Different Subtypes Of Attention-deficit/hyperactivity Disorder Research Articles

Impaired segregation of the attention deficit hyperactivity disorder related pattern in children

BackgroundInattention is a key characteristic of attention deficit hyperactivity disorder (ADHD). Specific brain abnormalities associated with this symptom form a discernible pattern related with ADHD in children (i.e., ADHD related pattern) in our earlier research. The developmental processes of segregation and integration may be crucial to ADHD. However, how brains reconfigure these processes of the ADHD related pattern in different subtypes of ADHD and across sexes remain unclear. MethodsNested-spectral partition method was applied to identify effects of subtype and sex on segregation and integration of the ADHD related pattern, using 145 ADHD patients and 135 typically developing controls (TDC) aged 7–14. Relationships between the measures and inattention symptoms were also investigated. ResultsChildren with ADHD exhibited lower segregation of the ADHD related pattern (p = 1.17 × 10−8) than TDCs. Only the main effect of subtype was significant (p = 1.14 × 10−5). Both ADHD-C (p = 2.16 × 10−6) and ADHD-I (p = 2.87 × 10−6) patients had lower segregation components relative to the TDC. Moreover, segregation components were negatively correlated with inattention scores. ConclusionsThis study identified impaired segregation in the ADHD related pattern of children with ADHD and found shared neural bases among different subtypes and sexes.

Response inhibition in children with different subtypes/presentations of attention deficit hyperactivity disorder: A near-infrared spectroscopy study.

Executive dysfunction in children with attention deficit hyperactivity disorder (ADHD) is thought to be closely related to the prefrontal cortex (PFC). However, there is controversy over the activation of the PFC in children with ADHD. Differences could be related to the subtype. Meanwhile, no study to date has used functional near-infrared spectroscopy (fNIRS) to explore the differences between subtypes. Thus, this study aimed to explore the activation of the PFC in children with different subtypes of ADHD during executive function task. Participants in this study include typically developing (TD) children (n = 28), ADHD-predominantly inattentive (ADHD-PI) (n = 39) and ADHD-combined (ADHD-C) (n = 24). To examine the executive function of ADHD, the Go/No-go task is chosen to assess the response inhibition function. The activation of PFC in all participants during the Go/No-go task was recorded by fNIRS. Meanwhile, behavioral data were also recorded. Both TD and ADHD children activated the right PFC [middle frontal gyrus (MFG) and inferior frontal gyrus (IFG)] during response inhibition. However, the range and degree of activation differed among these groups. Compared with TD children, those with ADHD-PI had a smaller extent of activation in the right PFC, and those with ADHD-C only had a tendency to enhance activation. In addition, children with ADHD-PI and ADHD-C had impaired activation of the temporal gyrus. Besides, compared with ADHD-C and TD, those with ADHD-PI also had impaired activation of the right precentral gyrus (PG), and the supplementary motor area (SMA). Compared with ADHD-PI, ADHD-C showed decreased activation of the right MFG. The activation of Ch34 (BA44, rPFC) in children with ADHD-PI and ADHD-C was negatively correlated with their clinical symptoms. The activation of the PFC in children with different subtypes of ADHD has both commonalities and differences. The degree of activation of the right PFC Ch34 in children with ADHD is negatively correlated with clinical symptoms. fNIRS could be served as a candidate hemodynamic biomarker for the diagnosis of ADHD.

School attendance problems in adolescent with attention deficit hyperactivity disorder.

Introduction: A link between having a neurodevelopmental disorder, such as attention deficit hyperactivity disorder (ADHD) and school absenteeism, has been found in previous studies. Why ADHD poses a risk for absenteeism remains unclear, and insight into the mechanisms of the association is needed. The aim of the present study was to investigate school attendance problems (SAP) and both the symptoms related and the perceived reasons for them, as reported by adolescents with ADHD (n = 95), compared with neurotypical adolescents (n = 1,474). Method: The current study (N = 1,569) was part of the School absence in Finland-project. SAPs were measured with the Inventory of School Attendance Problems (ISAP). The ISAP questionnaire contains a symptom scale (ISAP S) and a function scale (ISAP F), which shows if and how the symptoms impacts school attendance. A linear mixed effects model was used to analyze outcomes on the ISAP factors, controlling for background variables living status, gender, other diagnoses, highest level of education for the parent and age. Results: Results show that adolescents with ADHD had been more absent from school compared to neurotypical adolescents during the prior 12-weeks. Adolescents with ADHD showed significantly more symptoms of agoraphobia/panic, problems within the family and problems with parents than neurotypical peers. The symptoms separation anxiety, agoraphobia/panic, aggression, problems within the family and problems with parents more often were perceived as the reason for SAP (ISAP F). Discussion: The results are in line with our initial hypotheses and previous studies. Because of the low response rate on the ISAP F scale, the results regarding reasons for SAPs should be interpreted with caution. Future research could examine specific preventive actions of SAPs for adolescents with ADHD, and different subtypes of ADHD.

Quantitative tractography reveals changes in the corticospinal tract in drug-naïve children with attention-deficit/hyperactivity disorder

The specific role of the corticospinal tract with respect to inattention and impulsive symptoms in children with attention-deficit/hyperactivity disorder (ADHD) has been explored in the past. However, to our knowledge, no study has identified the exact regions of the corticospinal tract that are affected in ADHD. We aimed to determine comprehensive alterations in the white matter microstructure of the corticospinal tract and underlying neuropsychological substrates in ADHD. We recruited 38 drug-naïve children with ADHD and 34 typically developing controls. We employed a tract-based quantitative approach to measure diffusion parameters along the trajectory of the corticospinal tract, and we further correlated alterations with attention and response inhibition measures. Compared with controls, children with ADHD demonstrated significantly lower fractional anisotropy and higher radial diffusivity at the level of cerebral peduncle, and higher fractional anisotropy at the level of the posterior limb of the internal capsule in the right corticospinal tract only. As well, increased fractional anisotropy in the posterior limb of the internal capsule was negatively correlated with continuous performance test attention quotients and positively correlated with reaction time on the Stroop Colour–Word Test; increased radial diffusivity in the right peduncle region was positively correlated with omissions in the Stroop test. The sample size was relatively small. Moreover, we did not consider the different subtypes of ADHD and lacked sufficient power to analyze subgroup differences. Higher-order diffusion modelling is needed in future white matter studies. We demonstrated specific changes in the right corticospinal tract in children with ADHD. Correlations with measures of attention and response inhibition underscored the functional importance of corticospinal tract disturbance in ADHD.

Intelligibility of degraded speech and the relationship between symptoms of inattention, hyperactivity/impulsivity and language impairment in children with suspected auditory processing disorder

ObjectivesTo compare the sensitivity and specificity of Auditory Figure Ground sub-tests of the SCAN-3 battery, using signal to noise ratio (SNR) of +8 dB (AFG+8) and 0 dB (AFG0), in identifying auditory processing disorder (APD). A secondary objective was to evaluate any difference in auditory processing (AP) between children with symptoms of inattention versus combined sub-types of Attention Deficit Hyperactivity Disorder (ADHD). MethodsData from 201 children, aged 6 to 16 years (mean: 10 years 6 months, SD: 2 years 8 months), who were assessed for suspected APD were reviewed retrospectively. The outcomes of the SCAN-3 APD test battery, Swanson Nolan and Pelham-IV parental rating (SNAP-IV) and Children's Communication Checklist-2 (CCC-2) were analysed. ResultsAFG0 had a sensitivity of 56.3% and specificity of 100% in identifying children performing poorly in at least two of six SCAN-3 sub-tests or one of the two questionnaires, in contrast to 42.1% and 80% respectively for AFG+8. Impaired AP was mostly associated with symptoms of ADHD and /or language impairment (LI). LI was present in 92.9% of children with ADHD symptoms. Children with symptoms of combined ADHD plus LI performed significantly poorly (p < 0.05) compared to inattention ADHD plus LI in Filtered Words (FW) sub-test, but not in the rest of the SCAN-3 sub-tests. ConclusionSpeech in noise tests using SNR of 0 dB is better than +8 dB in assessing APD. The better FW performance of the inattention ADHD plus LI group can be speculated to be related to known difference in activity in a neural network between different sub-types of ADHD. The findings of the study and existing literature suggest that neural networks connecting the cerebral hemispheres, basal ganglia and cerebellum are involved in APD, ADHD and LI.

Motor Performance of Children With Attention Deficit Hyperactivity Disorder in Fourth to Sixth Grades: Differences Among Subtypes

Abstract Date Presented 3/30/2017 The aim of this study is to investigate motor performance in children with attention deficit hyperactivity disorder (ADHD). The findings regarding specific challenges in motor skills among children with different subtypes of ADHD will assist therapists to better understand and address these deficits in providing precise intervention. Primary Author and Speaker: Chien-Ying Yang Contributing Authors: Tsu-Hsin Howe

Comorbilidades asociadas al déficit de atención con hiperactividad

Introducción. Para diagnosticar trastorno por déficit de atención con hiperactividad (TDAH) en niños y adolescentes, es necesario identificar la coexistencia de comorbilidades afectivo-comportamentales que complejizan la evaluación y los procesos terapéuticos requeridos.Objetivo. Describir la comorbilidad afectivo-comportamental asociada a los diferentes subtipos del TDAH en niños y adolescentes de Manizales.Materiales y métodos. Muestra conformada por 141 participantes entre los 5 y 15 años de edad, escolarizados, diagnosticados con TDAH y coeficiente intelectual mayor o igual a 85. Estudio exploratorio-descriptivo, donde se analizaron registros de la entrevista neuropsiquiátrica internacional (MINI-KID).Resultados. El rango de edad con mayor asociación comórbida está entre los 8 y 11 años. El subtipo de TDAH predominante es el combinado. El 32% de la muestra presenta asociación comórbida afectivo-comportamental y los trastornos internalizantes tienen una representación del 68.8%, siendo la fobia simple específica la comorbilidad más preponderante en 80.6% de los casos.Conclusiones. La fobia simple específica es el trastorno internalizante más frecuente en el estudio en rangos de edad y subtipos de TDAH. El trastorno oposicionista desafiante es la comorbilidad externalizante predominante y exclusiva del subtipo de TDAH combinado.

P.1.a.018 - Gene expression in the brain of adult rats with behavioural alterations caused by neonatal exposure to the dipeptidyl peptidase-IV inhibitors diprotin A and sitagliptin

Attention deficit hyperactivity disorder (ADHD) is much more frequent in males than females, so several genes on the X chromosome (e.g., MAOA and MAOB) have been pursued as candidates for influencing risk for the disorder. HTR2C is also located on the X chromosome. In the current study, we examined the relationship between the C-759T and G-697C polymorphisms of HTR2C and ADHD in 488 Han Chinese families. Transmission Disequilibrium Test (TDT) analysis showed that the −759C allele, the −697G allele, and haplotype −759C/−697G were significantly over-transmitted to affected probands, while haplotypes −759C/−697C and −759T/−697C were under-transmitted. When families were divided into three subtypes according to the diagnosis of probands, the −697G allele and haplotype −759C/−697G were significantly over transmitted to ADHD-C probands, while haplotype −759T/−697C was under-transmitted to these individuals; however, no biased transmission of any allele or haplotype was observed for probands with ADHD-I, suggesting that different subtypes of ADHD have different genetic influences. Our findings highlight the need to explore the role of 5-HT2C receptor dysfunction in the pathogenesis of ADHD.

Sleep Parameters and Architecture in Children with Attention-Deficit/Hyperactivity Disorder: A Comparison with Typically Developing Peers and Across Subtypes

Sleep Parameters and Architecture in Children with Attention-Deficit/Hyperactivity Disorder: A Comparison with Typically Developing Peers and Across Subtypes Sleep problems in children with attention-deficit/hyperactivity disorder (ADHD) are common, yet poorly understood. Furthermore, little research has been done to compare sleep between subtypes of children with ADHD. The current study used polysomnography to investigate sleep architecture and sleep parameters in a rigorously diagnosed, medication-naive, age- and sex-matched sample of children with ADHD and their typically developing (TD) peers. Sleep was compared between 25 children with ADHD and 25 TD children between 6 and 12 years of age, and between children with different subtypes of ADHD. Results indicate that children with ADHD took longer to fall asleep than their TD peers; however, no other differences in sleep between the two groups were identified. Furthermore, no differences were found in any sleep parameters or sleep architecture variables between ADHD subtypes. Future research should continue to investigate sleep in children with ADHD by investigating additional sleep variables including nocturnal movement, stage shifts per hour, and sleep microstructure.

Pilot study: potential transcription markers for adult attention-deficit hyperactivity disorder in whole blood

Attention-deficit hyperactivity disorder (ADHD) is a common behavioural disorder that affects not only children and adolescents but also adults; however, diagnosis of adult ADHD is difficult because patients seem to have reduced externalized behaviour. ADHD is a multifactorial disorder in which many genes, all with small effects, are thought to cause the disorder in the presence of unfavourable environmental conditions. Therefore, in this pilot study, we explored the expression profile of a list of previously established candidate genes in peripheral blood samples from adult ADHD subjects (n = 108) and compared these results with those of healthy controls (n = 35). We demonstrate that combining the gene expression levels of dopamine transporter (SLC6A3), dopamine D5 receptor, tryptophan hydroxylase-1, and SNAP25 as predictors in a regression model resulted in sensitivity and specificity of over 80 % (ROC: max R(2) = 0.587, AUC = 0.917, P < 0.001, 95 % CI: 0.900-0.985). In conclusion, the combination of these four genes could represent a potential method for estimating risk and could be of diagnostic value for ADHD. Nevertheless, further investigation in a larger independent population including different subtypes of ADHD (inattentive, hyperactive, or combined type) patients is required to obtain more specific sets of biomarkers for each subtype as well as to differentiate between child, adolescent, and adulthood forms.

Self-perception in children with attention-deficit/hyperactivity disorder

The Attention-Deficit/ Hyperactivity Disorder (ADHD) is a disorder reflected by significant deficits in multiple domains, interfering with the self-perception and self-esteem of children. However, international studies that assess the relationship between ADHD and self-perception have come to contradictory results, and, in Portugal, this is still a little investigated subject. The present work aims to assess the self-perception in different areas (scholastic competence, athletic competence, physical appearance, behaviour and social acceptance) in children with ADHD, comparing them with a control group. This study also tries to analyze whether the subtype of ADHD could interfere differently with the self-perception. We studied 43 children who had a clinical diagnosis of ADHD and 59 children without this diagnosis. Children with ADHD were divided into different subtypes in view of the revised Conners Scales (Portuguese version). The children completed the Self-Perception Profile for Children. From the analysis of averages comparison, it was found that the group of children with ADHD showed lower values in the different domains of self-perception, with a statistically significant difference in scholastic and behaviour self-perceptions. Regarding the different subtypes of ADHD, the results were not statistically significant, but indicated that children perceive themselves differently depending on the subtype. This discrepancy was most evident in scholastic competence where children predominantly inattentive considered themselves less able than the others. In this study, as in the international literature, children with ADHD have an scholastic and behavioural self-perception statistically inferior to other children of the same age, needing every possible support for a healthy emotional development.

Effect of Methylphenidate on Intelligence Quotient Scores in Chinese Children With Attention-Deficit/Hyperactivity Disorder

Stimulants are the most effective drugs for attention-deficit/hyperactivity disorder (ADHD) symptoms. The purpose of this study was to explore the intervention effect of methylphenidate, a commonly used stimulant, on cognitive performance in ADHD children and whether the effect is associated with age, sex, different subtypes of ADHD, and drug dosage. Children with ADHD were divided into the following subtypes: combined type, predominantly inattentive type, and hyperactive/impulsive type. The intervention group consisted of 159 children treated with methylphenidate, and the control group consisted of 78 untreated patients. All 237 subjects were given a Wechsler Intelligence Scale for Children--Revised test at baseline, and 6 months later, they were retested. The scores of Verbal Intelligence Quotient (IQ) test, Performance IQ (PIQ) test, Full Scale IQ (FSIQ) test, and subtests were compared before and after the intervention. At baseline, scores were not statistically different between the 2 groups. After 6 months, PIQ and FSIQ scores of intervention group were higher than those of the control group (P < 0.05). Compared with baseline scores, the intervention group, but not the control group, showed significant increases in Verbal IQ (P < 0.05), PIQ (P < 0.01), and FSIQ (P < 0.01). In the intervention group, the 5 subtests scores of PIQ improved significantly (P < 0.01). In the control group, none of the scores from the subtests showed statistical differences. Furthermore, there was no statistical difference between the change of IQ scores and children's age, sex, different subtypes of ADHD, and drug dosage. Methylphenidate can enhance cognitive performance in ADHD patients thus evaluating their IQ scores, although the effect size seems to be relatively small. The result should not be indicated as an increase in intelligence.

Community survey of attention-deficit / hyperactivity disorder among primary school pupils in Benin City, Nigeria

Attention-deficit/hyperactivity disorder (ADHD) is a common childhood neuro-developmental condition with early onset. ADHD affects children worldwide. However, there is a variation in the prevalence across different countries. In Nigeria, there is paucity of information on the prevalence. To provide the relevant information, a cross-sectional study was conducted between February and August 2006 among 1473 public primary school pupils aged 6-12 years selected systematically among pupils in Egor Local Government Area of Edo State. All the 1473 pupils were screened with the Disruptive Behavior Disorder (DBD) Rating Scale to identify children who had ADHD symptoms as contained in the DSM -IV. Such children were compared with randomly selected controls. The academic records of both the groups were also compared. The prevalence of ADHD was 7.6%. The prevalence was higher in boys (9.4%) when compared to girls (5.5%) (P = 0.003). Of the three different subtypes of ADHD, the predominantly inattentive subtype (ADHD-I) was the most prevalent (47.3% of the ADHD population) followed by the combined type (ADHD-C; 31.3%), while the least prevalent was the hyperactive/impulsive subtype (ADHD-HI; 21.4%). There was no statistically significant difference in the prevalence of ADHD amongst the different age cohorts. The prevalence of ADHD was relatively high when compared to the figures available for other countries. For this reason, there is a need to pay increased attention to this condition in Nigeria. Community screening under the umbrella of the School Health Program could be of assistance.

Executive and intellectual functions in attention-deficit/hyperactivity disorder with and without comorbidity

Background: Recent neuropsychological theories have targeted deficient executive functions (EF) as the main characteristic of Attention Deficit Hyperactivity Disorder (ADHD), but the nature of the neuropsychological deficits remains elusive and findings are heterogeneous. In particular, it is still unclear whether ADHD subtypes and comorbidity affect intellectual and executive functioning, because large variability has been found in different patient populations. Furthermore, the role of IQ in EF deficits in individuals with ADHD has been debated. Methods: The aim of the present research was to study in detail the relationship between EF, ADHD subtypes, and comorbid diagnosis by taking into account the potential role of IQ. For this purpose, 23 children (aged from 5 to 16 years; 20 males and 3 females) with a diagnosis of ADHD were selected. Results: Data show no differences between children with different subtypes of ADHD on measures of EF, but they evidence differences on EF measures in children with different comorbidities (internalizing versus externalizing disorder). Namely, compared to the internalizing disorders group, the children with externalizing disorders obtained significantly lower scores on different measures of EF (i.e., verbal working memory and categorical fluency), but these differences were strictly dependent on IQ level. Conclusions: Comorbidity patterns, rather than ADHD subtypes, appear to be more valid for defining the neuropsychological features of the ADHD endophenotype. Moreover, general intelligence seems to play a substantial role in the cognitive processes underling the disorder, especially in relation to externalizing aspects.

Platelet monoamine oxidase activity in children with attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a highly heritable developmental disorder characterized by symptoms of impulsivity, hyperactivity and/or inattention, and associated with structural and biochemical abnormalities in cortical and limbic structures innervated by dopamine, noradrenalin and serotonin. The enzyme monoamine oxidase, type B (MAO-B), is expressed in platelets, and metabolizes endogenous amines. Its activity has been proposed to represent a peripheral marker of various traits and forms of psychopathology. This study evaluated platelet MAO activity with a spectrofluorimetric method in 72 boys and 12 girls with predominantly hyperactive, predominantly inattentive, and combined subtype of ADHD (DSM-IV criteria), and in 64 control children. The results showed significantly lower platelet MAO activity in children with hyperactive, inattentive, and combined subtype of ADHD than in control children. There was no significant association between platelet MAO activity and gender or age. The limitation of the study was in the small sample of girls with ADHD ( N = 12), and in the determination of only one peripheral marker. In line with hypotheses of lower platelet MAO activity in different types of psychopathology, children with different subtypes of ADHD had significantly lower platelet MAO-B activity than control children.

Screening signs of auditory processing problem: Does it distinguish attention deficit hyperactivity disorder subtypes in a clinical sample of children?

The aim of this study is to survey parental report of screening signs of auditory processing problem in attention deficit hyperactivity disorder (ADHD) children and its association with gender and co-morbidity with oppositional defiant disorder (ODD) and separation anxiety (SAD). 104 children and adolescents referrals to the child and adolescent psychiatry clinic were interviewed. The auditory processing problem checklist asked parents to indicate their child's reaction to sounds. It screens signs of two aspects of auditory processing problem including hypersensitivity to sounds (HES) (or auditory defensiveness) and hyposensitivity to sound (HOS) (under-registers). The mean age of the children was 8.5 (SD=1.8). Children with ODD had significantly higher HES, HOS and the whole checklist mean scores. Subtypes of ADHD were not associated with the auditory processing problems. ADHD children with ODD are likely to be at a significant risk for manifesting both of the auditory processing problem including defensiveness and auditory hyposensitivity to sounds. Auditory processing problems do not differentiate different subtypes of ADHD.

Association between polymorphisms in serotonin 2C receptor gene and attention-deficit/hyperactivity disorder in Han Chinese subjects

Attention deficit hyperactivity disorder (ADHD) is much more frequent in males than females, so several genes on the X chromosome (e.g., MAOA and MAOB) have been pursued as candidates for influencing risk for the disorder. HTR2C is also located on the X chromosome. In the current study, we examined the relationship between the C-759T and G-697C polymorphisms of HTR2C and ADHD in 488 Han Chinese families. Transmission Disequilibrium Test (TDT) analysis showed that the −759C allele, the −697G allele, and haplotype −759C/−697G were significantly over-transmitted to affected probands, while haplotypes −759C/−697C and −759T/−697C were under-transmitted. When families were divided into three subtypes according to the diagnosis of probands, the −697G allele and haplotype −759C/−697G were significantly over transmitted to ADHD-C probands, while haplotype −759T/−697C was under-transmitted to these individuals; however, no biased transmission of any allele or haplotype was observed for probands with ADHD-I, suggesting that different subtypes of ADHD have different genetic influences. Our findings highlight the need to explore the role of 5-HT2C receptor dysfunction in the pathogenesis of ADHD.

Gene–environment interaction in hyperkinetic conduct disorder (HD + CD) as indicated by season of birth variations in dopamine receptor (DRD4) gene polymorphism

Recently, an interaction between season of birth and the expression of candidate genes has been suggested. Season of birth variations in tryptophan hydroxylase (TPH), the serotonin transporter (5-HTTLPR) and the dopamine D4 receptor (DRD4) gene polymorphisms are different for affective disorders and schizophrenia. The DRD4 gene has been postulated as a candidate gene for attention-deficit-hyperactivity disorder (ADHD), equivalent to hyperkinetic disorder (HD). The seven-repeat long variant of this gene (DRD4*7) in comparison to the short repeat variants of the DRD4 gene polymorphism, has been found to be associated with ADHD. A seasonal pattern of birth has also been proposed for different subtypes of ADHD. Therefore, in a subgroup of children with HD and conduct disorder (CD) and in healthy controls, we investigated a possible association between the DRD4*7 allele and HD + CD in association with the season of birth. Supporting this hypothesis, we found an interaction between the seasons of birth and the expression of the DRD4 candidate gene in children with HD + CD as well as in controls, which differ significantly from each other. Depending on the season of birth, children carrying the DRD4*7R allele showed different relative risks for developing HD + CD.

Electroencephalogram differences in two subtypes of Attention‐Deficit/Hyperactivity Disorder

This study investigated EEG differences between children with two subtypes of Attention-Deficit/Hyperactivity Disorder (ADHD) and normal control subjects. EEG was recorded during an eyes-closed resting condition and Fourier transformed to provide absolute and relative power estimates for the delta, theta, alpha, and beta bands, and the mean frequency for each band was calculated. Ratio coefficients were also calculated between frequency bands. Mean group differences were found in the theta, alpha, and beta bands between all three groups. Similarly, differences were found between all three groups for the theta/alpha and theta/beta ratios and for the mean frequency of the total EEG. These results support a model of ADHD resulting from a developmental deviation rather than a maturational lag in the central nervous system. Differences between the clinical groups in frontal activity suggest that different neuroanatomical systems are involved in the different subtypes of ADHD.