Is autonomic nervous system function atypical in attention deficit hyperactivity disorder (ADHD)? A systematic review of the evidence.

Although arousal mechanisms have frequently been found to be atypical in ADHD, these findings usually emerged from indirect behavioural measures which give only a limited understanding of arousal dysregulation in this condition. To assess the hypothesis that functioning of the autonomic nervous system (ANS), one component of arousal, is atypical in ADHD, we carried out a systematic review of the literature on 55 studies investigating electro-dermal, heart rate and pupillometry measures under different experimental conditions (resting-state, cognitive tasks and in response to reinforcers or socio-emotional stimuli). Our literature review identified ANS dysfunction in individuals with ADHD, more often in the direction of hypo-arousal than hyper-arousal, particularly at rest and during tasks requiring response regulation and sustained attention. Almost half of the reported findings were null. Stimulant medications increased ANS activity and, in some studies, reinforcers and rewards produced a similar effect, suggesting that ANS function can be modified in ADHD. Further research is needed to assess the influence of comorbid symptoms and to explore methodological parameters that may influence findings.