Functional measures in spinocerebellar ataxia (SCA) – evaluation of a SCA Functional Composite Index

Functional measures are assumed to have superior metric properties as an outcome parameter to assess changes over time. We here report population norms and variability of a Composite Index assessed in 412 SCA patients (genotypes 1,2,3 and 6) in a large multinational trial. The Functional Index is formed of three functional measures: 8m walking time, 9-hole peg test and PATA repetition rate. Whereas PATA rate is found near normally distributed in our study sample (mean 21.7/10s, SD 7.7), time for 8m walk (mean 10.7s, SD 11.8) or 9-hole peg test (mean 47.2s, SD 35.7 in dominant and 52.2s, SD 41.8 in non-dominant hand) is skewed with the majority performing within the lower range. Near normal distrubtion can be achieved by reciprocal transformation of the raw scores. The then comparable and acceptable skewness of the data allow the formation of a composite functional index, as the arithmetic mean of the respective Z-scores of each measure. Z-scores of each component as well as he functional index correlate well with clinical measures of disease severity (Pearson coeff. -0,770), disease duration (-0,334) and patient self assessment of health status (0,363) in univariate analyses. A weaker correlation with age (-0,162) has to be considered as a confounding factor when using this variable in clinical trials.