While there is variability in the estimated prevalence of the disorder, most researchers agree that ADHD affects between 3% and 7% of the school-age population. The reported prevalence differences likely reflect variations in assessment methodologies, such as sampling procedures, assessment instruments, and diagnostic criteria. For example, the estimates of affected children tend to be inflated when sampling procedures do not assess for the degree of functional impairment. There is currently limited information regarding the prevalence of ADHD in adults, but based upon extrapolations from childhood estimates, some researchers have suggested that ADHD affects between 2% and 6% of the adult population.
Prevalence variation also appears to be related to gender. There is a large body of empirical evidence to suggest that males are much more likely to be diagnosed with ADHD than females. Estimated male-to-female ratios for the disorder range from 2:1 to 9:1 depending upon the setting (community sample vs. clinic referred sample). It also appears that the gender discrepancy is lower for individuals with the predominantly inattentive subtype of ADHD.
The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM—IV—TR) specifies 18 symptoms that compose the disorder. Nine of the criteria are related to symptoms of inattention, and nine are related to symptoms of hyperactivity/ impulsivity. Symptoms of inattention include failure to pay attention, having difficulty with sustained attention, not listening, not following through on instructions, poor organization skills, avoiding tasks requiring sustained mental effort, often losing things, being easily distracted, and being forgetful. Symptoms of hyper-activity include fidgeting often, getting out of one’s seat at inappropriate times, behaving or feeling restless, not being able to play quietly, being compelled to be active, and talking excessively. Symptoms of impulsivity include blurting out answers, having difficulty awaiting one’s turn, and interrupting others.
Based upon this cluster of symptoms, three primary subtypes of ADHD can be diagnosed. If an individual exhibits six of the nine inattention symptoms for a period of at least 6 months, and if these exhibitions are both maladaptive and inconsistent with the individual’s developmental level, the diagnosis of attention deficit/hyperactivity disorder, predominately inattentive type can be made. If an individual exhibits six of the nine hyperactivity/impulsivity symptoms for at least 6 months and to a maladaptive degree given the individual’s developmental level, the diagnosis of attention deficit/hyperactivity disorder, predominately hyperactive-impulsive type can be made. If an individual exhibits significant elevations on six of the inattention symptoms and six of the hyperactive-impulsive symptoms, he or she could be diagnosed with attention deficit/hyperactivity disorder, combined type. In all of the subtypes of ADHD listed above, some symptoms causing impairment must have been present before the age of 7, some impairment from symptoms needs to be present in two or more settings, and there needs to be clear evidence of significant impairment in social, academic, or occupational functioning. In cases where it is unclear if these criteria have been met, a diagnosis of attention-deficit/hyperactivity disorder, not otherwise specified could be made.
The final criterion to consider is that the symptoms could not be better accounted for by another mental disorder. This is a very important criterion to consider. Symptoms characterized by inattention, impulsivity, and hyperactivity can occur in numerous other mental disorders. The clinician needs to consider not only that the individual has some disorder other than ADHD, but that the individual has another disorder in addition to ADHD.
When making the diagnosis of ADHD, the clinician has a number of different tools to choose from. First are the DSM-IV ADHD rating scales. These scales take the DSM-IV criteria and have parents or teachers rate a child’s behavior on a Likert-type scale. These specific scales tend to be relatively brief and easy to administer and have adequate psychometric properties.
The second type of assessment instrument is the broad-band rating. These instruments assess for a broader array of behaviors than just those associated with ADHD. Examples of commercially available instruments that would fall into this category include the Child Behavior Checklist (CBCL) and the Behavior Assessment System for Children-2nd Edition (BASC-2). Both of these instruments assess for a broader range of symptoms, including depression, aggression, and somatic complaints. These instruments are well normed, have acceptable psycho-metric properties, and are available in parent, teacher, and self-report versions. Research suggests that youth self-report versions are rather poor at differentiating between children with ADHD and controls, suggesting that children may not be accurate informants of their behavior problems. In addition studies suggest that when there are discrepancies between parents’ and teachers’ perceptions on such instruments, teachers tend to be the more accurate informants. The reason for this is that teachers usually have a better understanding of child development and age-appropriate behavior than many parents do.
Structured and semistructured interviews are a third type of assessment tool that can be employed when making a diagnosis for ADHD. These instruments have advantages over rating scales in that the clinician can follow up on parent ratings and ensure that the parent understands the intent of the question. However, interviews are time intensive for both the examiner and the informant (usually the parent). The Diagnostic Interview for Children and Adolescents-Revised (DICA-R) and the Diagnostic Interview Schedule for Children-Version IV (DISC-IV) are two of the more widely used diagnostic interviews that have adequate psychometric properties.
Impairment rating scales are a fourth category of assessment for ADHD. Some measures provide documentation for global or overall impaired psychosocial functioning (e.g., the Children’s Global Assessment of Functioning), whereas others provide a multidimensional rating of impairment (e.g., the Child and Adolescent Functional Assessment Scale). Some of the more widely used impairment rating scales show good temporal stability and interrater reliability. In addition they also have evidence for convergent and concurrent validity.
A fifth category of assessment is observational measures. There is a long tradition of conducting observations for children with behavior disorders. Observations can occur in analogue or naturalistic settings. Observation measures can be very complex and assess multiple behaviors across multiple settings or can be relatively simple and involve the assessment of only a couple of behaviors (e.g., verbal intrusions and out-of-seat activity in the classroom). Studies have shown that when the behaviors for observation are carefully chosen, observation measures that involve relatively few behaviors can be as effective as the more comprehensive systems.
Physiological or laboratory measures form a final category of assessment. While no such measures can diagnose ADHD, some are widely used to aid in making the diagnosis. One group of such measures is the continuous performance tests (CPTs). There are many different types of CPTs available; some of the more common commercially available ones include the Test of Variables of Attention (TOVA), Conner’s CPT, and the Individual Variables of Attention (IVA). All CPTs attempt to provide a computerized assessment of inattention, response inhibition (hyperactivity), and reaction time (impulsivity). While studies have shown that individuals with ADHD do perform more poorly on these tests than do controls, individuals in other clinical groups (e.g., learning disabilities, depressed) also perform more poorly than controls.
Some researchers have argued that when assessing for ADHD, less emphasis should be placed on identifying symptoms that confirm the diagnosis and more emphasis should be placed on evaluating the function of the behavior that is causing the problems. Through functional behavioral analysis, clinicians are better equipped not only to recognize the maladaptive behaviors but also to implement a strategy to treat them.
When compared to children of similar age and gender, children with ADHD experience functional and adaptive impairment academically, behaviorally, and socially. The core symptoms of inattention, impulsivity, and hyperactivity can manifest in various ways. Attention is frequently defined by a set of observable behavioral characteristics. For example, children with ADHD may find it difficult to remain focused on an assignment with ongoing distractions. Each passing moment may present a challenge for them to focus on a particular stimulus. Furthermore, once distracted, these children are often slower to return to task. Factors such as background noises, their own thoughts, and activity of any kind present potential distractions, which may preclude the child from staying on task. Adults working with these children frequently report that the children appear to be daydreaming, lethargic, or prone to not listening to instructions.
Impulsivity represents the inhibitory deficits an individual exhibits. These problems are associated with the tendency to interrupt others, to call out answers before the speaker can finish a question, and to have trouble waiting one’s turn. Frequently, children with ADHD report acting without considering the consequences for their behavior. Problems with low frustration tolerance and temper outbursts may be related to impulsivity. Some researchers, such as Russell Barkley, consider impaired inhibition to be the paramount symptom associated with ADHD.
Hyperactivity is the third component of the ADHD triad. When compared to normal controls, children with ADHD are significantly more active. Often this activity appears aimless, as if the child is compelled to be doing something. This tendency may also result in the child engaging in more high-risk behaviors, such as running into the street without looking or jumping from a fast-moving object. These risk-taking behaviors appear to persist in adulthood. Adults with ADHD have been found to be more prone than controls to substance abuse, traffic accidents, and legal problems.
Frequently, a child with ADHD will first exhibit problematic behavior at school. Teacher-student relationships are often marked by strain and significant discord. Children with ADHD frequently engage in disruptive behavior, refuse to follow directions, and fail to complete assignments. Observers often misinterpret such a child’s dislike for tasks that require sustained attention as an indication that the child is lazy or immature. Additionally, when compared to peers, children with ADHD tend to lose necessary materials and have more organizational difficulties. Consequently parents and teacher may view them as being oppositional and defiant. These behaviors often result in increased disciplinary referrals, lower academic achievement, and more school failure.
Parents of children with ADHD frequently experience problems relative to parenting practices, parent-child interactions, and parenting stress. They frequently exhibit higher levels of stress than parents of controls. In addition, there is a greater chance of disagreement between parents relative to child-rearing practices. Mothers of children with ADHD are more likely to be depressed, and fathers are more likely to have substance abuse problems, than parents of controls.
Parent-child interactions are frequently marked by conflict. Research suggests that these parents are more likely to engage in punitive parenting practices. Perhaps in response to these conflicted interactions, children with ADHD become even more overactive, defiant, uncooperative, and impulsive. This further exacerbates the parent-child conflicts. There is some research to suggest that the use of harsh physical discipline is associated with the development of oppositional defiant disorder or a conduct disorder in children with ADHD. Older adolescents and young adults who received harsh physical punishment as children appear to be more impaired than their ADHD peers whose parents did not use such practices.
In addition to negative family interactions, peer relationships also tend to suffer. Children with ADHD interact less with playmates during conversation, tend to be more bossy, exhibit a diminished receptivity to social cues, and appear less likely to engage in close friendships. The aforementioned problems are exacerbated when there is a comorbid oppositional defiant or conduct disorder.
Low self-esteem has been observed more frequently in children with ADHD than in controls. In an effort to preserve their self-esteem, children with ADHD may overestimate their performance in the domains with greatest deficits. So, a child with ADHD who is struggling in school may exaggerate his or her academic achievement for the purpose of self-protection.
When a child has ADHD, he or she is more likely than not to have a comorbid disorder. Recent research suggests that up to 87% of children with ADHD have at least one comorbid condition and up to 67% have at least two. Approximately half of children with ADHD will go on to develop another behavior disorder such as oppositional defiant disorder or conduct disorder. Other common comorbid conditions include the following: learning disability (40% to 60%), depressive disorder (17% to 30%), anxiety disorder (20% to 43%) and substance abuse disorder (18% to 36%). There is some evidence to suggest children with ADHD may be more likely to develop juvenile-onset bipolar affective disorder. While half of the individuals with Tourette’s syndrome have ADHD, children with ADHD do not appear to be more likely to develop Tourette’s.
There are numerous hypotheses relative to the etiology of ADHD. However, few are supported with empirical research. One of the theories that has some support surmises a strong biological component. The evidence for a genetic influence in ADHD has been gathered from family studies, twin studies, and molecular genetics studies. From the research relative to families, there are data that suggest that parents with an ADHD diagnosis are more likely than non-ADHD parents to have children with the same disorder. In fact, some researchers assert that there is a 57% chance that parents with ADHD will have children with the same diagnosis.
Researchers have also looked at the prevalence of the disorder in parents of children who have been diagnosed with ADHD. These studies found that 15% to 20% of the mothers and 20% to 30% of the fathers of an affected child also have ADHD. There is also a 1 in 3 chance that a child with ADHD will have a sibling who also has the disorder. Identical twins also appear to have increased risk for ADHD. However, with these studies it is difficult to tease out environmental contributions to the expression of the disorder. Gonzales-Limas suggested that 70% to 95% of the trait variability among individuals with ADHD can be accounted for with genetic heritability. Researchers involved in molecular genetics research believe that several genes are related to risk for ADHD. These genes include DAT1, DRD4, and DHB.
Some studies have found that birth complications may increase the likelihood that a child will develop ADHD. In addition, several studies suggest a link between premature birth or low birth weight and ADHD. In fact, it has been estimated that low birth weight is associated with 14% of all ADHD diagnoses. Maternal behaviors such as smoking and using drugs or alcohol during pregnancy have also been linked to low birth weight and increased risk for ADHD.
Several environmental variables have been consistently implicated in the expression of ADHD. Correlations between exposure to environmental toxins, such as lead, and hyperactive behaviors have been reported in numerous studies. Other environmental variables may include malnutrition, disease, and trauma. Several large, well-controlled studies that systematically evaluated the diets of children for the effects of additives such as processed sugar, wheat germ, and food dyes on the development and expression of ADHD have failed to find significant differences between groups of children who were exposed to the additives and those who were not.
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