Preliminary design and validation of the "6-K-scale" for bulbar symptoms evaluation in SBMA.

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Spinal and bulbar muscular atrophy (SBMA) is a late onset, X-linked neuromuscular disease. Bulbar symptoms are a main characteristic of the disease but a tool for their clinical evaluation still does not exist. The aim of this study was to design and test a new scale (6-K-scale) for evaluation of bulbar function in SBMA. We considered 60 genetically confirmed SBMA patients and built a scale to evaluate the V, VII, IX, X, and XII cranial nerves (CN) and the ansa cervicalis. Functional status was evaluated through the Spinal and Bulbar Muscular Atrophy Functional Rating Scale (SBMAFRS), 6-min-walk-test (6MWT), Adult Myopathy Assessment Tool (AMAT) scale, and FVC%. Twenty patients underwent a re-test after 3weeks, while 31 were tested longitudinally after 6months. Validation of the scale included reliability assessment and factorial analysis. To evaluate convergent validity, correlations between the 6-K-scale and functional parameters were performed. Internal consistency as measured by Cronbach's alpha was high (0.85) as was test-retest reliability. Principal component analysis yielded a six-factor solution accounting for 71.7% of the variance. The scale score was strongly correlated with the functional parameters. In conclusion, we designed and validated a new scale for bulbar evaluation in SBMA patients. This scale will be a useful tool in the clinical practice as well as a possible outcome measure in clinical trials.