Movement Disorders in Adults With Intellectual Disability and Behavioral Problems Associated With Use of Antipsychotics?

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To the Editors
We read with great interest the article by Scheifes and colleagues1 about movement disorders in adults with intellectual disability (ID) and behavioral problems associated with the use of antipsychotics. The authors examined movement disorders in this population. The study shows that 44% of the study population with ID and behavioral problems had at least 1 movement disorder (parkinsonism, dyskinesia, akathisia, and dystonia). These results are in line with research in other populations. High prevalence of movement disorders among psychiatric patients using antipsychotics is already known.2–6 The conclusion of this study is that the prevalence of movement disorders in people with ID and behavioral problems is high, especially in ID patients using antipsychotics. Current use of antipsychotic drug and a dose in target range seems significantly associated with the risk of having these movement disorders.
However, in our opinion, there are limitations in the validity of the instruments and the design of the study that compromise the validity of the conclusions.
The best available instruments are used despite the limited number of standardized assessments for this population. Unfortunately, the validity of these instruments is unknown for people with ID.
In addition, there are avoidable omissions in other measurements that may have biased the results. Information about the medical history of the participants is missing; it is essential to know if the participants had the movement disorder at the moment of their admission in the centers. Other medication use is not included in the analysis, although it was reported. This information is important because it is also known that other medications, such as dopamine receptor blockers, antidepressants, and antiepileptics, can cause these movement disorders.7
Several cross-sectional studies about movement disorders in ID patients have been performed in the past few years (Table 1). None of these studies are generalizable. In the study population of Scheifes et al,1 the intelligence quotient (IQ) is much higher compared with the general ID population. It could be that the level of ID influences the prevalence of movement disorders.8,9
Because of a lack of valid measurements, several tools are used in different studies. Therefore, the comparison of the study results is difficult. This highlights the need for uniform instruments that are valid, reliable, practical, and feasible in the ID population. There are only 3 instruments developed and validated in people with ID, these are as follows: Akathisia Ratings of Movement Scale for akathisia (ARMS), Dyskinesia Identification System Condensed User Scale (DISCUS) for dyskinesia, and Matson Evaluation of Drug Side Effects (MEDS) for overall adverse effects of psychotropic drugs.11–13 In the case of other movement disorders, there are no instruments that are validated for the ID population. More research is needed to acquire valid instruments and subsequently reach consensus on recommended instruments.
In the study population of Scheifes et al,1 most patients have a Diagnostic and Statistical Manual of Mental Disorders IV (DSM IV) classification, and antipsychotics are used on-label, comparable with the studies of Matson et al and Fodstad et al.9,10 In the conclusion of Scheifes et al,1 the results are compared with the study of de Kuijper et al,8 but these participants use the antipsychotics off-label as shown in Table 1. Therefore, these populations are not comparable.
It is still unclear if antipsychotics cause movement disorders in the ID population. It could be a result of the ID itself,14 an adverse effect of the antipsychotics,8 or a result of the psychiatric disease.15 Studies done in this field are all cross-sectional (Table 1), and none of them examined the presence of movement disorders before the start of antipsychotics.

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