Noninvasive assessment of respiratory muscle strength and activity in Myotonic dystrophy

Abstract

Objective: To evaluate sensitivity/specificity of the maximum relaxation rate (MRR) of inspiratory muscles, amplitude of electromyographic activity of the sternocleidomastoid (SCM), scalene (SCA), parasternal (2ndIS) and rectus abdominis (RA) muscles; lung function and respiratory muscle strength in subjects with Myotonic dystrophy type 1 (DM1) compared with healthy subjects.

Design and methods: Quasi-experimental observational study with control group. MRR of inspiratory muscles, lung function and amplitude of the electromyographic activity of SCM, SCA, 2ndIS and RA muscles during maximum inspiratory pressure (PImax), maximum expiratory pressure (PEmax) and sniff nasal inspiratory pressure (SNIP) tests were assessed in eighteen DM1 subjects and eleven healthy.

Results: MRR was lower in DM1 group compared to healthy (P = 0.001) and was considered sensitive and specific to identify disease in DM1 and discard it in controls, as well as SNIP% (P = 0.0026), PImax% (P = 0.0077) and PEmax% (P = 0.0002). Contraction time of SCM and SCA was higher in DM1 compared to controls, respectively, during PImax (P = 0.023 and P = 0.017) and SNIP (P = 0.015 and P = .0004). The DM1 group showed lower PImax (P = .0006), PEmax (P = 0.0002), SNIP (P = 0.0014), and higher electromyographic activity of the SCM (P = 0.002) and SCA (P = 0.004) at rest; of 2ndIS (P = 0.003) during PEmax and of SCM (P = 0.02) and SCA (P = 0.03) during SNIP test.

Conclusions: MD1 subjects presented restrictive pattern, reduced respiratory muscle strength, muscular electrical activity and MRR when compared to higher compared to controls. In addition, the lower MRR found in MD1 subjects showed to be reliable to sensitivity and specificity in identifying the delayed relaxation of respiratory muscles.

Fig 1. Study flowchart.

Fig 2

Comparison between MRR (A), SNIP (B), PImax (C) and PEmax (D) for DM₁ and control group. Data and cutoffs of respiratory muscle weakness for PImax, PEmax and SNIP are presented in absolute values. The cutoff points established for male and female, respectively, were: 59.1 cmH₂O and 70.4 cmH₂O, for PImax; 58.2 cmH₂O and 53.9 cmH₂O, for SNIP; 101.1 cmH₂O and 69.2 cmH₂O for PEmax. All data showed parametric distribution and were compared using Unpaired t test. Differences were considered significant when P<0.05.

Fig 3

Surface electromyography during the electrical activity of the muscles SCM, SCA, RA and 2^ndIS at rest (A), SNIP (B), PImax (C) and PEmax (D). Values were normalized and are shown in normalized data (% of RMS at rest) in SCM, SCA and RA. The normalized data from 2^ndIS was done considering the RMS of maximal values of SNIP and PImax maneuvers of each muscle, during the maneuvers. The sEMG measure at 2^ndIS was normalized by SNIP and PImax, both maximal measures of inspiratory muscles. All data presented non-parametric distribution and were compared using the Mann-Whitney test. Differences were considered significant when P<0.05.

Fig 4

Time of electrical activity of SCM, SCA, RA and 2^ndIS muscles during SNIP (A) and PImax (B) assessment. Values are expressed in seconds. All data showed parametric distribution and were compared using the unpaired t-test with Welch’s correction. Differences were considered significant when P<0.05.

Fig 5. ROC curves of MRR, % predicted of SNIP, PImax e PEmax for DM₁ and control group.