In the last few years there have been large international efforts in order to identify outcome measures in DMD. The course of DMD is actually measured by serial clinical assessments of muscle strength, pulmonary function, and functional rating scales. A number of measures including the 6MWT and different timed items or functional scales have been proposed for ambulant DMD patients as part of safety studies or in early stages of treatment. These measures rely on patient effort or on subjective rating of function. However some treatments may be administered to specific groups of muscles, and we therefore need additional measures that may detect and monitor minimal changes in a single muscle or in muscle groups. For these reasons, quantitative assesment have been explored as adjuncts to the physical examination in the assessment of patients with neuromuscular disorders. In this study we assessed QMT using the Kin Com®
, an assessment already used in DMD boys in previous studies and with pediatric normative data. The results of our study show that this technique can be reliably used in DMD boys and that can assess changes over time. All our DMD boys were able to understand instructions to perform the quantitative assessments, showing good participation and motivation. Furthermore, although two patients lost ambulation in the course of the year and thus were unable to continue to perform functional measures, all patients completed the assessment with the Kin Com®
dynamometer. Having five time points (3 monthly spaced assessments over one year) we were also able to estimate a linear trend with time for each Kin Com®
variable. The deviations from these trends represent the normal random fluctuations that can be expected in a population of DMD patients [22
]. It is of interest that the results of both cross sectional and longitudinal assessment showed that the slope of deterioration in DMD boys occurs at approximately 7.5
years. These results are in agreement with our recent observation obtained in a multicentric study and in a much larger cohort using 6MWT and NSAA. Not surprisingly, the decline was more obvious after the age of 9, in all quantitative tests performed with the Kin Com®
dynamometer during right and left isometric and isokinetic flexion/extension of the knee and during isometric flexion/extension of the elbow. The strength value of lower and upper limbs obtained with Kin Com®
show no significant difference between right and left lower limbs or between right and left upper limbs. All the average one-year changes were significantly different than those experienced by the age matched controls. In order to have a more complete assessment, at baseline and 12
months the study cohort was also assessed using the NSSA and the 6MWT. There was a high degree of correlation between the functional tests and isokinetic knee extension tests, especially for correlation between isokinetic knee tests and value of North Star scale.
The results obtained indicate that the use of QMT, in addition to the functional scales, provides sensitive and objective information of muscle strength changes of DMD patients, suggesting that the Kin Com® dynamometer is a valid, sensible and reproducible tool to evaluate muscle strength in ambulant and allowed us to have some results also in patients who lost ambulation after baseline. Therefore, the combination of isometric/isocinetic QMT and functional measures should be regarded as a useful outcome measure for clinical trials in which the mechanism of the drug is expected to produce an increase in strength. Although the numbers were relatively small, our findings information expand the spectrum of our knowledge on individual measures but also provides more insights on their correlation as quantitative muscle strength assessments, 6MWT and functional measures have all been previously investigated in DMD but had never been assessed in the same cohort.