Background Inclusion body myositis (IBM) is a rare idiopathic inflammatory myopathy subtype. Our study aimed to describe the disease progression rate in IBM using measures of muscle strength and functional assessment tools.
Methods Retrospective analysis of quantitative muscle strength data and functional assessment tool results (IBM Functional Rating Scale [IBM-FRS] and the Neuromuscular Symptom and Disability Score [NSS]), systematically collected from verified IBM cases (July 2003 to March 2015) at a UK neuromuscular clinic. Yearly rate of strength change was estimated for each muscle group and associations between with the IBM-FRS/NSS were examined using simple linear regression.
Results Data from 80 IBM cases (44% female) were analysed, a total of 232 person-years and a median 2.3 years (IQR 0.6, 4.4) follow-up. Similar rates of deterioration were found for right and left sided muscle groups. Annual rate of strength loss was greatest for grip, pinch and knee extension (-5.5% [95% CI -6.3, -4.6], -16.0% [-18.4, -13.5] and -7.1% [-7.6, -6.5]). The rate of strength loss was greatest for males. Increased strength measurements were associated with improved IBM-FRS/NSS scores for most muscle groups.
Conclusion This is the first long-term study to comprehensively demonstrate quantitative strength change in a large IBM cohort. We provide valuable natural history data to support discussions regarding prognosis, focus effective choice of outcome measures and inform required study numbers for future clinical trials.
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