Psychology

ADHD is the commonest neurobehavioral syndrome of childhood and is associated with serious complications and significant morbidity. Several studies have been conducted worldwide to accurately assess its prevalence, which is estimated at 5.29. Research data consistently show that ADHD is manifested differently in males and females and in different age groups. Boys are more vulnerable to the disease and display greater symptom severity, more co-morbidities and greater psychosocial dysfunction compared to girls. Likewise, ADHD is a developmental syndrome and its epidemiology and clinical picture differs through time, from preschoolers to adolescents. In addition, modern research suggests that the disorder seems to persist into adulthood and continues to cause major suffering, emphasizing the need for formulating new diagnostic criteria applicable to adults.

Introduction
Attention deficit and hyperactivity disorder (ADHD) is a common neurodevelopmental syndrome with significant co-morbidity that causes major impairments in social and academic functioning in a sizeable percentage of children and adolescents (Smalley, 2007  Bell, 2010). Several studies have linked ADHD to various psychopathological entities, including learning disorders, oppositional and conduct disorder, aggression, anxiety and affective disorders, suicidal behaviors, substance abuse, personality disorders and criminal behaviors (Matthys, 1999). Although it has been traditionally considered a disorder of school-age boys, recent epidemiological and clinical data increasingly suggest that it can also affect girls, preschoolers, adolescents and young adults, raising much debate regarding disease epidemiology (Bell, 2010).

Prevalence definition and significance
Estimating the exact prevalence of a disorder is a matter of great importance for clinical, scientific and public policy reasons. Prevalence is an epidemiological term that describes the total number of cases in the population at a given time or time period, divided by the number of individuals in the population (Le, 1995). It represents the percentage of affected individuals in the general population and therefore constitutes a useful tool for clinicians in order to make a diagnosis and propose an appropriate treatment plan. In addition, knowing the exact epidemiological profil of a disorder can be an important factor when designing and conducting further scientific research on the field. Moreover, prevalence rates  are usually taken into serious consideration by policy makers when approving funding and implementing prevention and intervention strategies. When facing a highly prevalent health issue, authorities are more willing to take action to control its consequences. At this point, we should emphasize the fact that, despite ADHDs undoubted neurobiological origin and high prevalence, much discussion has been raised in the scientific community, the mass media and the public opinion regarding the use of pharmacotherapy, namely psycho-stimulants. Accurately evaluating the frequency of the disorder and delineating its clinical variations will probably lead to a more evidence-based management, relieving public worries about unnecessary pharmacological treatment and its potential side-effects and financial cost.

Overall ADHD prevalence
In the most recent edition of the Diagnostic and Statistic Manual (DSM-IV), there has been a major revision in ADHD diagnostic criteria that led to the description of three distinctive sub-types the inattentive, the hyperactive impulsive and the combined type. The application of these criteria has raised the prevalence of the disorder from 3-5 to 12 of school-age population (Baumgaertel et al., 1995 Wolraich et al., 1996).

There is a wide range in prevalence estimates reported by various researchers in studies conducted around the world. It seems that multiple parameters contribute to these differences, including demographic variables (gender, age), diagnostic criteria, type of informant (parent, teacher or subject), research instruments (behavior checklists or clinical interviews), the presence of mental retardation or other developmental disorders and whether impairment criteria were used (Scahill, 2000  Smalley, 2007). In the U.S.A., the mean prevalence of the disorder is 5 (Faraone, 2003), while Polanczyk et al (2007) in a meta-regression analysis found a worldwide prevalence of 5.29. In a similar vein, a systematic analysis of existing epidemiological studies, revealed that ADHD is a common behavioral disorder across countries and cultures (Faraone, 2003).  For example, Huss et al (2008) found a prevalence of 4.8 in German children aged 3-17 years old. In a large-scale epidemiological study conducted in Puerto Rico, ADHD was the commonest disorder with a prevalence of 8 (Canino, 2004). Likewise, a recent study reported that the prevalence of ADHD symptoms in Arab students ranges from 5.1 to 14.9, and is comparable to prevalence rates observed in the western world (Farah, 2009).

ADHD and age effects
As mentioned above, age is a demographic variable closely associated with the frequency of the disease and there seems to be an increase in disease prevalence from preschool years to school-age and subsequently a progressive slight decline through adolescence and adulthood (Schmidt, 2009). In a recent review, the authors report that ADHD is diagnosed in 2 of preschoolers, 3-15 of school-age children and adolescents and in 1-7 of adults (Schmidt, 2009). Moreover, clinical studies have shown that there are significant age differences between the three subtypes of ADHD. Hyperactivity and impulsivity prevail in preschoolers and tend to remit over time. In contrast, attentional problems become apparent in school-age years when academic demands are higher and tend to persist through adolescence and adulthood. As a consequence, the hyperactive impulsive type seems to be more common in preschoolers, the combined type in school-age children and the inattentive type in adolescents (Schmidt, 2009).

Hyperactivity and impulsivity are the symptoms that mostly disrupt behavior, create significant social difficulties and are related to aggression, substance abuse and criminality. Their tendency to improve with time (Schmidt, 2009), has mistakenly created the impression that ADHD remits in adolescence. However, the ongoing attentional deficits continue to impair academic, occupational and social functioning thus augmenting the psychosocial burden of the disease. Research in various populations has revealed that the majority (70-80) of children continue to fulfill the criteria for ADHD from school-age to adolescence, with a prevalence of 8.5 in Finland (Smalley et al., 2007), 3.3-7.5 in Taiwan (Gau et al., 2005) and 3.9 in Hong-Kong (Leung et al., 2008). Based on these observations, research has currently focused on the continuity of ADHD symptoms and impairment in adulthood (Pary, 2002  Wilens, 2002). It is now well-established that ADHD symptoms may persist through adulthood in 40-60 of patients (Schmidt, 2009), especially in cases with a strong familial component (Thapar, 2008). The presence of ADHD symptoms in adulthood has a major impact on patients social and occupational functioning. These people are usually underachievers and report lower income compared to the general population, suggesting that ADHD is associated with decreased work productivity and significant financial costs both for individual patients and the society (Biederman, 2006). In addition, ADHD patients, due to their profound impulsivity and inattention, are prone to accidental injuries and to legal offenses and are more likely to get arrested or incarcerated compared to non-ADHD offenders and the general population . Adult ADHD frequently escapes clinical attention, given that the current diagnostic criteria are children oriented and can hardly be applied to adults. Kessler et al (2006) found a prevalence of 4.4 in individuals aged 18-44 years old, which is comparable to the prevalence of childhood and adolescent ADHD. In addition, adult ADHD is usually associated with significant co-morbidities which may confuse the clinical picture. A significant percentage of children with the syndrome will be diagnosed with oppositional or conduct disorder in adolescence and some of them will present antisocial traits, substance abuse, personality disorders, criminal behavior and further psychosocial maladjustment in adult life (Kessler, 2006).

These severe and debilitating psychopathological entities usually dominate the clinical picture and ADHD may go undiagnosed.  In light of these observations, it is becoming increasingly urgent to reformulate the current diagnostic criteria through a more developmental perspective.

This effect of age on ADHD symptomatology, course and prognosis has prompted a wealth of research on disease etiology, focusing on certain brain structures which are involved in attention and self-regulation. According to that research, there is significant evidence to suggest that delayed brain maturation may underlie ADHD pathogenesis (Shaw, 2007, Stanley, 2008  McAlonan, 2009). Neuro-imaging studies have consistently revealed anatomical and physiological abnormalities in the prefrontal cortex, the basal ganglia, the cerebellum and the inferior parietal lobe of children with ADHD, which may reflect multiple cognitive deficits affecting executive functioning, inhibition control, perception, motivations and motor control (Stanley, 2008). Cognitive development normally runs an asynchronous course, with primary cortical motor and sensory areas reaching maturity prior to the maturation of higher order association areas. This finding is consistent with the observations of healthy young children who firstly develop independent sensory and motor functioning and secondly acquire more complex skills, such as language, behavioral and emotional regulation and motor coordination. ADHD children seem to follow the same developmental course, in a significantly slower pace (Shaw, 2007). Interestingly, Ptacek et al (2009) found that ADHD children lag behind in physical growth too, compared to healthy controls, adding further to the view that ADHD may be a syndrome of developmental immaturity. 

ADHD and gender effects
Another important parameter that should be taken into consideration when discussing ADHD prevalence and phenomenology is gender. ADHD is more common in males compared to females and several investigations have attempted to clarify the associations of sex with prevalence rates. Girls usually suffer from the inattentive type, while boys commonly present with the combined type (Biederman, 2002). In Taiwanese adolescents, the odds ratio between males and females ranged from 4.5 to 5.2 (Gau, 2005). In a similar study in Finland, the odds ratio were 5.2-6.1 (Smalley, 2007), while in Hong-Kong, similar research revealed that male adolescents ran a two-fold risk of suffering from the disorder compared to females (Leung, 2008). Studies conducted in Arab populations revealed a similar odds ratio of 2-31 between males and females (Farah, 2009). However, the gender differences in prevalence observed in adolescent samples are lower than the differences observed in school-age children (Smalley et al., 2007), probably due to the fact that girls usually suffer from the inattentive type (Biederman, 2002) and are referred for evaluation during adolescence when academic and societal demands are higher and functional impairment becomes more obvious. In a similar vein, no significant prevalence differences have so far been detected between males and females in adults with ADHD (Schmidt, 2009). These findings suggest that females may be affected by the disease and its associated functional impairment at an older age compared to males.

The observed gender differences in ADHD prevalence have not yet been causally explained, although gonadal hormones and neuro-endocrine mechanisms have been implicated in the pathogenesis of ADHD (Strous, 2001  Dean, 2008). It has been postulated that girls may be more resilient, given that more risk factors need to be present for the full clinical syndrome to emerge. However, empirical statistical data failed to verify this hypothesis. In contrast, genetic studies have shown that some ADHD risk genes may have different effects in males and females (Biederman, 2008). Apart from different prevalence rates, males and females with ADHD differ in symptom severity, co-morbidity, cognitive ability and degree of functional impairment. Boys report more severe symptoms, are more likely to manifest learning difficulties, disruptive behavior disorders and substance abuse and suffer from greater social and academic impairment. In contrast, girls with ADHD usually report lower self-esteem and poorer coping capacities and more internalizing symptoms. These discrepancies may probably explain the fact that male predominance is even greater in clinic-referred compared to community samples (101 vs 31, respectively) (Biederman, 2002). Further studies have attempted to detect gender differences in treatment responses but no significant findings emerged (Rucklidge, 2010).

Conclusion
In conclusion, current research increasingly supports the notion that ADHD is a neurobehavioral disorder which can be manifested quite differently between males and females and in different age-groups. Boys usually suffer from the typical syndrome with combined symptoms, externalizing behaviors, frequent co-morbidities and greater psychosocial impairment. Girls, on the other hand, are commonly classified as inattentive, exhibit more internalizing symptoms and lower self-esteem  and are usually referred for evaluation and treatment at an older age. In a similar vein, young children usually present with hyperactivity and impulsivity, while older children, adolescents and adults mainly display attentional difficulties. According to most epidemiological studies, ADHD prevalence rises from preschool years to school-age and than slightly decreases from adolescence to adult life.

Viewing ADHD as a purely developmental disorder has major implications for research and clinical practice. Instead of rigidly abiding by the current descriptive diagnostic formulation, which is applicable only to children, we should attempt to establish new developmentally oriented criteria, which can be applied throughout the lifespan. In this way, ADHD patients who previously were misdiagnosed thus being deprived of treatment opportunities, will eventually be provided with the proper intervention and care, a development that will hopefully lead to lightening the diseases psychosocial and financial burden and enhancing patients quality of life.