As for other psychiatric disorders occurring during the developmental age, the categorical and relatively simple symptomatological core of ADHD often does not appear alone.
Frequently, a wide variety of concurrent psychiatric disorders contribute to the psychopathological status of children and adolescents with ADHD, with a well-established consensus among authors that the presence of overlapping psychiatric disorders is more likely to be the rule than the exception.
The medical term commonly used for this concurrence is comorbidity, even though the early meaning of the word as a "distinct additional clinical entity occurring during the clinical course of a patient having an index disease".
The main disorders likely to co-occur with ADHD are oppositional defiant disorder (ODD) (50–60%), conduct disorder (CD) (20–50% in children and 40–50% in adolescents), depression (16–26%) and anxiety (10–40%) disorders, bipolar disorders (11–75%), tic disorders (20%), obsessive-compulsive disorders (6–15%), and autism spectrum disorders (65–80%).
Other comorbidities, however, have also been observed in ADHD children and adolescents.
A wide variety of learning difficulties are associated with ADHD, with over 45% having at least one or more significant impairments in reading, arithmetic, or spelling.
Several other problems, including, for example, social problems or sleep disturbances, may be more common in ADHD than in the population without this condition.
Comorbidity involving ADHD and learning disorders are frequent, ranging from 25 to 40%.
The overall prevalence of psychiatric disorders associated with ADHD in children and adolescents ranges from about 40 to 80% depending on the sample, with higher rates in clinically referred ADHD children (67–87%).
Reale, L., Bartoli, B., Cartabia, M., Zanetti, M., Costantino, M. A., … Bonati, M. (2017). Comorbidity prevalence and treatment outcome in children and adolescents with ADHD. European Child & Adolescent Psychiatry, 26(12), 1443–1457.