Steinert’s disease or Myotonic Dystrophy is one of the most common type of muscular dystrophy found in adolescent and adults, which necessitates a better understanding of the functioning of skeletomuscular system(de Araújo Evangelista et al., 2017).  Evangelista and colleagues conducted a study to test the efficacy of respiratory muscle in patients suffering from Myotonic dystrophy. Patients suffering from Myotonic Dystrophy suffer from gradual weakness and atrophy of skeletal muscles, cardiac problems, cognitive abnormalities and reduction in respiratory capacity.  One of the most important clinical symptom of myotonic dystrophy is delayed relaxation of contracted skeletal muscles. The maximum relaxation rate (MRR) has been proposed as an indirect marker for fatigue and to measure the strength of respiratory muscle. Evangelista and colleagues propose to measure the MRR of inspiratory muscles, and the electromyographic activity of the stemocleidomastoid (SCM), scalene (SCA), parasternal (2^nd IS) and rectus abdominis (RA) muscles between healthy individuals and patients with Myotonic Dystrophy.

This was a quasi-experimental observational study with a control group (n = 11) and a test group of patients (n= 18). The authors selected the sample size based on a power analysis to lower the probability of committing Type-I error below 0.05.  The authors assessed respiratory muscle strength was assessed from measurements of Plmax, PEmax and SNIP (Sniff Nasal Inspiratory Pressure) by digital manometer. Plmax was measured from residual volume and the PEmax was calculated from lung capacity. The mean age of patients was 42.3±11.3 years with mean time since diagnosis was 6.83±5.98 years.

The results of the study showed that Myotonic Dystrophy patients had significantly lower MRR compared to control subjects. The results conclusively show that Plmax, PEmax scores of the patients was significantly different from the control subjects. The authors conducted a ROC curve analysis and determined that the cutoff point was 5.65; it was found that the electrical activity of the accessory muscles involved in respiration was significantly higher in patients compared to controls with statistical significance < 0.05 in all cases viz., SCM, SCA, and RA.

The authors posit that similar studies have been conducted, wherein MRR has been used to assess the severity of neuromuscular disorders (García-Rio et al., 2006).  The findings of the present study are consistent with previous research on maximum relaxation rate of muscles involved in respiration. This study provides a better understanding of the various factors that may affect myotonic dystrophy using the functional paradigm of maximum relaxation rate.

As admitted by the authors, one of the major limitations of the study was the small sample size and the logistical inadequacies that prevented inclusion of a larger number of subjects in the study. However, the results of this study have been validated and may be used to create new evaluation methods to diagnose the onset, progression of neuromuscular diseases. The indices used in this study is not only applicable to the diseases of myotonic dystrophy but to other neuromuscular diseases as well.

References

de Araújo Evangelista, M., Dias, F. A. L., Júnior, M. E. T. D., do Nascimento, G. C., Sarmento, A., Gualdi, L. P., … de Freitas Fregonezi, G. A. (2017). Noninvasive assessment of respiratory muscle strength and activity in Myotonic dystrophy. PloS One, 12(6). https://doi.org/10.5061/dryad.dc464.Funding

García-Rio, F., Mediano, O., Pino, J. M., Lores, V., Fernández, I., Alvarez-Sala, J. L., & Villamor, J. (2006). Noninvasive measurement of the maximum relaxation rate of inspiratory muscles in patients with neuromuscular disorders. Respiration, 73(4), 474–480.