Muscular dystrophy (MD) is a group of inherited disorders characterized by progressive weakness and deterioration of skeletal muscles. Genetic defects cause muscle fibers to be fragile and easily damaged, leading to progressive muscle wasting. The heart, a specialized muscle, is frequently and severely affected by this multisystemic condition. Cardiac involvement is a major cause of illness and reduced lifespan for many individuals with MD.
Which Types of Muscular Dystrophy Affect the Heart
Cardiac complications vary by the specific genetic mutation but are significant in several forms of MD. Duchenne Muscular Dystrophy (DMD) and Becker Muscular Dystrophy (BMD) are the most common types linked to severe heart disease. These are known as dystrophinopathies because they result from mutations in the gene responsible for the dystrophin protein.
Virtually all individuals with DMD who survive into adulthood will develop cardiomyopathy. The cardiac issues in DMD are aggressive, often manifesting as dilated cardiomyopathy (DCM), where the heart becomes enlarged and weakened. BMD, a milder form, also causes cardiomyopathy, but the onset is later and the progression is slower than in DMD.
Myotonic Dystrophy Type 1 (DM1) has significant cardiac involvement, presenting differently than dystrophinopathies. DM1 primarily causes electrical conduction abnormalities, leading to a higher risk of heart blocks and arrhythmias, often before cardiomyopathy develops. Emery-Dreifuss Muscular Dystrophy (EDMD) is also strongly associated with cardiac conduction defects, and individuals often require a pacemaker due to the risk of sudden cardiac death.
The Mechanism of Cardiac Involvement
The damage mechanism is rooted in the function of the protein dystrophin. Dystrophin acts as a structural anchor, connecting the internal scaffolding of the muscle cell to the outer membrane. This connection stabilizes the muscle fiber during the mechanical stress of contraction and relaxation.
In DMD, the gene mutation results in a near-total absence of functional dystrophin. In BMD, the protein is often non-functional or present in reduced amounts. Without this stabilizing protein, heart cells (cardiomyocytes) are highly susceptible to damage every time the heart beats. This constant mechanical stress causes microscopic tears and damage to the cell membrane.
Damaged heart cells undergo repeated cycles of injury, inflammation, and repair. Over time, the body replaces functional heart muscle tissue with non-contractile scar tissue, a process called fibrosis. This fibrosis initially begins in specific areas, particularly the posterolateral wall of the left ventricle.
As fibrosis progresses, it reduces the heart’s ability to pump effectively, leading to dilated cardiomyopathy. The scar tissue disrupts the heart muscle structure, causing the left ventricle to stretch and thin, impairing its ability to contract forcefully. This structural damage is the primary cause of heart failure in dystrophinopathies.
Fibrosis also interferes with the heart’s electrical signaling system. When scar tissue replaces normal conductive pathways, it can create “short circuits” that lead to irregular heart rhythms, or arrhythmias. These electrical disturbances can cause life-threatening ventricular arrhythmias as the heart muscle weakens further.
Monitoring and Treatment Strategies
Given the high risk of heart complications, regular cardiac monitoring is essential for individuals with muscular dystrophy. Screening typically begins at diagnosis, often in early childhood for DMD, and includes non-invasive tests performed at least annually. These tests include an electrocardiogram (ECG) to assess electrical activity and an echocardiogram to evaluate the heart’s structure and pumping function.
Cardiac magnetic resonance imaging (CMR) is increasingly used because it can detect early signs of fibrosis in the heart muscle before changes are visible on an echocardiogram. Detecting this subclinical damage allows clinicians to initiate early treatment, which can significantly slow cardiomyopathy progression. More frequent monitoring, sometimes every six months, is necessary once cardiac changes are identified or as the disease advances.
Pharmacological treatment focuses on reducing the heart’s workload and preventing further damage. Angiotensin-converting enzyme (ACE) inhibitors or Angiotensin Receptor Blockers (ARBs) are the first-line medications used to prevent adverse remodeling of the heart muscle. Beta-blockers are often added to slow the heart rate and reduce blood pressure.
These medications are often started prophylactically, meaning they are given to high-risk patients, such as boys with DMD by age ten, even if they show no signs of heart failure. For those with advanced heart failure, diuretics may be prescribed to manage fluid retention. Individuals with Myotonic Dystrophy who develop significant conduction abnormalities may require a permanent pacemaker, while an implantable cardioverter-defibrillator (ICD) may be considered for those at high risk of sudden cardiac arrest due to malignant arrhythmias.