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The Epidemiology and Diagnostic Issues in Preschool Attention-Deficit/Hyperactivity Disorder

A Review

Egger, Helen Link MD; Kondo, Douglas MD; Angold, Adrian MRCPsych

Editor(s): WOLRAICH, MARK L.

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Abstract

ATTENTION-DEFICIT/HYPERACTIVITY DISORDER (ADHD) appears to be the most common diagnosis received by young children referred for mental health services (Gadow, Sprafkin, & Nolan, 2001; Keenan & Wakschlag, 2000; Wilens et al., 2002). For instance, 86% of 200 preschoolers consecutively referred to an academic child psychiatry clinic were diagnosed with ADHD (Wilens et al., 2002). While it is probably true that ADHD is second only to autism in being the most studied preschool psychiatric disorder, the research on preschool ADHD is still in its infancy, in comparison with the research on ADHD in older children (Angold & Egger, 2004; Pine et al., 2002). The central challenge is defining when preschoolers' inattention, hyperactivity, and impulsivity are developmentally appropriate as opposed to being clinically significant. While the DSM-IV-TR definition of ADHD symptoms states that the child's inattention and/or hyperactivity-impulsivity must be severe, frequent, persistent, and “inconsistent with developmental level” (American Psychiatric Association, 2000, p. 92) to be considered a symptom of ADHD, no guidance is given as to how to distinguish between normal preschool inattention, hyperactivity, and impulsivity and ADHD symptoms.

The purpose of this article is to review the research on preschool ADHD to address whether ADHD, as defined in DSM-IV-TR, is a valid and clinically relevant diagnosis for preschoolers. We will focus on studies conducted with community or primary care pediatric clinic samples, rather than those with subjects recruited from mental health clinics. Since patients attending specialty mental health clinics are known to be a biased subset of children with disorders in the general population (Costello & Janiszewski, 1990; Goodman et al., 1997), it is not possible to define the presentation or prevalence rates of ADHD in the community from studies conducted in these settings. There are relatively few nonspecialty clinic studies of preschool psychiatric disorders overall, or ADHD in particular. There are still fewer that use measures with good psychometric properties in this age group and/or sampling frames that result in a cohort actually representative of the population to be studied. Yet, despite these limitations, there is sufficient data from these studies to begin to address the following 7 key questions: (1) Can the symptoms of ADHD and ADHD diagnoses be reliably measured in young children? (2) Are the DSM-IV ADHD criteria actually “symptoms” in preschoolers or are they descriptions of normal preschool inattention, high activity, and impulsivity? (3) Using the DSM-IV-TR 6-symptom cutpoints, what is the prevalence of ADHD, overall and by subtype, in preschool children? (4) Are there significant differences between toddlers (2- and 3-year-olds) and older preschoolers (4- and 5-year-olds) or between boys and girls? (5) Are preschoolers with ADHD impaired? (6) Do preschoolers with ADHD have increased rates of other psychiatric disorders? (7) Are preschoolers with ADHD receiving mental health evaluations and, if warranted, treatment? We will discuss the implications of the answers to these questions for clinical practice, as well as future research.

METHODS

Medline searches using the search words, “ADHD,” “preschoolers,” “epidemiology,” and “community studies,” were conducted to identify the studies examining ADHD symptoms and diagnosis in preschool children. We also followed up any further relevant studies identified from their reference lists. For the purposes of this review, we define preschoolers as children aged 2 through 5 years. Table 1 includes the major studies examining individual ADHD symptoms in nonspecialty clinic populations, whereas Table 2 includes these studies examining the diagnosis of ADHD. In Table 2, these studies are divided into those using checklist measures and those using diagnostic measures. We also refer to case-control studies of preschool ADHD to address issues raised in these other studies (eg, Lahey et al., 1998, 2004; E. Sonuga-Barke, Dalen, Daley, & Remington, 2002; E. J. Sonuga-Barke, Dalen, & Remington, 2003). Only those studies that separately reported data on preschool children are included.

Table 1
Table 1:
Prevalence of ADHD symptoms in preschool children in community or pediatric clinic studies*
Table 1
Table 1:
Prevalence of ADHD symptoms in preschool children in community or pediatric clinic studies* (Continued)
Table 2
Table 2:
Prevalence of preschool ADHD, overall and by subtype, in community or pediatric clinic studies*

All of these studies have limitations: a number have high refusal rates and thus may not be representative of the population studied (eg, both Lavigne et al., 1996, and Gadow et al., 2001, have response rates of less than 50%); only one study used a diagnostic (as opposed to checklist) measure with proven psychometric properties in this age range (ie, Egger, Keeler, Angold, 2006); and most base the diagnosis of ADHD on parent reports, not on parent and teacher report (eg, Briggs-Gowan, Horwitz, Schwab-Stone, Leventhal, & Leaf, 2000; Earls, 1982; Egger et al., 2006; Keenan, Shaw, Walsh, Delliquadri, & Giovannelli, 1997). Of course, a challenge in assessing ADHD in preschoolers is that not all young children are in preschool or daycare. There is

also greater variety in the quality of caregivers and types of settings, making “teacher” assessments of ADHD symptoms in this age group difficult to obtain, and, perhaps, less useful than those from teachers in traditional school settings.

RESULTS

Can ADHD symptoms and ADHD diagnoses be reliably measured in preschool children?

Test-retest reliability demonstrates the degree to which the same informant provides the same information in separate administrations of a measure. Cohen's kappa (Cohen, 1960) is used to assess agreement on categorical variables, while the intraclass correlation coefficient (or, less satisfactorily, Pearson's r) is used to assess the agreement between syndrome scale scores (Shrout & Fleiss, 1979). A number of ADHD-specific checklist measures have been shown to be reliable for assessment of preschoolers, including the Connors Parent Rating Scale–Revised and the Connors Teacher Rating Scale–Revised (validated for 3 and older) (Conners, Sitarenios, Parker, & Epstein, 1998) and the ADHD Rating Scale (DuPaul, Power, Anastopoulos, & Reid, 1998; Gimpel & Kuhn, 2000). Although the SNAP-IV (Swanson, Nolan, and Pelham–IV questionnaire) and the SKAMP (Swanson, Kotkin, M-Flynn, and Pelham Rating Scale) were recently used in a multisite psychopharmacologic treatment study for preschool children with ADHD (Scott Kollins, Oral communication, 2005), to our knowledge there are no reported psychometric data on their use with children younger than 6 years. A few broad checklist measures of preschool symptoms have ADHD subdomains, which have been shown to be reliable. The “attention problems” syndrome of the Child Behavior Checklist 1½–5I has a Pearson r of 0.78, while the “Attention Deficit/Hyperactivity Problems” DSM-oriented scales has an r of 0.74 (Achenbach & Rescorla, 2000). The Early Childhood Inventory (ECI), a DSM-IV symptom checklist, is reported to have the following Pearson rs: ADHD inattentive type (r = 0.64), ADHD hyperactive-impulsive type (r = 0.72), and ADHD combined type (r = 0.67) (Gadow & Sprafkin, 1997).

The Preschool Age Psychiatric Assessment (PAPA) is the only structured parent interview assessing psychiatric symptoms and disorders in preschool children with reliability data (Egger & Angold, 2004; Egger, Erkanli, Keeler, Potts, Walter, & Angold, in press). In a test-retest study, the PAPA was administered twice, with a mean of 11 days separating the 2 interviews. The kappa for the diagnosis of ADHD was 0.74 and the intraclass correlation coefficient for an ADHD scale score was 0.80, demonstrating that ADHD diagnosis and symptom scales scores can be as reliably measured in preschoolers as in older children (Egger et al., in press).

The fact that ADHD can be reliably assessed in preschoolers using symptom checklists and structured diagnostic interviews means that it is possible to conduct epidemiologic studies and clinical studies of ADHD in young children. The reliability of both the checklist measures and PAPA is also an indicator of the coherence of the DSM-IV ADHD construct for young children.

Are the DSM-IV ADHD criteria “symptoms” in preschoolers or are they descriptions of normal preschool inattention, high activity, and impulsivity?

Preschool behavioral and emotional symptoms must be approached from a developmental perspective that takes adequate account of the rapid cognitive, motor, social, and emotional changes that occur from age 2 to 5 years (Kochanska, Coy, & Murray, 2001; Kochanska, Murray, & Harlan, 2000). To define whether a preschooler's inattention, hyperactivity, and impulsivity are “inconsistent with developmental level” as DSM-IV specifies (American Psychiatric Association, 2000, p. 92), we must determine how common these symptoms are in preschoolers. Table 1 presents data from a number of nonspecialty clinic studies on the prevalence of DSM-IV ADHD symptoms. What is immediately clear from these data is the wide range of prevalence rates. For example, rates of the symptom “on the go” range from a low of 2% to a high of 72%! All but one study used checklist measures without symptom definitions. The study with the highest rate used a categorical “yes/no” rating scale while the others used Likert scales, usually with 3 points, to distinguish between degrees of severity. In most of these studies, symptoms were counted as present if they occurred “often” or “very often.” Rates were lower, and perhaps more reflective of clinically significant behaviors, when only the highest point (eg, “very often” or “very much”) were counted.

In the PAPA test-retest (PTRT) study, an ADHD symptom was coded as present if it occurred in at least 2 activities and was, at least sometimes, uncontrollable by the child or by admonition. These criteria were included to distinguish between transient behaviors that can be controlled by the child or an adult and those that may be clinically significant. One way to define “developmentally inappropriate” behaviors is to identify those behaviors that occur in less than 10% of a given population (or at a level of frequency of the behavior that identifies the top 10% of children) (Angold & Costello, 1996). In the PTRT study, the majority of ADHD symptoms occurred in less than 10% of the children assessed, suggesting that ADHD symptoms are far from normative in preschoolers. Three of the hyperactivity/impulsivity symptoms (interrupts/intrudes, talks excessively, and often leaves seat) and 3 of the inattention symptoms (does not follow instructions, easily distracted, avoids sustained mental effort) were endorsed by more than 10% of the parents, with nearly 50% of parents reporting the presence of interrupting/intruding. However, every one of the ADHD symptoms was significantly more common in children who met criteria for ADHD than those without ADHD. For instance, even with the most common symptom, “interrupts/intrudes,” 44.7% of parents of preschoolers without ADHD endorsed this symptom, compared with 100% of parents of preschoolers who meet ADHD criteria (P < .0001). The other important finding was that each of the ADHD symptoms, even the more common symptoms, was significantly associated with impairment. For example, 64.6% of children who were reported to be “on the go” were impaired compared with only 17.5% without this symptom (OR = 8.6 (3.8, 20); P < .0001). Children with the relatively common symptom of “interrupting or intruding” were 5 times more likely to be impaired than those without this symptom (36% vs 10%; OR = 5.0 (2.5, 10); P < .0001) (Egger et al., 2006).

Another way to examine whether ADHD symptoms are normative in preschoolers is to look at the mean counts of ADHD symptoms in community samples of preschoolers. In Gadow and Sprafkin's study of more than 500 unreferred preschoolers using the Early Childhood Inventory, the mean symptom count for the 9-symptom inattention scales was 0.78 (SD = 1.72) for boys and 0.50 (SD = 1.26) for girls. The mean symptom count for the 9-symptom hyperactive-impulsive scale was 1.48 (SD = 1.72) for boys and 0.99 (SD = 1.60) for girls (Gadow & Sprafkin, 1997). In a study of 455 nonreferred preschoolers aged 3–5 years using the Conners Teacher Rating Scale, the mean score on the 10-item hyperactivity index was 0.54 (SD = 0.5) (Miller, Koplewicz, & Kelin, 1997). In the PTRT study, the mean number of hyperactive/impulsive symptoms was 1.3 (SD = 1.9) and inattentive symptoms 0.6 (SD = 1.5). A total of 14.2% had 1 or 2 inattentive symptoms, but only 2.8% had the 6 or more symptoms needed to meet the symptom count criterion for inattentive ADHD. A total of 36.7% of preschoolers had 1 or 2 hyperactive/impulsive symptoms but only 5.6% had 6 or more of these symptoms (Egger et al., 2006). These data suggest that while having 1 or 2 ADHD symptoms is common, and within the range of being developmentally normative, having multiple ADHD symptoms at the level specified by the DSM-IV-TR criteria is far from being in the normal range by any usual criterion for preschoolers.

Using the DSM-IV-TR 6-symptom cutpoints, what is the prevalence of ADHD, overall and by subtype in preschool children?

A further way to examine the appropriateness of DSM-IV definitions of ADHD is to determine the apparent prevalence in preschoolers using the usual DSM-IV rules and approaches to assessment. If rates in preschoolers were wildly out of line with the prevalence rates expected in middle or later childhood, they would raise questions about whether the nosology is working properly for younger children. Across a number of community studies (N > 17), prevalence rates of ADHD in older children range from 3% to 5%, although some studies, particularly those using DSM-III or DSM-III-R criteria, report rates as high as 12% (Angold, Costello, & Erkanli, 1999). Table 2 present data on the prevalence of ADHD, overall and by subtype, in community studies of preschoolers using checklist measures (top half of table) and diagnostic measures to identify ADHD (bottom half of table). The prevalence of any type of ADHD ranges from a high of 18.3% to a low of 2.0%, with relatively higher rates reported in the studies using checklist, rather than diagnostic measures (Briggs-Gowan et al., 2000; Earls, 1982; Egger et al., 2006; Gadow et al., 2001; Gimpel & Kuhn, 2000; Keenan et al., 1997; Lavigne et al., 1996; Pineda et al., 1999). Across all of the studies using diagnostic measures, the mean prevalence for any type of ADHD in preschoolers was 4.2%. Including only those studies that assessed children 5 years old or younger, the mean prevalence was 3.3%. In light of concerns about the overdiagnosis of ADHD in preschoolers, these prevalence rates should be quite reassuring both because they are comparable to (indeed, rather at the low end of) those reported for older children.

Fewer studies have examined the prevalence of subtypes of ADHD in preschoolers (Egger et al., 2006; Gadow et al., 2001; Gimpel & Kuhn, 2000; Pineda et al., 1999). The one study using a diagnostic interview reported lower rates for each subtype of ADHD than did the 3 studies using checklist measures. This may reflect the fact that checklists base the diagnosis on symptom counts alone and do not include other DSM criteria such as duration of symptoms. For example, in the PTRT study, there were no cases of pure inattentive type ADHD, while the studies using checklist measures reported rates from 1% to 2%. However, the lack of inattentive type ADHD was coupled with a rate of combined type ADHD of 1.5%, suggesting that preschoolers with clinically significant inattentive symptoms almost always present with clinically significant hyperactive-impulsive symptoms as well, which results in a combined type ADHD diagnosis. The low rates of inattentive type ADHD are consistent with studies in older children which have found that children with inattentive type ADHD were significantly older than children with other types of ADHD (Lahey et al., 1994, 2004).

The rate of hyperactive-impulsive type ADHD was 3.6% in 2 of the checklist studies, about double the rate of 1.8% found in the PTRT study. It is unclear why the rate of hyperactive-impulsive ADHD was 13.7% in the Columbian study but the rate was high as well for children aged 6–11 years (9.3%), suggesting that this might reflect a measurement bias. In the DSM-IV field trials, 3 quarters of the children who met criteria for hyperactive impulsive type ADHD were 6 years or younger (Lahey et al., 1994). These data raise the question whether the hyperactive impulsive criteria applied to young children are capturing children with a true disorder or an exuberant temperament that may be better controlled as the child develops. On the other hand, the relatively lower rates found in the other preschool studies suggest that the criteria are identifying children at the extreme end of the symptom distribution.

Clearly, further studies of the prevalence of the subtypes of ADHD in preschoolers are needed, particularly using diagnostic measures. Nonetheless, these initial studies suggest that hyperactive-impulsive and combined type ADHD are the 2 most common types of preschool ADHD with fewer children meeting criteria for pure inattentive type. Because studies in older children have shown significant familial clustering by ADHD subtype, as well as different genetic influences contributing to each subtype, it is critical that future studies of preschool ADHD assess the disorder by the DSM-IV subtypes (and the alternative latent class derived subtypes proposed by Todd et al., 2001), rather than by the presence or absence of ADHD overall alone (Rasmussen et al., 2002, 2004; Rohde et al., 2001).

Are there significant differences between toddlers (2- and 3-year-olds) and older preschoolers (4- and 5-year-olds) or between boys and girls?

Age effects

Few community studies of preschool ADHD report age differences within the preschool period. In the PTRT study, 3 inattention symptoms (does not listen, loses things, forgetful) increased linearly from ages 2 to 5 years. This may reflect (1) a true increase in inattention; (2) the fact that greater demands to listen, remember, and keep track of belongings are placed on children as they move from toddlerhood to the late preschool period, and/or; (3) the fact that inattentive symptom criteria are not developmentally sensitive and do not capture preschool problems with attention. Three other symptoms (difficulty doing things quietly, difficulty waiting turn, and talks excessively) showed an increase from age 2 to 3 years, then a gradual decrease, most likely reflecting increased inhibitory control as the child develops (Kochanska et al., 2000). The only ADHD diagnosis with significant age effects was combined type ADHD, which was more commonly diagnosed in older preschoolers, with no 2-year-olds and only 0.5% of 3-year-olds meeting criteria for this diagnosis (Egger et al., 2006). As noted above, the overall prevalence of preschool ADHD is similar to that reported for older children, with inattentive type probably being less common, and hyperactive-impulsive type probably occurring more commonly in younger children.

Gender effects

Table 3 presents the results on gender differences across the nonspecialty clinic studies of preschoolers with ADHD. In all but one, boys were more likely than girls to meet criteria for ADHD with about a 2:1 ratio. In population studies of older children, boys are 3 times more likely to meet criteria for ADHD than girls (Carlson, Tamm, & Gaub, 1997; Gaub & Carlson, 1997). Combined type ADHD also seems to be more common in preschool boys. In the PTRT study, boys were nearly 10 times as likely as girls to meet criteria for combined type ADHD. Results are split for hyperactive-inattentive type with some studies reporting higher rates in boys and others reporting no gender differences. The 3 studies using checklist measures did not find a gender difference for inattentive type ADHD. In a case-control study of ADHD in children aged 4 to 6 years, girls with ADHD were as impaired as boys with ADHD and had similar cognitive and academic deficits (Hartung et al., 2002).

Table 3
Table 3:
Gender effects in preschool ADHD, community or pediatric clinic studies

Are preschoolers with ADHD impaired?

The psychosocial impairment in relationships and functioning at home and at school, familiar from studies of older children with ADHD (Barkley, 1998; Barkley, Anastopoulos, Guevremont, & Fletcher, 1991), is already present during the preschool period. In the PTRT study, preschoolers with ADHD were 8 times more likely than children without ADHD to be impaired. On an impairment scale with a range of 0 to 30, children with ADHD had a mean score of 15.3 while those without ADHD had a mean score of 1.7. While the ADHD subtypes were all significantly associated with impairment, there were degrees of difference. Children with combined type ADHD were more severely impaired than those with hyperactive-impulsive type (combined: mean impairment score of 21.3; hyperactive-impulsive: mean impairment score of 10.2), and children whose ADHD was comorbid with other psychiatric disorders were more impaired than those with “pure” ADHD (comorbid ADHD: mean impairment score of 21.2; “pure” ADHD: mean impairment score of 8.1) (Egger et al., 2006). Gadow and Nolan have also reported that preschoolers with comorbid ADHD (ADHD + ODD) had significantly greater developmental deficits and greater peer conflicts than children with ADHD without ODD (Gadow & Nolan, 2002).

Preschoolers with ADHD are not impaired in just a single domain or relationship. In Lahey and colleagues' study of 126 preschoolers with ADHD and 126 matched controls, the children with ADHD (all subtypes) were significantly impaired on a range of measures obtained from both parents and teachers (Lahey et al., 1998). The children were impaired in global functioning, as well in specific social relationships, including those with peers, and in academic functioning (Lahey et al., 1998). A 3-year follow-up found that children with preschool ADHD continued to be functionally impaired across all settings and relationships (Lahey et al., 2004). Similarly, in the PTRT study, preschoolers with ADHD were significantly impaired across multiple relationships. More than half of parents felt that their child needed help for his or her behaviors, and a similar proportion were limited in their ability or willingness to take the child out to the store or a restaurant because of the child's inability to behave appropriately (Egger et al., 2006). Preschoolers with ADHD were also already impaired in their school/daycare functioning. More than 40% had already been suspended from school or daycare, compared with only 0.5% of children without ADHD. Nearly 16% had been expelled. All of those expelled had comorbid, combined type ADHD, which was associated with greater severity of impairment (Egger et al., 2006).

While the DSM-IV-TR 6-symptom cutpoint identifies a group of children who are significantly impaired, these children are different in degree, not type, from children with subthreshold ADHD. As in studies of older children, studies of preschool ADHD have found that subthreshold ADHD (the presence of ADHD symptoms below the diagnostic cutpoint of 6 symptoms) is associated with impairment (Egger et al., 2006; Keenan et al., 1997; Lahey et al., 1998, 2004), and a variety of neuropsychological deficits (E. Sonuga-Barke et al., 2002; E. J. Sonuga-Barke et al., 2003). In the PTRT study, there was a linear relationship between the number of ADHD symptoms and impairment. Each additional ADHD symptom doubled the chance that the child would also be impaired (Egger et al., 2006). Studies in older children have shown this same linear relationship between psychiatric symptoms and impairment, with children who missed the diagnostic threshold often as impaired as those who met full diagnostic criteria (Angold, Costello, Farmer, Burns, & Erkanli, 1999; Costello, Angold, & Keeler, 1999; Pickles et al., 2001). Lahey and colleagues found that preschoolers who missed formal diagnostic criteria for ADHD at their initial assessment had increased rates of ADHD over the next 3 years and were often more impaired than children who had initially met full diagnostic criteria (Lahey et al., 1998, 2004).

These data showing the strong association between DSM-IV-diagnosed ADHD in preschoolers and significant functional and psychosocial impairment are a strong indication of the validity of these diagnostic criteria for preschool children. These findings highlight the critical need for early evaluation and treatment, particularly when considered in light of studies in older children, which suggest that early onset of ADHD may be associated with worse outcomes including greater cognitive and language deficits, higher rates of psychiatric comorbidity, and greater psychosocial and academic impairment (McGee, Williams, & Feehan, 1992; Taylor, 1999; Willoughby, Curran, Costello, & Angold, 2000).

Do preschoolers with ADHD have other psychiatric disorders?

The rate of psychiatric comorbidity for preschoolers with ADHD is similar to that found in older children with ADHD (eg, Angold, Costello, & Erkanli, 1999; Ford, Goodman, & Meltzer, 2003). In Lavigne and colleagues' study, 90% of preschoolers with ADHD met criteria for at least 1 other DSM-III-R disorder (Lavigne et al., 1996). In the PTRT study, preschoolers with ADHD had an 8 times greater rate of ODD and nearly 30 times greater rate of CD. Preschoolers with ADHD had significantly higher rates of depression with an odds ratio of almost 9, yet there was no significant association between preschool ADHD and anxiety disorders (Egger et al., 2006). As we have already noted, preschoolers with comorbid ADHD have been found to be more impaired than those with “pure” ADHD (Egger et al., 2006; Gadow & Nolan, 2002; Lahey et al., 2004).

Do preschoolers with ADHD receive mental health evaluations and/or treatment?

Studies of preschoolers seen in mental health clinics have found high rates of ADHD (Wilens et al., 2002). However, the 2 studies of preschool psychopathololgy conducted in primary care pediatric clinics, rather than in tertiary mental health clinics, suggest that young children with ADHD, particularly toddlers, were not identified as needing help by their pediatricians, despite their high levels of impairment and high rates of parental concern. In the PTRT study, only 23% of preschoolers with any type of ADHD, 17% with combined type ADHD, and 8% with hyperactive-impulsive type ADHD had been referred for mental health evaluations (Egger et al., 2006). Although Lavigne and colleagues did not report rates of mental health referral by diagnosis, they also found that only about 25% of preschoolers with a DSM-III-R disorder had been referred for treatment (Lavigne et al., 1998). In that study, children with a diagnosis were more likely to be receiving mental health treatment if they were older (4 and 5 years old), white, and more impaired (Lavigne et al., 1998). Similarly, in the PTRT study, no 2- or 3-year-olds with ADHD had been referred for evaluation. In the PTRT study, only 2 children were on stimulants; both met criteria for ADHD. These data serve to counter the concern that preschoolers are being overprescribed stimulants, at least in this setting. Contrariwise, they suggest that preschoolers are being undertreated, because a number of studies have demonstrated the efficacy of stimulants for treating ADHD, at least in older preschoolers (Connor, 2002).

CONCLUSION

In this article, we examined the reliability and validity of the diagnosis of ADHD in preschoolers by addressing 7 questions. We conclude from this review that (1) ADHD can be reliably diagnosed in children as young as 2 years old; (2) most ADHD symptoms, when defined clearly, are not normative in preschool children; (3) the prevalence of ADHD in preschoolers is about 3.3%, a rate comparable to the low end of rates reported for older children. Combined type and impulsive-hyperactive subtypes are more common in this age group than inattentive type ADHD; (4) older preschoolers (4- and 5-year-olds) and preschool boys are more likely to meet criteria for ADHD than toddlers and preschool girls; (5) preschoolers with ADHD have clinically significant impairment across all relationships and settings; (6) preschool ADHD is highly comorbid with other psychiatric disorders, particularly disruptive behavior disorders and depression; (7) despite the validity of the diagnosis of preschool ADHD and the high degree of impairment, only about a quarter of preschoolers with ADHD are being referred for mental health evaluations and treatment. These findings suggest that pediatricians and child psychiatrists or psychologists should be alert for the signs of ADHD, and ensure that these young children receive a comprehensive psychiatric evaluation by a practitioner who has appropriate experience assessing young children. We also hope that these findings will lead to increased impetus for well-designed treatment studies for young children with ADHD, as well as for studies of early invention efforts that might decrease the psychosocial impact of ADHD in early and later childhood.

REFERENCES

Achenbach, T. M., & Rescorla, L. A. (2000). Manual for the ASEBA preschool forms and profiles: An integrated system of multi-informant assessment. Burlington: University of Vermont, Department of Psychiatry.
American Psychiatric Association. (2000). Diagnostic and statistical manual of mental disorders: Fourth edition text revision. Washington, DC: Author.
Angold, A., & Costello, E. J. (1996). Toward establishing an empirical basis for the diagnosis of oppositional defiant disorder. Journal of the American Academy of Child and Adolescent Psychiatry, 35, 1205–1212.
Angold, A., Costello, E. J., & Erkanli, A. (1999). Comorbidity. Journal of Child Psychology and Psychiatry, 40, 57–87.
Angold, A., Costello, E. J., Farmer, E. M. Z., Burns, B. J., & Erkanli, A. (1999). Impaired but undiagnosed. Journal of the American Academy of Child and Adolescent Psychiatry, 38, 129–137.
Angold, A., & Egger, H. L. (2004). Psychiatric diagnosis in preschool children. In R. DelCarmen-Wiggins & A. Carter (Eds.), Handbook of infant, toddler, and preschool mental health assessment (pp. 123–139). New York: Oxford University Press.
Barkley, R. A. (1998). Attention deficit-hyperactivity disorder—a handbook for diagnosis and treatment (2nd ed.). New York: Guilford Press.
Barkley, R. A., Anastopoulos, A. D., Guevremont, D. C., & Fletcher, K. E. (1991). Adolescents with ADHD: Patterns of behavioral adjustment, academic functioning, and treatment utilization. Journal of the American Academy of Child and Adolescent Psychiatry, 30, 752–761.
Briggs-Gowan, M. J., Horwitz, S. M., Schwab-Stone, M. E., Leventhal, J. M., & Leaf, P. J. (2000). Mental health in pediatric settings: Distribution of disorders and factors related to service use. Journal of the American Academy of Child and Adolescent Psychiatry, 39, 841–849.
Carlson, C. L., Tamm, L., & Gaub, M. (1997). Gender differences in children with ADHD, ODD, and co-occurring ADHD/ODD identified in a school population. Journal of the American Academy of Child and Adolescent Psychiatry, 36(12), 1706–1714.
Cohen, J. (1960). A coefficient of agreement for nominal scales. Educational Psychology Measurement, 20, 37–46.
Conners, C., Sitarenios, G., Parker, J., & Epstein, J. (1998). The revised Conners' Parent Rating Scale (CPRS-R): Factor structure, reliability, and criterion validity. Journal of Abnormal Child Psychology, 26(4), 257–268.
Connor, D. (2002). Preschool attention deficit hyperactivity disorder: A review of prevalence, diagnosis, neurobiology, and stimulant treatment. Journal of Developmental & Behavioral Pediatrics, 23, S1–S9.
Costello, E. J., Angold, A., & Keeler, G. P. (1999). Adolescent outcomes of childhood disorders: The consequences of severity and impairment. Journal of the American Academy of Child and Adolescent Psychiatry, 38, 121–128.
Costello, E. J., & Janiszewski, S. (1990). Who gets treated? Factors associated with referral in children with psychiatric disorders. Acta Psychiatrica Scandinavica, 81, 523–529.
DuPaul, G. J., Power, T. J., Anastopoulos, A. D., & Reid, R. (1998). Manual for the AD/HD rating scale–IV. New York: Guilford Press.
Earls, F. (1982). Application of DSM-III in an epidemiological study of preschool children. American Journal of Psychiatry, 139(2), 242–243.
Egger, H. L., & Angold, A. (2004). The Preschool Age Psychiatric Assessment (PAPA): A structured parent interview for diagnosing psychiatric disorders in preschool children. In R. DelCarmen-Wiggins & A. Carter (Eds.), Handbook of infant, toddler, and preschool mental assessment (pp. 223–243). New York: Oxford University Press.
Egger, H. L., Erkanli, A., Keeler, G., Potts, E., Walter, B., & Angold, A. (in press). The test-retest reliability of the preschool age psychiatric assessment. Journal of the American Academy of Child and Adolescent Psychiatry.
Egger, H. L., Keeler, G., & Angold, A. (2006). Preschool ADHD in prediatric primary care.
Ford, T., Goodman, R., & Meltzer, H. (2003). The British child and adolescent mental health survey 1999: The prevalence of DSM-IV disorders. Journal of the American Academy Child and Adolescent Psychiatry, 42, 1203–1211.
Gadow, K., & Nolan, E. (2002). Differences between preschool children with ODD, ADHD, and ODD+ADHD symptoms. Journal of Child Psychology and Psychiatry and Allied Disciplines, 43, 191–201.
Gadow, K. D., & Sprafkin, J. (1997). Early Childhood Symptom Inventory–4 norms manual. Stony Brook, NY: Checkmate Plus.
Gadow, K. D., Sprafkin, J., & Nolan, E. E. (2001). DSM-IV symptoms in community and clinic preschool children. Journal of the American Academy of Child and Adolescent Psychiatry, 40, 1383–1392.
Gaub, M., & Carlson, C. L. (1997). Gender differences in ADHD: A meta-analysis and critical review. Journal of the American Academy of Child and Adolescent Psychiatry, 36(8), 1036–1045.
Gimpel, G., & Kuhn, B. (2000). Maternal report of attention deficit hyperactivity disorder symptoms in preschool children. Child: Care, Health and Development, 26, 163–179.
Goodman, S. H., Lahey, B. B., Fielding, B., Dulcan, M., Narrow, W., & Regier, D. (1997). Representativeness of clinical samples of youths with mental disorders: A preliminary population-based study. Journal of Abnormal Psychology, 106, 3–14.
Hartung, C. M., Willcutt, E. G., Lahey, B. B., Pelham, W. E., Loney, J., Stein, M. A., et al. (2002). Sex differences in young children who meet criteria for attention deficit hyperactivity disorder. Journal of Clinical Child & Adolescent Psychology, 31(4), 453–464.
Keenan, K., Shaw, D. S., Walsh, B., Delliquadri, E., & Giovannelli, J. (1997). DSM-III-R disorders in preschool children from low-income families. Journal of the American Academy of Child and Adolescent Psychiatry, 36, 620–627.
Keenan, K., & Wakschlag, L. S. (2000). More than the terrible twos: The nature and severity of behavior problems in clinic-referred preschool children. Journal of Abnormal Child Psychology, 28, 33–46.
Kochanska, G., Coy, K. C., & Murray, K. T. (2001). The development of self-regulation in the first four years of life. Child Development, 72, 1091–1111.
Kochanska, G., Murray, K. T., & Harlan, E. T. (2000). Effortful control in early childhood: Continuity and change, antecedents, and implications for social development. Developmental Psychology, 36, 220–232.
Koot, H. M., & Verhulst, F. C. (1991). Prevalence of problem behavior in Dutch children aged 2–3. Acta Psychiatrica Scandinavica, 83(Supp. 367), 1–37.
    Lahey, B. B., Applegate, B., McBurnett, K., Biederman, J., Greenhill, L., Hynd, G. W., et al. (1994). DSM-IV field trials for attention deficit hyperactivity disorder in children and adolescents. American Journal of Psychiatry, 151(11), 1673–1685.
    Lahey, B. B., Pelham, W. E., Loney, J., Kipp, H., Ehrhardt, A., Lee, S. S., et al. (2004). Three-year predictive validity of DSM-IV attention deficit hyperactivity disorder in children diagnosed at 4–6 years of age. American Journal of Psychiatry, 161(11), 2014–2020.
    Lahey, B. B., Pelham, W. E., Stein, M. A., Loney, J., Trapani, C., Nugent, K., et al. (1998). Validity of DSM-IV attention-deficit/hyperactivity disorder for younger children. Journal of the American Academy of Child and Adolescent Psychiatry, 37, 695–702.
    Lavigne, J. V., Arend, R., Rosenbaum, D., Binns, H. J., Christoffel, K. K., Burns, A., et al. (1998). Mental health service use among young children receiving pediatric primary care. Journal of the American Academy of Child and Adolescent Psychiatry, 37(11), 1175–1183.
    Lavigne, J. V., Gibbons, R. D., Christoffel, K. K., Arend, R., Rosenbaum, D., Binns, H., et al. (1996). Prevalence rates and correlates of psychiatric disorders among preschool children. Journal of the American Academy of Child and Adolescent Psychiatry, 35(2), 204–214.
    McGee, R., Williams, S., & Feehan, M. (1992). Attention deficit disorder and age of onset of problem behaviors. Journal of Abnormal Child Psychology, 20(5), 487–502.
    Miller, L., Koplewicz, H., & Kelin, R. (1997). Teacher ratings of hyperactivity, inattention, and conduct problems in preschoolers. Journal of Abnormal Child Psychology, 25(2), 113–119.
    Pavuluri, M. N., Luk, S. L., & McGee, R. (1999). Parent reported preschool attention deficit hyperactivity: Measurement and validity. European Child & Adolescent Psychiatry, 8(2), 126–133.
      Pickles, A., Rowe, R., Simonoff, E., Foley, D., Rutter, M., & Silberg, J. (2001). Child psychiatric symptoms and psychosocial impairment: Relationship and prognostic significance. British Journal of Psychiatry, 79, 230–235.
      Pine, D. S., Alegria, M., Cook, E. H., Jr., Costello, E. J., Dahl, R. E., Koretz, D., et al. (2002). Advances in developmental science and DSM-V. In D. J. Kupfer, M. B. First, & D. A. Regier (Eds.), A research agenda for DSM-V (pp. 85–122). Washington, DC: American Psychiatric Association.
      Pineda, D., Ardila, A., Rosselli, M., Arias, B. E., Henao, G. C., Gomez, L. F., et al. (1999). Prevalence of attention-deficit/hyperactivity disorder symptoms in 4- to 17-year-old children in the general population. Journal of Abnormal Child Psychiatry, 27(6), 455–462.
      Rasmussen, E. R., Neuman, R. J., Heath, A. C., Levy, F., Hay, D. A., & Todd, R. D. (2002). Replication of the latent class structure of attention-deficit/hyperactivity disorder (ADHD) subtypes in a sample of Australian twins. Journal of Child Psychology & Psychiatry & Allied Disciplines, 43(8), 1018–1028.
      Rasmussen, E. R., Neuman, R. J., Heath, A. C., Levy, F., Hay, D. A., & Todd, R. D. (2004). Familial clustering of latent class and DSM-IV defined attention-deficit/hyperactivity disorder (ADHD) subtypes. Journal of Child Psychology & Psychiatry & Allied Disciplines, 45(3), 589–598.
      Richman, N., Stevenson, J. E., Graham, P. J., Ridgely, M. S., Goldman, H. H., & Talbott, J. A. C. (1974). Prevalence of behaviour problems in 3-year-old children: An epidemiological study in a London Borough. Journal of Child Psychology and Psychiatry, 16, 277–287.
        Rohde, L., Barbosa, G., Polancyzk, G., Eizirik, M., Rasmussen, E., Neuman, R., et al. (2001). Factor and latent class analysis of DSM-IV ADHD symptoms in a school sample of Brazilian adolescents. Journal of the American Academy Child and Adolescent Psychiatry, 46, 711–718.
        Shrout, P. E., & Fleiss, J. L. (1979). Intraclass correlations: Uses in assessing rater reliability. Psychological Bulletin, 86, 420–428.
        Sonuga-Barke, E., Dalen, L., Daley, D., & Remington, B. (2002). Are planning, working memory, and inhibition associated with individual differences in preschool ADHD symptoms? Developmental Neuropsychology, 21, 255–272.
        Sonuga-Barke, E. J., Dalen, L., & Remington, B. (2003). Do executive deficits and delay aversion make independent contributions to preschool attention-deficit/hyperactivity disorder symptoms? Journal of the American Academy of Child & Adolescent Psychiatry, 42(11), 1335–1342.
        Taylor, E. (1999). Developmental neuropsychopathology of attention deficit and impulsiveness. Development & Psychopathology, 11(3), 607–628.
        Todd, R. D., Rasmussen, E., Neuman, R., Reich, W., Hudziak, J., Bucholz, K., et al. (2001). Familiality and heritability of subtypes of attention deficit hyperactivity disorder in a population sample of adolescent female twins. American Journal of Psychiatry, 158, 1891–1898.
        Thomas, B. H., Byrne, C., Offord, D. R., & Boyle, M. H. (1991). Prevalence of behavioral symptoms and the relationship of child, parent, and family variables in 4- and 5-year-olds: Results from the Ontario Child Health Study. Journal of Developmental and Behavioral Pediatrics, 12, 177–184.
          Wilens, T. E., Biederman, J., Brown, S., Monuteaux, M., Prince, J., & Spencer, T. J. (2002). Patterns of psychopathology and dysfunction in clinically referred preschoolers. Journal of Developmental and Behavioral Pediatrics, 23, 531–537.
          Willoughby, M. T., Curran, P. J., Costello, E. J., & Angold, A. (2000). Implications of early versus late onset of attention-deficit/hyperactivity disorder symptoms. Journal of the American Academy of Child and Adolescent Psychiatry, 39, 1512–1519.
          Keywords:

          attention-deficit/hyperactivity disorder; DSM-IV; epidemiology; pediatric clinic sample; preschoolers; psychopathology

          ©2006Lippincott Williams & Wilkins, Inc.