Abstract
Objective: Obsessive-compulsive disorder (OCD) is known to frequently coexist with attention deficit hyperactivity disorder (ADHD). However, the two disorders are characterized by a variety of opposite clinical and biological features. We investigated whether different symptom dimensions of OCD might account for this intriguing comorbidity given the different phenomenological and neurobiological characteristics exhibited by patients with OCD.
Methods: The sample of this study comprised one hundred and ninety-six patients with OCD who were classified according to their principal obsession or compulsion: (1) contamination and washing; (2) taboo thoughts (sexual, aggressive, or religious obsessions); and 3) symmetry, counting, repeating, or ordering (SCRO). Our study examined whether or not ADHD accompanies any of the abovementioned symptom dimensions. The prevalence of ADHD was assessed by using the DSM-IV criteria. The prevalence of tic disorders, smoking, and alcohol or substance use disorder (SUD) was also examined. The Yale-Brown Obsessive Compulsive Scale (Y-BOCS), 17-Item Hamilton Depression Rating Scale (HDRS-17), Beck Anxiety Inventory (BAI), and Barratt Impulsiveness Scale Version 11 (BIS-11) were administered.
Results: The prevalence of ADHD in the following symptom dimensions was 11.6% (n=8) for washing, 20.4% (n=22) for taboo thoughts, and 26.3% (n=5) for SCRO. These differences, however, were not found to be significant. Additionally, neither the total BIS-11 scores nor its subscale scores concerning cognitive impulsiveness, motor impulsiveness, and non-planning differed significantly among the three symptom dimensions. Further, the scores of HDRS-17 and BAI and the prevalence of tic disorder, alcohol abuse, and SUDs were not significantly different among the symptom dimensions. On the other hand, the Y-BOCS scores including the scores of its subscales of obsession and compulsion were significantly higher in the washing symptoms group than the other two groups (p<0.0001), and the prevalence of smoking differed significantly among the three dimensions (p<0.004): 23.2% of the individuals with washing symptoms, 29.6% of the individuals with taboo thoughts, and 63.2% of those with SCRO symptoms were smokers.
Conclusions: The prevalence of ADHD was approximately twice as high in individuals with taboo thoughts and about two and half times as high in individuals with SCRO symptoms compared to individuals with washing symptoms. However, this obvious difference did not reach significance, possibly because of the small sample size. Significant differences in the prevalence of smoking exhibited the same pattern of distribution as the prevalence of ADHD among the symptom dimensions, suggesting that the relationship between OCD symptom dimensions and various aspects of ADHD requires further investigation. In addition, similar levels of impulsiveness were observed among the three symptom dimensions, contributing to the peculiarities in the co-occurrence of OCD, ADHD, and addiction. Further research involving more participants should shed further light on ADHD as a comorbidity with the different symptom dimensions of OCD.

Key words: Obsessive-compulsive disorder, symptom dimensions of obsessive-compulsive disorder, contamination and washing, taboo thoughts, aggressive obsessions, religious obsessions, blasphemous obsessions, symmetry/counting/repeating/ordering, attention deficit hyperactivity disorder, psychiatric comorbidity