages of spastic paresis, stretch-sensitive (spastic) muscle overactivity emerges as a third fundamental mechanism of motor impairment, along with paresis and soft tissue contracture. Part II of this review primarily addresses the pathophysiology of the various forms of spastic overactivity. It is argued that muscle contracture is one of the factors that cause excessive responsiveness to stretch, which in turn aggravates contracture. Excessive responsiveness to stretch also impedes voluntary motor neuron recruitment, a concept termed stretch-sensitive paresis. None of the three mechanisms of impairment (paresis, contracture, and spastic overactivity) is symmetrically distributed between agonists and antagonists, which generates torque imbalance around joints and limb deformities. Thus, each may be best treated focally on an individual muscle-by-muscle basis. Intensive motor training of the less overactive muscles should disrupt the cycle of paresis-disuse-paresis, and concomitant use of aggressive stretch and focal weakening agents in their more overactive and shortened antagonists should break the cycle of overactivity-contracture-overactivity.

Aftereffects of mechanical vibration and muscle contraction on limb position-sense

Mechanical vibration (MV) of a muscle causes position-sense errors during and after application. Isometric muscle contraction at a shorter (hold-short conditioning) or longer (hold-long conditioning) length causes limb position-sense errors after the muscle returns to its intermediate length by means of intrafusal muscle thixotropy. However, it is unclear whether MV enhances these thixotropic position-sense errors. We studied the after-effects of MV on position-sense errors induced by hold-short and hold-long conditioning in the biceps of 12 healthy men. After hold-short conditioning, subjects perceived that the conditioned forearm was placed in a more extended position than occurred in reality; after hold-long conditioning, a more flexed position was perceived. Use of MV with hold-short or hold-long conditioning enhanced both errors, which were most obvious at 100 HZ. These results suggest that MV and muscle conditioning work together efficiently to develop intrafusal muscle thixotropy. MV combined with hold-long conditioning may alleviate thixotropically increased muscle stiffness, such as in spastic hypertonia.

Further insights into post-exercise effects on H-reflexes and motor evoked potentials of the flexor carpi radialis muscles

The present study investigated the relative contribution of the cortical and spinal mechanisms for post-exercise excitability changes in human motoneurons. Seven healthy right-handed adults with no known neuromuscular disabilities performed an isometric voluntary wrist flexion at submaximum continuous exertion. After the subjects continued muscle contraction until volitional fatigue, the H-reflexes induced by an electric stimulation and motor evoked potentials (MEPs) induced by a transcranial magnetic stimulation (TMS) from a flexor carpi radialis (FCR) muscle were recorded 7 times every 20 s. The H-reflex was used to assess excitability changes at the spinal level, and the MEP was used to study excitability changes at the cortical level. Hreflexes showed a depression (30% of control value) soon after the cessation of wrist flexion and recovered with time thereafter. On the other hand, an early (short latency) MEP showed facilitation immediately after the cessation of wrist flexion (50% of control value) and thereafter decreased. A possible mechanism for the contradictory results of the 2 tests, in spite of focusing on the same motoneuron pool, might be the different test potential sizes between them. In addition, a late (long latency) MEP response appeared with increasing exercise. With regard to the occurrence of late MEP response, a central mechanism may be proposed to explain the origin-that is, neural pathways with a high threshold that do not participate under normal circumstances might respond to an emergency level of muscle exercise, probably reflecting central effects of fatigue.

The history of contraction of the wrist flexors can change cortical excitability

Voluntary contractions induce thixotropic changes in intrafusal muscle fibres and hence, by induction or removal of "slack", the background discharge and sensitivity of spindle endings to stretch is altered. This study assessed whether such changes also altered the "excitability" of the motor cortex. Eleven subjects performed a series of voluntary conditioning contractions of the wrist flexors designed to remove slack in the intrafusal fibres (contract and test at intermediate length, termed "contract-test") or to introduce slack (contract at long length and test at intermediate length, termed "contract-long"). Surface electromyographic recordings were made from one wrist flexor, flexor carpi radialis. Subjects relaxed after each contraction, and 10 s later a test stimulus was applied to elicit a tendon tap response, H-reflex, or motor-evoked potential (MEP) to transcranial magnetic stimulation in the flexor carpi radialis. Each of the three test stimuli was applied during 15 consecutive pairs of contractions ("contract-long" and "contract-test"). Three subjects repeated the protocol using transmastoid electrical stimulation as the test stimulus to evoke a cervicomedullary motor-evoked potential (CMEP). For the group of subjects, after conditioning contractions designed to induce slack there was a significant reduction in the amplitude of the tendon reflex, no significant change in the H-reflex, and a small but significant reduction in the amplitude of the MEP. In one subject the CMEP was significantly reduced, while it was unchanged in two others. In the absence of corresponding changes in the H-reflex (or CMEP), changes in the size of the response to motor cortical stimulation suggest that the level of motor cortical "excitability" changes according to naturally induced variations in the discharge of muscle spindle afferents.

Botulinum treatment of spasticity: why is it so difficult to show a functional benefit?

Clinical experience seems to indicate that botulinum toxin injections can, in selected patients with upper motor neurone syndrome, reduce spasticity and improve voluntary movement and active function. However, double-blind placebo-controlled trials have had difficulty showing active functional improvement, despite the clear ability of botulinum toxin to reduce spasticity. This prompts a re-analysis of the basic assumption that spasticity impairs voluntary movement and a review of the methodology of the clinical trials. Motor dysfunction is usually caused by weakness and the other "negative" features of upper motor neurone syndrome, rather than muscle overactivity. Recent research has explored the pathophysiological basis of the voluntary movement disorder, in particular the role of the various forms of motor overactivity, which might be amenable to botulinum toxin treatment. The failure of double-blind placebo-controlled clinical trials to show improvement in active function is, to a large extent, a result of their methodology, especially patient selection, injection protocols, and the choice of outcome measures. Clinical trials need to be re-designed and based upon expert experience and a better understanding of the pathophysiology of the motor disorder.

Short-term effects of dynamic lycra splints on upper limb in hemiplegic patients

OBJECTIVE

To assess acceptability, effects on swelling, resting posture, spasticity, and active (AROM) and passive range of motion (PROM) of individually tailored upper limb Lycra garments, designed as dynamic splints to exert directional pull on certain limb segments, when worn for 3 hours by hemiplegic patients.

DESIGN

Crossover trial.

SETTING

Outpatient and inpatient rehabilitation center.

PATIENTS

Convenience sample of 16 patients with hemiparesis and upper limb spasticity caused by a stroke more than 3 weeks before the study.

INTERVENTIONS

Assessments performed at the start and end of a 3-hour period during a standard rehabilitation day when the patients were and were not wearing the garment.

MAIN OUTCOME MEASURES

(1) Comfort assessed by questionnaire; (2) circumference of each limb segment; (3) resting posture at elbow and wrist; (4) spasticity at shoulder, elbow, and wrist using the Tardieu scale; and (5) AROM and PROM at shoulder, elbow, and wrist measured using a goniometer; (6) elbow proprioception using McCloskey's method; (7) visual neglect syndrome using the line bisection test. Differences between changes occurring with and without the garment were compared using Wilcoxon's signed rank test for ordinal variables (spasticity grading) and Student's t test for continuous variables (all other data).

RESULTS

During 3 hours, garments worn on the arm by patients with hemiplegia (1) were comfortable, (2) improved wrist posture and reduced wrist and finger flexor spasticity, (3) reduced swelling in patients with swollen limbs (digit circumference decreased by 4%; p<.01), (4) improved PROM at shoulder (mean increase in range, 4.1 degrees +/- 13.0 degrees per shoulder movement; p<.01); and (5) impaired ability to flex fingers (range of voluntary flexion of digit III reduced from 107.3 degrees +/-79.6 degrees to 91.4 degrees +/-74.1 degrees; p<.05).

CONCLUSION

Lycra garments, designed to produce continuous stretch of spastic muscles when worn for several hours each day, have rapid splinting and antispastic effects on wrist and fingers in patients with hemiplegia. These garments may help severely affected patients with major spasticity or painful swollen limbs.