Deriving Vignettes for the Rare Disease AADC Deficiency Using Parent, Caregiver and Clinician Interviews to Evaluate the Impact on Health-Related Quality of Life

Purpose: Aromatic l-amino acid decarboxylase (AADC) deficiency is a rare genetic condition, characterised by movement disorder, and speech and cognitive functioning impairment. To enable economic evaluation of treatments, health-related quality of life or utilities need to be derived. These are currently lacking in the literature. This is challenging, where patient numbers are small, particularly in paediatric populations. This study outlines the 5-stage development of vignettes describing AADC, for use in a subsequent health-state utility elicitation study, with an emphasis on caregiver and clinician engagement.

Methods: To align with the economic model, 5 vignettes describing 5 AADC deficiency motor milestones were developed, ranging from "bedridden" to "walking with assistance". Stage 1 comprised identification of symptoms/impairments from the literature and AADC deficiency charity websites. Stage 2 comprised group discussion with 3 caregivers. A symptoms matrix was developed, followed by draft vignettes (Stage 3). Eight clinicians reviewed these, alongside the same 3 caregivers via a survey (Stage 4). The vignettes were revised at stage 5.

Results: There was high consensus regarding symptoms at Stages 1 and 2, although the literature highlighted behavioural and autonomic symptoms, which caregivers did not. The symptoms were grouped into neuromuscular, autonomic, cognitive, behavioural and functional impairments. Clinician and caregiver vignette feedback highlighted the idiopathic nature of AADC. Despite this, clinicians suggested only 2 additional symptoms. Similarly, caregivers suggested 4 symptoms and a change to the wording used for the cognitive symptoms. Not all changes were included.

Conclusion: The differing focus of caregivers, clinicians and the literature reinforces the importance of patient/caregiver engagement. The vignettes need to comprehensively capture what it is like to live with AADC deficiency, in order for the subsequent utilities to be robust. A focus on evidence triangulation, especially for idiopathic conditions, and how engagement is sought from caregivers are important future avenues of research.