Spinal and bulbar muscular atrophy is an X-linked motor neuron disease caused by a CAG repeat expansion in the androgen receptor gene. To characterize the natural history and define outcome measures for clinical trials, we assessed the clinical history, laboratory findings and muscle strength and function in 57 patients with genetically confirmed disease. We also administered self-assessment questionnaires for activities of daily living, quality of life and erectile function. We found an average delay of over 5 years from onset of weakness to diagnosis. Muscle strength and function correlated directly with serum testosterone levels and inversely with CAG repeat length, age and duration of weakness. Motor unit number estimation was decreased by about half compared to healthy controls. Sensory nerve action potentials were reduced in nearly all subjects. Quantitative muscle assessment and timed 2 min walk may be useful as meaningful indicators of disease status. The direct correlation of testosterone levels with muscle strength indicates that androgens may have a positive effect on muscle function in spinal and bulbar muscular atrophy patients, in addition to the toxic effects described in animal models.

SummaryBackground-Spinal and bulbar muscular atrophy (SBMA) is caused by polyglutamine expansion in the androgen receptor, which results in ligand-dependent toxicity. Animal models have a neuromuscular deficit that is mitigated by androgen-reducing treatment.

We aimed to develop, validate, and evaluate a disease-specific outcome measure for SBMA: the Spinal and Bulbar Muscular Atrophy Functional Rating Scale (SBMAFRS). We examined the Japanese version (SBMAFRS-J) in 80 Japanese SBMA subjects to evaluate its validity and reliability. We then assessed this scale longitudinally in 41 additional SBMA subjects. The English version (SBMAFRS-E) was also tested in 15 US subjects. The total score of the SBMAFRS-J was distributed normally without an extreme ceiling or floor effect. For SBMAFRS-J, the high intra- and inter-rater agreement was confirmed (intra-class correlation coefficients [ICCs] 0.910 and 0.797, respectively), and internal consistency was satisfactory (Cronbach’s alpha 0.700–0.822). In addition, SBMAFRS-J demonstrated concurrent, convergent, and discriminant validity, except for the respiratory subscale. The inter-rater reliability and internal consistency of SBMAFRS-E were also satisfactory. Longitudinally, SBMAFRS-J showed a higher sensitivity to disease progression than the existing clinical measures. In conclusion, we developed and validated a disease-specific functional rating scale for SBMA in both Japanese and English versions, although it needs to be re-assessed in interventional studies with a larger sample size including English speaking subjects.