This cross-sectional study focused on force-generating capabilities of muscle in IIM measured with MMT. Our findings provided a data-driven description of weakness patterns in IIM and provided additional insights into associated muscle dysfunction.
The eleven bilateral muscle groups included in this study were found to be symmetric in degree of weakness for IIM clinical groups. We elected to organize the MMT subscores into regional and functional anatomic muscle groups to better understand the patterns of muscle weakness in IIM. While the traditional organization of MMT subscores into regional anatomic groups is used in the clinical diagnosis of IIM, these subgroups do not always reflect a functional organization of muscle groups within the context of basic and instrumental activities of daily living. We suggest that this functional organization of MMT scores may provide a construct for improving our understanding of strength function relationships in future studies (e.g. lower extremity strength influence on walking speed or chair rise ability). The severity of lower extremity and proximal muscle group weakness exceeded that of the upper extremity, axial and distal muscle groups, consistent with clinical insights of other investigators [1
]. However, our findings suggest that only the proximal and lower extremity MMT subscores were significantly different across all patient groups. Identifying and monitoring lower extremity weakness in patients with IIM is of clinical importance since impairments of these functional muscle groups have been associated with increased risk of falls and nursing home placement for older adults [40
While the proximal pattern of weakness in IIM is well known, less attention has been given to variations in the patterns of weakness among different IIM clinical groups. Although distal MMT subscores were higher for all patients, it may be a distinguishing feature for some IIM clinical groups. Our findings revealed significantly lower MMT grades for the wrist flexors in patients with DM and the ankle plantar flexors in patients with PM.
Individual muscle groups were stratified based on the 10-point MMT grading criteria (). This allowed us to make distinctions among clinical groups and provide a clinical context for the MMT results. The proportion of individual MMT grades in the ‘moderate’ and ‘severe’ range revealed that PM, DM and JDM share the five weakest muscle groups from both the axial and proximal regions: neck flexors, shoulder abductors, hip flexors, hip extensors and hip abductors. These findings provide data to further refine MMT use as a core set measure [5
] and clinical guidance regarding the targets of exercise interventions.
Improving our understanding of the distribution and severity of weakness in IIM is important, and raises a critical question: how does one interpret MMT scores in the context of functional performance? Strength demands of walking or rising from a chair require lower extremity muscle groups to be capable of not only moving the limbs against gravity, but also supporting and moving the body weight. Eriksrud and Bohannon [41
] reported that elderly subjects unable to complete the sit-to-stand task attained 64% of the maximum summed MMT grades for the knee extensors, whereas subjects with 89% of the maximum summed MMT grades could complete the task. Siegel et al.
] showed the number of lower extremity muscles with an MMT grade of <7 out of 10 predicted walking speed in patients with juvenile IIM. Our JDM patients were relatively high functioning and did not exhibit any lower extremity median MMT grades <7 out of 10. However, our patients with DM or PM exhibited median MMT grades of ≤7 or ≤5 out of 10, respectively, for the hip muscle groups, indicating considerable weakness. The increased proximal weakness observed at all three hip muscles in patients with PM compared with DM may reflect a critical threshold of function and may partially explain why our patients with PM scored lower on functional performance measures.
Although this is the largest study of specific patterns of weakness in IIM clinical groups using a standardized approach to assessing strength, there are limitations. This study included MMT data obtained from patients over a wide range in age, including four participants with JDM between 4.6 and 7 yrs of age and five patients between 7 and 8 yrs of age. Some investigators [28
] have cautioned against administering the MMT to patients <5 or 6 yrs of age. Complications encountered in the strength assessment of very young subjects may be related to levels of attention, motivation and comprehension during testing [43
]. In a study featuring 825 children with myelomeningocoel, McDonald and colleagues [44
] reported that serial MMT examinations yielded stable results in participants older than 5 yrs of age. Our investigation featured only three subjects aged 5 yrs old or younger. In addition, our paediatric therapists did not encounter any patients in this study in which MMT findings were affected by subject cooperation or behaviour.
We found significant association among the functional measures and the proximal MMT subscore, but our regression analyses did not provide any additional insights regarding the relationship of clinical course variables with the magnitude of weakness. It should be noted, however, that disease duration was greater in patients with PM compared with DM, which may have had an influence on the greater degree of proximal weakness seen in this clinical group. Additional study will be needed to better understand the relationship among weakness, functional performance and extra-musculoskeletal disease features. Also, different physical therapists tested the adult and paediatric patients. Nonetheless, we believe that use of multiple physical therapists had minimal impact on results based on prior examiner training and our observation of good inter-rater reliability of summed MMT scores [45
It is important to note that a general limitation of MMT is that it is unable to distinguish between strength impairments that result from disease activity vs
disease damage [8
]. IMACS and PRINTO have recognized the constraints of MMT as a primary therapeutic trial outcome and have recommended incorporating a physician global disease activity assessment to augment this core set measure [5
]. Finally, our reported findings regarding the distribution and severity of muscle weakness in our cohort of patients with IIM are constrained by our selection of muscle groups to include in the MMT. The results of our study do not suggest that other muscle groups not included in our MMT do not contribute to the functional limitations and disability observed in this patient population or that our entire group of muscles tested is recommended for inclusion in routine clinical testing or clinical trials.
In conclusion, our study provides the most detailed account of weakness in a cohort of patients with IIM. These findings suggest that PM is associated with greater proximal and lower extremity weakness than DM and JDM, and muscle groups are affected symmetrically across IIM groups. In addition, we have identified the five weakest muscle groups across IIM clinical groups, which may serve to influence the selection of end-points in future therapeutic trials and the targets of exercise intervention.