Attention Deficit Hyperactivity Disorder (ADHD) is the most common neurodevelopment disorder in childhood and may persist during adolescence and adulthood. ADHD is characterized by three main symptoms: inattention, hyperactivity and impulsivity (Conners, 1999; Homer et al., 2000; Mercugliano, 1999) . The disorder comprises impairments in behavioral inhibition, sustained attention, resistance to distraction, and in regulating the activity level of the person in specific situations, being frequently observed excessive and irrelevant motor activity to the task in execution (Barkley & Murphy, 2008; Farré & Narbonne, 2001) .
These features of the disorder imply losses in several areas of functioning, such as school, society, emotion, and the losses vary according to symptoms, age, and risk and protective factors (Araújo, 2002; Pondé & Freire, 2005; Rohde, Dorneles, & Costa, 2006) .
Tracking studies have been conducted with the aim of outlining the epidemiology of ADHD. Globally, the prevalence of the disorder varies from 5% to 8% (Barkley & Murphy, 2008; Rohde et al., 1998) . In Brazil, studies have found different results, nevertheless this may be related to the nature of the study, assessment instrument used, and sample (Andrade & Flores-Mendoza, 2010) . These investigations are important to trace the epidemiological framework of the disorder, functioning as starting point for the scientific development of treatment (Pastura, Mattos, & Araújo, 2007; Vasconcelos et al., 2003) .
Although the results are still heterogeneous in screening studies; this does not constitute lack of knowledge in the area, since the interest in ADHD is not a recent event. From the first assertion researchers have been conducting epidemiological studies to identify how this phenomenon affects the population. Thus, based on the notion that there is an accumulation of knowledge on the subject, this paper aims to review the literature systematically to describe the epidemiological framework of the disorder presented at national and international studies on ADHD, concerning on three main questions:
1) What’s the definition of ADHD?
2) What’s the Prevalence of ADHD?
3) What are the most widely used instruments for diagnostic assessment of ADHD?
331 articles were retrieved from the search in the database Periodicals CAPES, PsicInfo and MedLine, in which we used the combination of descriptors “Epidemiologia” [and] “Transtorno de Déficit de Atenção e Hiperatividade”, and its correlates in English (Epidemiology; ADHD) and Spanish (Epidemiología; Trastornopor Déficit de Atención/Hiperatividad). The retrieved articles were analyzed based on the following criteria: 1) Have been published between 2000 and 2013; 2) Set up as an empirical study; 3) Have used standardized test for sample evaluation. After additional criteria were applied:
1) Definition of ADHD: Studies were selected whose definition of the disorder was based on the diagnostic criteria of Diagnostic and Statistical Manual of Mental Disorders 3rd, 4th, and 5th Edition, and International Code of Diseases (ICD) 10th Edition.
2) Sample: Participants of both sexes with no age criteria. Regarding to the selection of the sample was not of convenience the study was included.
3) Source of information: Family members, caregivers, and teachers.
4) Applied Instruments: Studies using checklists, scripts history or standardized based on DSM III, DSM IV, DSM V and ICD 10. Their validity needed to be ratified in clinical and epidemiological studies, under specific protocols to guarantee the generalization of the finds.
The description of the selection stages of the articles that compose this study and that are analyzed in the results session, is described in the Figure 1.
3. Results and Discussion
3.1. Definition of ADHD
ADHD has become a focus of study to improve knowledge of professionals in health and education. The investigation of the disease occurs by the atypical conditions of development with higher incidence and prevalence in the population of school age around the world. Seems to be a consensus among researchers about the definition of ADHD, which is considered a disorder of multifactorial origin, whose main symptoms are motor hyperactivity, impulsivity and attention deficit. This major triad co-occurs with secondary symptoms such as emotional disorders, learning disabilities, oppositional defiant disorder, conduct disorder, depression and anxiety (Andrade & Flores-Mendoza, 2010; Aragonès et al., 2010; Oscar & Alma, 2010; Faraone, Sergeant, Gillberg, & Biederman, 2003; Lindblad ,
Figure 1. Description of the stages of selection of the articles.
Pondé and Freire (2007) define the disorder as follows:
The disorder is characterized by three groups of symptoms, the prevalence of each define disease subtype: 1) predominantly inattentive, 2) predominantly hyperactive-impulsive or 3) combined. Children with ADHD may have difficulty in school, relationship problems and low selfesteem. Comorbidity with other psychiatric disorders, according the authors, can result in serious social repercussions and exclusion. (p. 241)
In other analysis there is convergence as the chronicity of ADHD. Different from the previous procedures in the treatment two decades ago, currently, researchers have focused on developing procediments of intervention strategies on different stages of life. This is important to the fact that ADHD does not qualify as a disorder of childhood and adolescence, and may persist with significant functional impact on at least 50% of cases on adulthood. The symptoms of the disorder changes with advancing age, already possible to determine which features are more common in two main stages of life, childhood and adulthood (Aragonès et al., 2010; Blázquez-Almería et al., 2005; Faraone et al., 2003; Poeta & Rosa Neto, 2004) .
Fayyad et al. (2007) state that:
It has been observed through clinical studies in children with attention- deficit/hyperactivity disorder and hyperactivity symptoms often persist into adulthood deficit. (p. 402)
Childhood is the phase that commonly identifies ADHD-like symptoms. It can be observed more frequent in frequent exchange of activities; problems in academic organization; difficulty to maintaining friendship relationship with children of the same age; accumulation of different activities; motor disturbance; impulsivity; learning disability, and often school failure (Andrade & Flores- Mendoza, 2010; Poeta & Rosa Neto, 2004; Pondé & Freire, 2007) . There are even indications that the transition to adolescence, individuals with ADHD, especially those with comorbid conduct disorder, has increased to engage in delinquent behavior trend, substance abuse and sexual risk practices (Faraone et al., 2003) . Already in adulthood, lack of attention, impulsivity, irritability and low frustration tolerance mark the life of these people, as emphasized Poet and Rose (2004) .
The prevalence of the pathology in adults is approximately 4%. North- American researchers claim that the sequelae of ADHD affects between 2% to 2.5% of adults, whom have inattention, impulsivity, irritability, intolerance and frustration. (p. 151).
3.2. Prevalence of ADHD and Comorbidity with Psychiatric Disorders
If the definition of ADHD achieves a consensus in the scientific community, the same did not occur with the epidemiological findings. Some authors argue that the main problematic is the use of procedures not considering the diagnostic criteria recommended by the World Health Organization (WHO), the DSM IV, with inaccurate or nosological definitions for inadequacies in the methods of data collection (Andrade & Flores-Mendoza, 2010; Blasquez-Almería et al., 2005; Faraone et al., 2003; Pineda, Lopera, Palacio, & Castellanos, 2001) .
Regarding this matter Montiel-Nava, Peña and Montiel-Barbero (2003) highlight:
The prevalence of the disorder, attention deficit hyperactivity disorder (ADHD) is a condition with greater variability in different published epidemiological studies. Data on the prevalence of ADHD vary substantially from country to country and offer a range of heterogeneous data that describe the same clinical syndrome in children. Several epidemiological studies in different countries that use different classification and diagnosis (ICD-9, ICD-10, DSM-III-R, DSM-IV) systems, data indicated prevalence controversial. (p. 815)
The data on the prevalence of ADHD change according to the region where the survey was conducted, analytical and collected. Barkley and Murphy (2008) estimate that the disorder reaches 3% - 7% of school-age children. Holmes et al. (2002) argue that this rate is actually 10%. Brazelton and Sparrow (2003) present another estimation, that the disorder affects 5% of school-age children. Regarding this diversity of results, Golfeto and Barbosa (2003) informs that differences in epidemiological data can vary from 1% to 20%, requiring a careful analysis of the studies. This is corroborated with the studies contained in Table 1, where one can observe a great disparity between the data on the prevalence of ADHD, ranging from 0.04% in the study of Aragonès et al. (2010) to 24.5% in Azevedo, Caixeta, Andrade and Bordin (2010) .
3.3. Diagnostic Assessment of ADHD
The assessment for the diagnosis of ADHD has been the key point for the design of epidemiological profile of the disorder (Barkley & Murphy, 2008; Vasconcelos et al., 2003; Pastura, Mattos, & Araújo, 2007) . Three main aspects affect the quality of these finds: 1) ADHD Definition; 2) Respondent and 3) Instruments used to collect the results.
On this behalf Andrade and Flores-Mendoza (2010) state:
Although it is a recurrent disorder with personal and social negative consequences, its epidemiological research and determination are searched by means of various evaluation methods and informants, which does not provide precise data on its prevalence in the general population. (p. 17)
ADHD is frequently confused with other developmental disorders, episodic diseases, and behavior problems without any organic component or environmental situations, such as home education without objective rules (Andrade & Flores-Mendoza, 2010; Cardoso, Sabbag, & Beltrame, 2007) . It is necessary to
Table 1. Synthesis of epidemiological studies.
consider also cultural patterns that interfere significantly in establishment of pattern behaviors that are considered appropriate and inappropriate in a specific reunion.
Montiel-Nava, Peña-Barbera and Montiel (2003) discuss this issue as follows:
In recent decades, changes in psychiatric nosology systems on the conceptualization of the disease, which affected the number and the combination of signals required for the diagnosis of ADHD. These changes contribute greatly to the disparity in prevalence rates reported in several studies. Other factors are related to the methods used, type of sample studied (clinical or community), the source of communication (parents, teachers, children), and sociocultural characteristics. Culture is one of the most powerful influencers in the normal development of a child or the occurrence of psychopathological variables, such as expectations and standards associated with adequate performance and children that vary from country to country behaviors. (p. 815)
The quality of information is directly related to the source. That means, depending on the level of relationship between those responding to the protocols and participants (with ADHD) behavior can be observed under different analyzes. The level of understanding of the respondent on the ADHD phenomenon is an important matter to discuss. Moreover, the validity of the instruments used to collect information plays decisive role for the establishment of epidemiological data (Molinero, Villalobos, Redondo, Martín Rivera, & Sanz, 2009; Montiel- Nava, Peña, & Montiel-Barbero, 2003) .
Studies of this nature that use procedures of in their own scales in detriment of the diagnostic criteria of DSM IV they avoid to discuss and to analyze critically consistent characteristics with ADHD in a range of situations that can assign or delete the final diagnosis. It is necessary to consider the age of simptomatological onset, frequency, damaged areas and levels of prejudice. It is this set of information that will ensure greater reliability to the results. In most studies, it is observed that these variables are not considered, and assigned a diagnosis of ADHD to all those who filled out a number compatible with the 1st criteria of DSM IV symptoms, without considering the other. Thus, multidimensional investigation is abandoned.
Molinero et al. (2009) state that:
The variability in prevalence figures is influenced by the determination of the sample, clinical and/or psychometric strategy, the cut used in the scales, the informant, age, origin, geographical location, diagnostic criteria and whether or not the proper definition of dysfunction. All this suggests that the comparison of the values obtained in different studies is not straightforward and should be done carefully because we are aware that the use of older samples, include measures of impairment or use two instead of one informant tend to offer a lower prevalence, while use against DSM-IV and DSM-IIIR criteria ICD-10 improves prevalence. (p. 253)
Although the criteria for diagnostic assessment of ADHD is increasingly standardized based on DSM IV. Some researchers have extrapolated the use of a single checklist to ensure the highest reliability of the data obtained. As observed in studies of Graaf et al. (2008) , Kessler et al. (2006) , Londoño, Cifuentes and Lubert (2011) , Molinero et al. (2009) , Montiel-Nava, Montiel-Barbero and Peña (2007) , Montiel-Nava, Peña and Montiel-Barbero (2003) , Pineda, Lopera, Henao, and Palacio Castellanos (2001) , Scandar (2003) and Vasconcelos et al. (2003) .
In general, we can arrange the instruments used in data collection in three main groups:
Behavioral Rating Scales: Checklists compounds of descriptive items of topography and frequency of behaviors. Likerts scales have been widely used for initial screening of repertoires consistent with the clinical picture of ADHD. These scales were developed with the primary basis of the diagnostic criteria of DSM IV and are usually applied in the form of interviews with parents and teachers. The adoption of instruments based on DSM IV strengthens the predictive quality and reliability of the data obtained, considering that these scales are based on observation of pattern behaviors, as well as the reports of others about the development of the person assessed. Among the articles that make up this work identified themselves quite often the scales (Table 2).
Neuropsychological Assessment Scales: Neuropsychological assessment is the use of tests to assess the level of cognitive development, as well as identify possible (functinal) areas affected by ADHD. In the studies reviewed here, the scales were used (Table 3).
Table 2. Behavioral assessment scales used in the 21 articles.
Clinical Interview: The medical protocols have been important in identifying the developmental pathway of the subject. Thus, it is possible to exclude episodic diseases of atypical developmental conditions that have chronic nature, such as ADHD. In general, care services or health research groups in which the designs have been developed own history, which, unlike the scales for behavioral and neuropsychological assessment, do not go through the process of validation protocols. Some clinical routes used are listed in Table 4.
The findings of this study corroborate previous findings in the literature regarding the difficulty of establishing the prevalence of ADHD. Epidemiological studies are very important to emerge as the starting point for the development of intervention technology for this audience; however, the designs are presenting many different patterns to achieve the results, making it impossible for the data to be a crossed view of the complete picture about the disorder.
Among the main criticisms are the different definitions used for ADHD; instruments used, which are not infrequently inadequate public studied with respect
Table 3. Neuropsychological assessment scales used in 21 articles.
Table 4. Clinical interview guide used in the 21 articles.
to age, gender, culture and the data source, who are usually parents and teachers. Furthermore, the selection of instruments inspired by the DSM IV criteria, which have not gone through validation, makes the fragile quality of the information be obtained, which can generate incidence data below what is real.
In general, one can observe that for definition data of safer prevalence, it is important to extend the sample to statistical significance thresholds, selecting the instruments that have been tested and finally efficiency draws a parallel between the data obtained from different sources of information, since it is necessary to discard information that refer to behaviors that are manifested in specific environmental conditions. Thus, studies of an epidemiological nature should consider the multidimensional character of ADHD, considering then, instruments that reach this condition and allow the researcher the direct contact with historical family, developmental and clinical variables.