Quantifying changes in material properties of stroke-impaired muscle

Abstract

Background: Material properties of muscles are clinically important parameters for evaluating altered muscle function. Stroke survivors display motor impairments almost immediately after the vascular event, and then gradually develop altered muscle properties. Little is known about the magnitude of these changes in muscle material properties, specifically stiffness. Previous measures of stiffness are limited to estimates of joint stiffness or groups of muscles. Thus, our aim was to determine changes in passive muscle stiffness and composition by measuring: (1) shear wave speed using shear wave ultrasound elastography and (2) echo intensity of the B-mode ultrasound images of the biceps brachii muscle in individuals who have had a stroke.

Methods: Shear wave ultrasound elastography and B-mode ultrasound images of the biceps brachii muscle of the paretic and non-paretic limbs of sixteen stroke survivors were captured at rest.

Findings: Our main results show that shear wave speed and echo intensity of the paretic side were on average 69.5% and 15.5% significantly greater than those of the non-paretic side, respectively. Differences in shear wave speed between the non-paretic and the paretic muscles were strongly correlated with differences in echo intensity, time since stroke, and with Fugl-Meyer scores.

Interpretation: Muscle stiffness and muscle composition, as indicated by SW speed and echo intensity, may be altered in stroke-impaired muscle at rest. These findings highlight the potential for SW elastography as a tool for both investigating the fundamental mechanisms behind changes in stroke-impaired muscle, and for evaluation of muscle mechanical properties as part of clinical examination.

Keywords: Echo intensity; Muscle; Stiffness; Stroke; Ultrasound.

Conflict of interest statement

Conflict of interest

The authors have nothing to disclose.

Copyright © 2015 Elsevier Ltd. All rights reserved.

Figures

Fig. 1.

Shear wave ultrasound (SW) elastography images of the non-paretic and paretic biceps brachii muscles of a representative subject. The squares represent the region of interest from which SW speed was measured. In these images, the non-paretic muscle has a mean SW speed of 2.5 m/s, whereas the paretic muscle has a mean SW speed of 4.7 m/s.

Fig. 2.

B-mode ultrasound images of the non-paretic and paretic biceps brachii muscles of a representative subject. The scale of the figure refers to the echo intensity in that pixels that are black have a value of 0, whereas pixels that are very bright, or white, have a value of 255. The squares represent the region of interest from which echo intensity was calculated. The higher the echo intensity, the brighter or “whiter” the pixel appears. In these images, the non-paretic muscle has a mean echo intensity of 75 whereas the paretic muscle has a mean echo intensity of 91.

Fig. 3.

Mean shear wave speeds and standard deviation of the non-paretic (black bars) and paretic (gray bars) biceps brachii muscle of individual subjects. All but one subject (S14) has greater SW speed in the paretic muscle.

Fig. 4.

Mean echo intensity values and standard deviations of the non-paretic (black bars) and paretic (gray bars) biceps brachii muscle of individual subjects. All but two subjects (S3 and S7) have greater echo intensity in the paretic muscle. Echo intensity values range from 0 to 255, indicating black to white, respectively.

Fig. 5.

Relationship between the differences in shear wave speed and echo intensity of the non-paretic and paretic biceps brachii muscle.

Fig. 6.

Relationship between the difference in shear wave speed of the non-paretic and paretic biceps brachii muscle and time since stroke.

Fig. 7.

Relationship between the difference in shear wave speed of the non-paretic and paretic biceps brachii muscle and Fugl–Meyer test score. Lower Fugl–Meyer scores indicate more severe functional impairments.