An Investigation Into Etiological Pathways of DCD and ADHD Using a Monozygotic Twin Design

Prevalence and associated factors of developmental coordination disorder in primary school children

Developmental coordination disorder (DCD) is one of the most frequently observed movement disorders in childhood, yet data on its prevalence are still unclear. This two-stage epidemiological study aims to determine the prevalence of DCD in children between 6 and 10 years of age according to the diagnostic criteria of the Diagnostic and Statistical Manual of Mental Disorders-5 (DSM-5). In the first stage, the Developmental Coordination Disorder Questionnaire (DCDQ'07) were given to primary school students. In the second stage, clinicians conducted psychiatric interviews with children who had an indication of DCD or were suspected of having DCD according to the DCDQ'07 and their parents. The interviews utilized the DSM-5 diagnostic criteria for DCD and applied the Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime Version-Turkish Adaptation (K-SADS-PL) and Wechsler Intelligence Scale for Children (WISC-R) to identify co-occurring disorders. In the first stage, 2,306 children were evaluated, and 205 were invited for a clinical interview. Of the 198 children interviewed, 49 met the diagnostic criteria for DCD. The prevalence of DCD in Türkiye was found to be 2.1%. High parental education level, a previous psychiatric admission of the child, mother's postpartum depressive symptoms, co-occurring disorders, and co-occurring attention deficit hyperactivity disorder (ADHD) were significantly more common among children with DCD. Potentially associated factors were evaluated by logistic regression analysis. The mother's education level, the presence of postpartum depressive symptoms, and co-occurring disorders in the child were found to be associated factors.

The ability to maintain rhythm is predictive of ADHD diagnosis and profile

Attention deficit hyperactivity disorder (ADHD) is the most prevalent neuropsychiatric disorder in the world. Currently, the diagnosis is based mainly on interviews, resulting in uncertainties in the clinical assessment. While some neuropsychological tests are used, their specificity and selectivity are low, and more reliable biomarkers are desirable. Previous research indicates that ADHD is associated with morphological changes in the cerebellum, which is essential for motor ability and timing. Here, we compared 29 children diagnosed with ADHD to 96 age-matched controls on prism adaptation, eyeblink conditioning, and timed motor performance in a finger tapping task. Prism adaptation and timing precision in the finger tapping task, but not performance on eyeblink conditioning, differed between the ADHD and control groups, as well as between children with and without Deficits in Attention, Motor control, and Perception (DAMP) - a more severe form of ADHD. The results suggest finger tapping can be used as a cheap, objective, and unbiased biomarker to complement current diagnostic procedures.

Effects of Comorbid Developmental Coordination Disorder and Symptoms of Attention Deficit Hyperactivity Disorder on Physical Activity in Children Aged 4-5 Years

Developmental coordination disorder (DCD) is often comorbid with attention-deficit/hyperactivity disorder (ADHD). While children with DCD engage in less moderate-to-vigorous physical activity (MVPA) compared to typically developing (TD) children, research pertaining to how ADHD affects this relationship is limited. We investigated the effect of ADHD on MVPA among children at risk for DCD (DCDr). 507 children aged 4-5 years (DCDr = 233, TD = 274) participated. Motor skills were assessed using the Movement Assessment Battery for Children-2nd edition (DCDr; ≤ 16th percentile), ADHD symptoms were assessed using the Child Behaviour Checklist, and Actigraph accelerometers measured MVPA over seven days. DCD did not negatively affect MVPA, however, after adjusting for ADHD symptoms, the effect of DCD became significant and was driven by symptoms of inattention. Symptoms of ADHD may be suppressing the negative effects of DCD on MVPA, highlighting the importance of assessing and controlling for ADHD symptoms in this population.

The adult developmental coordination disorders/dyspraxia checklist - German: adapted factor structure for the differentiation of DCD and ADHD

BACKGROUND

The Adult Dyspraxia/DCD Checklist (ADC) is the only existing self-report questionnaire to screen adults for potential Developmental Coordination Disorder (DCD). It was developed in English and Hebrew, however, its factor structure has not yet been assessed.

AIMS

The goals of the current study were to (1) develop and refine a German translation, (2) explore the emerging factors in a new and restructured ADC in German and apply this structure to an English ADC, and (3) explore its potential for distinguishing DCD versus ADHD.

METHOD

In a series of three studies, we assessed comprehensibility of the translation and revisions to the questionnaire. We further examined subscale structure in a sample of N = 148 individuals with DCD or ADHD and retested it in an English-speaking sample (N = 134).

RESULTS

Overall, we found decent reliability and construct validity for the German ADC. Three components emerged with themes of fine motor coordination, gross motor coordination, and executive functions which had strong psychometric properties in German and English.

CONCLUSIONS

The studies collectively highlight the German translation is effective and has strong potential to differentiate DCD and ADHD. Most notably, there are unique symptom profiles in motor and executive functioning difficulties in adults with DCD or ADHD. WHAT THIS PAPER ADDS?: The translation and preliminary validation of the German ADC in this study has the potential to screen for probable DCD in German-speaking adults for the first time. In addition, the new subscale structure can be generalized to the English version as well and allows for the potential assessment of several key symptomatic patterns in the realms of gross motor, fine motor, and executive functioning skills that differ between adults with DCD and ADHD. These differences were fairly consistent between English- and German-speaking samples, indicating potential for more widespread assessment and differentiation of DCD and ADHD.

Developmental Coordination Disorder (DCD): Relevance for clinical psychologists in Europe

Background

Developmental Coordination Disorder (DCD) is a common neurodevelopmental disorder primarily characterized by fine and gross motor coordination difficulties. Yet, many aspects remain unclear regarding the clinical presentation of secondary symptoms and their implications for Clinical Psychology. Therefore, the purpose of this review is to provide an update about the current understanding of DCD for clinical psychologists and psychotherapists across Europe, particularly based on new insights stemming from the last decade of research.

Method

We provide a narrative review of articles published in the last decade on the topic of DCD, and relevant aspects to clinical psychologist, including lesser known aspects of DCD (e.g., executive functions, psychological consequences, and adult DCD).

Results

DCD is a highly prevalent, disruptive, and complex disorder, which should be investigated further in many areas (e.g., co-occurrence to ADHD). Existing evidence points toward a key role of executive functioning difficulties at all ages. Most patients report secondary psychological problems, but little headway has been made in examining the effectiveness of psychotherapy for DCD.

Conclusions

Insights and remaining research gaps are discussed. It is critical for psychologists and clinical researchers to raise awareness for DCD, take note of the growing literature, and foster continued interdisciplinary approaches to research and treatment of DCD.

Awareness about Developmental Coordination Disorder (DCD) is low among some European Psychologists.

Growing knowledge about DCD should be disseminated among psychotherapists.

Association between General Movements Assessment and Later Motor Delay (excluding Cerebral Palsy) in Low-Birth-Weight Infants

The general movements (GMs) assessment is useful for the prediction of cerebral palsy (CP) and other developmental disorders. Developmental coordination disorder (DCD) is highly prevalent in low-birth-weight (LBW) infants. We investigated the association between aberrant GMs during early infancy and later motor development in LBW infants. The study included infants who fulfilled the following criteria: GMs assessed at 9–20 weeks post-term age; developmental quotient (DQ) assessed at 3 years of age using the Kyoto Scale; intelligence quotient (IQ) assessed at 6 years of age. Participants with normal IQs at 6 years of age without a diagnosis of CP (14 males and 37 females, 23–36 weeks gestation with birth weights of 492–1498 g) were categorized into normal (n = 39) and aberrant (n = 12) groups based on GMs assessment; DQ was compared between the groups. We investigated the items in the DQ assessment and found that the infants in the aberrant group were more frequently unable to perform. Infants in the aberrant group showed a significantly lower DQ in the ‘postural-motor domain’, and were more frequently unable to ‘climb the stairs with alternating legs’ and ‘Jump from a 15–20 cm platform’. This study highlights that GMs aberrancy in early infancy is associated with a delayed gross motor development, even in children with a typical development. The GMs assessment may be useful for the prediction of DCD.

Nonshared environmental factors in the aetiology of autism and other neurodevelopmental conditions: a monozygotic co-twin control study

Background

A significant proportion of variation in likelihood of neurodevelopmental conditions (NDCs) has been attributed to nonshared environmental (NSE) factors, although it remains unclear which NSE factors pose specific risks for certain NDCs.

Methods

A monozygotic co-twin design was applied in a sample of 224 twins (mean age = 17.70 years, SD = 6.28) controlling for confounders such as genes and shared environment. Generalized estimating equation models were fitted, using perinatal and postnatal indications of NSEs as exposure, operationalized both as separate risk factors and as cumulative risk loads. Categorical and dimensional operationalizations of autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), intellectual disability and other NDCs were used as outcomes.

Results

Birth weight discordance was associated with dimensional autism and ADHD for the smaller twin, and medication during infancy was associated with dimensional autism. Among postnatal factors scarlet fever during early childhood was associated with lower IQ. Especially autism was associated with a greater cumulative perinatal or postnatal risk load.

Limitations

When exploring the associations between each condition and specific NSEs the risk of being statistically underpowered increases. Hence, we limit the reported findings on specific indicators of NSEs to trait levels and present descriptive data for categorical NDCs.

Conclusions

The findings support previous research by indicating an association between exposure to perinatal and postnatal risks and subsequent NDCs within twin pairs and suggest that autism may be especially linked to accumulative early environmental risks. The findings are potentially important for developmental outcomes prognoses and may inform targeted prevention and early interventions.

Investigating the Association Between Exposure to Second Hand Smoke in utero and Developmental Coordination Disorder

Background: Developmental coordination disorder (DCD) and attention-deficit hyperactivity disorder (ADHD) are highly comorbid in children. There is evidence linking second hand smoke (SHS) exposure in utero to ADHD; however, it's relation to DCD is unknown. The purpose of this study was to examine the effect of SHS exposure in utero in children with and without DCD. Methods: This study was a cross-sectional examination of 122 children from the District School Board of Niagara (72 males, 50 females, M _age = 12.9 years) who were part of a larger, prospective cohort study. Participants were assessed for motor proficiency and intelligence and were screened for symptoms of ADHD using the Bruininks-Oseretsky Test of Motor Proficiency-Short Form, the Kaufman Brief Intelligence Inventory, 2nd edition, and the Conners' Parent Rating Scales-Revised: Short Form, respectively. Parent questionnaires were used to determine SHS exposure in utero as either yes or no. Multinomial logistic regression was used to examine the relationship between SHS exposure and DCD risk. Results: Children exposed to SHS in utero were significantly more likely to be at high risk for DCD than children who were not (OR = 3.33, p = 0.004), and children exposed to SHS in utero were more likely to be at moderate risk for DCD in the presence (OR = 3.57, p = 0.025) or absence of ADHD (OR = 2.38, p = 0.042). However, statistical adjustment for birth weight, socioeconomic status, age, and sex reduced this effect to non-significance in the moderate risk group. Conclusion: Results suggest exposure to SHS during pregnancy increases the chances of a child developing high risk DCD. While SHS exposure may increase DCD risk with and without ADHD, this effect may be explained by covariates and confounding factors. Further study is needed to examine the mechanisms linking SHS exposure in utero to motor coordination problems in children.

Impact of Cerebral Visual Impairments on Motor Skills: Implications for Developmental Coordination Disorders

Cerebral visual impairment (CVI) has become the primary cause of visual impairment and blindness in children in industrialized countries. Its prevalence has increased sharply, due to increased survival rates of children who sustain severe neurological conditions during the perinatal period. Improved diagnosis has probably contributed to this increase. As in adults, the nature and severity of CVI in children relate to the cause, location and extent of damage to the brain. In the present paper, we define CVI and how this impacts on visual function. We then define developmental coordination disorder (DCD) and discuss the link between CVI and DCD. The neuroanatomical correlates and aetiologies of DCD are also presented in relationship with CVI as well as the consequences of perinatal asphyxia (PA) and preterm birth on the occurrence and nature of DCD and CVI. This paper underlines why there are both clinical and theoretical reasons to disentangle CVI and DCD, and to categorize the features with more precision. In order to offer the most appropriate rehabilitation, we propose a systematic and rapid evaluation of visual function in at-risk children who have survived preterm birth or PA whether or not they have been diagnosed with cerebral palsy or DCD.

Coordination difficulty and internalizing symptoms in adults: A twin/sibling study

Increased anxiety and depression symptoms have been reported in individuals with neurodevelopmental disorders, and have been found to be associated with motor coordination difficulties, but little is known about the etiology of these associations. This study aimed to assess genetic, shared (making twins/siblings alike) and non-shared (individual-specific) environmental influences on the association between poor coordination and symptoms of anxiety and depressed mood using a sample of adult twin and sibling pairs. Participants were asked about their coordination skill and anxiety and depression symptoms. About half of the variance in coordination difficulty was explained by familial (combined genetic and shared environmental) influences, with the remaining variance explained by non-shared environmental influences. Phenotypic associations between coordination and anxiety (r=.46) and depression symptoms (r=.44) were largely underpinned by shared familial liability for the three traits. Non-shared environment accounted for about a third of the phenotypic association. Results suggest that both familial and non-shared environmental influences play a role in the etiology of coordination difficulty and its association with internalizing symptoms. The current study highlights that both biological and environmental pathways shared between these symptoms should be examined in future research to inform prevention and treatment approaches in clinical settings.

Developmental coordination disorder: core sensori-motor deficits, neurobiology and etiology

Among developmental disorders, DCD is one of the least studied and less understood one (Bishop, 2010). This review summarizes the current understanding of developmental coordination disorder in neuropsychology with a focus mainly on high level sensorimotor impairments, its etiology and its neural bases. We summarize these core deficits in the framework of an influent motor control model (Blakemore et al., 2002). DCD has several environmental risk factors which probably interplay with genetic factors but those have not been sufficiently identified. High-level sensori-motor deficits are probably multifactorial in DCD and involve predictive coding deficits as well as weaknesses in perceptual and sensory integration. At the brain level, DCD is associated with impaired structure and functions within the motor network. Throughout the review we highlight exciting new findings as well as potential future lines of research to provide a more comprehensive understanding of this disorder.

The Prevalence of ADHD in a Population-Based Sample

OBJECTIVE

Few studies of ADHD prevalence have used population-based samples, multiple informants, and Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV) criteria. Moreover, children who are asymptomatic while receiving ADHD medication often have been misclassified. Therefore, we conducted a population-based study to estimate the prevalence of ADHD in elementary school children using DSM-IV criteria.

METHOD

We screened 7,587 children for ADHD. Teachers of 81% of the children completed a DSM-IV checklist. We then interviewed parents using a structured interview (DISC). Of these, 72% participated. Parent and teacher ratings were combined to determine ADHD status. We also estimated the proportion of cases attributable to other conditions.

RESULTS

Overall, 15.5% of our sample met DSM- (4th ed.; text rev., DSM-IV-TR) criteria for ADHD (95% CI [14.6%, 16.4%]); 42% of cases reported no previous diagnosis. With additional information, other conditions explained 9% of cases.

CONCLUSION

The prevalence of ADHD in this population-based sample was considerably higher than 3% to 7%. To compare study results, the DSM criteria need standardization.

Developmental coordination disorder and overweight and obesity in children: a systematic review

Children with developmental coordination disorder (DCD) find themselves less competent than typically developing children with regard to their physical abilities and often experience failure. They are therefore likely to avoid physical activity. Physical inactivity is considered an important risk factor for developing overweight and obesity. The aim of this study is to assess the association between DCD and overweight and obesity in children and whether this association is influenced by age and/or gender. Six electronic databases were systematically searched. Titles and abstracts were screened for relevance. Remaining studies were subjected to full paper review. The quality of the included articles was assessed and relevant data were extracted for comparison. The search yielded 273 results. Twenty-one studies, based on 10 cohorts, were included. Participants' ages ranged from 4 to 14 years. In all cohorts, children with DCD had higher body mass index scores, larger waist circumference and greater percentage body fat compared with controls. Seven studies assessed the effect of gender and four studies provided information on the effect of age. Children with DCD seem to be at greater risk for overweight and obesity. This risk may be higher for boys and seems to increase with age and with the severity of motor impairment.

The Roots of Autism and ADHD Twin Study in Sweden (RATSS)

Neurodevelopmental disorders affect a substantial minority of the general population. Their origins are still largely unknown, but a complex interplay of genetic and environmental factors causing disturbances of the central nervous system's maturation and a variety of higher cognitive skills is presumed. Only limited research of rather small sample size and narrow scope has been conducted in neurodevelopmental disorders using a twin-differences design. The Roots of Autism and ADHD Twin Study in Sweden (RATSS) is an ongoing project targeting monozygotic twins discordant for categorical or dimensional autistic and inattentive/hyperactive-impulsive phenotypes as well as other neurodevelopmental disorders, and typically developing twin controls. Included pairs are 9 years of age or older, and comprehensively assessed for psychopathology, medical history, neuropsychology, and dysmorphology, as well as structural, functional, and molecular brain imaging. Specimens are collected for induced pluripotent (iPS) and neuroepithelial stem cells, genetic, gut bacteria, protein-/monoamine, and electron microscopy analyses. RATSS's objective is to generate a launch pad for novel surveys to understand the complexity of genotype-environment-phenotype interactions in autism spectrum disorder and attention-deficit hyperactivity disorder (ADHD). By October 2013, RATSS had collected data from 55 twin pairs, among them 10 monozygotic pairs discordant for autism spectrum disorder, seven for ADHD, and four for other neurodevelopmental disorders. This article describes the design, recruitment, data collection, measures, collected pairs' characteristics, as well as ongoing and planned analyses in RATSS. Potential gains of the study comprise the identification of environmentally mediated biomarkers, the emergence of candidates for drug development, translational modeling, and new leads for prevention of incapacitating outcomes.

Genome-wide association study of motor coordination problems in ADHD identifies genes for brain and muscle function

OBJECTIVES

Motor coordination problems are frequent in children with attention deficit/hyperactivity disorder (ADHD). We performed a genome-wide association study to identify genes contributing to motor coordination problems, hypothesizing that the presence of such problems in children with ADHD may identify a sample of reduced genetic heterogeneity.

METHODS

Children with ADHD from the International Multicentre ADHD Genetic (IMAGE) study were evaluated with the Parental Account of Children's Symptoms. Genetic association testing was performed in PLINK on 890 probands with genome-wide genotyping data. Bioinformatics enrichment-analysis was performed on highly ranked findings. Further characterization of the findings was conducted in 313 Dutch IMAGE children using the Developmental Coordination Disorder Questionnaire (DCD-Q).

RESULTS

Although none of the findings reached genome-wide significance, bioinformatics analysis of the top-ranked findings revealed enrichment of genes for motor neuropathy and amyotrophic lateral sclerosis. Genes involved in neurite outgrowth and muscle function were also enriched. Among the highest ranked genes were MAP2K5, involved in restless legs syndrome, and CHD6, causing motor coordination problems in mice. Further characterization of these findings using DCD-Q subscales found nominal association for 15 SNPs.

CONCLUSIONS

Our findings provide clues about the aetiology of motor coordination problems, but replication studies in independent samples are necessary.

Motor disorder and anxious and depressive symptomatology: a monozygotic co-twin control approach

The aim of this study was to investigate the relationship between poor motor ability and anxious and depressive symptomatology in child and adolescent monozygotic twins. The co-twin control design was used to explore these mental health issues in MZ twins concordant and discordant for a motor disorder, and controls. This methodology offers the unique opportunity to control for genetic effects and shared environmental influences, and permits the investigation of non-shared environmental influences. The Developmental Coordination Disorder Questionnaire was used to identify 23 sets of twins discordant for a motor disorder, 23 sets concordant for a motor disorder, and 773 sets of twins with no motor disorder from a total sample of 2122 Australian sets of twins. The Strengths and Weaknesses of ADHD Symptoms and Normal Behaviour questionnaire was used to exclude participants with high Attention Deficit Hyperactivity Disorder symptomatology. Anxious and depressive symptomatology were assessed using Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR) based questionnaires on Generalised Anxiety Disorder and Sad Affect. Results indicated significantly higher levels of anxious and depressive symptomatology in twins with a motor disorder in discordant pairs compared to their co-twins without a motor disorder, and controls. There were significantly higher levels of anxious symptomatology in twins with a motor disorder in discordant sets than in sets of twins concordant for a motor disorder. There were significantly higher levels of anxious symptomatology in concordant twins than in controls. Implications of these findings are discussed with emphasis on understanding and recognising the relationship between a motor disorder and anxious and depressive symptomatology in clinical practice for children and adolescents with these disorders.

Comorbid ADHD and DCD: examining cognitive functions using the WISC-IV

This study explored the cognitive performance of children with Attention Deficit/Hyperactivity Disorder (ADHD) and/or Developmental Coordination Disorder (DCD) using the Wechsler Intelligence Scale for Children-IV. Participants were 62 children with ages between 9 years 8 months and 12 years 7 months. These children were placed into one of the four groups: Comparison (n=26), ADHD (n=14), DCD (n=11), and ADHD+DCD (n=11) groups. The ADHD symptoms were assessed using the Australian Disruptive Behaviours Scale, and motor ability was assessed using the McCarron Assessment of Neuromuscular Development (MAND). Significantly poorer perceptual reasoning ability was seen in DCD and ADHD+DCD groups but not in the ADHD group. The findings provide evidence that a deficit in visuo-spatial ability may underlie DCD but not ADHD. These findings revealed different cognitive profiles for ADHD and/or DCD, thus the current study does not lend support to the common aetiology hypothesis in understanding the basis of ADHD and DCD comorbidity.

Physical activity and fitness in children with developmental coordination disorder: a systematic review

Developmental coordination disorder (DCD) is a neurodevelopmental condition characterized by poor motor proficiency that interferes with a child's activities of daily living. Activities that most young children engage in such as running, walking, and jumping are important for the proper development of fitness and overall health. However, children with DCD usually find these activities challenging. A systematic review of the literature was conducted to synthesize the recent available data on fitness and physical activity in children with DCD, and to understand the extent of the differences between children with DCD and their typically developing peers. Systematic searches of electronic databases and reference lists identified 40 peer-reviewed studies meeting the inclusion criteria. These studies were reviewed in terms of: (a) study design, (b) population, (c) assessment tools, (d) measures, and (e) fitness and physical activity outcomes. It has been demonstrated that body composition, cardiorespiratory fitness, muscle strength and endurance, anaerobic capacity, power, and physical activity have all been negatively associated, to various degrees, with poor motor proficiency. However, differences in flexibility were not conclusive as the results on this parameter are mixed. Studies' limitations and the impact of results on future work are discussed.

Monozygotic twins concordant and discordant for DCD: two sides to the story

Being an identical twin does not necessarily mean having identical perceptions of family functioning, nor of the twin relationship. Using the co-twin control design, the aim of this study was to explore perceptions of family dynamics and the twin relationship in monozygotic (MZ) twins discordant and concordant for Developmental Coordination Disorder (DCD). It was hypothesized that, as has been found in twins discordant for cerebral palsy, twins without DCD would perceive family functioning as less healthy than would their co-twins with DCD. It was also hypothesized that the twin relationship would be regarded generally as mutually supportive. Questionnaire data on 866 sets of MZ twins aged 6 to 17 years were used to identify seven sets discordant, and two sets concordant for DCD. Quantitative (General Functioning Scale of the Family Assessment Device - FAD), and qualitative (semi-structured interview) measures were used to assess family dynamics and the twin relationship. In discordant sets, six of seven twins without DCD rated family functioning at a less healthy level than did their co-twins with DCD. All twins in the DCD concordant sets rated their family functioning at a healthy level. From the semi-structured interviews, emergent themes included friendship, support, minimal sibling rivalry, and minor difficulties. It was concluded that, overall, the twin relationship was regarded as close and mutually supportive, with an ambivalent polarity between the best and most difficult aspects of being an identical twin. Implications for research, policy and clinical practice are discussed.

Motor coordination and emotional-behavioral problems in children

PURPOSE OF REVIEW

To summarize recent research on developmental coordination disorder (DCD), with particular attention to comorbidity and related questions of etiology.

RECENT FINDINGS

Although a general consensus on the disorder definition exists, case identification in research studies remains problematic. Despite this, recent research has reported high levels of attention deficit/hyperactivity disorder and internalizing disorders among children with poor motor coordination. These findings offer some support for the longstanding view that DCD may be one facet of a broader syndrome that includes learning difficulties and deficits in attention. 'Pure' cases are common, however, and other work suggests that DCD and attention deficit/hyperactivity disorder have distinct causes. There is also some evidence that internalizing disorder may be a consequence of DCD.

SUMMARY

Measurement issues in DCD persist, whereas findings on comorbidity have both illuminated the nature of the disorder and heightened debate on its usefulness as a distinct diagnostic entity.