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 < 0.01; spasticity -0.236 to -0.376 p < 0.05; tongue movement 0.437-0.818 p < 0.01; pulmonary function 0.485-0.577 p < 0.01). ALSAQ-40 and Borg score correlated highly with the corresponding ALSFRS-EX items. The German self-report version of the ALSFRS-EX possesses very good psychometric properties similar to the original scale including high internal consistency and test-retest reliability as well as excellent convergent validity.