![]() ![]() Despite weak evidence for deficits in these neurotransmitters, their role is substantiated by their distribution in those areas of the brain involved in ADHD and the positive response of ADHD patients to medications that modulate the neurotransmission of catecholamines ( 36). Activity among these areas depends on catecholaminergic brain circuitry ( 35). Based on volumetric ( 32) and functional MRI studies ( 33), differences are found in the structural development and functional activation in the prefrontal cortex, basal ganglia, anterior cingulate cortex, and cerebellum ( 34). Neuroimaging studies point to ADHD as a disorder of early brain development. Except for children experiencing exceptional early deprivation ( 28, 29), a causal relationship between family environment and psychosocial adversity and ADHD is unclear ( 20, 30, 31). Hypoxic–anoxic brain injury ( 21), epilepsy disorders ( 22), and traumatic brain injury ( 23–25) also contribute to ADHD risk.Įxposure to environmental toxins (specifically lead, organophosphate pesticides, and polychlorinated biphenyls) has been linked to ADHD symptoms ( 20, 26, 27). The contribution of pregnancy and birth complications is mixed, but strong evidence supports greater ADHD risk following in utero exposure to alcohol or tobacco ( 18) and low birth weight (<2,500 g) ( 19, 20). Noninherited neurological factors affecting brain development or resulting in brain injury have been implicated in ADHD pathogenesis. Ongoing pharmacogenetics research aims to identify genes involved in medication response with ADHD ( 17). (For more information, see the companion statement on special populations in this issue). The identification of gene sets affecting neurotransmitter pathways in the brain ( 15) has suggested that rare copy number variants or the accumulation of larger deletions and duplications influencing gene transcription are more commonly found in individuals with ADHD ( 16). Gene variants predicting risk for ADHD are important for brain development, cell migration, and encoding for catecholamine receptor and transporter genes ( 13, 14). Combinations of genetic, neurological, and environmental factors contribute to pathogenesis and its heterogeneous phenotype ( 12).Įvidence from family, twin, and adoption studies has suggested strongly that ADHD is a highly hereditary, polygenic disorder ( 13). Predictors of persistence include combined inattention/hyperactivity, increased symptom severity, comorbid major depressive or other mood disorder, high comorbidity (>3 additional DSM disorders), parental anxiety, and parental antisocial personality disorder ( 11).ĪDHD is a disorder with multiple etiologies. The presence of these comorbid conditions and ADHD is associated with increased mortality risk ( 10).Ĭurrent estimates show that 50% of children with ADHD continue to have symptoms of ADHD in adolescence and adulthood. Risk for substance abuse increases further when ADHD is comorbid with conduct disorder (CD) ( 8, 9). ![]() ADHD is associated with significant adverse outcomes in childhood and adolescence: educational problems (resulting in low rates of high-school graduation and completion of postsecondary education) ( 6), difficult peer relationships ( 7) and increased rates of motor vehicle accidents, accidental injuries, and substance misuse. Applying the definition of ADHD from the International Classification of Diseases, 10th edition, yields a lower prevalence rate compared with the DSM-5, because the ICD-10 requires criteria be met for both inattention and hyperactivity ( 5). Worldwide, ADHD is the third-most-common mental health disorder, after depression and anxiety, affecting an estimated 3.4% of children and youth ( 2).ĪDHD prevalence in the paediatric population has been stable over the past three decades ( 3) except in the USA ( 4), where it has increased. ![]() and negatively impacts directly on social and academic/occupational activities’ ( 1). The Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5) classifies attention-deficit hyperactivity disorder (ADHD) as a neurodevelopmental disorder and defines it as ‘a persistent pattern of inattention and/or hyperactivity-impulsivity that interferes with functioning or development…. ![]()
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