Obsessive-compulsive disorder (OCD) encompasses a relatively frequent anxiety disorder, characterized by repetitive thoughts, impulses, or images (obsessions), and repetitive behaviors or mental acts (compulsions) that cause marked distress (APA 1994). OCD is highly debilitating, and the World Health Organization has found it to be among the 10 most disabling medical conditions (Angst et al. 2004; Weissman et al. 1994; Murray and Lopez 1996). Hollander and colleagues have suggested that OCD is part of the “obsessive compulsive spectrum disorders” (Hollander and Wong 1995; Hollander et al. 1996). Disorders belonging to this spectrum are thought to share similarities in clinical symptoms, associated features, etiology, and response to treatment. The obsessive-compulsive spectrum disorders include neuropsychiatric conditions such as: Tourette’s syndrome, Sydenham’s chorea, and autism.

Most of the research into the relationship between autism and OCD has been performed in pediatric patients with autism spectrum disorders (ASD). Children with ASD have shown increased rates of obsessive-compulsive (OC) symptoms (Leyfer et al. 2006). Reversely, pediatric OCD patients show increased frequencies of autism symptoms (6.2%; Ivarsson and Melin 2008). Considering the frequency of adults with an early onset form of OCD (about 50%; Maina et al. 2008), it is likely that this association with ASD also holds for adults with OCD. Co-morbid tics (especially in girls with OCD) and pathological doubt seems to be the strongest predictor of autism symptoms in pediatric OCD (Ivarsson and Melin 2008). In adult OCD patients, up to 20% of the patients exhibited autistic traits (Bejerot et al. 2001). However, much less research has been carried out on symptom profiles of ASD in adults with OCD. This might partly be due to a lack of familiarity of adult clinicians with childhood-onset disorders with the consequence that ASD symptom are often missed (Cath et al. 2008). Nonetheless, knowledge about the nature of the co-morbidity between OCD and both ASD and ADHD is of great importance to the adult clinician, since it might explain non compliance to treatment and treatment resistance. Repetitive behaviors are among the core features of both ASD and OCD, and comparison of ASD with OCD has demonstrated more similarities than differences in OC symptom characteristics between these groups. Some difference can be noted: ASD patients demonstrate more hoarding, touching, tapping and self-damaging behaviors, less checking, repeating and counting behavior, and less aggressive and somatic obsessions than OCD patients (McDougle et al. 1995; Russell et al. 2005). Interestingly, adult patients with comorbid ASD and OCD report more sexual obsessions than patients with one of these disorders alone (Zandt et al. 2007; Russell et al. 2005; Cath et al. 2008). Besides symptom overlap, similar executive function impairments related to obsessions and compulsions have been found in patients and first degree relatives in both disorders (Delorme et al. 2007). Based on this overlap, Bejerot (2007) has proposed a new OCD subtype, with an autistic dimension characterized by either paranoid, schizoid, schizotypal, avoidant or obsessive-compulsive traits.

Attention-deficit/hyperactivity disorder (ADHD) is not included in the obsessive-compulsive spectrum theory. However, Palumbo and colleagues (Palumbo et al. 1997) have suggested that ADHD, OCD and autism share etiological overlap, constituting a group of developmental basal ganglia disorders. Elevated rates of ADHD symptoms have been found in OCD, with percentages up to 30% of comorbid ADHD in children and adolescents (Hanna 1995; March and Leonard 1996). Reversely, percentages of OCD in children with ADHD are estimated at 8% (Geller et al. 2000).

According to DSM IV criteria, when ADHD symptoms exclusively occur in the course of pervasive developmental disorders (PDD), a co-morbid diagnosis of ADHD is not allowed. Yet, recent clinical as well as epidemiological studies indicate that ADHD symptoms are present in up to 75% of children with PDD (Goldstein and Schwebach 2004; Sturm et al. 2004; Yoshida and Uchiyama 2004). Reversely, characteristics of ASD (such as difficulties in social interactions, and problems in verbal and nonverbal communication) have been found in children with ADHD (Clark et al. 1999), suggesting a strong interrelatedness between ASD and ADHD, as a consequence of symptom overlap, of commonalities in etiology or both. In addition to these findings, one study suggested that the number of autism symptoms in children with OCD is related to comorbid ADHD (Ivarsson and Melin 2008).

These findings suggest that autism, ADHD, and OCD are interrelated. However, little research has been conducted to further elucidate the nature of this relationship at a symptom level. Insight in the association of co-morbid ADHD and autism symptoms might impact both etiological and treatment studies. Therefore, we aimed at extending current knowledge on symptom overlap between OC, ADHD and autism symptoms in adult OCD patients by investigating (1) whether autism and ADHD symptoms occur more frequently in OCD patients than in controls; (2) to what extent ADHD co-morbidity and autism symptom profiles in OCD predict symptom severity, and (3) to what extent ADHD and autism symptoms are related to specific OC symptom dimensions.

A substantial relationship between ADHD (particularly the inattentive and combined subgroups) and autism symptoms has not only been found in clinical OCD samples (as in the present study) but also in ADHD clinical as well as population-based samples (Reiersen et al. 2007). Family research has indicated that autism traits, particularly in males with ADHD, might represent a familial trait that is partly shared with ADHD (Mulligan et al. 2009). Thus, although according to DSM IV classification, ADHD and autism diagnoses are considered to be mutually exclusive, a growing body of literature (including the present small scale study) indicates that this classification rule does not hold. Moreover, autism and ADHD symptoms seem to share commonalities with OCD dimensions as well.

The interrelatedness between ADHD and autism symptoms within our OCD sample might reflect symptom overlap across the co-morbid disorders. Particularly the AQ attention switching subscale was highly correlated with the inattention subdomain of ADHD. Inspection of the wording of the items revealed seemingly contradictory relations between AQ attention switching and ADHD inattention: whereas inattention is expressed by the difficulty to sustain attention on a given task (e.g. “easily distracted because of external stimuli”), attention switching problems are expressed by the difficulty to divert attention between tasks (e.g. “I frequently get so strongly absorbed in one thing that I lose sight of other things”). Although seemingly contradictory, ADHD patients have repeatedly been found to exhibit both difficulty in attention switching (related to hyperfocusedness) and attention problems (Cepeda et al. 2000; Oades and Christiansen 2008). In this study, AQ problems with attention switching and ADHD inattention proved to be the strongest predictor for OC symptom severity, and of all OC symptom dimensions except hoarding. Therefore, AQ attention switching problems and ADHD inattention might well be at two ends of the same underlying attention problems.

The social skills subscale of the AQ showed significant correlations with both ADHD scales and OC symptom dimensions. Deficits in social skills have been found, both in OCD (Cath et al. 2008) and in ADHD (Reiersen et al. 2007). Thus, apart from the potential artifacts introduced by symptom overlap in measurements between the co-morbid disorders, ASD, ADHD and OCD might share substantial etiology. Therefore, it would be worthwhile to reconsider the DSM-IV exclusionary classification of these disorders. In support of this, recent comparative studies between children with ASD and ADHD indicated that ASD children show differences as well as similarities on executive function when compared to ADHD and OCD children. Executive function refers to those neuropsychological processes that involve planning, strategic organization of complex processes, flexible adaptation to novel stimuli, ability to shift mental sets, to inhibit irrelevant (motor) responses, and to engage in trial and error learning (Purcell et al. 1998). Although substantial overlap existed between ASD and ADHD children, in general ASD children performed worse than ADHD children on tasks of planning and flexibility, whereas ADHD children were worse on motor inhibition tasks and spatial working memory tasks, (Sinzig et al. 2008). Interestingly, OCD patients (Greisberg and McKay 2003; van den Heuvel et al. 2006) and in some instances their first degree relatives show problems in both domains; planning deficits comparable to ASD (Delorme et al. 2007), and impaired motor inhibition on similar tasks as found in ADHD patients (Sinzig et al. 2008; Chamberlain et al. 2007). Thus, OCD patients and their relatives seem to share executive function problems, both with ADHD and ASD patients, strongly suggesting overlapping etiologies between the disorders. Therefore, to study common etiologies among OCD, ADHD and ASD patients, an endophenotypic family-based approach is warranted in which executive function associated with underlying genetic mechanisms is studied in OCD, ADHD and ASD in concert.

Our hypothesis that hoarding symptoms were related with autism symptoms was not confirmed. Although significant, correlations between hoarding and AQ scores were lower than between other OC dimensions and AQ. Further, autism scores did not predict hoarding symptoms, but hoarding seemed to be related with inattentiveness and with older age rather than with autism symptoms, which is in line with the literature. Problems with focused attention, coupled with problems in decision making, categorization, and planning, have frequently been described in hoarders (Saxena 2008). With respect to age, the frequency and recognition of hoarding behavior is substantially increased among persons over 40 years of age (Samuels et al. 2008). Thus, this co-morbidity pattern, i.e. the association between attention problems rather than autism symptoms and hoarding on the other hand, suggests that—in line with the literature—hoarding constitutes a phenotype that is in several (but not all) aspects discrete from the other OC symptom dimensions (Abramowitz et al. 2008; An et al. 2009). However, no definite conclusions can be drawn from this purely descriptive study, since the hoarding subscale entails two Y-BOCS symptoms only, which hampers the sensitivity to pick up real differences.

The finding that the AQ lack of social skills scale was associated with OC symptom severity was in line with previous findings (Cath et al. 2008). Lack of social skills in OCD might reflect subtle impairments in theory of mind related functioning, a core feature in autism patients. Interestingly, theory of mind dysfunction is related to dysfunctions in set shifting and planning skills in children with autism (Pellicano 2007), and—although not investigated—might also be present in OCD patients at the severe end of the spectrum.

Contrary to our expectation, the AQ attention to detail subscale exhibited low correlations with OCD, and did not turn out to be a significant predictor of OC symptoms or severity. Apparently, the AQ attention to detail subscale taps on a trait that differs from perfectionism, a trait known to be closely related to (some forms of) OCD (Myers et al. 2008). Although patients are known to show enhanced attention for details related to OCD-relevant topics, these data suggest that in general, rather than scanning stimuli with great care to detail, OCD patients suffer from attention problems.

Recent research on memory distrust in OCD has found that OCD patients exhibit lack of confidence in their own perception, attention and memory, without clear evidence for an objective memory deficit (Hermans et al. 2008, 2003; Radomsky et al. 2001; Radomsky and Rachman 1999). Moreover, the repetitive behaviors of OCD seem to weaken memory trust (Van den Hout and Kindt 2004). However, the cause of the initial low confidence of OCD patients in their own memory capacities is unclear. Our findings of inattention and attention switch problems in OCD patients raise an interesting possibility, i.e. that these problems may, at least partially, be responsible for the low confidence of OCD patients in their cognitive processes. Difficulties in the management of attention may serve as a common factor between OCD, ADHD and ASD symptoms and explain the pathological doubt/uncertainty that seems to characterize these disorders.

These findings have several implications for treatment. First, considering that inattention as found in this study is central to OC symptoms, one would expect that addition of Methylphenidate to treatment in OCD patients with co-morbid ADHD symptoms will enhance treatment efficacy. Future treatment studies should be directed at exploring this. Further, several behavioral strategies such as task concentration training that have been found to improve attention in ADHD patients (O’Connell et al. 2008, 2006; White and Shah 2006) could be added to behavior therapy in OCD patients, and might improve treatment efficacy particularly in OCD + ADHD patients. Another implication for clinical practice involves the importance of assessment of autism symptoms in the treatment of OCD. Possibly, increased autism symptoms complicate treatment and limit treatment gains. Cognitive-behavior therapy with patients with increased autism symptoms might demand longer duration, and require adaptations of standard treatment protocols. Further research of the effect of autism symptoms on treatment response to OCD is needed to confirm these hypotheses.

Such ideas are further strengthened by the lack of social skills as reported by the OCD patients, a finding that implies that adding a social skills improvement module to standard cognitive behavior therapy for OCD might increase overall treatment efficacy in OCD, as was recently suggested by Cath and colleagues (Cath et al. 2008).

Limitations of the present study include its descriptive nature, which precludes causal conclusions. Further, each symptom domain was measured by a single screening instrument. The measurement of ASD symptoms through self report alone is particularly problematic. Patients with complaints in the domain of self-reflection might be less able to reflect on their behavior. Further research should be conducted using multiple measures of the symptom domains of OCD, ADHD, and autism. Moreover, we suggest the use of more elaborated hoarding scales in future research, to further study relationship between autism and hoarding. The relations between pathological doubt and attention difficulties also need future exploration.