In their combined systematic review and meta-analysis Subara-Zukic et al «Subara-Zukic E, Cole MH, McGuckian TB, ym. Behavio...»1 aimed to provide a synthesis of the experimental work on mechanisms of DCD.
A multi-database systematic literature search was conducted from September 2016 to April 2021, using eight electronic databases (Medline Complete, APA PsycINFO, APA PsycArticles, Psychology and Behavioral Sciences, CINAHL Complete, Scopus, Embase, and Web of Science).
Study inclusion criteria were as follows: an experimental case-control, cross-sectional or longitudinal study; reported measures of behavioral performance (motor control, motor learning, cognition); reported sufficient statistical information to calculate effect sizes; DSM-5 diagnosed DCD or probable DCD (low level of motor competence on a standardized test, plus confirmation of motor difficulties by teacher/parent); DCD group compared with typically developing (TD) controls, in any age. DCD groups with co-occurring disorders were not excluded. The first author executed the searches through each database and the articles were double screened by her and the last author. Quality of studies were assessed utilizing a modified version of the Critical Appraisal Skills Programme for case-control studies (CASP) by two independent authors. Calculation of effect size (Cohen's d) was conducted for each DCD-TD comparison. Meta-analyses were conducted using multi-level random-effects models.
A total of 100 articles were identified and categorized by task domain for synthesis. The median age of DCD and TD control samples was 9.8 and 10 years, respectively. A total of 76% of studies reported that DSM criteria for DCD were met, 22% reported DCD as severe (≤5th percentile). 50% of studies excluded participants with intellectual disabilities, 41% those with co-occuring ADHD. 64% of studies were of high quality, 23% moderate, and 13% low. A total of 1,374 effect sizes were entered into the multi-level meta-analysis. The overall effect was moderate-to-large (d = 0.789, 95% CI = [0.688, 0.890]), indicating a generalized level of deficit in DCD groups relative TD group.
Sensori-Perceptual Function
There were 14 studies (13 high quality and 1 low) that contributed effect sizes in the sensori-perceptual task category, the magnitude of which was large overall (d = 0.86, 95% CI [0.48, 1.25]), but with substantial heterogeneity (I² = 87.2%).
A very large effect was found for visual perception of depth (d = 3.70, 95% CI [2.65, 4.76]). Large effects were also found for visuo-spatial integration (d = 0.92, 95% CI [0.27, 1.58]), visuo-perceptual processing with motor involvement (d = 0.76, 95% CI [0.50, 1.02]) and for tactile and haptic perception (d = 0.83, 95% CI [0.47, 1.19]), and moderate-to-large for kinaesthetic perception of distal-limb (d = 0.70, 95% CI [0.31, 1.08]).
Executive Functions and Intelligence Factors
There were 21 studies (15 high quality, 4 moderate, and 2 low) that assessed aspects of executive function and intelligence, with a moderate overall effect (d = 0.49, 95% CI [0.40, 0.59]).
Significant moderate-to-large effects were found for inhibitory control (d=0.46, 95% CI [0.32,0.59], working memory (d=0.75, 95% CI [0.50,1.01], and executive attention (or set shifting) (d = 0.61, 95% CI [0.39, 0.83]). For inhibitory control, the strongest effect was observed for interference control (d = 0.80, 95% CI [0.55, 1.04]). For working memory, significant moderate-to-large effects were observed for visual (d = 0.79, 95% CI [0.55, 1.02]) and verbal (d = 0.57, 95% CI [0.26, 0.89]) processing. For intelligence, moderate-to-large effects were shown for Full Scale IQ (d = 0.66, 95% CI [0.27, 1.04]), Non-verbal IQ (d = 0.63, 95% CI [0.24, 1.01]), and indices of Working Memory (d = 0.67, 95% CI [0.12, 1.22]) and Processing Speed (d = 0.81, 95% CI [0.25, 1.36]).
Dynamical/Ecological Paradigms—Rhythmic Coordination and Ecological Perception
A total of six studies (4 high quality and 2 moderate) were included in the dynamical/ecological domain, based on rhythmic coordination and ecological perception tasks, with a moderate overall effect (d = 0.65, 95% CI [0.38, 0.92]). For rhythmic coordination, moderate-to-large effects were found for visual-bimanual coupling (d = 0.73, 95% CI [0.03, 1.42]), auditory-verbal sequencing errors (d = 0.88, 95% CI [0.05, 1.70]), and auditory-visual-verbal sequencing stability (d = 1.07, 95% CI [0.05, 2.10]) outcomes.
Motor Imagery, Action Observation, Imitation, and Gesture Production
Both implicit (automatically activated) and explicit (imagined) MI were investigated in seven studies (6 high quality and 1 moderate), with a moderate overall effect (d = 0.61, 95% CI [0.35, 0.88]). A large, significant effect was found for implicit MI (d = 0.81, 95% CI [0.43, 1.19]).
For the action observation (AO), imitation and gesture production category, there were nine papers (7 high quality, 1 moderate and 1 low). The production of meaningful gestures to verbal command or pictured object, imitation of complex novel gestures and object assembly had the largest, significant effects (d = 0.82–1.12).
Oculomotor Control
The two included studies on oculomotor control (1 high quality and 1 moderate) examined saccadic control, smooth pursuit, and fixation using conventional eye-movement paradigms. There was a very large overall effect (d = 1.11, 95% CI [0.47, 1.75]), but with substantial heterogeneity (I² = 79.1%). For memory-guided saccades, there were very large effects on saccade latency (d = 1.91, 95% CI [0.60, 3.22]) and speed of eye movement (d = 1.70, 95% CI [1.03, 2.37]), and for antisaccades on speed (d = 2.03, 95% CI [1.39, 2.66]) and accuracy (d = 0.93, 95% CI [0.26, 1.60]), and for delayed-saccade speed (d = 1.54, 95% CI [0.69, 2.39]). Large effects were observed on fixation time (d = 0.82, 95% CI [0.15, 1.48]) and smooth pursuit performance (d = 0.99, 95% CI [0.39, 1.59]).
Reaching and Manual Control
Goal-directed reaching was assessed in four studies (2 high quality, 2 moderate), manual tracking in one, and force control in another. There was a large overall effect (d = 0.87, 95% CI [0.76, 0.98]). A large, significant effect for manual stacking was reported under unimanual, hand movement conditions (d = 1.40, 95% CI [0.53, 2.27]). A manual tracking (wire maze) assessment showed a large, significant effect (d = 0.90, 95% CI [0.10, 1.70]). On force control tasks, large effects were shown under conditions of visual feedback (d = 1.17, 95% CI [0.34, 1.99]) and when visual feedback was withdrawn (d = 0.92, 95% CI [0.08, 1.76])
Internal Modeling (Including Prospective Reaching/Grasping)
Aspects of internal modeling were examined in eight studies (7 high quality and 1 moderate) using a bimanual unloading paradigm, visuomotor adaptation paradigm involving target-directed ball throwing, delayed visual-feedback detection in a hand-movement task, and end-state-comfort tasks. There was a moderate overall effect (d = 0.56, 95% CI [0.45, 0.67]) The visuomotor task adaptation phase showed a significant, moderate effect size (d = 0.60, 95% CI = [0.03, 1.16]).
Catching
For the catching category, two studies (1 high quality and 1 moderate) focused on visual behavior during catching and the interception of virtual objects. There was a moderate overall effect (d = 0.61, 95% CI [0.41, 0.82]). Very large effects were revealed for task success, measured through the number of objects caught (d = 2.15, 95% CI [0.88, 3.42]) and intercepted (d = 1.15, 95% CI [0.13, 2.17].
Gait (Including Visual Control of Gait and Clinical Assessment)
Gait parameters were examined under different terrain and task conditions in 11 papers (all high quality). The overall effect for all gait outcomes was moderate-to-large (d = 0.70, 95% CI [0.46, 0.94]). Compared to TD peers DCD groups showed slower walking speed (d = 0.88, 95% CI [0.12, 1.64]), preferred walking speed on a treadmill with a very large effect (d = 2.33, 95% CI [0.91, 3.76]), and significantly poorer gait coordination (d = 1.57, 95% CI [0.78, 2.37] under different sensory conditions. DCD groups showed different gaze patterns when surveying their surroundings (d = 0.51, 95% CI [0.03, 1.00]); a greater proportion of time fixating the intended target during locomotor pointing (d = 0.61, 95% CI [0.09, 1.13]), and less time fixating more distal areas of interest (d = 0.55, 95% CI [0.05, 1.05]).
Postural Control
Five studies (3 high quality, 1 moderate and 1 low) assessed differences in static postural control during unipedal and bipedal stance on both firm and compliant surfaces. Under all testing conditions, children with DCD exhibited significantly poorer outcomes than TD group for static postural control, regardless of whether vision was fully available, partially available, or completely obstructed (d ≥ 0.93). These group differences were most pronounced during unipedal stance when visual feedback was reduced or completely removed (d = 3.04, 95% CI [2.29, 3.78]).
Four papers (2 high quality and 2 low) assessed dynamic postural stability using traditional clinical assessments, interactive gaming methods, and immersive virtual reality environments. For most measures of dynamic postural control, large and significant effects were observed in favor of TD group (each d ≥ 0.73).
Dual-Tasking
For dual-tasking, there was one, high quality study that showed significant, and consistently moderate-to-large effects across locomotor-cognitive and manual-cognitive dual-tasks (d = 0.67– 1.22). The overall effect was large to very large (d = 0.92, 95% CI [0.54, 1.30]).
Attentional Focus and Motor Learning
Two studies (both high quality) investigated the impact of attentional focus instructions on motor performance. Small, nonsignificant effects were found, indicating no appreciable differences in performance between DCD and TD groups, regardless of the attentional focus used.
The process of motor learning was evaluated by three studies (two high quality, one low). All effects were significant and of moderate-to-large magnitude. The magnitude of the difference between DCD and TD groups varied according to the phase of learning: large for consolidation (d = 1.17, 95% CI [0.80, 1.55]), and moderate for retention of learning (d = 0.69, 95% CI [0.32, 1.05]).
Authors' Conclusion:
While the quality of recent studies has improved (64% of high-quality in the review), sample size, use of strict DSM-5 criteria for DCD and control of co-occuring conditions require closer attention in future studies. Taking into account of the shortcomings relating to the recent studies included in the meta-analysis deficits of moderate-to-large magnitude found in DCD across outcome measures suggest the complex and interactive nature of motor control, learning, and cognition, and fundamental deficits in visual-motor mapping and cognitive-motor integration, but also areas of pragmatic compensation for motor control deficits.