Nocturnal Autonomic Nervous System and Jaw Muscle Activities and Ramus Height in Children

Mandibular growth is influenced by function. This study tested for correlation amongst nocturnal autonomic nervous system (ANS) and jaw muscle activities and mandibular ramus height. According to Institutional Review Board oversight, children with skeletal Class II malocclusions were enrolled in this observational study. Subjects had cone-beam computed tomography images, participated in protocols to quantify jaw muscle activity per bite-force, and were trained to use portable recorders to collect jaw muscle and heart rate variability (HRV) data over four nights at home. Night-time ultradian cycling of ANS activity was characterised by fitting a polynomial to the HRV measures of sympathovagal tone (low/high frequency spectral powers, LF/HF). Twenty-minute windows around each inflection point of this polynomial were identified, within which HRV measures and jaw muscle activities (Duty Factor, DF) were quantified. DF versus HRV measures (DF/HRV) were plotted for each subject, and regression slopes calculated. Non-linear regression analysis was used to test for correlation between DF/HRV slopes, age (years), and ramus height (mm). Thirteen (eight males, five females) of 15 children enrolled completed protocols. DF versus LF/HF had average R2 values of 0.66 ± 0.22 for masseter muscles (left and right), and 0.57 ± 0.19 and 0.55 ± 0.17 for left and right temporalis muscles, respectively. Regression analysis demonstrated that approximately 42% of the variance in mandibular ramus height could be explained by the combined effects of age and masseter muscle DF versus LF/HF. Mandibular ramus height may reflect the influence of sympathovagal tone on nocturnal jaw muscle activity in children.