Effect of aging on autonomic function in individuals with severe motor and intellectual disabilities

Autonomic nervous function is often abnormally regulated in individuals with severe motor and intellectual disabilities (SMID). In this study, we assessed autonomic nervous function of patients with SMID and determined how it was influenced by age. The study included 21 individuals with SMID and 15 healthy adolescents (control). To determine the effect of aging, the patients with SMID were divided into an older age group (Old) and younger age group (Young). Autonomic nervous function was assessed using power spectral analysis of heart rate variability (HRV) for 24-h Holter electrocardiogram recordings. The low- and high-frequency components (LH and HF) of HRV were calculated. The ratio between LF and HF (LF/HF) was used as an indicator of sympathetic modulation, while HF alone was used as an indicator of parasympathetic modulation. The LH/HF in the control group was higher in the daytime than at nighttime, while HF had an opposite pattern of change. Therefore, circadian rhythms were observed in the control group for both sympathetic and parasympathetic nervous activities. In contrast, the LF/HF in patients with SMID had no circadian rhythm. The HF was higher in the Old SMID group than in the Young group, and it exhibited a circadian rhythm in eight patients in the Old SMID group versus none of the patients in the Young group. These findings suggest that the sympathetic nervous system is suppressed in both old and young patients with SMID. However, while the parasympathetic nervous system is suppressed in younger patients with SMID, it is activated with increasing age.