ABSTRACT
Attention Deficit Hyperactivity Disorder (ADHD) is a heterogeneous neurodevelopmental disorder. It is characterized by symptoms of inattention and/or hyperactivity-impulsivity that interferes with the functioning and development of affected individuals. The disorder affects 5.3–7.1" of school age children worldwide. Population-based twin and epidemiological studies report symptom overlap and high rates of ADHD with many psychiatric conditions. The two most commonly diagnosed comorbidities with ADHD are oppositional defiant disorder (ODD) and conduct disorder (CD). The high rates of comorbidity between psychiatric conditions generally suggest a common spectrum of psychopathology. Complications during pregnancy and birth and exposure to environmental toxins can increase risk for psychiatric conditions through disturbance of common pathway(s) in the nervous system. Accumulating evidence from neuropsychological, genetic and neurochemical studies has implicated deficits in fronto-striatal neural networks as an important contributing factor in the pathophysiology of ADHD. As with many neuropsychiatric conditions, heterogeneity in ADHD is the rule rather than the exception, and dimensional rather than categorical approaches.
