Factors Influencing Disease Progression in Autosomal Dominant Cerebellar Ataxia and Spastic Paraplegia

Objectives: To evaluate disease progression and determine validity of clinical tools for therapeutic trials. Design: Prospective cohort study (36 months). Setting: Referral center. Patients: One hundred sixty-two patients with autosomal dominant cerebellar ataxia and 64 with hereditary spastic paraplegia. Main Outcome Measures: The quantitative Composite Cerebellar Functional Severity Score with the writing test (CCFSw) and Scale for the Assessment and Rating of Ataxia (SARA) score. Results: Disease worsened in patients with SCA1, SCA2, and SCA3 mutations (mean [SE] increase in CCFSw, +0.014 [0.005] to +0.025 [0.004] per year), improved in patients with SPG4 mutations (mean [SE] increase in CCFSw, -0.012 [0.003] per year; P=.02), and remained stable in patients with SCA6, SCA7, or other SCA mutations (mean [SE] increase in CCFSw, -0.015 [0.011] to +0.009 [0.013] per year) or hereditary spastic paraplegia with other SPG mutations (mean [SE] increase in CCFSw, -0.005 [0.005] per year). Progression was faster in patients with SCA2 mutations and normal alleles with 22 or fewer repeats (P=.02) and in patients with SCA3 mutations with parkinsonism and/or dystonia at baseline (P=.003). Whereas CCFSw distinguished between patients with ataxia and spasticity, SARA scores increased in both groups. A 2-arm trial with SARA score as the outcome measure would require 57 patients with SCA2 mutations, 70 with SCA1 mutations, and 75 with SCA3 mutations per group to detect a 50% reduction in disease progression (power, 80%; alpha=.05). Conclusions: Disease progressed faster in SCAs with polyglutamine expansions in SCA1, 2, and 3 than the other groups. Both outcome measures are suitable for therapeutic trials; SARA requires fewer patients to attain the same power, but CCFSw needs less stratification. We demonstrate that the choice of clinical outcome measure is critical for reliable evaluation of progression in neurodegenerative diseases.