TreatHSP Platform: Adaptive Natural History Platform for Ataxias, HSPs, and Spastic Ataxias

Inclusion Criteria: General inclusion criteria: \- Age 5 or older Cohort 1: Affected * Clinical diagnosis of neurodevelopmental or neurodegenerative ataxia, spastic ataxia, HSP or related phenotype AND * Alternative causes of phenotype excluded Cohort 2: Presymptomatic mutation carriers \- Premanifest mutation carrier of (likely) pathogenic variant(s) in a disease gene associated with ataxia, spastic ataxia, HSP or related phenotype Cohort 3: Family controls - 1st or 2nd degree relative of a person with a clinical or genetic diagnosis of ataxia, spastic ataxia, HSP or related phenotype Cohort 4: Community controls \- Healthy individual unrelated to a person with neurodevelopmental or neurodegenerative ataxia, spastic ataxia, HSP or related phenotype Exclusion Criteria: * Presence of an alternative neurological or systemic condition that sufficiently explains the phenotype and is incompatible with ataxia, spastic ataxia, hereditary spastic paraplegia, or a related disorder. * Severe comorbidity or unstable medical condition that substantially interferes with study participation or interpretation of neurological and functional assessments. * Inability to comply with study procedures or follow-up requirements. * Lack of informed consent, including absence of consent by a legally authorized representative where required. * Current participation in an interventional clinical trial that may interfere with the objectives or outcome assessments of this observational study.