The Contribution of Decreased Muscle Size to Muscle Weakness in Children With Spastic Cerebral Palsy

Abstract

Muscle weakness is a common clinical symptom in children with spastic cerebral palsy (SCP). It is caused by impaired neural ability and altered intrinsic capacity of the muscles. To define the contribution of decreased muscle size to muscle weakness, two cohorts were recruited in this cross-sectional investigation: 53 children with SCP [median age, 8.2 (IQR, 4.1) years, 19/34 uni/bilateral] and 31 children with a typical development (TD) [median age, 9.7 (IQR, 2.9) years]. Muscle volume (MV) and muscle belly length for m. rectus femoris, semitendinosus, gastrocnemius medialis, and tibialis anterior were defined from three-dimensional freehand ultrasound acquisitions. A fixed dynamometer was used to assess maximal voluntary isometric contractions for knee extension, knee flexion, plantar flexion, and dorsiflexion from which maximal joint torque (MJT) was calculated. Selective motor control (SMC) was assessed on a 5-point scale for the children with SCP. First, the anthropometrics, strength, and muscle size parameters were compared between the cohorts. Significant differences for all muscle size and strength parameters were found (p ≤ 0.003), except for joint torque per MV for the plantar flexors. Secondly, the associations of anthropometrics, muscle size, gross motor function classification system (GMFCS) level, and SMC with MJT were investigated using univariate and stepwise multiple linear regressions. The associations of MJT with growth-related parameters like age, weight, and height appeared strongest in the TD cohort, whereas for the SCP cohort, these associations were accompanied by associations with SMC and GMFCS. The stepwise regression models resulted in ranges of explained variance in MJT from 29.3 to 66.3% in the TD cohort and from 16.8 to 60.1% in the SCP cohort. Finally, the MJT deficit observed in the SCP cohort was further investigated using the TD regression equations to estimate norm MJT based on height and potential MJT based on MV. From the total MJT deficit, 22.6-57.3% could be explained by deficits in MV. This investigation confirmed the disproportional decrease in muscle size and muscle strength around the knee and ankle joint in children with SCP, but also highlighted the large variability in the contribution of muscle size to muscle weakness.

Keywords: cerebral palsy; muscle size; muscle volume; muscle weakness; selective motor control; ultrasound.

Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2021 Hanssen, Peeters, Vandekerckhove, De Beukelaer, Bar-On, Molenaers, Van Campenhout, Degelaen, Van den Broeck, Calders and Desloovere.

Figures

Figure 1

Relationship between muscle volume and maximal joint torque for four lower limb joint movements in children with a typical development (TD) and with spastic cerebral palsy with gross motor function classification scale (GMFCS) levels I to III. (B) Regression formula parameters for the estimation of maximal joint torque based on muscle volume as shown in (A). SE, standardized error. The outliers indicate the cases that were removed based upon the standardized residuals or Cook's distance.

Figure 2

(A) Fictive example of the calculation of the different maximal joint torques (MJT) that were measured or estimated as a percentage of the expected score in typically developing children, their relative representation and their corresponding deficits. MJTpotential was based on muscle volume and the relationship with MJT in the typically developing cohort. The MJTnorm was based on the relationship of MJT with height in the typically developing cohort. The blue part indicates the relative MJTmeasured. The red part indicates the relative MJTdeficitMV, i.e., the deficit in strength that is proportional to the deficit in muscle volume. The orange part of the graph indicates the relative MJTdeficitother, representing the deficit in MJT that comes from other factors than the decrease in muscle volume. (B) Representation of the measured MJT and deficits relative to the expected score in typically developing children for four lower limb joint movements.