Subtypes of attention-deficit/hyperactivity disorder (ADHD): distinct or related disorders across measurement levels?

The aim of this literature review is to assess the current state of knowledge regarding differences and similarities between the inattentive (IA) and combined (C) subtypes of Attention-Deficit/Hyperactivity Disorder (ADHD) in order to detail challenges concerning further conceptualization, diagnostics, and treatment. The literature on ADHD-IA and ADHD-C was reviewed and contrasted across genetic, neuroanatomical, neurophysiological/ neurochemical, neuro(psycho)logical, and clinical psychiatric measurement levels. It was found that the more fundamental the measurement level, the less unambiguous evidence is found for subtype differences. Only on the clinical psychiatric diagnostic level, do more or less clear-cut differences in cognitive, social, academic, and behavioural functioning emerge. In conclusion, fundamental research that compares ADHD-IA and ADHD-C is relatively rare. At this point, only irrefutable phenomenological evidence of subtype differences seems to be available, even in attention problems which are presumed to be identical. The question as to whether both subtypes should be considered as two independent disorders was not adequately resolved.