THU0449 Utility of Ultrasound (US) in Assessing Skeletal Muscle Architecture in Idiopathic Inflammatory Myopathies (IIM)
Authors: B. Y. Hanaoka, L. C. Cleary, D. Long, G. S. Chleboun, C. A. Peterson, C. P. Starnes, L. J. Crofford
Journal: Annals of the Rheumatic Diseases (2013)

<h3>Background</h3> IIM are systemic autoimmune diseases characterized by chronic inflammation in skeletal muscle leading to proximal muscle weakness. In rheumatoid arthritis, muscle weakness has been linked to specific changes in muscle architecture that are measureable using ultrasound (US).<sup>1</sup> However, data regarding the contribution of muscle architecture to muscle weakness in IIM is lacking. US is a non-invasive, relatively inexpensive and validated method of assessing skeletal muscle architectural parameters [i.e. anatomic cross sectional area (ACSA), fascicle length (Lf), pennation angle (θ) and muscle thickness].<sup>2,3</sup> US determination of L<sub>f</sub> and θ allows calculation of the physiological cross sectional area of muscle, which is a better predictor of intrinsic muscle force compared to ACSA or volume.<sup>4</sup> <h3>Objectives</h3> To test the utility of US in determining skeletal muscle architecture in patients with IIM. To investigate associations between rectus femoris muscle ACSA (RF<sub>ACSA</sub>), muscle peak torque generation, and other clinical outcomes in patients with IIM. <h3>Methods</h3> Clinical data were obtained from chart reviews of IIM cases seen in the Rheumatology clinic at the University of Kentucky from May/2006 until Jan/2013. Participant body composition (DXA), mid-thigh bilateral RF<sub>ACSA</sub> and right vastus lateralis fascicle length(VL<sub>Lf</sub>) were measured with the knee extended and muscle relaxed. Right knee extensor and elbow flexor muscle-specific peak torques were measured using a Biodex dynamometer. Data were analyzed using descriptive statistics and Spearman rank correlation coefficient. <h3>Results</h3> 12 patients with IIM and overlap myositis (OM) were included: 2 polymyositis (PM), 4 dermatomyositis (DM), 5 sporadic inclusion body myositis (sIBM) and 1 OM. In 1 PM and 3 sIBM patients, muscle architecture was so disrupted that VL<sub>Lf</sub> could not be measured. Right and left mean RF<sub>ACSA</sub> were correlated (<i>p</i>=0.02). Mean RF<sub>ACSA</sub> was inversely correlated with total body and thigh fat (<i>p</i>&lt;0.05), and positively correlated with trunk lean mass and elbow flexor maximal voluntary isometric contraction (MVIC) (<i>p</i>&lt;0.05). However, RF<sub>ACSA</sub> did not correlate well with knee extensor MVIC. VL<sub>Lf</sub> was measured in 3 DM patients (mean=7.92cm, SD=2.03) and 1 PM patient (6.79cm), which is in the described range.<sup>2</sup> <h3>Conclusions</h3> RF<sub>ACSA</sub> was associated with body composition. It is possible that in IIM, quadriceps muscle is weaker than predicted by RF<sub>ACSA</sub> or lean body mass. In severe cases of IIM, in particular sIBM, substantial disruption of muscle architecture could be detected by US. <h3>References</h3> Matschke V, Murphy P, Lemmey AB, Maddison P, Thom JM. Skeletal muscle properties in rheumatoid arthritis patients. Medicine and science in sports and exercise 2010;42:2149-55. Chleboun GS, France AR, Crill MT, Braddock HK, Howell JN. In vivo measurement of fascicle length and pennation angle of the human biceps femoris muscle. Cells, tissues, organs 2001;169:401-9. Herbert RD, Gandevia SC. Changes in pennation with joint angle and muscle torque: in vivo measurements in human brachialis muscle. J Physiol 1995;484 ( Pt 2):523-32. Narici MV, Landoni L, Minetti AE. Assessment of human knee extensor muscles stress from in vivo physiological cross-sectional area and strength measurements. Eur J Appl Physiol Occup Physiol 1992;65:438-44. <h3>Acknowledgements</h3> This study was supported by the Arthritis Foundation, the Center for Clinical and Translational Science (CCTS) at the University of Kentucky, the University of Kentucky College of Medicine Clinical Scholars Program and Research Data Capture (REDCap). <h3>Disclosure of Interest</h3> None Declared