What can ADHD without comorbidity teach us about comorbidity?

Neuropsychiatric comorbidity in ADHD is frequent, impairing and poorly understood. In this report, characteristics of comorbid and comorbid-free ADHD subjects are investigated in an attempt to identify differences that could potentially advance our understanding of risk factors. In a clinically-referred ADHD cohort of 449 youths (ages 6-18), age, gender, IQ, SES and ADHD symptoms were compared among ADHD comorbid free subjects and ADHD with internalizing and externalizing disorders. Logistic regression analyses were also carried out to investigate the relationship between comorbidity and parental psychiatric status. Age range was younger in the ADHD without comorbidity and older in ADHD+internalizing disorders. No significant difference in IQ or SES was found among ADHD comorbid and comorbid-free groups. ADHD with internalizing disorder has a significantly greater association with paternal psychiatric conditions. After matching by age, gender, IQ and SES, ADHD with externalizing disorders had significantly higher total ADHD, hyperactivity/impulsivity score and single item score of difficulty awaiting turn than ADHD without comorbidity and ADHD with internalizing disorders. Older age ranges, ADHD symptom severity and parental psychopathology may be risk factors for comorbidity.