Background
Mutations in the X-linked gene encoding dystrophin cause skeletal and cardiac muscle diseases in men. Female “carriers” also can develop overt disease. The purpose of this study was to ascertain the prevalence of cardiac contractile abnormalities in dystrophinopathy carriers.
Methods
Twenty-four dystrophinopathy heterozygotes and 24 normal women each underwent standard exercise stress echocardiography.
Results
Heterozygotes demonstrated mildly lower left ventricular ejection fractions (LVEFs) at rest compared with controls (0.56 ± 0.10 vs 0.62 ± 0.07, P = .02). After exercise, the mean LVEF fell to 0.53 ± 0.14 in heterozygotes but rose to 0.73 ± 0.07 in controls ( P < .001). Twenty-one of 24 dystrophinopathy heterozygotes demonstrated ≥1 of the following: abnormal resting LVEF, abnormal LVEF response to exercise, or exercise-induced wall motion abnormality.
Conclusions
Women heterozygous for dystrophinopathy demonstrate significant left ventricular systolic dysfunction, which is unmasked by exercise. This finding has mechanistic implications for both inherited and acquired cardiac disease states.
In men, mutations in the X-linked gene encoding dystrophin, a component of muscle cytoskeleton, cause muscular dystrophy, cardiomyopathy, or both. Acquired abnormality of the cellular localization of dystrophin is a component of cardiomyopathy due to enteroviral myocarditis, congenital heart disease, and end-stage ischemic congestive heart failure.
Women heterozygous for a disease-associated dystrophin allele are also susceptible to the development of heart failure, and there is uncertainty about the optimal strategy for identifying individuals at risk. Hemodynamic stress, applied prior to the onset of overt cardiomyopathy, causes myocardial injury in homozygous dystrophin-deficient mice. We hypothesized that exercise stress would similarly unmask left ventricular dysfunction in asymptomatic women heterozygous for a disease-associated dystrophin mutation.
Methods
Subject Recruitment
The University of Iowa’s institutional review board approved all procedures. Dystrophinopathy heterozygotes were recruited from the Pediatric Neuromuscular Diseases Clinic at the University of Iowa, on the basis of their familial relationship to a boy with Duchenne or Becker muscular dystrophy. Dystrophin status was confirmed by deoxyribonucleic acid testing or through position in a pedigree. Potential subjects were excluded if they had any of the following risk factors for coronary artery disease: family history of myocardial infarction prior to 50 years of age, tobacco use, hypertension, diabetes, and hyperlipidemia. Potential subjects underwent directed medical histories and physical examinations and were excluded if they had any of the following potential symptoms or signs of cardiac or muscle disease: chest pain or dyspnea on exertion, orthopnea, unexplained syncope or symptomatic palpitations, peripheral edema, muscle pain or weakness, systolic blood pressure > 140 mm Hg, diastolic blood pressure > 90 mm Hg, jugular venous pressure > 8 cm H 2 O, carotid bruit, pulmonary crackles or wheeze, cardiac murmur > 1/6 in severity, cardiac gallop, hepatomegaly, or the use of medications known to affect cardiovascular function. A total of 50 potential subjects were offered participation. Thirteen declined, and 13 others had ≥1 exclusion criteria upon initial screening. The remaining 24 asymptomatic women heterozygous for dystrophinopathy mutations provided written informed consent. All 24 participating subjects described their lifestyles as “active”; none was a competitive athlete or participant in a structured physical training program.
Normal subjects (n = 24) included sibling control subjects (n = 5) recruited on the basis of their familial relationships to participating dystrophinopathy carriers. Each had undergone genetic testing and was known to lack the familial dystrophin mutation. An additional group of normal volunteers (n = 19) was recruited from the community at large via word of mouth and posted notices, with recruitment targeted to the following attributes: female gender; age similar to dystrophinopathy heterozygotes; and absence of history, symptoms, and risk factors of cardiovascular disease. Exclusion criteria were the same as for dystrophinopathy heterozygotes. All 24 normal subjects described their activity levels as “active”; none was a competitive athlete or participant in a structured physical training program, and all provided written informed consent.
Stress Testing
All stress echocardiographic procedures were performed using the Bruce protocol and were personally supervised by one of the authors (R.M.W., R.E.K., or K.S.S.), who was blinded with respect to subjects’ dystrophin status.
Echocardiography
Resting two-dimensional echocardiograms were obtained in standard parasternal long-axis and short-axis views and apical 2-chamber and 4-chamber views, using a Sonos 5500 or 7500 sonograph (Philips Medical Systems, Andover, MA) fitted with a 3-MHz sector-array probe. Image acquisition was resumed a few seconds after the cessation of exercise. For each echocardiographic image plane, the earliest technically acceptable cine clip was saved and used for subsequent offline quantitative analysis. Technically adequate images were acquired in all subjects, without need for echocardiographic contrast administration.
A subset of 10 dystrophinopathy heterozygotes underwent the assessment of left ventricular diastolic function at rest. Pulse-wave Doppler interrogation of mitral inflow was achieved by placing depth gates in the left ventricle, just beneath the mitral valve, in the apical 4-chamber view. Tissue Doppler acquisition was performed with regions of interest in the septal and lateral aspects, respectively, of the mitral annulus in the apical 4-chamber view.
Echocardiographic Analysis
Image analysis was conducted in blinded fashion with respect to subject dystrophin status and exercise time. Resting left ventricular end-diastolic chamber dimension and wall thickness were measured using electronic calipers at the level of the chordae tendineae in a parasternal long-axis view, using the leading-edge convention. The left atrial anteroposterior dimension was assessed at the time corresponding to ventricular end-systole. Rest and exercise ejection fractions were calculated from apical 4-chamber and 2-chamber views, using the biplane method of discs. End-diastolic and end-systolic silhouettes were identified by a single author (J.K.J.), who was blinded with respect to genotype. Regional systolic function was evaluated using the standard 17-segment model. A new wall motion abnormality was deemed present when ≥1 myocardial segment demonstrated systolic function worse by ≥1 grade than the majority of left ventricular segments, during exercise only. Regional function was evaluated by consensus of 3 authors (R.M.W., R.E.K., and P.D.L.), blinded with respect to genotype. Diastolic function was computed offline. Peak early mitral inflow (E) and peak inflow during atrial contraction (A) were recorded and expressed as a ratio (E/A). Peak early diastolic mitral annular excursion was measured for septal and lateral locations and was averaged (E′).
Statistical Analysis
Group data are reported as mean ± SD. Comparisons between groups using continuous quantitative variables used analysis of variance. Comparisons between groups of the frequency of occurrence of a binary variable (presence or absence of a finding) were performed using comparison of proportions. Statistical significance was deemed present for P values < .05.
Results
Study Population
Age and body morphometric data for women with heterozygous dystrophinopathy are compared with those for normal women in Table 1 . Age was similar in the two groups, but dystrophinopathy heterozygotes had higher body mass and body mass indexes.
| Variable | Normal subjects | Patients with heterozygous dystrophinopathy | P |
|---|---|---|---|
| Age (y) | 41 ± 10 | 39 ± 8 | .45 |
| Height (m) | 1.66 ± 0.05 | 1.64 ± 0.09 | .34 |
| Body mass (kg) | 70.7 ± 15.4 | 81.0 ± 18.2 | .04 |
| Body mass index (kg/m 2 ) | 25.6 ± 4.5 | 30.4 ± 7.4 | .008 |
| Left atrial dimension (cm) | 3.5 ± 0.4 | 3.7 ± 0.5 | .13 |
| LV end-diastolic dimension (cm) | 4.7 ± 0.4 | 4.8 ± 0.5 | .45 |
| LV end-systolic dimension (cm) | 3.0 ± 0.4 | 3.4 ± 0.6 | .009 |
| LV fractional shortening | 0.36 ± 0.07 | 0.30 ± 0.09 | .013 |
| Posterior wall thickness (cm) | 0.8 ± 0.2 | 0.8 ± 0.1 | 1 |
| Septal wall thickness (cm) | 0.8 ± 0.1 | 0.8 ± 0.1 | 1 |
| LV ejection fraction | 0.62 ± 0.07 | 0.56 ± 0.10 | .02 |
Resting echocardiographic data are shown in Table 1 . There were no significant differences between dystrophinopathy heterozygotes and normal subjects with respect to left atrial size, left ventricular end-diastolic dimension, or end-diastolic wall thickness. However, the dystrophinopathy heterozygotes demonstrated higher mean end-systolic left ventricular internal dimension, consequently lower fractional shortening, and lower left ventricular ejection fractions compared with normal controls. Five of the 24 dystrophinopathy heterozygotes had left ventricular ejection fractions < 0.48, which was >2 SDs below the mean for the normal group, whereas all 24 in the normal group had ejection fractions within 2 SDs of the mean (normal group range, 0.48-0.75). Linear regression analysis demonstrated no correlation between resting ejection fraction and body mass index ( r 2 = 0.001, P = .86).
The subset of 10 dystrophinopathy heterozygotes who underwent assessments of resting left ventricular diastolic function did not differ from the group as a whole with respect to age (40 ± 6 years), left ventricular ejection fraction (0.49 ± 0.10), end-diastolic dimension (4.5 ± 0.4 cm), or wall thickness (0.8 ± 0.10 cm). Results for this subset are shown in Table 2 . Only 1 subject had an E/A ratio < 1.0, and no subjects had E/E′ ratios > 8.0.
| Variable | Value |
|---|---|
| E (cm/s) | 73 ± 12 (50-90) |
| A (cm/s) | 62 ± 11 (51-75) |
| E/A | 1.2 ± 0.2 (0.9-1.6) |
| E′ (cm/s) | 12.9 ± 2.8 (8-17) |
| E/E′ | 5.9 ± 1.4 (2.9-8.0) |
Response to Exercise
Heart rate and blood pressure at rest and during peak exercise were similar between dystrophinopathy heterozygotes and normal subjects. However, dystrophinopathy heterozygotes had significantly lower exercise times compared with normal subjects. Linear regression analysis indicated that exercise time was negatively correlated with body mass index in dystrophinopathy heterozygotes ( r 2 = 0.43, P = .001; Table 3 ).
| Variable | Normal subjects | Patients with heterozygous dystrophinopathy | P |
|---|---|---|---|
| Resting heart rate (beats/min) | 75 ± 13 | 81 ± 15 | .15 |
| Peak heart rate (beats/min) | 170 ± 13 | 168 ± 15 | .62 |
| % maximum predicted heart rate | 95 ± 6 | 93 ± 10 | .41 |
| Resting systolic blood pressure (mm Hg) | 119 ± 9 | 120 ± 17 | .8 |
| Peak systolic blood pressure (mm Hg) | 169 ± 21 | 158 ± 21 | .08 |
| Peak heart rate × blood pressure (mm Hg/min) | 28,350 ± 3393 | 26,434 ± 4137 | .09 |
| Exercise time (s) | 615 ± 144 | 460 ± 140 | <.001 |
| Exercise left ventricular ejection fraction | 0.73 ± .07 | 0.53 ± 0.14 | <.001 |
| Change in ejection fraction | 0.11 ± 0.05 | −0.03 ± 0.15 | <.001 |
Ejection fraction response to exercise was markedly abnormal in the heterozygous dystrophinopathy group. Whereas exercise increased left ventricular ejection fractions in all 24 normal subjects (range, +0.02 to +0.22), dystrophinopathy heterozygotes, as a group, demonstrated decreased ejection fractions (range, −0.46 to +0.22) ( P < .001). Ejection fraction data for individuals are shown in Figure 1 . Thirteen of 24 individual dystrophinopathy heterozygotes, including 11 with normal resting ejection fractions, exhibited decreases in ejection fractions with exercise, a distinctly abnormal response. Linear regression analysis demonstrated no significant correlation between ejection fraction response to exercise and body mass index among dystrophinopathy heterozygotes ( r 2 = 0.03, P = .45). Neither resting ejection fraction nor the ejection fraction response to exercise correlated well with exercise time in dystrophinopathy heterozygotes ( P = .48 and P = .71, respectively).
Regional Left Ventricular Function
Thirteen of 24 dystrophinopathy heterozygotes developed new exercise-induced wall motion abnormalities in ≥1 segment (range, 0-4 per subject), including 5 of 8 subjects who had normal resting ejection fractions and who also had increased global ejection fractions with exercise. Twenty-one new regional wall motion abnormalities were identified in those 13 subjects and were preferentially located in the mid (n = 13) or apical (n = 8) third of the left ventricle but did not demonstrate a preference for any circumferential region (eg, anterior vs inferior). Only 1 of 24 control subjects developed a new wall motion abnormality with exercise ( P = .003 for the proportion vs dystrophinopathy heterozygotes). Still-frame images from a dystrophinopathy heterozygote, depicting exercise-induced worsening of inferior wall motion, are shown in Figure 2 . Moving images can be viewed in Videos 1 and 2 . ( View video clips online).
