Validity, Intra-Rater Reliability and Normative Data of the Neuroflexor™ Device to Measure Spasticity of the Ankle Plantar Flexors after Stroke

Abstract

Objective: Quantification of lower limb spasticity after stroke and the differentiation of neural from passive muscle resistance remain key clinical challenges. The aim of this study was to validate the novel NeuroFlexor foot module, to assess the intrarater reliability of measurements and to identify normative cut-off values. Methods: Fifteen patients with chronic stroke with clinical history of spasticity and 18 healthy subjects were examined with the NeuroFlexor foot module at controlled velocities. Elastic, viscous and neural components of passive dorsiflexion resistance were quantified (in Newton, N). The neural component, reflecting stretch reflex mediated resistance, was validated against electromyography activity. A test-retest design with a 2-way random effects model permitted study of intra-rater reliability. Finally, data from 73 healthy subjects were used to establish cutoff values according to mean + 3 standard deviations and receiver operating characteristic curve analysis. Results: The neural component was higher in stroke patients, increased with stretch velocity and correlated with electromyography amplitude. Reliability was high for the neural component (intraclass correlation coefficient model 2.1 (ICC2,1) ≥ 0.903) and good for the elastic component (ICC2,1 ≥ 0.898). Cutoff values were identified, and all patients with neural component above the limit presented pathological electromyography amplitude (area under the curve (AUC) = 1.00, sensitivity = 100%, specificity = 100%). Conclusion: The NeuroFlexor may offer a clinically feasible and non-invasive way to objectively quantify lower limb spasticity. LAY ABSTRACT Spasticity is a sensorimotor impairment, which often occurs after stroke as well as after other injuries to the central nervous system. Spasticity is characterized by increased resistance to passive stretch of weak muscles due to increased reflex activity. Spasticity is currently measured clinically while the examiner passively stretches a muscle. However, the clinical method cannot differentiate resistance due to increased reflex activity from resistance due to muscle stiffness, which can develop over time in weakened muscles. The aim of this study was to evaluate the novel NeuroFlexor foot module, which was developed to quantify and distinguish nerve and muscle components of resistance during passive stretching of the lower limb muscles. By quantifying these factors, one can obtain more reliable information than the clinical examination allows. NeuroFlexor measurements in 15 patients in the chronic stage after stroke and 18 healthy individuals allowed the validity of the method to be evaluated by assessing the relationship with velocity of stretch and by simultaneously examining the reflex activity using surface electromyography. The reliability of NeuroFlexor measurements was studied by comparing repeated measurements. Finally, the study established normal NeuroFlexor values from 73 healthy individuals. The results suggest that the NeuroFlexor foot module may be a valid, reliable and easy-to-use objective method to quantify lower limb spasticity.

Keywords

• stroke
• muscle spasticity
• stretch reflex
• outcome assessment
• lower extremity