This is a single-center, natural history study of subjects with Sanfilippo syndrome type D. This study will combine a retrospective review of medical records and an ongoing collection of clinical data on an observational basis from participants with Sanfilippo syndrome type D.~Participants will be asked to attend clinic visits in person on a yearly basis. In clinic visits will include neurocognitive, developmental, behavioral, biochemical, imaging measures as well as retrospective medical record analysis. At 6-month intervals between clinic visits, participants will be contacted remotely for some safety assessments (e.g., collecting information about concomitant medications, concurrent illnesses, and procedure-related adverse events [AEs] and serious AEs [SAEs] since last clinic visit). At 6-month intervals, caregivers will complete questionnaires and record home video assessments using a dedicated, compliant study application (app) called RARE (Recording Application for Real-World Evidence). The RARE app consists of 9 tasks of Activities of Daily Living and socialization as well as two parent reported questionnaires.~No formal power or sample size calculations will be performed. At least 4 and up to 10 patients living with Sanfilippo syndrome type D will be enrolled in the prospective and retrospective study (group/cohort 1). An additional 5 patients deceased or living will be enrolled for retrospective medical record collection only (group/cohort 2).~The objectives of this study are:~To enhance the understanding of the natural history and progression of Sanfilippo syndrome type D.~To define and categorize clinical endpoints that may be used in future clinical trials.
Sanfilippo syndrome type D is a ultra rare syndrome with limited available natural history data. This study is planned to document, through retrospective and prospective data collection, syndrome progression in children and young adults with Sanfilippo syndrome type D. The results from this study may inform future clinical studies in targeted therapies for patients with Sanfilippo syndrome type D and may serve as an external control since there are very few patients with Sanfilippo syndrome type D.