One of the more common monogenic causes of autism spectrum disorders (ASD) is a deletion or mutation of the SHANK3 gene on chromosome 22. Deficiency of the SHANK3 gene, also called Phelan-McDermid Syndrome (PMS), accounts for 0.5-1 percent of cases of ASD. Children with PMS show autistic symptoms including speech abnormalities, developmental delay, and motor deficits. Many children with PMS undergo oral-motor therapy to improve difficulties in chewing, swallowing, and sucking, issues that can lead to life-threatening complications in this population. Motor abnormalities are clinically reported in PMS and ASD; however, research that analyzes motor deficits associated with PMS or ASD is limited.
In order to better understand the development of motor abnormalities such as poor muscle tone in children with PMS and the broader autism spectrum, the present study sought to compare the muscular activation of one of the main muscles responsible for chewing, the masseter muscles. We aim to compare 15 children with PMS, to 15 children with ASD, to 15 typically-developing children. Specifically, the current investigation aimed to quantify empirically-based differences in the process of mastication in PMS as described by clinically-reported hypotonia and, if these differences exist, to see whether or not the results extend to the broader ASD population.
Participants included children ages 6-12 years who had a primary diagnosis of PMS, ASD, or typically-developing children. Muscular activation of the muscles used for chewing, the masseter muscles, was measured using electromyography (EMG), an instrument that detects the electrical activity produced by skeletal muscles. Each of the three groups underwent a surface EMG. Three electrodes were attached bilaterally above the right and left masseter muscles. The data was encoded using a ProComp Infiniti system. During the procedure, participants chose a solid food item, e.g. bite-sized pieces of potato chips. Each child completed an initial baseline and five trials of chewing followed by a recovery baseline period. Using Biograph Infiniti software, minimum, maximum, and mean baseline and chewing scores were calculated.
Preliminary results demonstrate that children with PMS have lower mean masseter muscle baselines, lower maximum peaks, and overall lower mean electrical activation of their masseter muscles compared not only to the typically-developing children, but also to the broader ASD group. Furthermore, children with ASD have lower muscle activation compared to typically-developing children.
The present study is the first scientific investigation to specifically explore motor abnormalities in PMS. We found that children with PMS and/or ASD show hypotonia, having lower muscle activation as compared to typically-developing children. Future research should increase the number of muscle groups and motor activities measured. The current findings of lower muscle activity in PMS and ASD have clinical implications; for example, muscular activation could be used as a biomarker in treatment studies.
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