Prevalence and Age-Modulated Presentation of Subthreshold Attention Deficit and Hyperactivity Disorder Comorbidity in Young People with Autism Spectrum Disorders

Background: There is high prevalence of comorbid Attention Deficit and Hyperactivity Disorder (ADHD) symptoms in Autism Spectrum Disorders (ASD).  It is still under debate whether these manifestations represents i) core ASD symptoms, ii) a real ADHD comorbidity or iii) the observable symptoms of another comorbid diagnosis that may mimic ADHD (e.g. anxiety, sleep apnea..). Recently, Gillberg et al., [1] have proposed that the ASD-ADHD co-occurrence may represent a dynamic phenotype that would change according to age. Few studies have investigated differential presentation of ASD subjects with and without ADHD traits. 

Objectives: We aim to measure frequency of ADHD traits in ASD clinical population, its evolution depending on age and the clinical factors associated.

Methods: Children and adolescents (2-17 years old) consecutively referred to an ASD specialized outpatient clinic (AMITEA) between January of 2012 and June of 2014 and meeting ASD criteria (DSM-IV-TR) comprised the study sample (N=371).  ADHD and ASD diagnoses were based on best clinical judgment of experienced child developmental psychiatrists. Autistic Diagnostic Interview- Revised (ADI-R) and Autism Diagnostic Observation Schedule-Generic ADOS-G were used when clinicians deemed it necessary. Symptoms of inattention and hyperactivity were also recorded. Additional socio-demographic and clinical information (including other psychiatric and medical comorbidities) was also collected. Clinical and socio-demographic features were compared between subjects with (ASD_ADHD) and without comorbid ADHD symptoms (ASD_NoADHD). Chi Square test was used to analyze correlation between comorbidity profile and age (ranges of age). All statistical analyses were performed using SAS (9.0).

Results: 371 patients (371 patients (8.2±3.9 years, 81.9% male) were included. Of them, 44.7% met criteria for typical Autism, 43.4% for Pervasive Developmental Disorder NOS, 10.5% for Asperger Syndrome, three cases met criteria for Rett Syndrome and two for Disintegrative Disorder criteria. Intellectual disability was present in 32.6% of the subjects (N=121).

Ten percent (N=37) of the subjects met diagnosis criteria for comorbid ADHD, and 29.7% (N=110) presented with ADHD traits. The ASD_ADHD group included more patients with a PPD NOS while ASD_ADHD traits group included more patients with typical Autism. No significant differences were found between groups regarding intellectual disability and epilepsy.We found a differential comorbidity profile depending on subject’s age. ADHD comorbidity was present in 28% of children between 2 and 6 years (N=118), in 48.7% of those 7-11 years (N=18), and in 37.8% of those 12-17 years (N=14). ADHD symptoms not meeting ADHD full diagnosis were also more frequent in subjects  7-11years (45.5%, N=50) and 2-6 years (48.7%, N=18); 37.3% of subjects aged 12-17 years with ASD and comorbid ADHD (N=41) present with other psychiatric diagnosis.

Conclusions: ADHD traits are highly prevalent in ASD population, especially between ages 7 and 11 years. In addition, our findings support that ADHD symptoms and other psychiatric comorbidity depend on age, thus modeling ASD clinical presentation. Future studies should address how this dynamic phenotype may impact treatment and outcome of ASD population.