In this study, using a 0 – 10 point MMT scale (8
) to test 24 bilateral proximal, axial, and distal muscles, we preliminarily validated the Total and Proximal MMT scores by demonstrating internal reliability and consistency, rater reliability, convergent construct validity, and responsiveness in assessing strength in patients with adult PM/DM and JIIM. Most therapeutic PM/DM trials have used unvalidated five-point MMT scales that include varying sets of proximal muscles and some distal muscle groups (6
). The lack of consistency in scoring or in the content of specific muscle groups makes it difficult to compare data across studies. More recent trials used the 0 – 10 point MMT of a total score of muscles, because this expanded scale is thought to enhance the sensitivity of strength testing, particularly in the fair to good muscle strength range (8
). Inclusion of a uniform set of muscle groups with standardized scoring (5
) that has been validated should provide clinicians and researchers with more reliable, standardized MMT data in patients with adult and juvenile DM/PM.
Total MMT testing has several disadvantages, including that testing of 24 muscle groups is time consuming, patients often fatigue during the testing, occasionally experience muscle pain, making muscle testing unpleasant and stressful, and children frequently are not able to cooperate for the entire 24 muscle group test, resulting in incomplete results or inconsistent strength evaluation (18
). Testing numerous muscles and changing test positions can add to overall and local muscle fatigue and decrease test reliability due to decreased muscle force. Documenting patterns and symmetry of weakness in our previous study in juvenile and adult DM/PM (5
) allowed us to evaluate Total and Proximal MMT scores as measures of strength testing in patients with myositis. In the present study, to approximate the Total MMT score while reducing its disadvantages, we also examined unilateral muscle subsets of six or eight muscle groups that included proximal, distal and axial muscles and more emphasis on proximal and lower extremity muscle groups. We postulated that testing a subset of muscles might be more sensitive to change over time and have better reliability. A validated abbreviated MMT score that includes fewer muscles and three instead of five test positions is not only more efficient but might also improve testing reliability.
The explicit purposes of developing these abbreviated muscle subsets are for enhancing consistency among international myositis therapeutic trials with adult and pediatric patients and for brief clinical follow-up visits. By NGT consensus formation, we rank-ordered the best eight muscle group subsets containing unilateral representative axial, as well as upper and lower extremity proximal and distal muscle groups (). These MMT subsets had results comparable to those of Total and Proximal MMT scores for internal reliability, consistency, and construct validity. These eight muscle group subsets were in a comparable range for rater reliability in JIIM patients. They were slightly more responsive than the Total or Proximal MMT score, which is especially important for therapeutic trials. The eight muscle group subsets performed better than the six muscle group subsets, and no six muscle group subsets achieved consensus for replacing the Total MMT score. The top-rated eight muscle group subsets also had acceptable face validity, included frequently involved muscle groups, and were felt to be easier to test, even in patients with joint contracture or calcinosis. However, they require prospective validation, including in therapeutic trials, to further define their performance characteristics in other populations.
MMT has been widely used to test muscle strength in the IIM (25
), and is a preferred method of testing strength in patients with IIM because weakness encompasses the full range, from no to full strength, and MMT tests this full range; it is simple, easy to use and available for all examiners internationally; and validated, standardized MMT scales allow comparison between studies (18
Although validation of the Total MMT and MMT8 subsets in this study is well substantiated, the study has some limitations. Although we included the most common forms of IIM (adult and juvenile PM/DM), we did not include patients with inclusion body myositis and myositis associated with malignancy. Twenty-four of the 45 adult patients were accepted into therapeutic trials for refractory disease and may have been weaker than those regularly encountered in clinical practice. Also, we did not include recently diagnosed, possibly weaker, patients. However, the remaining patients were either adults who did not meet weakness criteria for therapeutic trials or JIIM patients who were enrolled in a natural history study. These patients might be more representative of patients at other centers. Another potential limitation is that our patients enrolled from rheumatology centers that specialize in IIM, where the therapists have many years of myositis testing experience, which could result in better quality data than if they were enrolled at clinics serving fewer IIM patients.
Although the MMT8 has several advantages, the top-rated MMT8 subsets might not yet contain the most representative muscles. The highest ranking subset contained four of the five weakest muscles identified in the IIM (neck flexor, deltoid, gluteus maximus, and gluteus medius), but it did not include hip flexors, the other weakest muscle group in adult and JIIM patients (5
). It did include the proximal quadriceps and two distal muscles (ankle dorsiflexors and wrist extensors), which are functionally important (26
). Clinically, some muscles are easier to test than others; those that require position changes for testing are generally not as popular with clinicians. More research is needed to determine and validate the best MMT8 subsets.
The current study is the only one to validate the bilateral Total MMT score consisting of 24 muscles, a Proximal MMT score including 14 muscle groups, and unilateral eight muscle group MMT subsets, chosen by consensus formation, for patients with adult and juvenile DM/PM. The MMT8 subsets, which take less time and involve less patient effort, performed as well as or better than Total MMT in terms of responsiveness, content validity, and construct validity. This preliminary validation of Total MMT, Proximal MMT, and an abbreviated subset of eight key muscles can also help standardize the testing and reporting of MMT, an important measure of strength and a core outcome measure (9
) for myositis clinical studies.