Background and aim. Children and adolescents with cerebral palsy (CP) are at high risk for pain and their pain is frequently underestimated and undermanaged. They often experience a substantial pain burden from the pain itself and its influence on all aspects of their life, for example ability to perform daily activities, participate socially, sleep, or on overall quality of life. The overarching aim of the CPPain-program is to reduce the pain burden in children and adolescents living with CP.

Study design. The CPPain has a prospective cohort comparative design and will include before- and after measurements and process evaluation of a nested intervention. This registration concerns the CPPain baseline and includes a multinational survey (the CPPain survey) and qualitative interviews with children and adolescents with CP, their parents/caregivers and health care professionals. The nested intervention will later be developed based on findings from the baseline.

Qualitative data collection (the CPPain survey). The study population for the CPPain survey will include children younger than 18 years with a CP diagnosis (=primary participants) and their siblings (12-17 years) as controls. Parents of all primary participants will provide proxy report. Children with CP \> 8 years of age and cognitively able (approx. 50%) and siblings (12-17 years) will provide self-report. The survey will include a set of previously published and validated measures that assess aspects of pain (experience, interference and management), performance of daily activities, mental health, Health-Related Quality of Life (HRQoL), and participation. These measures include modified versions of Dalhousie Pain Interview (DPI) and Brief Pain Inventory (BPI), Pain Catastrophizing Scale (PCS), The Caregiver Priorities and Child Health Index of Life with Disabilities (CPCHILD), Revised Children's Anxiety and Depression Scale (RCADS-25), KidScreen, and Child and Adolescent Scale of Participation (CASP). We have also added a researcher-developed questionnaire on COVID-19 and pain. We will have three versions of the survey: Parent proxy, Self-report - full (12-18), and self-report - abbreviated (8-12 and older children with cognitive limitations). In addition to self- and/or proxy-reported survey data, demographic and disability-related data already registered in either national registers or electronic medical charts will be collected. Survey data will primarily be collected using an electronic survey. The Clinical Data Management System (CDMS) used for the eCRF in this study is Viedoc. The setup of the study specific eCRF in Viedoc will be performed by Research Support Services CTU, Oslo University Hospital (OUS).

Qualitative data collection. Potential participants for the child and parent interviews will be identified from the CPPain survey or clinicians in the child habilitation services. Inclusion criteria are school-aged children and adolescents (8-18 years) with cerebral palsy who have pain, determined from their or their parents' responses to the modified Dalhousie Pain Interview. If the child is unable to self-report or do not wish to participate only the parent/caregiver will be included.A purposeful sampling strategy will be used to ensure a diverse sample concerning child and parent age and gender, as well as the child's level of physical and cognitive disability and severity of pain burden. We estimate that we will include 15-20 child/parent dyads, but the final number will depend upon the richness of the material and data saturation.Both individual semi-structured interviews and focus groups will be used to answer the research objectives. Interview guides cover experiences with pain and its impact on life and living for children and parents respectively, parents' perception of their child's pain burden and its impact, as well as how pain is managed, including a detailed mapping of both the current and ideal help network. From the existing help network, identified by interview participants, health care professionals will be identified and recruited for a second round of interviews.

Data analysis. Quantitative questionnaire and register data will be analyzed using appropriate statistical methods for comparing groups and analyzing time trends in close cooperation with the study's statistician. Stratification will be used to investigate sub groups. Analysis will be adjusted for appropriate sociodemographic variables. SPSS will be the main tool for statistical analysis, while some of the more complex analysis will require the use of R. Other specialized software will be applied as needed. Qualitative data from interviews and from unstructured responses to survey questions will be analyzed using appropriate qualitative methods; primarily content, thematic and/or network analysis. NVivo will be the main qualitative analysis tool. Qualitative and quantitative data will be combined and analyzed from either a quantitative or qualitative perspective to provide a more comprehensive understanding of the phenomenon.