The relationship between executive functions (EF), ADHD subtypes, and comorbid diagnoses, and the potential role of IQ were investigated in the Child Neuropsychiatry Unit, Children's Hospital, Rome, Italy. In a cohort of 23 children (aged 5 to 16 years, 20 males and 3 females) with a diagnosis of ADHD, no differences were found between children with different subtypes of ADHD on measures of EF. Differences on EF measures were observed in children with different comorbidities (internalizing vs externalizing disorders). Compared to the internalizing disorders group (depression, phobia, and anxiety), children with externalizing disorders (oppositional defiant or conduct disorders) obtained significantly lower scores on different measures of EF (i.e. verbal working memory and categorical fluency), but the differences were dependent on IQ level. Children with externalizing disorders exhibited significantly lower IQ scores than those with internalizing disorders. Comorbidity patterns, rather than ADHD subtypes, define the neuropsychological features of the ADHD phenotype. General IQ plays a role in influencing the cognitive processes underlying the disorder. 
COMMENT. In an Editorial, Inagaki M  discusses the problems in executive function (EF) experienced by children with ADHD. The cognitive functions involved in goal-directed behavior are impaired, including inhibition, planning, set-shifting, working memory, and fluency, and reflect a dysfunction of fronto-subcortical pathways. The neuropsychological deficits in ADHD are heterogeneous. Although the majority of ADHD children have a deficit on at least one EF measure, the same is true for nearly half a control group (Nigg JT et al, 2005). Different populations of children with neuro-developmental disorders have deficits in EF similar to those found in children with ADHD (Sergeant JA et al, 2002). The EF deficits with ADHD are nonspecific and not diagnostic. In the Multimodal Treatment Study of Children with ADHD, a 14-month trial of treatment strategies (1999), 40% of the sample (n=579) had a comorbid ODD that could modulate or interact with the intellectual and executive profile. The role of IQ in prognosis of ADHD alluded to in this review is well known. Risk factors for a poor prognosis include a low average or borderline IQ, and comorbid oppositional and conduct disorders.