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.
MUNE, a technique used in ALS clinical trials to quantitatively assess motor neuron loss, should also be valuable in assessing progression in spinal bulbar muscular atrophy (SBMA), an x-linked neuronopathy. In ALS, instability of single motor units (SMUP) prompted Shefner9 to modify the statistical MUNE method to exclude SMUPs ≤ 40μV. It is unknown if there is a similar SMUP instability in the more chronic degenerative disease of SBMA.In this study, the standard parameter of excluding SMUP < 10 μV was compared with the exclusion of SMUP < 40 μV in the calculation of the statistical MUNE. The mean statistical MUNE, using the standard method and the Shefner method, was 60±21 to 47±23, respectively. Similar to ALS, SBMA showed an increased proportion (17%) of individual SMUPs ≤ 40 μV compared to normal controls.In conclusion, excluding SMUPs ≤ 40 μV from the statistical MUNE calculations is appropriate for SBMA subjects because their SMUPs characteristics are similar to ALS. Exclusion of the low amplitude SMUPs reduces the calculated MUNE.
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