Validity of balance measures in cerebellar ataxia: A prospective study with 12-month follow-up

Introduction: Balance deficits are common in cerebellar ataxia. Determining which balance outcome measures are psychometrically strong for this population remains an unmet need. Objective: To evaluate the validity and responsiveness of two clinic-based balance measures (Berg Balance Scale [BBS] and balance sub-component of the Scale for the Assessment and Rating of Ataxia [SARA-bal]) and two laboratory-based balance measures (Sensory Organization Test [SOT] and Limits of Stability [LOS]) in cerebellar ataxia. Design: Prospective cohort study. Setting: Institutional study assessing 40 participants with cerebellar ataxia at baseline, and 6 and 12 months. Main outcome measures: Balance was assessed using the BBS, SARA-bal, SOT, and LOS; disease severity was assessed using the SARA; and Patients' Global Impression of Change (PGIC) was used to estimate responsiveness to disease progress at 6 and 12 months. Results: BBS and SARA-bal (Spearman's correlation coefficient, ρS = −0.89, p <.01) demonstrated strong criterion validity. Convergent validity was moderate to high (ρS range: −0.75 to 0.92) and external validity was low (ρS range: −0.75 to 0.11). Composite SOT scores (SOT-COM; ρS = 0.29, p <.01) and maximal excursion (MXE-LOS) in the forward (F) and right (R) directions of the LOS (ρS = 0.18, p <.01) demonstrated moderate to low criterion and convergent validity. The area under the receiver-operating characteristic curve (AUC _ROC) and its effect size (standard response mean [SRM]) for categorizing “stable” and “worsened” patients at 6 and 12 months were satisfactory for the BBS (AUC _ROC: 0.75; SRM-Stable: 1.06; SRM-Worsened: 1.16), SARA-bal (AUC _ROC: 0.76; SRM-Stable: 0.86; SRM-Worsened: 0.85), and MXE-LOS(R) (AUC _ROC: 0.29; SRM-Stable: 0.41; SRM-Worsened: 1.39). Conclusion: BBS and SARA-bal have moderate to strong criterion and convergent validity and adequate responsiveness to balance changes. Both laboratory-based measures (SOT and LOS) demonstrated a high floor effect. The SOT-COM and MXE-LOS(R) demonstrated moderate to low criterion validity, with only the MXE-LOS(R) displaying adequate responsiveness to balance changes after 6 and 12 months.