Assessing Cerebral Hemodynamic Stability After Brain Injury.

OBJECTIVE: Following brain injury, unstable cerebral hemodynamics can be characterized by abnormal rises in intracranial pressure (ICP). This behavior has been quantified by the RAP index: the correlation (R) between ICP pulse amplitude (A) and mean (P). While RAP could be a valuable indicator of autoregulatory processes, its prognostic ability is not well established and its validity has been questioned due to potential errors in measurement. Here, we test (1) whether RAP is a consistent measure of intracranial hemodynamics and (2) whether RAP has prognostic value in predicting hemodynamic instability following brain injury. MATERIALS AND METHODS: RAP was tested in seven brain injured patients treated in a surgical intensive care unit. A sample of ICP data was randomly chosen and segmented into 1 hour periods. Hours were then categorized as either stable, which contained no sharp rises in ICP, or unstable, which contained ≥1 sharp rise-where a sharp rise is defined as ICP exceeding a mean slope of 0.15 mmHg/s. Equal numbers of stable and unstable segments were then selected for each patient. RAP was calculated as the Pearson's correlation coefficient between ICP pulse amplitude (AMP) and mean (mICP), determined in 6 second windows, according to established methods. RESULTS: Results showed that (1) average AMP and ICP levels were similar between stable and unstable periods and (2) unstable periods were identified by RAP values exceeding 0.6 with an average positive predictive value of 74%. CONCLUSIONS: We conclude that RAP can provide a valid measure of ICP dynamics, is not affected by sensor drift, and can better distinguish periods of instability than ICP or AMP alone.

Trending autoregulatory indices during treatment for traumatic brain injury.

Our goal is to use automatic data monitoring for reliable prediction of episodes of intracranial hypertension in patients with traumatic brain injury. Here we test the validity of our method on retrospective patient data. We developed the Continuous Hemodynamic Autoregulatory Monitor (CHARM), that siphons and stores signals from existing monitors in the surgical intensive care unit (SICU), efficiently compresses them, and standardizes the search for statistical relationships between any proposed index and adverse events. CHARM uses an automated event detector to reliably locate episodes of elevated intracranial pressure (ICP), while eliminating artifacts within retrospective patient data. A graphical user interface allowed data scanning, selection of criteria for events, and calculating indices. The pressure reactivity index (PRx), defined as the least square linear regression slope of intracranial pressure versus arterial BP, was calculated for a single case that spanned 259 h. CHARM collected continuous records of ABP, ICP, ECG, SpO2, and ventilation from 29 patients with TBI over an 18-month period. Analysis of a single patient showed that PRx data distribution in the single hours immediately prior to all 16 intracranial hypertensive events, significantly differed from that in the 243 h that did not precede such events (p < 0.0001). The PRx index, however, lacked sufficient resolution as a real-time predictor of IH in this patient. CHARM streamlines the search for reliable predictors of intracranial hypertension. We report statistical evidence supporting the predictive potential of the pressure reactivity index.

Clinical Risk Factors for Orthostatic Hypotension: Results Among Elderly Fallers in Long-Term Care.

BACKGROUND: Patients at greatest risk for fall-related injuries are older adults with orthostatic hypotension (OH), a condition which drops blood pressure. This study sought to determine salient demographic and patient-level factors increasing risk for OH among a sample of elderly fallers. METHODS: Data analysis for this retrospective study sought to assess the relationship between various demographic and clinical risk factors and the likelihood of OH. Because fallers could experience multiple falls, generalized estimating equations were used to account for patient-level correlations. RESULTS: One hundred seventeen falls occurred in 47 patients, who were primarily female with a mean age of 90.7 years. Falls resulted in 18 cases of OH. Orthostatic hypotension cases were less likely to have a gait in a steady line (5.6% vs 55.6%, P = 0.001). Patients with decreased muscular coordination were almost 5 times more likely to experience OH than those with no coordination problems (odds ratio = 4.86, P = 0.02). Patients with gait in a steady line were less likely to experience OH after a fall (OR = 0.06, P = 0.006). CONCLUSIONS: Orthostatic hypotension is potentially modifiable once detected. Evidenced-based protocol for assessment and management of OH among patients with gait and balance impairment is presented.

Inter-limb transfer of learned ankle movements.

Inter-limb learning transfer (ILT) between the upper-limbs has been well documented, but no corresponding study of the lower limbs has been done. We investigated ILT in the lower limbs of subjects who learned to move a cursor toward targets within 800 ms using ankle movements: plantar/dorsi-flexion and inversion/eversion. Twenty-two healthy right-dominant subjects were divided into two groups: half performed the tasks first using the right foot (group RL), and the other half performed it first with the left foot (group LR). Targets appeared on a computer screen at head-height while subjects were seated with one foot on a goniometric ankle platform. Subjects were required to move the cursor toward one of three randomly appearing targets under two conditions: (1) neutral or no visual motor rotation, and (2) with a 30 degrees visuo-motor rotation. Performance was quantified by computing the z-score for direction and position errors for each subject and ILT was assessed by comparing group performances for each foot. Results demonstrated that group LR but not group RL experienced significant ILT of directional as well as positional information in both tasks in a manner reflective of the distinctly different functional roles played by the upper and lower limbs.

Spatial resolution of spontaneous accelerations in reaching tasks.

Reaching tasks are considered well-executed if they appear "smooth," a quality that is typically quantified by its opposite, jerk, the rate of change of acceleration. While jerk is a theoretically sound measure, its application to spastic individuals sometimes yields counter-intuitive results, and does not reveal motor impairment across the workspace. To more generally quantify spontaneous accelerative transients (SATs) within a movement, a pseudo-wavelet transform was devised that iteratively compared angular trajectories to a series of straight-line approximants. Cumulative linear fit errors were expressed in terms of flexion angle, yielding an SAT map of the entire motion. To compare SAT maps with traditional smoothness measures, two scalar indices were extracted from them: residual excursion deviation (RED), representing the integral over Deltatheta and the ratio of peak error to mean error (PEME) on the map. Fifteen subjects, including five subjects with chronic stroke performed elbow flexions throughout their entire ranges of motion, Deltatheta, at a comfortable pace with their arms supported in the transverse plane. Maps revealed that stroke subjects were significantly less coordinated than controls, as measured both by RED: 8.0+/-2.9 x 10(-3) versus 3.1+/-0.8 x 10(-3) and PEME: 6.6+/-0.9 versus 12.1+/-1.9, both P<0.001. Comparable jerk metrics, including integrated average jerk, did not report a significant performance deficit at the P<0.05 level. Map metrics for all subjects were independent of average velocity (correlation with theta : rho0.31), but jerk-based metrics for stroke subjects were spuriously co-variant with velocity rho=0.85, which may relate to the significantly higher mean arrest period ratio in stroke subjects (0.26+/-0.19 versus 0.09+/-0.08, P<0.001). We conclude that SAT maps provide reliable information on regional movement impairments at a wide range of proficiency levels.

Biomechanical analysis of spasticity treatment in patients with multiple sclerosis.

OBJECTIVES: New metrics for clinical spasticity are needed to assess motor performance, since scales such as the Ashworth and Tardieu are unreliable. Here, we assessed outcomes of baclofen treatment in patients with multiple sclerosis (MS) using biomechanical analysis of voluntary movements. METHODS: Patients with MS and symptomatic limb spasticity were recruited for a pre-post baclofen titration study, along with age-matched healthy controls. Oral baclofen was titrated to optimize spasticity symptoms in all MS cases over 4 weeks. Clinical assessments included the Modified Ashworth Scale (MAS), Tardieu Scale (TS); elbow kinematics were measured via the Transient Acceleration Measurement Interface (TAMI); performance was measured as the score at 4 weeks minus the baseline score in all measures. Movement proficiency within TAMI was quantified through a scale-free smoothness measure, according to the regional excursion deviation (RED) from a constant-velocity approximant. RESULTS: Twelve patients with MS [age: 47.8 ± 9.8 years; women: 4; disease duration: 20 ± 10 years; disease-modifying therapy use: 7; Expanded Disability Status Scale (EDSS): 6.8 ± 1.4] and eight age-matched healthy controls were evaluated concurrently (mean age: 49.5 ± 13.1 years; women = 3). In MS cases, no significant improvement in arm spasticity was observed with main effects: MAS: -41.6 ± 72.6 (p = 0.09); EDSS: -1.6 ± 10.4% (p = 0.49); and TS: -8.3 ± 2.1% (p = 0.32), -24.9 ± 63.6% (p = 0.42), and -30.7 ± 79.9% (p = 0.06), at slow, moderate, and fast speeds, respectively. However, voluntary motion smoothness, as measured by TAMI: RED, decreased significantly: 0.62 ± 0.08 versus 0.54 ± 0.09, p < 0.001, indicating significant increase in movement smoothness post treatment. CONCLUSION: A simple biomechanical analysis of voluntary movements revealed a significant reduction of spasticity after 30 days of baclofen therapy in patients with MS that was not detected by clinical assessments.

Proceedings of the first workshop on Peripheral Machine Interfaces: going beyond traditional surface electromyography.

One of the hottest topics in rehabilitation robotics is that of proper control of prosthetic devices. Despite decades of research, the state of the art is dramatically behind the expectations. To shed light on this issue, in June, 2013 the first international workshop on Present and future of non-invasive peripheral nervous system (PNS)-Machine Interfaces (MI; PMI) was convened, hosted by the International Conference on Rehabilitation Robotics. The keyword PMI has been selected to denote human-machine interfaces targeted at the limb-deficient, mainly upper-limb amputees, dealing with signals gathered from the PNS in a non-invasive way, that is, from the surface of the residuum. The workshop was intended to provide an overview of the state of the art and future perspectives of such interfaces; this paper represents is a collection of opinions expressed by each and every researcher/group involved in it.

Piezoelectric substrates promote neurite growth in rat spinal cord neurons.

We tested the possibility that exogenous electrical activity from a piezoelectric substrate could influence neuronal structure in cultured spinal cord neurons. Oscillating electrical fields were delivered to rat neurons via substrates consisting of poly(vinylidene fluoride) film, both in its piezoelectric (PZ) and non-piezoelectric (PV) forms. To induce oscillating electrical fields at the film surfaces, a 50 Hz mechanical vibration was applied. After 4 days of mechano-electrical stimulation, neuronal densities were increased by 115% and neurons grew 79% more neurites, with more than double the branch points, compared with neurons grown on non-stimulated PZ films (p < 0.001). The effects were due to electrical field, because vibration applied to non-PZ films did not increase neurite growth. We conclude that the oscillating electric fields produced from PZ polymer substrates can induce plastic changes in neurons of the central nervous system and herein we show its influence on neurite growth and branching.

Improving fine motor function after brain injury using gesture recognition biofeedback.

PURPOSE: We developed a gesture recognition biofeedback (GRB) device for improving fine motor function in persons with brain injury using surface muscle pressures of the forearm to provide real-time visual biofeedback. The GRB apparatus is easy to don by moderately impaired users and does not require precise placement of sensors. METHOD: The efficacy of GRB training with each subject was assessed by comparing its effectiveness against standard repetitive training without feedback. The outcome was measured using a nine-hole peg test (HPT) administered before and after each condition, in a cross-over study design. RESULTS: GRB was shown to be effective for short-term improvement of fine motor function of 12 impaired participants, reducing their average time to completion of the HPT by 15.5% (S.D. 7.14%). In a subset of impaired subjects, this effect was significant in comparison to similar training without biofeedback (p < 0.05). Control subjects experienced negligible change in HPT time. CONCLUSIONS: This pilot study of a heterogeneous group shows that GRB may offer a simple means to help impaired users re-learn specified manual tasks.

Reformulation in the phase plane enhances smoothness rater accuracy in stroke.

To improve the characterization of motor impairment, we compared the sensitivities of a phase plane metric with temporal domain measures derived from integrated squared jerk (ISJ). Five subjects with stroke and a cohort of 21 neurologically intact volunteers performed self-paced, isolated elbow flexions. Analysis of angular trajectories from the stroke group revealed that temporal domain metrics failed to detect a performance deficit at the p < .05 level, while the phase plane metric did resolve a deficit (p < .01). When applied to a subset of movements with arrest periods, the phase measure also uniquely identified impairment (Wilcoxon rank-sum test, p < .001). Finally, when tested on a data-driven model, the phase measure, but not temporal metrics, increased monotonically with the severity of trajectory distortions. We conclude that motion smoothness can be accurately measured in the phase plane.

Relating biceps EMG to elbow kinematics during self-paced arm flexions.

Repetitive reaching movements to a fixed target can be generally characterized by bell-shaped velocity profiles and sigmoidal trajectories with variable morphologies across multiple repetitions. A neuromuscular correspondence of these kinematic variations has thus far eluded electromyographic (EMG) analysis. We recorded EMG and elbow kinematics from fourteen healthy individuals performing repetitive, self-paced, isolated elbow flexions, with their arms supported against gravity. The global kinematic pattern of each flexion was classified as either sigmoidal (S) or non-sigmoidal (NS), based on goodness of fit with analytical curves. Ten of the fourteen subjects generated an approximately equal number of S and NS types (383 movement cycles). Trajectories of the other four subjects were not classifiable or did not vary sufficiently and were excluded from subsequent analysis. A post hoc predictor of trajectory type was derived by testing linear support vector machines trained with a strategically selected 3-feature sub-space of the early phase of enveloped biceps EMG during a leave-one-out cross-validation paradigm. Results showed that EMG features predicted kinematic morphology with sensitivity and specificity both exceeding 80%. The high predictive accuracy suggests neuromotor signals coding for subtle variations in elbow kinematics during self-paced, unloaded motions, can be deciphered from the biceps EMG.

Surface muscle pressure as a measure of active and passive behavior of muscles during gait.

While surface electromyography (SEMG) can accurately register electrical activity of muscles during gait, there are no methods to estimate muscular force non-invasively. To better understand the mechanical behavior of muscle, we evaluated surface muscle pressure (SMP) in conjunction with SEMG. Changes in anterior thigh radial pressure during isometric contractions and gait were registered by pressure sensors on the limb. During isometric knee extensions by a single subject, SMP waveforms correlated well with SEMG (r=0.97), and SEMG onsets preceded those of SMP by 35-40 ms. SMP and SEMG signals were simultaneously recorded from the quadriceps of 10 healthy subjects during gait at speeds of 0.4, 0.8, 1.1, 1.4 and 2.2m/s. Muscle activity onset and cessation times were objectively determined for both modalities, and results showed high intra-class correlations. SMP waveforms were highly consistent from stride to stride, while SEMG waveforms varied widely. SEMG waveforms were typically brief, while SMP waveforms tended to be biphasic and outlasted the SEMG by approximately 40% of gait cycle at all speeds. These results are consistent with mechanical models of muscle, and demonstrate the use of SMP to estimate the timing of knee extensor muscle stiffness during gait.

Training grip control with a Fitts' paradigm: a pilot study in chronic stroke.

STUDY DESIGN: A clinical measurement study. PURPOSE: To test the applicability of Fitts' paradigm to grasping tasks in individuals with chronic stroke. INTRODUCTION: Fitts' Law relates the time of target achievement to task difficulty in repetitive motor tasks. METHODS: Six male chronic stroke patients performed repetitive actuation of a grip force dynamometer with their affected hands for 12 sessions over four to six weeks. RESULTS: Movement times followed Fitts' behavior with correlations of R(2)>0.8 for all subjects. Grasp control improved during training, as indicated by an average decrease in Fitts' slope of 26% at high difficulty levels (p<0.05), and decreases in the number of force corrections and in jerkiness, both at p<0.001 level. CONCLUSIONS: The Fitts' grip force targeting protocol provides an objective standardized instrument for grasp proficiency quantification and a potentially efficacious platform for hand training for persons with stroke. LEVEL OF EVIDENCE: N/A.

Pressure signature of forearm as predictor of grip force.

We studied the relationship between grip force and external forearm pressure in nondisabled subjects using force myography (FMG). FMG uses a sensorized cuff surrounding the forearm to register the distributed mechanical force, detecting pressure on the sensors generated by the volumetric changes of the underlying musculo-tendinous complex. Each of nine nondisabled subjects donned the FMG cuff and applied grip forces to a cylindrical dynamometer; grip forces ranged from 0% to 100% of the subjects' maximum voluntary contraction. The cuff was positioned with seven force sensors located on both the anterior and posterior surfaces of the proximal forearm, but no attempt was made to match sensor placement with particular muscles or sites. Grip prediction was simply encoded as the rectified sum of the FMG sensor outputs. During grip and release cycles, the FMG waveforms of each subject correlated closely with his or her force waveforms (r > 0.89). FMG also correlated highly with the timing of grip onset and release (intraclass correlation coefficient (ICC(A,2)) = 0.99) and time to peak (ICC(A,2) = 0.91), with negligible lag. These results demonstrate that when applied to the forearm, FMG represents a grip force signature that may be useful for near-real-time proportional control of upper-limb prosthetic devices.

The bionic man: restoring mobility.

Bionics engineers are making increasingly bold and successful use of their tools to restore mobility to persons with missing or nonfunctional limbs. These tools include the latest materials, minielectronics and megacomputers, advanced robotic mechanisms, and algorithms. With crucial help from their pioneering users, they are learning how and where the residual sensorimotor system can be tapped in order to transmit its intents to replacement or reactivated body parts.

Power spectral analysis of heart rate in relation to sleep position.

We used spectral analysis of heart rate variability, as a measure of autonomic tone, to determine spectral power differences in infants sleeping supine and prone. We studied 29 infants with a birth weight of 1,915 +/- 939 g, at the postconceptional age of 36 +/- 2 weeks. We then calculated total power (TP), low-frequency power (LF, 0.03-0.15 Hz), and high-frequency power (HF, 0.5-1.0 Hz). TP corresponds to overall heart rate variability, LF to both sympathetic and parasympathetic activity, and HF to parasympathetic activity only. Median (25th, 75th percentile) TP (beats/min2) in the supine position was 32.60 (23.12, 59.90), which was significantly higher than the prone position of 25.87 (14.94, 35.57). Similarly, LF (beats/min2) in the supine position of 13.82 (8.63, 23.31) was significantly higher than the prone position of 9.79 (5.46, 14.33). No significant difference was seen in the HF. We conclude that the prone position is associated with decreased heart rate variability and probably decreased sympathetic tone, which imply decreased autonomic stability in this position.