GriFT: A device for quantifying physiological and pathological mirror movements in children

Abstract

Goal: Mirror movements (MM) typically occur during unilateral actions and manifest as involuntary muscle activity of the passive limb, which “mirror” voluntary actions executed by the contralateral homologous body part. They are a normal motor feature in young children and gradually disappear during the first decade of life. In children suffering from neurological disorders, e.g. due to an early brain lesion as in unilateral cerebral palsy, the amplitude and occurrence of MM has been proposed to yield relevant information for diagnosis and therapy. However, in clinical practice MM are typically assessed using an ordinal rating scale, which provides mainly qualitative information. In contrast, there is no validated procedure that allows a quantitative assessment that might offer more objective and detailed information regarding the occurrence and amount of MM. Here we introduce the Grip Force Tracking Device (GriFT Device), a portable system to quantitatively assess MM during repetitive unimanual squeezing whilst playing a computer game. The GriFT Device consists of two identical equally sized handles equipped with two compressive force sensors (range 0-23kg, Fz 1000Hz). Children had to complete three trials of unimanual squeezing, whereby the visual display on the screen determined the rhythm of squeezing (0.67Hz at 15% maximum voluntary contraction, force level adjusted per hand). MM were characterized based on their frequency, strength, and temporal features (synchronization and time lag). These MM characteristics differed significantly between children with different clinical MM scores, and MM frequency and strength were most discriminative. Further categorization of physiological MM based on their frequency and strength proved highly sensitive (89-97%). We demonstrated feasibility and validity of the GriFT Device in a large cohort of typically developing children aged 5 to 15 years, as well as its clinical applicability in children with unilateral cerebral palsy with various levels of hand function. The quantification of MM as proposed in the current study is a promising tool to further investigate and categorize MM in children with unilateral cerebral palsy.