Centre for Youth Mental Health - Theses
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ItemNormative modelling in large-scale multi-site neuroimaging data sets to investigate brain abnormalities in depression( 2023-09)The search for diagnostic biomarkers using cortical thickness alterations in depression has been impeded by a combination factors, including small sample sizes underpowered to detect small effect sizes in cortical thickness differences, clinical and pathogenic between-subject heterogeneity, group average comparisons and brain ageing effects. Sample sizes in clinical neuroimaging can be increased by creating data sets by pooling. However, pooling studies across acquisition sites can create site effects, which stem from differences in during image acquisition and pre-processing. Site effects can induce biases into the estimates of cortical thickness measures and can interact with the effects of additional variables on cortical thickness, which can make their removal difficult. The aim of this thesis is to 1) Provide an educational review of retrospective site effect correction methods, their benefits, drawbacks and use cases, including normative modelling 2) Develop and test a normative model based on the covariates age, sex and site that allows to correct for site effects on cortical thickness measures in a pooled, public normative modelling data set 3) apply the best performing normative model to cortical thickness data of a large pooled depression neuroimaging data set, in order to compare z-score deviations of depressed individuals to those of healthy controls and link those deviations to clinical characteristics. Regarding aim 1) I summarise several retrospective site effect correction methods that have been published. The evaluation of the statistical foundation of each method reveals that each method has different used cases, advantages and disadvantages that the user should be aware of when choosing a method. To address aim 2), linear and non-linear versions of a normative model based on Hierarchical Bayesian Regression were developed and tested against alternative common site-effect correction methods. All models were evaluated based on their interference with making predictions from cortical thickness measures in a test set containing 35 cortical measures (34 bilateral regions and one whole brain average). ComBat 1–3, regressing out site and predictions from raw data led to a shrinkage of variance when predicting cortical thickness measures from the test set, which suggests the removal of both site variance and shared co-variation with other variables, such as age and sex. Normative modelling, in contrast, was able to retain a larger spectrum of variation. 3) Finally, the non-linear version of the normative model was applied to a large, pooled neuroimaging data set in that contained the same 35 cortical thickness measures of 5300 healthy individuals (training set: n = 3181, test set: n = 2119) and 3645 individuals with depression. The results show large between subject variability in cortical thickness alterations within depression and a large overlap of alterations with healthy controls. This thesis highlights the large between -subject variability in brain measures in clinical cohorts, that may be partially due to site effects in pooled neuroimaging studies. The findings of this thesis stress the need for methods and models in clinical neuroimaging that allow for individualised predictions and for site effect correction, stepping beyond the average patient. Last, the large overlap in the distribution of cortical thickness measures between individuals with depression and healthy controls suggests that cortical thickness might not be a suitable marker for the diagnosis of depression.
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ItemPsychological Interventions for Interpersonal Trauma in Young People with symptoms of Posttraumatic Stress, Anxiety and Depression( 2022-08)Worldwide exposure to potentially traumatic events is highly prevalent among young people aged 12 to 25 years, with international data indicating that approximately two-thirds of adolescents experience exposure to a traumatic event before age 16. In addition to interpersonal trauma inflicted by a caregiver, such as abuse (sexual, physical, and emotional), neglect, and maltreatment, young people are often exposed to family violence, physically assaulted or intimidated by siblings or peers, and are bullied. Unfortunately, subsequent exposure to a different interpersonal trauma type is not uncommon, with nearly 65% of young people experiencing multiple exposures once exposed to one interpersonal trauma. Exposure to interpersonal trauma during this developmentally sensitive period is associated with pervasive and long-lasting psychological, physical, behavioural, social, and economic costs accounting for between 28% and 45% of the population attributable risk for the early onset of youth psychological disorders. In addition to posttraumatic stress, anxiety, depression, problematic substance use, emotional dysregulation, lack of impulse control, poor interpersonal relationships, dissociation, as well as attention and cognition, dysfunctions are highly prevalent and associated with increased incidents of self-harming behaviours and suicidal thoughts and behaviours. Although evidence-based interventions and guideline recommendations exist for the treatment of PTSD in children/adolescents and adults, none of the recommended interventions has been developmentally adapted for transitional-aged youth aged 15 to 25 years exposed to interpersonal trauma, and it remains unclear what the most effective and safe treatment is for these young people. This thesis sought to address this significant gap in the literature with the ultimate aim of evaluating the evidence for psychological interventions for posttraumatic stress disorder (PTSD) and comorbidities in young people exposed to interpersonal trauma and understanding if trauma-focused cognitive behavioural therapy ([TF-CBT]; Cohen et al., 2017), specifically are potentially suitable for this group of young people. The research reported in this thesis had three main aims. Aim 1: To collect, interpret and synthesise quantitative research about the efficacy of psychological interventions for treating anxiety, depression, and substance use in addition to posttraumatic stress symptoms in young people exposed to interpersonal trauma. Aim 2: To understand if trauma-focused cognitive behavioural therapy (TF-CBT) is feasible, acceptable, and potentially clinically effective for young people impacted by interpersonal trauma and symptoms of posttraumatic stress disorder (PTSD). Aim 3: To understand if TF-CBT is safe and tolerable, with a specific focus on determining whether the exposure component of TF-CBT (known as trauma narration) is associated with an elevation in distress or an increase in self-harming behaviours or suicidal thoughts and behaviours. Three studies were conducted to address these aims. Study 1: To achieve the first aim, Study 1 used a meta-analytical approach to evaluate the efficacy of psychological interventions in reducing PTSD, anxiety, depression, and substance use symptoms in young people exposed to interpersonal trauma. Until now, the largest meta-analysis of young people is the study conducted by Gutermann et al. (2016). Unfortunately, the results of this study were affected by incomplete age-data (i.e., no mean age or age range) and poorly reported trauma data (i.e., reported mixed types, not specific types). I was interested in overcoming these two issues in order to provide specific evidence of treatment effects for young people (aged 12–25 years) and those exposed to a wide range of interpersonal trauma types (i.e., beyond physical and sexual abuse). Therefore, I conducted a new review, selecting only those studies where participants’ mean age fell between 12 and 25 years, with at least 80% of the sample exposed to one or more interpersonal traumas. Large significant effect sizes were observed for psychological interventions versus controls for outcomes of PTSD, and there were small significant effect sizes for anxiety and small trend-level non-significant effect sizes for depression. TF-CBT outperformed other treatments in the sub-group analysis for PTSD. However, results need to be interpreted within the context of the small sample size and heterogeneity. Study 2: To achieve the second aim, Study 2 employed a single-arm pre vs post study design, with two additional assessment points at the start and end of the trauma narration phase of TF-CBT for transitional-aged young people aged 15- 25. The sample included 20 young people (65% female, n = 13) who participated in up to 20 sessions of TF-CBT over 30 weeks. Two female participants dropped out of treatment (one after the first session and the other before the start of trauma narration. The remaining participants attended a mean of 15 sessions of TF-CBT over 25 weeks. TF-CBT was implemented with fidelity, and young people reported that they would recommend the intervention to a friend experiencing a similar issue. Quantitative data supported the relative clinical efficacy of TF-CBT. At the end of treatment, only one of the 16 participants with a baseline PTSD diagnosis met the diagnostic criteria. Significant improvements were also noted in self-reported PTSD, anxiety, and depression outcomes. Study 3: To achieve the third aim, distress, self-harm and suicide from Study 2 were analysed and interrogated to investigate if TF-CBT was safe and tolerable. The young people enrolled in Study 2 completed the Subjective Units of Distress Scale (SUDS) at the start and end of each session and were also asked question 6 of the C-SSRS, which inquired about their suicidal thoughts and behaviours in the week between therapy sessions. In addition, data from the Deliberate Self Harm Inventory (DSHI) and Adult Suicide Ideation Questionnaire (ASIQ) were also analysed. Across the 279 sessions of TF-CBT (M = 15.5 sessions), there were 16 incidents of elevated distress in seven participants, 15 incidents of self-harming behaviour in seven participants, and one of both elevated distress and suicide ideation. Results indicated there might be a relationship between distress and self-harming behaviours. Conclusion: This thesis used quantitative methods to understand the relative efficacy of psychological interventions for PTSD, anxiety, and depression in transitional-aged young people exposed to interpersonal trauma. The results of the meta-analysis and data from the pilot study demonstrated the potential safety and efficacy of TF-CBT for transitional-aged young people. This new knowledge is timely given the expansion in specialised mental health service delivery and the need to better accommodate the needs of transitional-aged young people with a history of trauma exposure.
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ItemCharacteristics and Predictors of Suicidality in Young People with Depressive Disorders( 2023)Depression is one of the most prevalent and disabling mental health conditions among young people worldwide. Suicidality and depression are closely intertwined, yet the specific factors that contribute to the nature and severity of suicidality, or changes in suicidality over time, are not perfectly understood. Factors other than depressive symptom severity, such as comorbid psychopathology and personality traits, might be important contributors. In order to reduce the burden of suicidality in young people with depression, we need to improve our understanding of its underlying constructs and contributory elements. This has the potential to contribute to improved prevention and early intervention efforts across multiple stages of suicidality, in addition to informing more targeted clinical treatment approaches. Aims and Objectives The broad aim of this research program was to contribute towards an improved understanding of suicidal thoughts and behaviours in young people with clinically diagnosed depressive disorders. More specifically, the aim was to identify modifiable intervention targets, which could inform clinical treatment approaches, and suicide prevention and early intervention efforts more broadly. Methods This research program comprises four studies each addressing different research questions. Study 1 is a comprehensive systematic review of contributors to suicidality in young people with unipolar and bipolar depression. Study 2 is an analysis of the dimensionality of a widely used measure of suicidal ideation, including associations between this measure’s latent factors and actual suicidal behaviour in young people with major depressive disorder (MDD). Study 3 is an investigation of how different dimensions of social support are associated with suicidal ideation in a treatment seeking cohort of young people with MDD; and Study 4 is a longitudinal analysis of associations between a range of psychosocial correlates and suicidal ideation severity in this same cohort of young people MDD, assessed over a 12-week period. Main Results Several key themes can be drawn from the findings of this research program. First, there is a lack of consensus regarding how the construct of suicidality should be defined, highlighting the need for international collaboration in the development of a standardised, validated classification system for suicidal ideation and suicidal behaviours. The second key finding is that suicidality in young people with depressive disorders is multidimensional in nature. That is, the way in which suicidality manifests in an individual is multifaceted. Suicidality is comprised of multiple constructs encompassing both active and passive ideation, intrapersonal cognitions such as hopelessness and lack of self-worth, and interpersonal factors such as perceived burdensomeness. The third key finding is that there are multiple determinants of suicidality in young people with depressive disorders; in addition to depressive symptoms, there are numerous other predictors of the nature and severity of suicidality. Notably, familial support is an important protective factor, while psychopathological features such as state and trait anxiety contribute to suicidality severity. Discussion Outcomes of this research program reinforce the notion that suicidality is complex and multideterminant in nature. Depression symptomatology is an important contributor, suggesting that regular monitoring of depression symptom severity should be a core aspect of the clinical management of suicidality in young people with depressive disorders. Suicidality is not driven by depression symptomatology alone and it is clear that suicide prevention and intervention efforts need to go beyond simply reducing depression severity. Psychiatric comorbidity with depression, particularly comorbid anxiety, is an important determinant of suicidality. Anxiety and depression, in particular, share many clinical features and risk factors. Interventions targeting transdiagnostic features could have clinical utility in reducing the burden of suicide in young people. In addition, methodological assessment of personality features and carefully targeted intervention approaches such as dialectal behaviour therapy, or mentalization-based therapy, could be a beneficial component of the clinical management of depression and suicidality in young people. An important clinical implication of this research program is that there is likely substantial variability underlying the mechanisms for suicidality from one depressed young person to the next. This suggests that there is perhaps a similarly high degree of variability with respect to potentially effective treatment targets for suicidality. It raises the question of how useful it is to consider a diagnosis of depression as a specific risk factor for suicidality in young people. Alternative approaches to identifying aetiological mechanisms of suicidality, such as a specific symptom approach, could be warranted. It is crucial to develop and employ early intervention approaches for suicidality in young people which focus on the earliest stage of suicidality. Potential targets for early intervention, such as increasing adaptive social support to reduce severity of suicidal ideation, are likely to be beneficial in preventing transition to suicidal behaviour. This highlights the need to assess and monitor suicidality early in young people presenting with mental health symptoms, irrespective of the specific diagnosis. This requires the use of age-appropriate suicidality assessment tools designed for use in young people. Given the fluctuating nature of suicidality, real-time symptom monitoring could perhaps be implemented as part of routine clinical care. The use of transdiagnostic interventions aimed at modifying common cognitive processes underlying depression, anxiety, and suicidality could be an effective treatment approach. Although suicidality is a complex phenomenon and no single approach to prevention or intervention is likely to be universally effective, the findings of this research program do have the potential to help reduce the suicidality-related health burden in this particularly vulnerable population.
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ItemNo Preview AvailableUsing Machine Learning to Disentangle Heterogeneity Within and Between Psychosis and Depression: Improving Pathways for Precision Medicine in Psychiatry( 2022)The aim of this PhD was to examine the clinical and biological -primarily structural brain- heterogeneity within and between depression and psychosis and provide tools for the improvement of diagnosis and targeted treatment. In chapter 3, a systematic review of structural neuroimaging studies in depression and psychosis identified potential transdiagnostic patterns of gray matter volume (GMV) and white matter volume (WMV) reductions in areas including the middle frontal gyrus, hippocampus, and left-sided posterior subgenual prefrontal cortex. In chapter 4, clinical/neurocognitive and neuroanatomical support vector machine (SVM) learning models demonstrated separability of prototypic depression from psychosis. Psychosis patients with affective comorbidity aligned more strongly to depressive rather than psychotic disease processes. In chapter 5, we identified two transdiagnostic neuroanatomically informed clusters which are clinically and biologically distinct, challenging current diagnostic boundaries in recent onset mental health disorders. In chapter 6, five clusters of schizophrenia with distinct immune signatures, associated with differing GMV and neurocognitive function were identified, with potential to inform the development of novel, targeted treatments. Overall, machine learning was utilised to elucidate and reduce heterogeneity within and between psychosis and depression, and identify biologically relevant and transdiagnostic subtypes that could become potential candidates for targeted treatment. The results are promising and challenge the current nosological system.
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ItemAntecedents of incident bipolar disorder in youth( 2017)Background: Bipolar disorder (BD) is a serious mental illness characterised by episodes of mania and depression. The disability associated with this disorder and the observation that at least a sub-proportion have a progressive course suggests that early or preventive interventions may be an effective strategy to minimise the disability. However, prevention efforts for BD require characterisation of targets for such interventions. Aims and objectives: Thus, the overall aim of this research program was to describe the pre-onset illness stages related to the development of incident (hypo)manic episodes and their associated functional impairment. Specifically, we aimed to examine clinical populations where preventive efforts may be more feasible. The objectives included identification of i) baseline characteristics associated with later BD among non-bipolar help-seeking youth; ii) rates and predictors of transition from major depressive disorder (MDD) to BD in previously published studies; iii) instruments that have prospective predictive validity in identifying BD and iv) the precursors of functional impairment in the post-illness period. Methods: This thesis comprises five studies that have examined these issues using diverse methodologies – using systematic review, meta-analyses and longitudinal cohort designs. Three studies involved examining baseline characteristics associated with the development of later BD from non-bipolar states. One study identified the instruments that have been used to predict the later onset of BD using a systematic review, while the final study examined the pre-onset predictors of later functioning among young people with first episode BD. Main results: The characteristics associated with later BD in the two cohort studies included subthreshold manic symptoms, comorbid substance use, severity of depression, antidepressant use and lower functioning. Meta-analyses identified that family history of BD, comorbid psychotic symptoms and lower age of onset of depression was predictive of transition from MDD to BD. The systematic review identified few instruments with prospective validity for predicting BD onset that had been replicated in clinical cohorts. However, instruments with validity in non-clinical cohorts, or those without replication were described. Across the first four studies, combinations of risk factors were associated with a greater risk of transition to BD. Poor premorbid adjustment in the pre-onset phase was predictive of later functioning among youth with first episode mania. Discussion: The findings of these studies point to the need to use combinations of risk factors identified using validated instruments, particularly in young people to predict the onset of BD. This may then help develop preventive interventions that may be tested in studies that are feasible and have adequate statistical power. Incorporating functional precursors into pre-illness stages may help with prevention of functional impairments. A putative instrument which may decrease measurement bias is also proposed. The primary limitation of the included studies was in the post-hoc nature of analyses and the associated lack of availability of all possible baseline confounders. Additionally, low statistical power limited the ability to examine certain associations. Future studies should examine multiple confounding variables in longitudinal cohorts of youth and young adults. Larger cohorts that are enriched for multiple risk factors may help improve statistical power.
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ItemAn examination on cognitive functioning following a first episode of mania in people treated with lithium and quetiapine monotherapy: a 12-month follow-up study( 2016)Cognitive impairments that persist during remission in bipolar disorder have received intensive investigation over the past couple of decades. Nevertheless, the onset and extent of cognitive deficits that occur either prior to, or following, a first episode of mania (FEM), and the trajectory of cognitive functioning in the early stages of illness remain unclear. Moreover, the effect of treatment medication on cognitive functioning in people with bipolar disorder warrants systematic investigation. Specifically, lithium carbonate and quetiapine fumurate have shown to be effective in the treatment of mania and bipolar depression and are both postulated to have neuroprotective properties. The aims of this research program were to: (i) examine cognitive functioning in people following FEM; (ii) assess cognitive changes following FEM over a 12-month period; and (iii) compare the effects of lithium and quetiapine monotherapy on cognitive functioning over the same 12-months. A systematic review of all peer-reviewed studies on cognitive functioning in people with FEM during both the acute and remission phases of the illness was conducted. This involved a systematic search of the literature from three electronic databases from 1980 to 2014. The search identified seven studies on cognitive functioning in people with FEM. The limited studies in the acute phase focussed only on aspects of executive functioning, with findings of impairment in cognitive flexibility, but not in response inhibition or verbal fluency. Although deficits in several cognitive domains were identified during the remission phase, the findings between studies were largely inconsistent. Nevertheless, the most consistent finding during remission was a deficit in working memory, and that verbal fluency and nonverbal memory were not affected. The systematic review highlighted the need for further studies and clarification on the extent of cognitive impairment following FEM, and assisted in the formation of the hypotheses of the empirical studies. For the empirical studies, a total of 61 participants with FEM randomised to lithium or quetiapine monotherapy, and 21 demographically matched healthy controls (HCs) were recruited. FEM participants and HCs were compared on cognitive functioning using an extensive cognitive battery over a 12-month follow-up period. FEM participants were assessed on cognitive functioning at baseline, 3-months, and 12-months, whereas, cognitive functioning of HCs was assessed at baseline and 12-months. The cognitive assessment included measurements of processing speed, attention, sustained attention, verbal learning and memory, nonverbal memory, working memory, verbal fluency, executive functioning, and intelligence. The first empirical study involved a cross-sectional analysis comparing cognitive functioning in FEM participants following stabilisation relative to HCs. Although the groups were matched in age, sex and premorbid intelligence, the findings revealed that FEM participants had significantly lower full-scale IQ (FSIQ) and education level than HCs. However, the difference between groups in FSIQ was no longer significant after controlling for premorbid intelligence. FEM participants displayed cognitive deficits of medium to large effect in processing speed, verbal learning and memory, and working memory compared to HCs. There were no significant differences between groups on other measures of cognition after controlling for FSIQ, education and premorbid intelligence. The second empirical paper involved a longitudinal analysis that assessed the trajectory of cognitive functioning in the FEM participants relative to HCs over 12-months. The findings revealed a significant group by time interaction in one measure of processing speed (Trail Making Test – Part A), and immediate verbal recall (Rey Auditory Verbal Learning Test – trial 1), with a significant improvement observed in the FEM group relative to HCs over time. On the contrary, a significant group by time interaction was observed in a processing speed measure of focussed reaction time (CogstateTM Detection), with FEM participants showing a slower performance relative to HCs over time. A significant group by time interaction was also observed in one aspect of executive functioning - effortful inhibitory control (Stroop effect), revealing that HCs improved in performance over 12-months, whereas the FEM participants did not. There were no other group by time interactions for other measures of cognition. The final empirical study was a randomised-controlled trial, which examined the effects of lithium and quetiapine monotherapy in people following FEM over a 12-month period. The results showed a significant group by time interaction in phonemic verbal fluency, with an improved performance in lithium-treated participants compared to quetiapine-treated participants over time. There were no other significant group by time interactions after controlling for multiple comparisons. In conclusion, the findings from this research program revealed that FEM participants had poorer global intelligence relative to HCs, as well as impairments in some but not all cognitive domains following stabilisation from FEM. There appears to be stability in cognitive functioning for most domains over the 12-month period following FEM. However, an improvement was observed in immediate verbal memory and one measure of processing speed over time in FEM participants relative to HCs, although FEM participants performed slower over time on a simple processing speed test of focused reaction time. In addition, there may be a developmental arrest in effortful inhibitory control, as HCs showed an improvement in this function over the 12-month period that was not evident in the FEM group. There were no significant differences between lithium-treated and quetiapine-treated participants on most cognitive domains, apart from phonemic verbal fluency, in which lithium-treated participants showed a significant improvement relative to quetiapine-treated participants over the 12-month period. These findings suggest that neuroprotective properties of lithium may benefit aspects of cognitive functioning when commenced in the early stages of illness. The presence of cognitive deficits in FEM signifies that cognitive changes may occur very early in the illness course. The findings from this research program did not provide evidence for a progressive deterioration in cognitive functioning following FEM, although more rigorous longitudinal studies involving subgroup analysis are warranted. Future studies should examine the relationship between brain abnormalities and cognitive functioning in people following FEM, as well as assess the effects of lithium and quetiapine monotherapy on clinical and neuroanatomical changes over time.
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ItemPromotion of self-help strategies for sub-threshold depression: an e-mental health randomised controlled trial( 2012)Background: Sub-threshold depression refers to clinically relevant depressive symptoms that fall short of a diagnosis of major depression. Sub-threshold depression is very common in the general population, impairs functioning, increases the risk of developing major depression, and poses a significant burden at the population level. Public promotion of greater use of effective self-help strategies for depression has been proposed as one technique to reduce this population burden. Improving self-help strategies could reduce symptoms of sub-threshold depression and prevent major depression. Depression experts have identified several self-help strategies that are helpful for sub-threshold depression. The aim of the study was to develop messages based on these strategies that could be promoted to members of the public with sub-threshold depression, and to test whether their promotion was effective. Method: Twelve email messages (‘Mood Memos’) were developed, based on self-help strategies endorsed as effective and feasible by depression experts. The email messages were developed with reference to theories of behaviour change, persuasion, and health communication, in order to persuade recipients to engage in the self-help behaviours. The effectiveness of these emails was evaluated in a randomised controlled trial. Recruitment was via internet-based sources and participants joined the study by visiting the website www.moodmemos.com. Adults aged 18+ with sub-threshold depression who were not receiving professional treatment for depression were eligible to participate. Participants were randomly allocated to receive emails twice weekly for six weeks in a fully automated intervention. The active group received emails based on the effective self-help strategies and the control group received emails containing general information about depressive disorders. Assessment points were at baseline, midway through the intervention and at the end of the intervention (6 weeks post-baseline). The primary outcome was depression symptom score on the Patient Health Questionnaire-9 (PHQ-9). Secondary outcomes were psychological distress, assessed with the ten-item Kessler Psychological Distress Scale (K10), and level of functioning, assessed with the Work and Social Adjustment Scale. The primary hypotheses were that the emails containing self-help strategies would reduce depression symptoms and reduce the incidence of major depression more than the control emails post-intervention. Results: The study recruited an international sample of 1,326 adults with sub-threshold depression. There was a small, significant difference in depression symptoms post-intervention, favouring the active group (Cohen’s d = 0.17, 95% CI: 0.01 to 0.34). There was also a higher, though non-significant, risk of major depression in the control group (Relative Risk = 1.32, 95% CI: 0.89 to 1.98). A similar effect was found for psychological distress (d = 0.22, 95% CI: 0.05 to 0.38), but effects on functioning were less strong, with no significant difference between the active and control groups (d = 0.12, 95% CI: -0.05 to 0.28). A mediation analysis indicated that the effect of the emails on depression symptoms was completely mediated by the use of the self-help strategies promoted in the emails. Discussion: Overall, the results indicate that promoting effective self-help strategies to the public via automated emails was effective for sub-threshold depression. The improvement in depression was associated with use of the self-help strategies promoted in the emails. The delivery of self-help messages via email is a scalable, easily disseminated intervention. The study is a novel contribution with potential to reduce the large population burden of depression.
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ItemTreatment decision making for young people diagnosed with major depressive disorders( 2011)Adolescence is a crucial period of risk for depression, with one in every five people experiencing a depressive episode before the time they are 18 years old. Engaging young people in effective treatment during this time is essential to prevent potential long-term negative impact. Guidelines advocate for young people to be involved in treatment decision making, both in terms of receiving information about treatment options, and also making choices about their own care. These recommendations are in line with a model of decision making called ‘shared decision making’ (SDM), one of several models of medical decision making. Yet little is known about processes related to treatment decision making in this age group or young people’s preferred model of treatment decision making. What little literature that exists suggests young people would value involvement in treatment decision making and that such involvement may enhance engagement. In order to address this gap in our understanding of treatment decision making in young people, semi-structured, qualitative interviews were conducted with clients (n=10), caregivers (n=5) and clinicians (n=22) about their experiences and beliefs about treatment decision making for young people diagnosed with major depressive disorder (MDD). Thematic analysis was used to identify key themes in the data. Clients and caregivers reported a range of experiences regarding how involved they were in treatment decision making, yet, consistent with the small body of literature identified, they all wanted involvement of some sort. Clinicians also wanted clients, and caregivers as appropriate, to be involved. All participants (clients, caregivers and clinicians) reported a lack of information exchange (e.g. information about potential risks and benefits of different treatment options) and wanted resources to fill this gap. Overall, the findings from these interviews indicated a preference for involvement in treatment decision making that was in line with a SDM model. In response, an evidence-based decision aid that facilitates SDM was developed for young people diagnosed with MDD who are faced with the decision about which treatment option is best for them. The decision aid was developed according to international standards, and included field-testing with clients (n=5) and clinicians (n=3), who all found the tool acceptable and useful. The current study provides the basis from which an understanding of treatment decision making for young people diagnosed with MDD can be further built, and from which additional resources can be developed and tested in order to contribute to the emerging field of youth SDM. Approaches that support young people to make evidence-based and preference-based treatment decisions have the potential to increase guideline-concordant care, satisfaction, adherence and clinical outcomes.