Subtypes of Attention-Deficit/Hyperactivity Disorder (ADHD): Distinct or Related Disorders Across Measurement Levels?

Abstract

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.

Heterogeneity in Clinical Symptoms and Cognitive Functioning of Children with Hyperactivity-Impulsivity and Inattention: Dimensional and Person-Centered Perspectives

“…In accordance with previous findings on predominant hyperactive-impulsive subtype (Baeyens et al, 2006;Milich et al, 2001), HI group is characterized by a higher intensity of externalizing symptoms and a lower intensity of internalizing symptoms, except of anxiety depression observed by teachers. As was noted earlier, this subscale seems to be related to mood liability, which was found to be associated with hyperactivity-impulsivity (Sobanski et al, 2010).…”

“…The C group manifests the highest severity of psychopathological symptoms and difficulties in cognitive functioning. These results are consistent with the findings of studies concerning the combined subtype of ADHD, which is characterized by severe difficulties in functioning across domains and poor executive functioning (Baeyens et al, 2006;Chhabildas et al, 2001;Gambin & Święcicka, 2009;Geurts et al, 2004;Milich et al, 2001). …”

“…Children from HI group, characterized by low emotional control, high intensity of rule-breaking and anxious-depressive problems observed by teachers, as well as a high severity of aggression observed by teachers and parents, do not display difficulties in cognitive functioning. Similarly, previous studies have shown that children with a predominantly hyperactiveimpulsive subtype of ADHD do not exhibit deficits in executive functioning (Baeyens et al, 2006;Milich et al, 2001). The C group manifests the highest severity of psychopathological symptoms and difficulties in cognitive functioning.…”

“…The co-occurrence of comorbid disorders -both internalizing (e.g., anxiety and depression) and externalizing (e.g., aggressive and delinquent behaviors) -with ADHD is a consistently replicated finding (Baeyens, Roeyers & Walle, 2006;Milich, Balentine & Lynam, 2001). Previous studies shown that it is important to look for higher-order patterns among symptoms of different disorders (Hudziak, Althoff, Derks, Faraone, & Boomsma, 2005;Hudziak, Achenbach, Althoff & Spine, 2007;Stevenson et al, 2005).…”

“…We have included symptoms that most often co-occur with ADHD, including both internalizing (anxiety and depression) and externalizing (aggressive and rule-breaking behaviours) symptoms (Baeyens et al, 2006;Milich et al, 2001), as well as symptoms observed by different informants: parents and teachers. We took into account executive functions that were found to be the most impaired in children with ADHD symptoms: response inhibition, planning and working memory (Willcutt, Doyle, Nigg, Faraone, & Pennington, 2005) and we have chosen commonly used executive function tests: Stop Signal Task, Tower of Hanoi, and Digit Span (Klinberg et al, 2005;Nigg, Blaskey, Huang-Pollock, & Rappley, 2002).…”