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Brief report: Follow-up outcomes of multisystemic therapy for adolescents with an intellectual disability and the relation with parental intellectual disability.

Research on follow-up outcomes of systemic interventions for family members with an intellectual disability is scarce. In this study, short-term and long-term follow-up outcomes of multisystemic therapy for adolescents with antisocial or delinquent behaviour and an intellectual disability (MST-ID) are reported. In addition, the role of parental intellectual disability was examined. Outcomes of 55 families who had received MST-ID were assessed at the end of treatment and at 6-month, 12-month and 18-month follow-up. Parental intellectual disability was used as a predictor of treatment outcomes. Missing data were handled using multiple imputation. Rule-breaking behaviour of adolescents declined during treatment and stabilized until 18 months post-treatment. The presence or absence of parental intellectual disability did not predict treatment outcomes. This study was the first to report long-term outcomes of MST-ID. The intervention achieved similar results in families with and without parents with an intellectual disability.

Therapists completed a questionnaire at the start and at the end of the treatment. Parents answered questionnaires during home visits by the research team at the start and at the end of the treatment and were contacted by an independent call centre for a telephone interview at 6-month, 12-month and 18-month follow-up. At 6-month follow-up, 40 parents (73% of the total sample) participated in the interview, at 12-month follow-up 33 parents participated (60%), and at 18-month follow-up 27 parents (49%) participated in the interview. Data from the start and end of treatment as well as 6-month follow-up have previously been discussed in an earlier study (Blankestein et al., 2019). All 12-month and 18-month follow-up data are thus newly collected data. Parents were contacted at each point in time, unless they withdrew their consent to partake in the study.
The study was approved by the Internal Review Board of one of the participating mental healthcare agencies.

| MST-ID
MST is aimed at families with adolescents who display antisocial or delinquent behaviour and are at risk of out-of-home placement (Henggeler, Schoenwald, Borduin, Rowland, & Cunningham, 2009). Treatment sessions are conducted at home with a focus on increasing parental skills and parental empowerment. MST-ID has been adapted to suit the needs of adolescents with an intellectual disability and their parents. Specific attention is paid to the generalization of new knowledge and skills and the promotion of the active involvement of the social network. The mean treatment duration seen in this study was 5.1 months (range 2 to 8 months).

| Screening for intellectual disability
Parents were asked to complete the Dutch Screener for Intelligence and Learning Disabilities 18+ (SCIL 18+;Nijman, Kaal, Van Scheppingen, & Moonen, 2016). The screener provides a valid indication of whether a person's IQ is below 85 (Nijman et al., 2016).
For adolescents, unless IQ scores were already known, IQ was assessed using a short form of the Dutch Wechsler Intelligence Scale for Children (WISC-III-NL;Wechsler, 2005) in adolescents aged < 17 years.

| Behavioural problems: Rulebreaking behaviour
Parents reported on adolescent problem behaviour using the subscale "Rule-breaking behaviour" of the Child Behaviour Checklist (CBCL 6-18; Achenbach & Rescorla, 2001). T-scores were computed and used in analyses.

| Ultimate outcomes: Police contact, school or work, living at home
The three main outcomes of MST-ID were assessed at all time points: (a) the adolescent is living at home (yes/no), (b) the adolescent attends school or works for at least 20 hours a week (yes/no), and (c) the adolescent has not been involved with the police since the start of treatment (post-treatment)/in the previous six months (follow-up) (yes/no).

| Missing data
Families without missing follow-up interviews (n = 23) were compared on baseline characteristics to families with at least one missing follow-up interview (n = 32). Independent t tests were calculated for continuous variables and chi-squares for categorical and dichotomous variables. Results revealed that families without missing follow-up interviews reported more parenting stress at the start of treatment (M = 70.26,SD = 8.15) than families with one or more missing follow-up interview(s) (M = 64.13, SD = 12.36; t(53) = 2.217, p = .031). Families with and without follow-up data did not differ with regard to age, gender, IQ score, treatment duration, externalizing problem behaviour, rule-breaking behaviour, educational level, country of birth, living at home, engagement in school or work or police contact of the adolescent or SCIL score, parenting stress, educational level or country of birth of the parent.
Missing data were imputed 40 times using the predictive mean matching method (PMM) in SPSS version 25. PMM only imputes values that have been observed for that variable in other cases. As such, all imputed values are realistic values. All variables reported in this study were imputed. The analyses mentioned below were performed separately on the imputed data sets and on the original data set. Two-sided analyses were performed with a 95% confidence interval (p = .05).

| Analyses over time
Dependent (paired samples) t tests were used to assess whether continuous outcomes changed significantly over time. Treatment outcomes at the end and at follow-up were compared to these variables at the start of treatment. For dichotomous outcomes, analyses over time could not be conducted as pooled estimates could not be calculated (Li, Raghunathan, & Rubin, 1991). Nevertheless, descriptive results of dichotomous outcomes are reported.

| Parents with and without intellectual disability
(Logistic) Regression analyses were performed to explore if the presence of parental intellectual disability affected the treatment outcomes. Regression analyses were used to examine the relation between parental intellectual disability and continuous outcomes, and logistic regression analyses were used to examine the relation between parental intellectual disability for dichotomous outcomes.
Analyses were conducted separately for adolescents who did and did not demonstrate certain outcome measures at start (e.g. police contact). Thus, logistic regression analyses for police contact were conducted separately for adolescents with and without police contact at the start of treatment. This was not the case for the variable "living at home" because all adolescents had to be living at home at the start of treatment to receive MST-ID.

| Outcomes over time
The results of the imputed data (see Table 1) indicated that rulebreaking behaviour declined significantly between start and end of treatment, between start and 12-month follow-up and between start and 18-month follow-up (small effect sizes (ES); Cohen's d = between −0.29 and −0.44). Results did not differ between start and 6-month follow-up (small ES; Cohen's d = −0.21).
The results of the original data showed a similar pattern, although the decline in rule-breaking behaviour between start and end of treatment, between start and 12-month follow-up and between start and 18-month follow-up showed larger effect sizes (medium ES; Cohen's d = between −0.50 and −0.68). Contrary to findings from the imputed data sets, rule-breaking behaviour declined significantly between start and 6-month follow-up (p < .01; medium ES; Cohen's d = −0.61).
Descriptive percentages of the other treatment outcomes are depicted in Table 1. Results of the imputed data sets suggest that successes achieved at the end of treatment were not maintained up to 18-month follow-up. Results of the original data suggest that the outcomes "no police contacts of adolescents" and "adolescents living at home" were maintained until 12-month follow-up and 6-month follow-up, respectively.

| Parental intellectual disability
Analyses on the imputed and original data sets showed no significant differences in outcomes for parents with or without intellectual disability (see Table 2).

| D ISCUSS I ON
The current study provides insight into long-term outcomes of MST-ID for families with adolescents with an intellectual disability and antisocial or delinquent behaviour, and parents with or without an intellectual disability. Families with and without missing data differed on levels of parenting stress at the start of treatment, but no other differences were found. Findings indicate that Rule-breaking behaviour declined during treatment and that this was sustained until 18 months after treatment. Although over 75% of adolescents had no police contact, were in school or work or lived at home at the end of the treatment, the percentages of adolescents without police contact, percentages of adolescents engaged in school or work, and percentages of adolescents living at home were lower at 18-month follow-up than at the end of treatment. This finding emerged in both the original and imputed data sets, suggesting that imputation of the missing data did not affect these results.
It may seem surprising that positive changes were found for rule-breaking behaviour, but not for other outcomes such as police contact. Previous studies comparing standard MST to other treatments also found a reduction in problem behaviour at the 18-month time-point, but not for recidivism or re-arrest (see e.g. Weiss et al., 2013). This finding may be explained by the way rule-breaking behaviour was measured in the present study. It includes different forms of rule-breaking behaviours such as having bad friends,  ta-analysis showed that several systemic interventions targeting antisocial behaviour produce positive long-term effects, it did not distinguish between adolescents with and without an intellectual disability (Sawyer, Borduin, & Dopp, 2015). It is, therefore, still unclear what the long-term follow-up results are of systemic interventions for adolescents with antisocial or delinquent behaviour and an intellectual disability (Crnic et al., 2017;Sawyer et al., 2015). As the current study did evaluate outcomes up until 18-month follow-up, it adds to a small body of research and may serve as a point of reference for future studies.
Parental intellectual disability did not influence adolescents' treatment outcomes. This could indicate that MST-ID achieves similar results for families with parents with as well as without an intellectual disability. Further research is needed to establish if these findings can be replicated, especially since the current analyses pertain to a relatively small sample of 55 families.
TA B L E 2 Treatment outcomes in subgroups of parents with (n = 32) and parents without an intellectual disability (n = 23)

Variable
Original data Imputed data

Rule-breaking behaviour (CBCL)
At the end of treatment − A first limitation is that the study did not employ a control group, and thus outcomes cannot be ascribed to the treatment condition. Future research, therefore, should include a control group, for instance families with adolescents with an intellectual disability and antisocial or delinquent behaviour receiving a different type of treatment.
A second limitation is the amount of missing data seen in the original data set. To deal with missing data, the current study employed multiple imputation. In general, the results from the imputed data showed smaller effect sizes and outcomes were less positive than results in the original data. It is hypothesized that parents experiencing more difficulties might drop-out at follow-up more often.
Therefore, it might not be surprising that the imputed data had less favourable outcomes than the original data. Since imputation allows for the discussion of long-term outcomes of all 55 families in this study, it is believed the imputed data are of substantial added value to this study.
A third limitation is that the authors did not know if all parents of the adolescents had an intellectual disability, since only one parent per family was involved in the research. Subsequently, families may have been categorized as not having a parent with an intellectual disability although the parent's partner did have an intellectual disability. To develop a more comprehensive understanding of the family situation, future research may need to assess parental intellectual disability of all caregivers involved.
A fourth limitation is that participants with mild intellectual disability (IQ score 50-69) or borderline intellectual functioning (IQ score 70-85) were brought together in one target group.
In the Netherlands, individuals with mild intellectual disability or borderline intellectual functioning may be admitted to the same (healthcare) organizations for treatment and care (Seelen- de Lang et al., 2019). Henceforth, the present authors defined intellectual disability as an IQ score of between 50 and 85. As this definition may differ across international studies, it affects the generalizability of results.

| CON CLUS ION
The current study is one of few-insofar the present authors are aware-studies looking into the follow-up outcomes of an intervention for adolescents with an intellectual disability and antisocial or delinquent behaviour and parents with or without an intellectual disability. As generalization and the sustainability of treatment results is difficult for these families, it is imperative that intervention studies employ follow-up data more often.