Dynamical complexity of postural control system in autism spectrum disorder: a feasibility study of linear and non-linear measures in posturographic analysis of upright posture

Abstract

Background: Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder, characterized by impairments in social interaction and communication with restricted and repetitive behavior. Postural and motor disturbances occur more often in ASD, in comparison to typically developing subjects, affecting the quality of life. Linear and non-linear indexes derived from the trajectory of the center of pressure (COP) while subjects stand on force platforms are commonly used to assess postural stability. The aim of the present feasibility study was to investigate whether combining linear and non-linear parameters of the COP during stance in subjects with ASD, could provide insight on specific features of motor dysfunction possibly linked to ASD cognition and clinical characteristics.

Methods: Twenty-two males, aged 10-15 years, including subjects with ASD and healthy controls (N = 11, respectively), were studied. They all had normal cognitive level and independent walking ability. A piezoelectric force platform was used to evaluate posture over three feet positions, with eyes open, closed and during visually-guided saccades. Linear (sway path, total area and root mean square) and non-linear parameters (fractal dimension and sample entropy) of the COP were measured to determine postural stability and the complexity and regularity of the COP signals. GLMM analyses were performed to assess COP parameter changes across experimental conditions and subject groups. Finally, Spearman correlations evaluated the significance of potential relationships between linear and non-linear measures as well as between non-linear parameters and clinical data in patients with ASD.

Results: Compared to controls, subjects with ASD showed reduced postural stability and complexity, with higher regularity of COP trajectories, particularly in the most unstable feet positions, during visually-guided saccades and in the medial-lateral direction. Spearman correlations indicated that, in the patients' group, postural instability was associated with a decrease in the geometric complexity and an increase in the regularity of the COP trajectory. Moreover, the increase in regularity of the COP trajectory was associated to the severity of restricted and repetitive behavior.

Conclusions: The results of this study highlight the importance of combining linear and non-linear measures in evaluating postural control in patients with ASD, also with respect to the outcome of interventions on these patients targeting postural balance.

Keywords: Autism spectrum disorder; Complexity; Postural control system; Postural stability; Posturographic analysis; Restricted and repetitive behavior.

Fig. 1

Representation of the experimental conditions of the study and of COP measures. A force platform (A) was used to measure ground reaction forces and to obtain the trajectory and time related data of the center of pressure (COP). Schematic representation of the three feet positions used for the postural tests are depicted in panel (B). For each foot position, the postural tests were performed with eyes open, closed and during visually-guided saccades. The COP sway path is represented as temporal oscillations along AP and ML directions (C) and as the COP spatial trajectory (D)

Fig. 2

Distributions of COP Sway Path and Area values across experimental conditions and subject groups. Box and whisker plots report the median value (horizontal line within the box) and the variability represented as interquartile range (vertical length of the box), as well as the highest and the lowest value (lines above and below the box), of the total SP (A), AP and ML components of the SP (B and C, respectively) and Area, computed separately for ASD patients (red) and controls (blue). Abbreviations FP, foot position; ASD, Autism Spectrum Disorder; SP, sway path; AP, anterior-posterior; ML, medial-lateral; EC, eyes closed; EO, eyes open; Sac, visually-guided saccades

Fig. 3

Distributions of COP RMS values across experimental conditions and subject groups. Same layout as Fig. 2, for the RMS of the AP and ML components (A and B, respectively). Abbreviations FP, foot position; ASD, Autism Spectrum Disorder; RMS, Root Mean Square; AP, anterior-posterior; ML, medial-lateral

Fig. 4

Distributions of COP nonlinear parameters across experimental conditions and subject groups. Same layout as Fig. 2, for the FrDim of the AP and ML components (A and B, respectively), and for the SampEn of the AP and ML components (C and D, respectively). Abbreviations FP, foot position; ASD, Autism Spectrum Disorder; FrDim, Fractal Dimension; SampEn, Sample Entropy; AP, anterior-posterior; ML, medial-lateral

Fig. 5

Correlations between COP nonlinear parameters and COP linear parameters and between COP nonlinear parameters and clinical scales. Correlations between nonlinear (SampEn in A, FrDim in B) and linear postural parameters (Area in A, RMS in B), and between nonlinear parameters (SampEn) and clinical scales (SPM-2 in C, RBS-R in D-F) in subjects with ASD (red circles) and controls (blue circles). Abbreviations ASD, Autism Spectrum Disorder; SampEn, Sample Entropy; RMS, Root Mean Square; SPM-2, sensory profile measure 2; RBS-R, repetitive behavior scale revised; ML, medial-lateral