Background
Cardiac disease is a major cause of death in patients with muscular dystrophies. The use of feasible and reproducible echocardiographic measures of cardiac function is critical to advance the field of therapeutics for dystrophic cardiomyopathy.
Methods
Participants aged 8 to 18 years with genetically confirmed Duchenne muscular dystrophy (DMD), Becker muscular dystrophy, or limb-girdle muscular dystrophy were enrolled at five centers, and standardized echocardiographic examinations were performed. Measures of systolic and diastolic function and speckle-tracking echocardiography–derived cardiac strain were reviewed independently by two central readers. Furthermore, echocardiographic measures from participants with DMD were compared with those from retrospective age-matched control subjects from a single site to assess measures of myocardial function.
Results
Forty-eight participants (mean age, 13.3 ± 2.7 years) were enrolled. Shortening fraction had a greater interobserver correlation (intraclass correlation coefficient [ICC] = 0.63) compared with ejection fraction (ICC = 0.49). One reader could measure ejection fraction in only 53% of participants. Myocardial performance index measured by pulse-wave Doppler and Doppler tissue imaging showed similar ICCs (0.55 and 0.54). Speckle-tracking echocardiography showed a high ICC (0.96). Focusing on participants with DMD ( n = 33), significantly increased mitral A-wave velocities, lower E/A ratios, and lower Doppler tissue imaging mitral lateral E′ velocities were observed compared with age-matched control subjects. Speckle-tracking echocardiography demonstrated subclinical myocardial dysfunction with decreased average circumferential and longitudinal strain in three distinct subgroups: participants with DMD with normal shortening fractions, participants with DMD aged < 13 years, and participants with DMD with myocardial performance index scores < 0.40 compared with control subjects.
Conclusions
In a muscular dystrophy cohort, assessment of cardiac function is feasible and reproducible using shortening fraction, diastolic measures, and myocardial performance index. Cardiac strain measures identified early myocardial disease in patients with DMD.
Cardiomyopathy causes significant morbidity and mortality in multiple forms of muscular dystrophy (MD) affecting children, including Duchenne MD (DMD), Becker MD (BMD), and subtypes of autosomal-recessive limb-girdle MD (LGMD). The prevalence of cardiomyopathy increases with age and is currently underdiagnosed in patients with DMD. Therefore, it is increasingly important to study cardiac disease in muscular dystrophies to determine the best diagnostic and treatment modalities.
Adequately powered pharmaceutical studies for the treatment of MD require the collection of reproducible data across multiple clinical sites because of the rarity of these diseases. Several studies have researched cardiac end points, but comparisons across studies are hindered by different study designs and functional measures, which include percentage shortening fraction (SF%), percentage ejection fraction (EF%), mitral inflow velocities, myocardial performance index (MPI), and myocardial strain. These measures are also affected by technical limitations in patients with MD, including scoliosis, barrel chest deformities with lung hyperinflation, and limited mobility.
The purpose of this study was to assess the feasibility and reproducibility of noninvasive echocardiography-based functional cardiac measures in a multicenter cohort of participants with MD and determine which measures can detect early subclinical changes in myocardial function. The Cooperative International Neuromuscular Research Group (CINRG) is a coalition of academic clinical centers dedicated to MD research. CINRG has validated skeletal muscle functional measures for multisite DMD studies. The results of this study will help direct the selection of cardiac measurements for future clinical trials and enhance consistency within the field of cardiomyopathies in patients with MD.
Methods
This was a multicenter, prospective study investigating different echocardiographic parameters with participants enrolling at five institutions in the CINRG network, all also part of the Clinical and Translational Science Award network ( http://www.ctsacentral.org ). The study was approved by the institutional review board at each institution. Written informed consent and assent were obtained from all participants and their parents or legal guardians. The study was registered with ClinicalTrials.gov (identifier NCT01066455 ). Participants had confirmed genetic diagnoses of DMD, BMD, or an autosomal-recessive subtype of LGMD (LGMD2C, LGMD2D, LGMD2E, LGMD2F, or LGMD2I). Participants were excluded if they had histories of congenital cardiac defects or other cardiac diseases unrelated to MD. Retrospective control subjects for participants with DMD were patients referred to the cardiology clinic at one of the five participating CINRG clinical sites (Children’s National Health System) for cardiac murmur or chest pain and were determined to have normal results on cardiac evaluation with normal conventional echocardiographic parameters. Race and ethnicity were self-reported and not required for control subjects.
Methods to Determine the Feasibility and Reproducibility of Noninvasive Echocardiography-Based Functional Cardiac Measures
A central sonographer traveled to each participating CINRG site to review and train the site personnel on the use of a centralized protocol using standard imaging planes and recording three-beat loops saved in Digital Imaging and Communications in Medicine format. Echocardiographic studies were performed with participants in the supine position on an examination table or seated in a power wheelchair if transfer to an examination table was not possible. These digital loops were interpreted by two pediatric cardiologists and one pediatric cardiology fellow. There were two readers at the Washington site (C.F.S. and S.J.G. [fellow]) and one reader at the Pittsburgh site (F.M.M.).
All measurements and calculations, including SF%, EF% using the single-plane modified Simpson protocol, wall stress, velocity of circumferential shortening, rate-corrected velocity of circumferential shortening, MPI (which includes measures of ejection time [ET], isovolumic contraction time [IVCT], and isovolumic relaxation time [IVRT]) using both pulse-wave Doppler (PWD) and Doppler tissue imaging (DTI), mitral inflow, and left ventricular peak E-wave velocities, were made according to standards of the American Society of Echocardiography. Each reader (C.F.S. and F.M.M.) measured two-dimensional and Doppler values over three cardiac cycles (or the maximum feasible when imaging was of limited quality), and the average was used for analysis. To assess intraobserver variability, 47 echocardiograms were reassessed by the same observer (C.F.S.) after a period of ≥2 weeks. Interobserver variability was assessed by having a different reader (F.M.M.) perform all the measures on the conventional echocardiographic images independently.
Methods to Determine Early Subclinical Myocardial Changes
Cardiac strain was measured using speckle-tracking echocardiography (STE) on the subset of participants with DMD from all five participating CINRG centers and historical control subjects from a single institution (Children’s National Health System), using proprietary software (Syngo Velocity Vector Imaging; Siemens Medical Solutions USA Inc, Mountain View, CA). Speckle-tracking echocardiographic analysis was performed by two readers (C.F.S. and S.J.G.). For STE, endocardial tracings of the left ventricle were manually performed in the apical four-chamber view (for longitudinal measurements) and the parasternal short-axis view at the level of the midpapillary muscles (for circumferential measurements). A single cardiac beat with the best-appearing image quality was used. Tracking was automatically performed by the software, and the analysis was accepted as satisfactory only after visual inspection. Endocardial tracing and automatic tracking were performed twice on each beat, and the average of the two measurements was recorded for each variable. If tracking was suboptimal, the endocardial border was retraced until either satisfactory tracking was accomplished or 5 min had passed, in which case the view was excluded from analysis. Measured deformation parameters included average peak systolic longitudinal strain (LS), average peak systolic longitudinal strain rate, average peak systolic circumferential strain (CS), and average peak systolic circumferential strain rate (CSR). Segmental data were not analyzed. Mathematically, all of these parameters are negative (indicating shortening), and a smaller absolute value indicates worse ventricular function. To assess intraobserver variability for STE, a randomly selected set of 11 echocardiograms (six from patients with DMD, five from control subjects) were reassessed by the same observer (S.J.G.) after a period of ≥2 weeks. Interobserver variability was assessed by having a different observer (C.F.S.) perform speckle-tracking echocardiographic measurements on all possible DMD echocardiograms using the same beat as the original observer.
Statistical Analysis
Descriptive measurements, including demographic characteristics (age, ethnicity, and race), blood pressure, heart rate, and body mass index, are summarized as mean ± SD or as ranges or frequencies appropriate for each data type. Age, blood pressure, and heart rate were compared between the overall MD group and the DMD control subjects using t tests, while body mass index required log-transformed data for t tests. Both the number of participants and the distribution of diagnosis (DMD, BMD, and LGMD) were compared by medication use (glucocorticoids, cardiac medications, coenzyme Q 10 ) with exact χ 2 analysis. Mean age in the different medications use group was compared with a t test. Race and ethnicity were not consistently captured for the control subjects and hence were not compared with these variables among patients with MD.
Nineteen continuous echocardiographic parameters are summarized as mean ± SD and range for each reader. Agreement between echocardiographic measurements was assessed by calculation of the intraclass correlation coefficient (ICC) both between the two readers (interobserver) and between repeated measurements from reader 1 (intraobserver). Bland-Altman plots were produced showing the mean bias and 95% limits of agreement. EF% and SF% were compared using t tests, while MPI required log-transformed data for the t test. Left ventricular EF%, SF%, and MPI measurements were also dichotomized between normal and abnormal on the basis of the following criteria: EF% (normal, ≥55; abnormal, <55), SF% (normal, ≥28; abnormal, <28), and DTI and PWD MPI (normal, <0.40; abnormal, ≥0.40). These dichotomized variables were then compared pairwise to evaluate agreement in normal or abnormal designation using a McNemar test.
Continuous measurements from STE, along with basic demographic characteristics, are summarized as mean ± SD and were compared between patients with DMD and control subjects using t tests. Subgroup analyses were performed in the same manner on those with normal SF% or EF%, those aged < 13 years, and those with normal MPI values (<0.40).
All analyses used Stata version 13 (StataCorp LP, College Station, TX), and P values < .05 were considered to indicate statistical significance. No adjustments for multiple tests were done, because of the observational focus of this study.
Results
Patient Characteristics
Information for all participants (those with MD and retrospective control subjects) is included in Table 1 . Fifty-two participants with MD provided consent at five sites, 48 of whom completed the study. Thirty-three control subjects without MD were used for subset analysis of participants with DMD. Participants with MD had genetically confirmed diagnoses of DMD (73%), BMD (23%), or LGMD (4%). The mean age in the MD cohort was 13.3 ± 2.7 years, with a range of 8.4 to 17.7 years, and the mean age in the control cohort was 12.8 ± 2.8 years, with a range of 8.7 to 17.9 years. Information on other patient characteristics, including demographics, vital signs, body mass index, and past and current medications (glucocorticoids, cardiac medications, and coenzyme Q 10 ), for the MD cohort are shown in Table 1 .
| Characteristic | Muscular dystrophy cohort | Controls for DMD cohort | P | |||
|---|---|---|---|---|---|---|
| DMD | BMD | LGMD | All | |||
| Number | 35 (73%) | 11 (23%) | 2 (4%) | 48 (100%) | 33 (100%) | |
| Age (y) | 13 ± 3 | 14 ± 3 | 14 ± 2 | 13 ± 3 | 13 ± 3 | .43 |
| Ethnicity | ||||||
| Hispanic | 5 (14%) | 0 (0%) | 1 (50%) | 6 (13%) | NA | |
| Non-Hispanic | 30 (86%) | 11 (100%) | 1 (50%) | 42 (88%) | NA | |
| Race | ||||||
| Caucasian | 29 (83%) | 10 (91%) | 1 (50%) | 40 (83%) | NA | |
| Asian | 2 (6%) | 0 (0%) | 0 (0%) | 2 (4%) | NA | |
| African American | 3 (9%) | 1 (9%) | 1 (50%) | 5 (10%) | NA | |
| Other | 1 (3%) | 0 (0%) | 0 (0%) | 1 (2%) | NA | |
| Vital signs | ||||||
| SBP (mm Hg) | 109 ± 13 | 111 ± 9 | 95 ± 3 | 109 ± 12 | 115 ± 10 | .02 |
| DBP (mm Hg) | 65 ± 8 | 64 ± 9 | 60 ± 5 | 65 ± 7 | 64 ± 7 | .76 |
| Heart rate (beats/min) | 92 ± 14 | 79 ± 12 | 93 ± 1 | 89 ± 15 | 67 ± 13 | <.001 |
| BMI (kg/m 2 ) | 24 ± 7 | 20 ± 3 | 18 ± 2 | 23 ± 7 | 22 ± 6 | .68 |
| Glucocorticoid use | .003 | |||||
| Past users | 6 (17%) | 0 (0%) | 1 (50%) | 7 (15%) | NA | |
| Current users | 24 (69%) | 4 (36%) | 1 (50%) | 29 (61%) | NA | |
| Never used | 5 (14%) | 7 (64%) | 0 (0%) | 12 (25%) | NA | |
| Cardiac medication use | .76 | |||||
| Past users | 1 (3%) | 0 (0%) | 0 (0%) | 1 (2%) | NA | |
| Current users | 13 (37%) | 4 (36%) | 0 (0%) | 17 (36%) | NA | |
| Never used | 21 (60%) | 7 (64%) | 2 (100%) | 30 (63%) | NA | |
| CoQ 10 use | .99 | |||||
| Past users | 3 (9%) | 2 (18%) | 0 (0%) | 5 (10%) | NA | |
| Current users | 8 (23%) | 1 (9%) | 0 (0%) | 9 (19%) | NA | |
| Never used | 24 (69%) | 8 (73%) | 2 (100%) | 34 (71%) | NA | |
Feasibility
On the basis of image quality, only one study could not be read by both readers (C.F.S. and F.M.M.), and some measures were not obtained in all studies by both readers. For EF%, reader 1 was able to confidently trace the endocardial border in only 25 of 48 studies (52%) and reader 2 in 43 of 48 studies (90%). For reader 1, the average age of participants in whom EF% could not be measured was 14.0 ± 2.7 years, compared with 12.7 ± 2.6 years for studies with measured EF% ( P < .08). Six participants could not transfer out of their wheelchairs, and reader 1 was unable to measure EF% in four of these participants.
Reproducibility
The means, ranges, and interobserver ICCs for all echocardiographic measures from readers 1 and 2 are presented in Table 2 . Using SF%, 40 participants (85%) showed normal systolic function and seven participants (15%) had decreased systolic function. M-mode measures of left ventricular internal diameters in diastole and in systole used for SF% had two of the three highest ICC levels for all conventional echocardiographic measures. Although there was no single case in which EF% was normal and SF% was abnormal, there were six cases with decreased EF% values and normal SF% values. SF% measurements also showed high agreement between repeated measurements by the same reader (intraobserver ICC = 0.73). We did not observe any pattern of differences in SF% values between two readers ( Figure 1 A) or by diagnosis ( Figure 1 B). The variability of SF% remained consistent over the range of observed magnitudes ( Figure 2 A) and over the age range of participants in this study ( Figure 2 B), although the two readers showed some differences in range for the youngest age group.
| Cardiac measure | Reader 1 | Reader 2 | Readers 1 and 2 | ||||
|---|---|---|---|---|---|---|---|
| n | Mean ± SD | Range | n | Mean ± SD | Range | Interobserver ICC (95% CI) ∗ | |
| Conventional | |||||||
| LVIDd (mm) | 47 | 4.5 ± 0.6 | 3.3–5.9 | 47 | 4.4 ± 0.6 | 3.2–5.8 | 0.88 (0.80–0.93) |
| LVIDs (mm) | 47 | 3.0 ± 0.6 | 2.2–4.9 | 47 | 3.0 ± 0.6 | 2.1–4.6 | 0.89 (0.82–0.94) |
| SF% | 47 | 33 ± 6 | 17–46 | 47 | 32 ± 5 | 18–43 | 0.63 (0.42–0.77) |
| EF% | 25 | 61 ± 8 | 44–79 | 43 | 54 ± 8 | 24–64 | 0.49 (0.19–0.70) |
| WS (g/cm 2 ) | 46 | 56 ± 16 | 20–112 | 47 | 61 ± 19 | 27–116 | 0.38 (0.12–0.59) |
| VCF (circ/s) | 46 | 1.3 ± 0.3 | 0.7–2.1 | 46 | 1.2 ± 0.3 | 0.6–1.8 | 0.31 (0.05–0.54) |
| VCFc (circ/s) | 46 | 1.1 ± 0.2 | 0.6–1.5 | 46 | 1.0 ± 0.2 | 0.5–1.4 | 0.35 (0.07–0.58) |
| PWD | |||||||
| ET (msec) | 47 | 256 ± 26 | 200–310 | 46 | 250 ± 20 | 210–290 | 0.71 (0.50–0.83) |
| IVRT (msec) | 47 | 51 ± 11 | 30–80 | 46 | 53 ± 10 | 33–77 | 0.43 (0.16–0.63) |
| IVCT (msec) | 47 | 49 ± 10 | 30–80 | 46 | 49 ± 11 | 30–75 | 0.41 (0.14–0.62) |
| MPI | 46 | 0.39 ± 0.09 | 0.26–0.67 | 46 | 0.41 ± 0.08 | 0.28–0.60 | 0.55 (0.27–0.73) |
| MV E wave (cm/sec) | 46 | 86 ± 15 | 52–124 | 46 | 91 ± 14 | 63–123 | 0.82 (0.53–0.92) |
| MV A wave (cm/sec) | 46 | 49 ± 10 | 27–78 | 46 | 54 ± 10 | 36–75 | 0.63 (0.17–0.82) |
| DTI | |||||||
| ET (msec) | 46 | 266 ± 25 | 210–330 | 46 | 266 ± 22 | 210–313 | 0.92 (0.85–0.95) |
| IVRT (msec) | 46 | 49 ± 9 | 30–70 | 46 | 47 ± 12 | 30–70 | 0.61 (0.40–0.76) |
| IVCT (msec) | 46 | 52 ± 11 | 30–80 | 46 | 48 ± 11 | 20–73 | 0.57 (0.30–0.74) |
| MPI | 46 | 0.39 ± 0.09 | 0.26–0.63 | 46 | 0.36 ± 0.09 | 0.23–0.60 | 0.54 (0.28–0.83) |
| Septal LV peak E′ velocity (cm/sec) | 46 | 12 ± 3 | 3–17 | 46 | 14 ± 3 | 7–19 | 0.61 (0.02–0.83) |
| Lateral LV peak E′ velocity (cm/sec) | 46 | 15 ± 5 | 0.9–32 | 46 | 18 ± 5 | 7–34 | 0.72 (0.12–0.89) |
| STE | |||||||
| LS (%) | 30 | −14 ± 4 | −21 ± 7 | 30 | −15 ± 4 | −23 ± 7 | 0.90 (0.77–0.95) |
| CS (%) | 28 | −20 ± 5 | −33 ± 11 | 28 | −20 ± 5 | −31 ± 12 | 0.96 (0.92–0.98) |
| LSR (sec −1 ) | 30 | −1.1 ± 0.4 | −2.4 ± 0.5 | 30 | −1.3 ± 0.5 | −2.7 ± 0.5 | 0.64 (0.35–0.82) |
| CSR (sec −1 ) | 28 | −1.9 ± 0.5 | −2.7 ± 0.9 | 28 | −2.1 ± 0.7 | −3.4 ± 0.8 | 0.78 (0.47–0.91) |
Early Subclinical Myocardial Involvement
Both DTI and PWD methods were used to determine MPI as a measure of subclinical myocardial disease. Between the two readers, the MPI measured via PWD had a similar ICC compared with the MPI measured via DTI ( Table 2 ). Looking at specific components of the MPI measure (IVCT, IVRT, and ET), the PWD MPI measures consistently had lower correlations compared with the DTI MPI measures, and the ICC for ET measured in DTI tracings showed the highest correlation of all measures. In our MD cohort, PWD- and DTI-derived MPI values demonstrated agreement in only 29 of 46 of the studies (63%) when using an MPI value of 0.40 as a cutoff between normal and abnormal ( Figure 3 ). In cases in which SF% was ≥28, PWD MPI was ≥0.40 in 23 of 38 (61%), and DTI MPI was ≥0.40 in only 14 of 38 (37%). Thus, DTI MPI showed a lower rate of defining subclinical disease compared with PWD MPI when SF% was normal. To better understand the differences between PWD and DTI measures of MPI, we performed a subanalysis on the 15 cases in the upper left quadrant of Figure 3 with DTI MPI values <0.40 and PWD MPI values ≥0.40. In these cases, DTI measured significantly longer ETs (267 ± 19 msec via DTI vs 239 ± 17 msec via PWD, P < .001) and shorter IVRTs (48 ± 8 msec via DTI vs 52 ± 9 msec via PWD, P < .001), leading to a lower calculated DTI MPI. There were no significant differences in IVCT measurements. For ET and IVRT measures, a higher ICC was observed for each using DTI compared with PWD (see Table 2 ).
