Comorbidity and Neuroimaging in Attention-Deficit Hyperactivity Disorder
Hechtman, Lily, Canadian Journal of Psychiatry
It is clear that attention is not a unidimensional entity. We are all aware of the different aspects and functions of attention, such as focusing, executing, sustaining, and shifting attention. It has also been well documented that different aspects of attention are associated with different brain regions (cortical, subcortical, and midbrain), which are interconnected and organized into an attentional system.1,2 This attentional system is so widespread that it is very vulnerable to damage and dysfunction. The extensiveness of the attentional system and its vulnerability may account for the very high incidence of attention-deficit hyperactivity disorder (ADHD) in the population (5% to 10% in children,3 and about 4.4% in adults4).
ADHD is a condition with high rates of comorbidity. 4^6 Generally, the rate of comorbidity increases with age.7 Neuroimaging studies of ADHD are thus complicated by the extensiveness of the attentional system and the high rate of comorbidity because the various comorbid conditions exert their own impact on different brain regions and functions.
The 2 In Review papers and 1 Original Research paper address some of the challenges of adult comorbidity, as well as recent neuroimaging findings in ADHD.
Ms Mariya V Cherkasova and I8 review neuroimaging findings in ADHD. Areas of the brain that are reviewed are the frontostratial circuit, the cerebellum, and the parietal and temporal lobes. For each area, structural and functional (at rest and during cognitive tasks) findings are presented. Although there is strong evidence suggesting that frontostratial dysfunction is central to the pathophysiology of ADHD, there is currently growing evidence that other regions, such as the cerebellum, the parietal lobes, and the temporal lobes may also have an important role in this condition. Findings generally suggest deficits in the brain areas mentioned above, with decreased volumes and decreased perfusions. However, it is also evident that some areas show enlargement (for example, the anterior hippocampus) or hyperfusion (for example, the temporal areas) when they are brought into play to compensate for areas showing deficits.
Dr F Xavier Castellanos, a leading expert in neuroimaging in ADHD, and coauthors, Dr Clare Kelly and Dr Michael P Milham,9 present a review that highlights recent advances in the conceptualization of ADHD, which is emerging from neuroimaging and endophenotypic approaches. Their review focuses on recent published literature on the phenomena of resting-state, rntrasubject variability, and diffusion tensor imaging pertaining to ADHD. These approaches are an attempt to address the complex heterogeneity of ADHD and to develop testable models of pathophysiology. These models focus on intrasubject variability, intrinsic brain activity, and reward-related processing.
Last but not least, Dr Lucy Cumyn and coauthors10 review findings regarding the comorbidity in adults with ADHD and report on a large study of adults (n = 447) referred for ADHD assessment in our Original Research paper. Axis I and II disorders are compared in adults with and without ADHD. Even though both groups are clinically referred, the ADHD group has significantly higher rates of Axis I (7 1 .9%, compared with 55.5%) and II (50.2%, compared with 38%) disorders. The most common Axis I disorders in the ADHD group include mood, anxiety, conduct, and substance use disorders. The most common Axis II disorders in the ADHD group include obsessive-compulsive, passive-aggressive, depressive, narcissistic, and borderline personality disorders. …