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This presentation shall discuss on the concept of impulsivity in Attention Deficit Hyperactivity Disorder (ADHD), neurobiology of impulsivity in ADHD and disorders significant for differential diagnosis and comorbidity in adult ADHD due to the impulsivity symptom. A literature review shall be made in light of the concepts of impulsivity and impulse control disorder, impulsivity in and neurobiology of adult ADHD, differential diagnosis and comorbidity with other psychiatric disorders with impulsivity symptoms. ADHD is a chronic, developmental psychiatric disorder which begins in early childhood, with continuing basic symptoms of which are inattention, hyperactivity and impulsivity, throughout adulthood as well. In DSM-V, 18 symptoms are listed for attention deficit and hyperactivity/impulsivity areas just as in DSM-IV, and meeting of minimum six symptoms in one area is required for an ADHD diagnosis. Among these symptoms, those stating as “Usually replies before the question is completed”, “Usually experiences difficulty as to waiting for his/her turn” and “Usually interrupts other people’s speeches or interferes in what they are doing” point out to impulsivity symptoms. There are 3 subtypes of ADHD: predominantly Inattentive, predominantly Hyperactive-Impulsive, or the two Combined. Combined subtype is the most frequently observed during childhood, where hyperactivity and impulsivity symptoms are reported to be significantly decreased compared to attention deficit during adolescence and young adulthood. Executive functions such as sustaining concentration and attention, perseveration, resistance to interfering impacts, ability to change categories, sustaining goal oriented behavior, ability to suppress/inhibit the tendency to react which is inconvenient for the moment are known to be impaired in children and adults with ADHD as compared to the healthy population. Executive function disorders lead to impulsive behavior and indifferent attitudes lacking empathy. Executive functions are defined as the functions of frontal area. Dorsolateral prefrontal cortex (DLPFC) is responsible for attention, working memory, planning, organization of a task, functions related to processing new information, whereas orbitofrontal cortex (OFC) ensures regulation of emotional stimuli and inhibitor control. In DLPFC, the damage leads to apathy, lack of motivation, disinterest, insufficiency in planning and behavioral şexibility, where in OFC, it leads to symptoms such as socially inappropriate behavior, increase in motor activities, being inconsiderate to others, being affected by environmental stimulants and removal of sexual inhibition. In many cases where orbitofrontal lesions are involved, impulsive and antisocial behaviors are observed. It has been reported that OFC dysfunction can exist in predominantly hyperactive-impulsive subtype of ADHD, and DLPFC dysfunction in predominantly inattentive subtype of ADHD. Although impulsiveness can be observed in anybody, whether a DSM-V axis I or II diagnosis exists, it is likely to be more frequently observed in people associated with specific psychiatric diagnosis such as ADHD, substance addiction, personality disorders, etc. The relation between these disorders and impulsivity can be related with behavioral inhibition deficit, which is partially a part of the foregoing disorders. Impulsivity can be described as a process including a rapid action without a conscious judgment, acting without adequate consideration and a tendency to act with less common sense despite the existence of a normal intelligence level. Impulse control disorder is a class of psychiatric disorders characterized by impulsivity – failure to resist a temptation, urge or impulse that may harm oneself or others. ADHD differential diagnosis together with bipolar disorder, substance abuse, behavioral addictions, cluster B personality disorders, where impulsivity appears as a symptom, as well as impulse control disorder is significant. Comorbid cases with these disorders can lead to overlook an ADHD diagnosis. Lifetime expansion of bipolar disorder type II & I have been found to be 10% in people in both genders who has an adult ADHD. Bipolar disorder and ADHD can demonstrate similar symptoms such as hyperactivity, inattention, emotional lability, impulsivity etc. Symptoms such as excessive spending, deliriums and other psychotic symptoms, grandiosity, acceleration in thinking and decrease in sleep need assist differentiation of mania or hypomania cases from ADHD. There exist evidence as to increase of borderline personality disorders in adults with ADHD and existence of ADHD as a co-diagnosis in the subgroup of people with borderline personality disorder. Borderline personality disorder is characterized with impulsivity, mood lability and hostility such as in ADHD, however, these symptoms are episodic, shorter and less serious in persons with ADHD. Moreover, ADHD is not characterized with dichotomic thoughts, fears of abandonment and self mutilating behavior as in borderline personality disorder. Anti-social personality disorder can be seen with a frequency of 10-23%. Hyperactive-impulsive and combined ADHD types can be associated with aggression, committing crimes, being opponent-opposing, and anti social behavior. Antisocial personality disorder shares the impulsivity and effective liability symptoms with ADHD. Behaviors observed in antisocial personality disorder such as a history of arrest, lack of empathy and lack of prick of conscience might be of help to distinguish between the two disorders. ADHD double folds substance addiction risks compared to society in general. Co-diagnosis of substance addiction in ADHD is 40-50% whereas co-diagnosis of ADHD in substance addicts remains at 15-25% level. Higher ADHD symptoms have been detected in adolescents with internet addiction. There are studies available showing associations between pathological gambling and childhood ADHD symptoms as well as adult ADHD symptoms. In such a study, an ADHD history was found in 25% of people who have pathological gambling and at-risk gambling and people with an ADHD history have more serious gambling problems, higher level of gambling-related cognitions, more frequent psychiatric comorbidity and increased suicide risks. It was shown that higher impulsivity proposes a risk factor for ADHD comorbidity. Higher ADHD symptoms were reported in people with compulsive buying disorder as compared to control group. Frequent association with other psychiatric disorders might lead to overlook ADHD. Being aware of the comorbid conditions with psychiatric disorders with which it shares similar symptoms such as impulsivity and differential diagnosis against these disorders are significant factors for efficient treatment of these so called disorders.