The use of muscle strength assessed with handheld dynamometers as a non-invasive biological marker in myotonic dystrophy type 1 patients: a multicenter study

Abstract

Background: Myotonic dystrophy type 1 (DM1) is a multisystem disorder that demonstrates variable symptoms and rates of progression. Muscle weakness is considered one of the main problems with a clinical picture that is characterized by distal weakness of the limbs progressing to proximal weakness. The main objective of this study was to characterize the maximal strength of ankle eversion and dorsiflexion in DM1 patients. Manual and handheld dynamometer (HHD) muscle testing were also compared.

Methods: The maximal strength of 22 patients from Quebec (mean age = 41,1 +/- 13,8) and 24 from Lyon (mean age = 41,6 +/- 10,2) were compared to 16 matched controls.

Results: With the use of HHD, an excellent reproducibility of the torque measurements was obtained for both centers in eversion (R2 = 0,94/Quebec; 0,89/Lyon) and dorsiflexion (R2 = 0,96/Quebec; 0,90/Lyon). The differences between 3 groups of DM1 (mild, moderate, severe) and between them and controls were all statistically significant (p < 0,001). No statistical differences between sites were observed (p > 0.05). The degree of muscle strength decline in dorsiflexion (eversion) were 60% (47%), 77% (71%), and 87% (83%) for DM1 with mild, moderate, and severe impairments, respectively. The smallest mean difference between all DM1 patients taking together was 2.3 Nm, a difference about twice than the standard error of measurement. There was a strong relationship between eversion and dorsiflexion strength profiles (R2 = 0,87;Quebec/0,80;Lyon). Using a 10-point scale, manual muscle testing could not discriminate between the 3 groups of DM1 patients.

Conclusions: The HHD protocol showed discriminative properties suitable for multicentre therapeutic trial. The present results confirmed the capacity of quantitative muscle testing to discriminate between healthy and DM1 patients with different levels of impairments. This study is a preliminary step for the implementation of a valid, reliable and responsive clinical outcome for the measurement of muscle impairments with this population.

Figure 1

The maximum isometric muscle strength of the ankle evertors (A) and dorsiflexors (B and C) was assessed with a Chatillon push-pull hand-held dynamometer. The positioning of the subject was standardised ensuring that «gravity eliminated» positions were used for each muscle group.

Figure 2

Reproducibility of muscle strength at baseline between Day 1 and Day 21 for the ankle evertors and dorsiflexors for Quebec (n = all trials of 21 patients and 7 controls) and Lyon (n = all trials of 24 patients and 9 controls). The equation and coefficients of determination R² are indicated for each muscle group and both centres.

Figure 3

a and b. The top figure shows the mean peak torque and SD of ankle evertors and dorsiflexors of controls (G0) and DM1 patients as classified into Group 1 (G1, mild), Group 2 (G2, moderate), and Group 3 (G3, severe) according to the severity of their muscle impairment. The bottom figure illustrates the same data pooled for the two study sites.

Figure 4

Relationships between the maximal strength (Day 1 and D 21 values combined; n = 180 trials) of the ankle evertors and dorsiflexors for Quebec and Lyon as estimated with simple linear regression. The equation and coefficients of determination R² are indicated for each centre.

Figure 5

Correspondence between the peak torques for the evertor (dotted lines) and dorsiflexor (continuous lines) muscles obtained with quantitative muscle testing (QMT) and manual muscle testing (MMT) scores obtained using the Modified-modified medical research council scale (MM-MRCS). For each MMT score from 0 to 10, the number of trials used (n) for comparisons is indicated. The horizontal arrows are few examples that show, for successive peak torque of 2.5, 5, 10, 15, 25, and 35 Nm, the numerous corresponding MMT scores that could be obtained according to the QMT. The empty and full diamonds in line with each MMT score indicate the mean QMT value, and the related two-tail T bar line the highest and lowest values for that specific MMT score.