Background/Purpose: Idiopathic inflammatory myopathies (IIM) is a group of systemic autoimmune diseases characterized by proximal muscle weakness, which is often assessed clinically using manual muscle testing (MMT). Studies have shown that patient’s weight and examiner’s experience and strength may play important roles in grading muscle strength with MMT. Therefore, more objective tools are required to quantify muscle strength in order to optimize treatment response in clinical trials and practice. Hand-held dynamometry (HHD) is a quantitative, inexpensive, compact device with reliability in several neuromuscular diseases. We aimed to assess reliability, validity and responsiveness of HHD in IIM.

Methods: The 6 validated myositis core set measures (CSM; manual muscle testing [MMT], physician global disease activity (MD-GDA), patient global disease activity (PT-GDA), extra-muscular global disease activity (EM-GDA), HAQ and muscle enzymes) and 3 functional measures (six-minute walk [6MWD], timed up-and-go [TUG] and sit-to-stand tests [STS]) were completed on patients with IIM [DM, PM, necrotizing myopathy (NM) or anti-synthetase syndrome] at baseline, 3 and 6 months. At each visit strength was assessed using HHD (Micro FET2, Hoggan Health Industries, Draper, UT) on 3 consecutive attempts. HHD was validated and compared with MMT as well as other CSMs and functional tests. Spearman correlations were used with rho >0.5 considered strong, 0.35-0.5 moderate, and 0.2-0.35 weak. We examined the test-retest reliability as well as HHD responsiveness.

Results: Fifty patients [mean age, 53.6 (±14.6); 30 females/20 males] were studied. Eleven of 50 had PM, 28 had DM, 7 had NM and 4 had anti-synthetase syndrome. HHD showed strong test-retest reliability (Rho: 0.96) and excellent internal consistency (Cronbach-α: 0.95). HHD correlated moderately with MMT score (R:0.44, p=0.003). HHD showed moderate to strong correlation with muscle disease activity (R:-0.5, p:0.0008) and MD-GDA (R:-0.4, p:0.006), HAQ (R:-0.5, p:0.0005), STS (R:0.5, p:0.0002), and 6MWD (R:0.4, p:0.005) (Table 1).  Longitudinal change in HHD strongly correlated with total improvement score (R:-0.6, p=0.01) at 6 months. HHD scores changed significantly in physician-reported “moderately-better” and “a-little-better” groups (p=0.01) demonstrating responsiveness to change. HHD did not change longitudinally in patients who had stable disease demonstrating reliability. MMT and HHD showed similar correlations with the conventional CSM, however, effect size and standardized response mean (SRM) of HHD was higher than MMT (0.56 vs 0.20; 0.33 vs 0.20).

Conclusion: HHD demonstrates good test-retest reliability, construct validity and responsiveness in a large cohort of patients with IIM and a better effect size and SRM than MMT.