Attention deficit hyperactivity disorder (ADHD) is one of the most common neurodevelopmental problems in children. In Hong Kong, the prevalence of ADHD among primary one Chinese schoolboys has been reported to be 8.9%.1 Associated neurodevelopmental co-morbidities and mental health problems are frequently found among individuals with ADHD. The majority (67%) have at least one co-morbidity and the degree of functional impairment increases stepwise with the number of associated co-morbidities that includes dyslexia, other specific learning disorders, motor incoordination, anxiety, depression, oppositional defiance disorder, tics, and Tourette syndrome.2 3 A diverse group of motor problems has been found to be co-morbid with ADHD, including an increase in associated movements.4 Individual subtests of the Motor Function Neurological Assessment reveal that 80% to 96% of children with ADHD, compared with 0% to 44% of a control group, demonstrate moderate-to-severe problems in motor inhibition and proximal truncal stabilisation.5 Children with ADHD also experience greater difficulties in handwriting and penmanship, which is independent of other motor problems associated with the disorder.6 7

Developmental coordination disorder (DCD) is a well-recognised motor disability in an otherwise healthy individual. The Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5) defines DCD as marked impairment in performance of motor skills, significantly interfering with daily activities and/or academic achievements.8 The degree of coordination deficit is not consistent with the child’s intellectual ability and is not caused by pervasive developmental disorder or general medical condition. The prevalence of DCD in the normal population varies from 4% to 19%.9 10 11 12 Clinically, DCD is a heterogeneous condition with coordination deficits involving gross motor skills, fine motor skills, or both. Affected children show difficulties and restricted participation in daily living, eg self-care activities, academic achievement, leisure, and sports. Internalising symptoms and motor coordinating problems frequently co-occur in these children but their causal relationship is unclear.13 This condition is believed to be lifelong and the majority of affected children will not outgrow the problems. They will continue to experience motor difficulties, poor self-concept, and various kinds of problems at school.14 Children with DCD benefit from accommodation in school and multidisciplinary interventions that focus on task-specific training activities relevant to daily living and function.15 16 17

The prevalence of DCD in children with ADHD has been reported to be as high as 30% to 50%, depending on case definitions.18 19 The presence of ADHD co-morbid with DCD carries the worst prognosis and predicts poor psychosocial function in early adulthood.20 Therefore early identification and intervention in DCD is important in this group of children who are already adversely affected by ADHD.

Local data on the occurrence of motor problems in children with ADHD are not available but an understanding of this important issue may enable better planning of medical services. As such, the aim of this study was to estimate the prevalence of motor problems in a sample of children with ADHD at a public hospital in Hong Kong.

The participants were selected from patients referred to the Joint Paediatric and Child Psychiatric ADHD Program, a collaboration of Paediatricians and Child Psychiatrists of the New Territories East Cluster (NTEC) of Hong Kong over a period of 6 months from 1 July to 31 December 2011. The hospitals in NTEC serve a population of approximately one million and this ADHD Program is the only public service provided for children with ADHD within the cluster. Hospital records of patients aged 6 to 9 years at their first visit were reviewed. Potential candidates were those with a diagnosis of ADHD clearly documented in their record and in whom medication for ADHD was indicated. This study also included children with confirmed ADHD but whose parents had declined drug treatment. The diagnosis of ADHD was based on DSM-IV through clinical judgement of individual physicians during the clinic visit. Rating scales, such as Strengths and Weaknesses of ADHD symptoms and Normal Behavior Scale, were used in some patients as a reference but the diagnosis of ADHD remained clinical in our daily practice. In order to recruit patients with a more definitive diagnosis of ADHD, only those in whom drug treatment was indicated were included as potential candidates. This sample should therefore represent most children diagnosed with ADHD in a clinic setting. Patients were excluded when hospital records documented the presence of intellectual disability, features of autistic spectrum disorder, or medical conditions that could affect motor performance, such as cerebral palsy, hemiplegia, or a neuromuscular condition. This was consistent with the exclusion criteria for DCD in DSM-IV.

Movement Assessment Battery for Children (MABC) is commonly used as a standardised tool to diagnose DCD in both clinical and research settings.21 It provides an objective and quantitative measure of the motor performance of children between 4 and 12 years of age. There is good concurrent validity between MABC and Bruininks-Oseretsky test22 and a local study has also shown that this instrument provides satisfactory inter-rater and test-retest reliability.23 The assessment comprises eight test items related to three motor domains, namely manual dexterity, ball skills, and balance (static and dynamic). The sum of scores from the eight test items generates a total impairment score (TIS) and categorises children into one of the three groups—no, borderline, or definite motor problems. The Developmental Coordination Disorder Questionnaire (DCDQ) is a parent-reporting questionnaire first developed in Canada as a reliable and valid screening tool for DCD.24 25 It comprises statements by which parents rate their child’s motor performance in comparison with other children of the same age for ball games, balance, and handwriting skills across home, school, and play environments. A Chinese version of the DCDQ (DCDQ-C) has been validated in Taiwan as a screening tool for DCD in a Chinese-speaking community.26 Parents responded to each of the 15 questions on a 5-point Likert scale to generate a total raw score which was then converted into a probability of having DCD. A score of >40% chance is suggestive of DCD while 25% to 40% chance indicates a suspected case. In this study, DSM-IV was used to diagnose DCD and the questionnaire provided information on one of the diagnostic criteria about any impairment in daily functions related to motor performance.

The selected candidates were invited to attend a one-to-one study session that lasted approximately 30 minutes. The investigator performed MABC on the children after obtaining consent from the parents/carers. Baseline information was also collected and included age, gender, primary school level, ADHD medication use, drugs taken before assessment, gestational age, birth weight, and socio-economic group.

Statistical analysis was performed using the Statistical Package for the Social Sciences (Windows version 16.0; SPSS Inc, Chicago [IL], US). The result of MABC was used as the diagnostic standard for calculation of DCD prevalence in children with ADHD. A diagnosis of DCD was made when the TIS was ≥10, indicating ‘borderline’ (TIS=10 to 13.5) or ‘definite’ (TIS >13.5) motor problems, and representing the 15th or 5th percentile of TIS in a general population, respectively. The cut-off at either the 5th or 15th percentile varied among published data. As diagnosis of DCD requires fulfilling other criteria, such as impairment of daily activities, raising the cut-off to the 15th percentile in MABC could improve the sensitivity of the test. Nonetheless, use of the DCDQ-C avoided the risk of overdiagnosis. Prevalence was expressed as the percentage of DCD cases in participants with ADHD who underwent MABC assessment. The Chi squared test was applied to detect any difference in characteristics of participants with and without DCD. Analysis of variance was used to detect any significant difference in TIS of the MABC between DCDQ-C–defined motor performance groups. The concurrent validity was investigated by testing the probability score of DCDQ-C against TIS of MABC with Pearson’s correlation coefficient. Sensitivity, specificity, and positive and negative predictive values of DCDQ-C were calculated.

Ethical approval was obtained from the Joint Chinese University of Hong Kong and NTEC Clinical Research Ethics Committee.

From 1 July to 31 December 2011, 304 new cases were referred to the Joint Paediatric and Child Psychiatric ADHD Program of NTEC. Of the 195 potential candidates aged 6 to 9 years, diagnosis of ADHD was unconfirmed in 38. After exclusion of 21 patients with autistic features, one patient with moderate intellectual disability, one patient with left hemiplegia, and one patient with possible neurological deficit following cardiopulmonary resuscitation, 133 children were eligible for this study. Of the 132 parents who were successfully contacted, 107 agreed to participate. The MABC was tested on 102 candidates and five defaulted. The results of seven participants were rejected as they were uncooperative, making MABC scoring unreliable. Of the 133 eligible candidates, statistical analysis was performed on results from 95 (71.4%), with 63 males and 32 females and a mean age of 7.8 years. The Figure summarises the workflow of this study and the candidate recruitment process.

Table 1 shows the results of MABC performed on the 95 participants. Based on the TIS, three motor performance groups were identified: no motor problem (n=63; mean TIS=4.75 with 95% confidence interval [CI] of 4.1-5.4), borderline motor problem (n=15; mean TIS=11.53 with 95% CI of 11.0-12.1), and definite motor problem (n=17; mean TIS=18.53 with 95% CI of 16.6-20.5). Using the 15th percentile of motor performance in MABC as a cut-off, the prevalence of motor problems in this group of children with ADHD was 33.7% (95% CI, 24.2%-43.2%).

Table 2 shows the baseline information about the two groups with motor problems (borderline and definite) and the group with no motor problems and includes sex, perinatal history, socio-economic background, drug treatment, and previous motor training. There was no statistical difference between these two groups, except for a history of receiving motor training at a younger age (P=0.002).

Two carers who accompanied a child to the study session could not read Chinese, therefore 93 completed DCDQ-C were analysed. The questionnaire identified 71 and 22 participants with no motor problems and suspected motor problems, respectively, but none was found to have definite DCD (>40% chance) [Table 3]. For correlation between MABC and DCDQ-C, the Kappa value was 0.228 indicating a low agreement between the two instruments (P=0.023). Using MABC as a standard measure for motor performance, sensitivity and specificity of DCDQ-C on identification of DCD was 37.5% and 83.6%, respectively. The positive predictive value of DCDQ-C was 54.5% and the negative predictive value was 71.8%. Thus DCDQ-C could quite reliably exclude DCD but was rather insensitive when identifying motor problems in children with ADHD in Hong Kong.

In this clinic sample of children with ADHD, the results of MABC revealed that 15.8% had borderline and 17.9% had definite motor problems. Overseas studies have often used the 15th percentile of TIS in MABC as the cut-off for identification of DCD.17 19 25 By using the same standard here, participants who belonged to the borderline and definite motor problem groups could be potentially diagnosed as having DCD. A diagnosis of DCD, however, requires confirmation of motor problems and impaired daily functions. Initially, DCDQ-C was intended to provide the impairment criteria for a DCD diagnosis but it could not be reliably applied here due to the significant lack of agreement with MABC in this study sample. We therefore estimated that DCD may occur in up to one third of patients with ADHD, a figure that is comparable with the literature.

In our clinical experience, motor difficulties are usually not a common presenting symptom during a medical consultation for ADHD. There are a few possible reasons why motor performance may be overlooked. First, the parents of these children are often overwhelmed by the symptoms of ADHD and attribute all difficulties to a single diagnosis. In addition, it may be difficult for parents to differentiate symptoms related to motor impairment from those related to ADHD, for examples, bumping into objects, poor postural stability, and illegible handwriting. Second, cultural influences may play a part. Hong Kong children adopt a very sedentary lifestyle and are probably the most physically inactive students in the world.27 Sports skills are not essential for most local children in the school and social environment. Third, there was less diversity for participation in leisure activities among ADHD children compared with their normal peers.28 Children with ADHD were encouraged to participate in tutorial lessons to support academic achievement, rather than other non–academic-related activities. They spent many hours after school every day for completion of homework and revision, leaving little time for sports or other leisure activities. Limited participation in physical activities masks underlying motor problems and conversely means any motor talent goes unrecognised. Therefore, clinicians who manage children with ADHD should be aware of these issues and need to consider motor problems (or DCD) as a factor that causes persistent impairment, especially when the symptoms of ADHD improve.

The prevalence of DCD in children born extremely premature (<29 weeks) or with extremely low birth weight (<1000 g) has been reported to be high, which was around 42% in one study.29 A recent meta-analysis of studies in school-aged children with very low birth weight (VLBW)/very preterm reported an odds ratio (OR) of up to 8.66.30 In our study, 6.5% and 10.8% of children had a history of prematurity or low birth weight, respectively, but none was born very preterm or with VLBW. It was therefore not surprising to see no significant difference between the groups with and without motor problems in terms of maturity and birth weight. Local figures published in 1998 show that the incidence of very preterm delivery (<34 weeks) and very/extremely low birth weight (<1500 g) was 2.22% and 1.25%, respectively.31 The sample size of the current study was simply not large enough to include these children.

There are studies that show improved motor performance and quality of life in children with co-morbid ADHD and DCD following treatment with methylphenidate.32 33 34 As shown in Table 2, a lower prevalence of motor problems was found in children who were prescribed regular medication or who had taken medication prior to the assessment (OR=0.6 and 0.5, respectively) but this was not statistically significant (P=0.28 and 0.18, respectively). This may be because we recruited some children who had been recently diagnosed with ADHD and medication was not yet optimised at the time of MABC testing, or simply because of a lack of statistical power due to the small number of subjects. To better understand this issue, further studies should be carried out to specifically examine the effect of ADHD medication on motor performance.

Of the 15 participants who had received previous motor training, 11 had motor problems. This group was quite heterogeneous and, interestingly, many parents could not recall the exact reason for the motor training. As mentioned before, DCD is a relatively stable condition and intervention should focus on specific motor skills. Previous training does not preclude children from having future motor problems as demand for activities, such as handwriting and participation in sports, increases when children progress from preschool to primary school. It is therefore vital to determine whether motor skills are at an age-appropriate level for both academic and extracurricular activities and provide task-specific training whenever indicated.

Although the DCDQ-C has been validated for use in Taiwan, which is a Chinese community similar to Hong Kong, the questionnaire was not appropriate for local Hong Kong children. This demonstrates the need to be cautious when adopting an assessment tool from overseas without local validation, even from an area with comparable cultural and socio-economic background. Furthermore, a questionnaire cannot replace detailed history taking in clinical practice that is indispensable when making a diagnosis of neurodevelopmental disorders, such as ADHD and DCD.

Since DCDQ-C could not reliably reflect motor performance in this group of children, the degree of impairment in daily activities was not adequately assessed to make a definite diagnosis of DCD. A local study of the prevalence of DSM-IV disorders in Chinese adolescents pointed out that figures would be overestimated if the impairment criteria were not taken into account.35 Although MABC used alone would probably overdiagnose DCD by not considering the impairment factors, it is worth noting that 17.9% of our study candidates had definite motor problems. This is actually below the 5th percentile of the general population. Thus the magnitude of motor difficulties is substantial and motor problems (with or without a diagnosis of DCD) should not be overlooked in children with ADHD.

There are some limitations to this study. Parents who perceived their children to have motor problems were more keen to participate, leading to a selection bias in the recruitment of children. Although all children were diagnosed with ADHD, they were not a homogeneous group. They were diagnosed by different physicians and were not at a uniform stage of drug treatment. Even though MABC is a widely used tool in Hong Kong, the lack of a local norm might still affect the validity of this study.

Motor problems in children with ADHD are as common in Hong Kong as in other countries and DCD may have been present in up to 33.7% of this clinic sample. Acknowledgement of their own strength and weakness will enable patients to better plan future goals. Provision for assessment and management of DCD and other motor problems should be a fundamental part of a comprehensive programme to manage ADHD.

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