Validation of the German version of the extended ALS functional rating scale as a patient-reported outcome measure

The revised Amyotrophic Lateral Sclerosis Functional Rating Scale (ALSFRS-R) is a well-established rating instrument to assess the functional status of ALS patients. A recent innovation was the addition of three further items designed to improve its sensitivity at lower levels of physical function (ALSFRS-Extension, ALSFRS-EX). Neither the ALSFRS-R nor the ALSFRS-EX has been validated in German yet. The aim of the present study was the validation of the German version of a self-administered form of the ALSFRS-EX. Seventy-six patients participated in the study. Psychometric analysis included reliability assessment and factorial analysis. To evaluate convergent validity, correlations between ALSFRS-EX items and the MRC score, spasticity, tongue movement, pulmonary function, ALSAQ-40 and Borg dyspnoea scales (upright and supine) were performed. Internal consistency as measured by Cronbach’s alpha (total scale 0.868, subscales 0.690–0.938) and corrected item to total correlations (all above 0.50) was high. Test–retest reliability assessed by Spearman’s rho (0.882–0.972) and Cohen’s Kappa (0.63–0.92) was also high. Principal component analysis with varimax rotation yielded a four-factor solution accounting for approximately 79 % of the variance. Clinical parameters were strongly correlated with respective items and subscores of the ALSFRS-EX (muscle strength 0.568–0.833 p