Dilated cardiomyopathy is 1 of the 3 traditional classes of cardiomyopathy, along with hypertrophic and restrictive cardiomyopathy. However, the classification of cardiomyopathies continues to evolve, based on the rapid evolution of molecular genetics as well as the introduction of recently described diseases.
Multiple causes of dilated cardiomyopathy exist, one or more of which may be responsible for an individual case of the disease (see Etiology). All alter the normal muscular function of the myocardium, which prompts varying degrees of physiologic compensation for that malfunction.
The degree and time course of malfunction are variable and do not always coincide with a linear expression of symptoms. Persons with cardiomyopathy may have asymptomatic LV systolic dysfunction, LV diastolic dysfunction, or both. When compensatory mechanisms can no longer maintain cardiac output at normal LV filling pressures, the disease process is expressed with symptoms that collectively compose the disease state known as chronic heart failure (CHF).
Continuing ventricular enlargement and dysfunction generally leads to progressive heart failure with further decline in LV contractile function. Sequelae include ventricular and supraventricular arrhythmias, conduction system abnormalities, thromboembolism, and sudden death or heart failure–related death.
Cardiomyopathy is a complex disease process that can affect the heart of a person of any age, but it is especially important as a cause of morbidity and mortality among the world's aging population. It is the most common diagnosis in persons receiving supplemental medical financial assistance via the US Medicare program.
Nonpharmacologic interventions are the basis of heart failure therapy. Instruction on a sodium diet restricted to 2 g/day is very important and can often eliminate the need for diuretics or permit the use of reduced dosages. Fluid restriction is complementary to a low-sodium diet. Patients should be enrolled in cardiac rehabilitation involving aerobic exercise.
For patient education information, see the Heart Center, as well as Congestive Heart Failure.
Pathophysiology
Dilated cardiomyopathy is characterized by ventricular chamber enlargement and systolic dysfunction with greater LV cavity size with little or no wall hypertrophy. Hypertrophy is judged as the ratio of LV mass to cavity size; this ratio is decreased in persons with dilated cardiomyopathies.
The enlargement of the remaining heart chambers is primarily due to LV failure, but it may be secondary to the primary cardiomyopathic process. Dilated cardiomyopathies are associated with both systolic and diastolic dysfunction. The decrease in systolic function is by far the primary abnormality. This leads to an increase in the end-diastolic and end-systolic volumes.
Progressive dilation can lead to significant mitral and tricuspid regurgitation, which may further diminish the cardiac output and increase end-systolic volumes and ventricular wall stress. In turn, this leads to further dilation and myocardial dysfunction.
Early compensation for systolic dysfunction and decreased cardiac output is accomplished by increasing the stroke volume, the heart rate, or both (cardiac output = stroke volume ´ heart rate), which is also accompanied by an increase in peripheral vascular tone. The increase in peripheral tone helps maintain appropriate blood pressure. Also observed is an increased tissue oxygen extraction rate with a shift in the hemoglobin dissociation curve.
The basis for compensation of low cardiac output is explained by the Frank-Starling Law, which states that myocardial force at end-diastole compared with end-systole increases as muscle length increases, thereby generating a greater amount of force as the muscle is stretched. Overstretching, however, leads to failure of the myocardial contractile unit.
These compensatory mechanisms are blunted in persons with dilated cardiomyopathies, as compared with persons with normal LV systolic function. Additionally, these compensatory mechanisms lead to further myocardial injury, dysfunction, and geometric remodeling (concentric or eccentric).
Neurohormonal activation
Decreased cardiac output with resultant reductions in organ perfusion results in neurohormonal activation, including stimulation of the adrenergic nervous system and the renin-angiotensin-aldosterone system (RAAS). Additional factors important to compensatory neurohormonal activation include the release of arginine vasopressin and the secretion of natriuretic peptides. Although these responses are initially compensatory, they ultimately lead to further disease progression.
Alterations in the adrenergic nervous system induce significant increases in circulating levels of dopamine and, especially, norepinephrine. By increasing sympathetic tone and decreasing parasympathetic activity, an increase in cardiac performance (beta-adrenergic receptors) and peripheral tone (alpha-adrenergic receptors) is attempted.
Unfortunately, long-term exposure to high levels of catecholamines leads to down-regulation of receptors in the myocardium and blunting of this response. The response to exercise in reference to circulating catecholamines is also blunted. Theoretically, the increased catecholamine levels observed in cardiomyopathies due to compensation may in themselves be cardiotoxic and lead to further dysfunction. In addition, stimulation of the alpha-adrenergic receptors, which leads to increased peripheral vascular tone, increases the myocardial workload, which can further decrease cardiac output. Circulating norepinephrine levels have been inversely correlated with survival.
Activation of the RAAS is a critical aspect of neurohormonal alterations in persons with CHF. Angiotensin II potentiates the effects of norepinephrine by increasing systemic vascular resistance. It also increases the secretion of aldosterone, which facilitates sodium and water retention and may contribute to myocardial fibrosis.
The release of arginine vasopressin from the hypothalamus is controlled by both osmotic (hyponatremia) and nonosmotic stimuli (eg, diuresis, hypotension, angiotensin II). Arginine vasopressin may potentiate the peripheral vascular constriction because of the aforementioned mechanisms. Its actions in the kidneys reduce free-water clearance.
Natriuretic peptide levels are elevated in individuals with dilated cardiomyopathy. Natriuretic peptides in the human body include atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide. ANP is primarily released by the atria (mostly the right atrium). Right atrial stretch is an important stimulus for its release. The effects of ANP include vasodilation, possible attenuation of cell growth, diuresis, and inhibition of aldosterone. Although BNP was initially identified in brain tissue (hence its name), it is secreted from cardiac ventricles in response to volume or pressure overload. As a result, BNP levels are elevated in patients with CHF. BNP causes vasodilation and natriuresis.
Counterregulatory responses to neurohormonal activation involve increased release of prostaglandins and bradykinins. These do not significantly counteract the previously described compensatory mechanisms.
The body's compensatory mechanisms for a failing heart are evidently shortsighted. Compensation for decreased cardiac output cannot be sustained without inducing further decompensation. The rationale for the most successful medical treatment modalities for cardiomyopathies is therefore based on altering these neurohormonal responses.
Circulating cytokines as mediators of myocardial injury
Tissue necrosis factor-alpha (TNF-alpha) is involved in all forms of cardiac injury. In cardiomyopathies, TNF-alpha has been implicated in the progressive worsening of ventricular function, but the complete mechanism of its actions is poorly understood. Progressive deterioration of LV function and cell death (TNF plays a role in apoptosis) are implicated as some of the mechanisms of TNF-alpha. It also directly depresses myocardial function in a synergistic manner with other interleukins.
Elevated levels of several interleukins have been found in patients with left ventricular dysfunction. Interleukin (IL)–1b has been shown to depress myocardial function. One theory is that elevated levels of IL-2R in patients with class IV CHF suggest that T-lymphocytes play a role in advanced stages of heart failure.
IL-6 stimulates hepatic production of C-reactive protein, which serves as a marker of inflammation. IL-6 has also been implicated in the development of myocyte hypertrophy, and elevated levels have been found in patients with CHF. IL-6 has been found to correlate with hemodynamic measures in persons with left ventricular dysfunction.
Etiology
Cardiomyopathy has many causes, including inherited disease, infections, and toxins. Finding a specific cause for an individual case may be difficult, especially in patients with multiple risk factors.
Causes of dilated cardiomyopathy include the following:
- Genetics
- Secondary to other cardiovascular disease: ischemia, hypertension, valvular disease, tachycardia induced
- Infectious: viral, rickettsial, bacterial, fungal, metazoal, protozoal
- Probable infectious: Whipple disease, Lyme disease
- Metabolic: endocrine diseases (eg, hyperthyroidism, hypothyroidism, acromegaly, myxedema, hypoparathyroidism, hyperparathyroidism), diabetes mellitus, electrolyte imbalance (eg, potassium, phosphate, magnesium)
- Nutritional: thiamine deficiency (beriberi), protein deficiency, starvation, carnitine deficiency
- Toxic: drugs, poisons, foods, anesthetic gases, heavy metals, ethanol
- Collagen vascular disease
- Infiltrative: hemochromatosis, amyloidosis, glycogen storage disease
- Granulomatous (sarcoidosis)
- Physical agents: extreme temperatures, ionizing radiation, electric shock, nonpenetrating thoracic injury
- Neuromuscular disorders: muscular dystrophy (limb-girdle [Erb dystrophy], Duchenne dystrophy, fascioscapulohumeral [Landouzy-Dejerine dystrophy]), Friedreich disease, myotonic dystrophy
- Primary cardiac tumor (myxoma)
- Senile
- Peripartum
- Immunologic: postvaccination, serum sickness, transplant rejection
The frequency of different causes of cardiomyopathy varies with geographic location. Traditionally, ischemic cardiomyopathy is listed as the most common cause of cardiomyopathy in North America and Europe. In Africa, idiopathic congestive cardiomyopathy and cardiomyopathy from endomyocardial fibrosis and from rheumatic heart disease all are more prevalent than cardiomyopathy caused by atherosclerotic coronary artery disease.
In many cases of dilated cardiomyopathy, the cause remains unexplained. However, some idiopathic cases may result from failure to identify known causes such as infections or toxins. The idiopathic category should continue to diminish as more information explaining pathophysiologic mechanisms, specifically genetic-environmental interactions, becomes available.
Toxins are a significant cause. Almost a third of cases may result from severe ethanol abuse.
Viral myocarditis
Viral myocarditis is an important entity within the category of infectious cardiomyopathy. Viruses have been implicated in cardiomyopathies as early as the 1950s, when coxsackievirus B was isolated from the myocardium of a newborn baby with a fatal infection. Advances in genetic analysis, such as polymerase chain reaction testing, have aided in the discovery of several viruses that are believed to have roles in viral cardiomyopathies.
Viral infections and viruses associated with myocardial disease may be caused by the following:
- Coxsackievirus (A and B)
- Influenza virus (A and B)
- Adenovirus
- Echovirus
- Rabies
- Hepatitis
- Yellow fever
- Lymphocytic choriomeningitis
- Epidemic hemorrhagic fever
- Chikungunya fever
- Dengue fever
- Cytomegalovirus
- Epstein-Barr virus
- Rubeola
- Rubella
- Mumps
- Respiratory syncytial virus
- Varicella-zoster virus
- Human immunodeficiency virus
Viral myocarditis can produce variable degrees of illness, ranging from focal disease to diffuse pancarditis involving myocardium, pericardium, and valve structures. Viral myocarditis is usually a self-limited, acute-to-subacute disease of the heart muscle. Symptoms are similar to those of CHF and often are subclinical. Many patients experience a flulike prodrome.
Confirming the diagnosis can be difficult because symptoms of heart failure can occur several months after the initial infection. Patients with viral myocarditis (median age, 42 years) are generally healthy and have no systemic disease.
Acute viral myocarditis can mimic acute myocardial infarction, with patients sometimes presenting in the emergency department with chest pain; nonspecific electrocardiographic (ECG) changes; and abnormal, often highly elevated serum markers such as troponin, creatine kinase, and creatine kinase-MB.
The diagnosis of viral myocarditis is mainly indicated by a compatible history and the absence of other potential etiologies, particularly if it can be confirmed with acute or convalescent sera. An ECG demonstrates varying degrees of ST-T wave changes reflecting myocarditis and, sometimes, varying degrees of conduction disturbances. Echocardiography is a crucial aid in classifying this disease process, which manifests mostly as a dilated type of cardiomyopathy.
Myocarditis is almost always a clinically presumed diagnosis because it is not associated with any pathognomonic sign or specific, acute diagnostic laboratory test result. In the past, percutaneous transvenous right ventricular endomyocardial biopsy has been used, but the Myocarditis Treatment Trial revealed no advantage for immunosuppressive therapy in biopsy-proven myocarditis, so biopsy is not routinely performed in most cases.
If a patient is thought to have viral myocarditis, the initial diagnostic strategies should be to evaluate cardiac troponin I or T levels and to perform antimyosin scintigraphy. Positive troponin I or T findings in the absence of myocardial infarction and the proper clinical setting confirm acute myocarditis. Negative antimyosin scintigraphy findings exclude active myocarditis.
The exact mechanism for myocardial injury in viral cardiomyopathy is controversial. Several mechanisms have been proposed based on animal models. Viruses affect myocardiocytes by direct cytotoxic effects and by cell-mediated (T-helper cells) destruction of myofibers. Other mechanisms include disturbances in cellular metabolism, vascular supply of myocytes, and other immunologic mechanisms.
Viral myocarditis may resolve over several months during the treatment of left ventricular systolic dysfunction. However, it can progress to a chronic cardiomyopathy. The main issue in recovery is ventricular size. Reduction of ventricular size is associated with long-term improvement; otherwise, the course of the disease is characterized by progressive dilation.
Because of an immunologic mechanism of myocyte destruction, several trials have investigated the use of immunomodulatory medications. (Other trials are currently being conducted.) According to Mason et al in 1995, the Myocarditis Treatment Trial demonstrated no survival benefit with prednisone plus cyclosporine or azathioprine in patients with viral (lymphocytic) myocarditis. Randomized trials are under way to evaluate intravenous immunoglobulin as treatment for viral myocarditis.
Familial cardiomyopathy
Familial cardiomyopathy is a term that collectively describes several different inherited forms of heart failure. Familial dilated cardiomyopathy is diagnosed in patients with idiopathic cardiomyopathy who have 2 or more first- or second-degree relatives with the same disease (without defined etiology). Establishing a diagnosis with more-distant affected relatives (third degree and greater) simply requires identifying more family members with the same disease. Genetic screening has been recommended for patients fulfilling the above criteria.
A study by van Spaendonck-Zwarts et al suggested that a subset of peripartum cardiomyopathy is an initial manifestation of familial dilated cardiomyopathy. This may have important implications for cardiologic screening in such families.
Several forms of familial cardiomyopathy have been described, and theories postulate its association with other causes of cardiomyopathy. Inheritance is autosomal dominant; however, autosomal recessive and sex-linked inheritance have been reported.
Several different genes and chromosomal aberrations have been described in studied families. One example is the gene that codes for actin, a cardiac muscle fiber component. Other forms of familial cardiomyopathy involve a strong association with conduction system disease. As research continues, the knowledge database regarding familial cardiomyopathies is likely to expand.
Doxorubicin-induced cardiomyopathy
Anthracyclines, which are widely used as antineoplastic agents, have a high degree of cardiotoxicity and cause a characteristic form of dose-dependent toxic cardiomyopathy. Both early acute cardiotoxicity and chronic cardiomyopathy have been described with these agents. Anthracyclines can also be associated with acute coronary spasm. The acute toxicity can occur at any point from the onset of exposure to several weeks after drug infusion. Radiation and other agents may potentiate the cardiotoxic effects of anthracyclines.
Cardiac injury occurs even at doses below the empiric limitation of 550 mg/m2. However, whether injury results in clinical CHF varies. The development of heart failure is very rare at total doses less than 450 mg/m2 but is dose dependent.
The history of these patients, in addition to having classic heart failure symptoms or symptoms of acute myocarditis, involves a previous history of malignancy and treatment with doxorubicin.
Anatomically, these patients' hearts vary from having bilaterally dilated ventricles to being of normal size. The mechanism of myocardial injury is related to degeneration and atrophy of myocardial cells, with loss of myofibrils and cytoplasmic vacuolization. The generation of free radicals by doxorubicin has also been implicated. Progressive deterioration is the norm for this toxic cardiomyopathy.
Prevention is based on limiting dosing after 450 mg/m2 and on serial functional assessments (ie, resting and exercise evaluation of ejection fraction). The drug should be discontinued if the ejection fraction is less than 0.45, if it falls by more than 0.05 from baseline, or if it fails to increase by more than 0.05 with exercise. Dexrazoxane is an iron-chelating agent approved by the FDA to reduce toxicity; however, it increases the risk of severe myelosuppression.
Cardiomyopathy associated with collagen-vascular disease
Several collagen-vascular diseases have been implicated in the development of cardiomyopathies. These include the following:
- Rheumatoid arthritis
- Systemic lupus erythematosus
- Progressive systemic sclerosis
- Polymyositis
- HLA-B12–associated cardiac disease
Diagnosis is based on identification of the underlying disease in conjunction with appropriate clinical findings of heart failure.
Granulomatous cardiomyopathy (sarcoidosis)
Endomyocardial biopsy may be helpful in establishing the diagnosis, especially in sarcoidosis in which the myocardium may be involved. Involvement may be patchy, resulting in a negative biopsy finding. The diagnosis can also be made if some other tissue diagnosis is possible or available in conjunction with the appropriate clinical picture for heart failure. Cardiac involvement in sarcoidosis reportedly occurs in approximately 20% of cases.
Patients have signs and symptoms of sarcoidosis and CHF. Patients rarely present with CHF without evidence of systemic sarcoid. Bilateral mediastinal, paratracheal, and/or hilar lymphadenopathy may be evident.
Noncaseating granulomatous infiltration of the myocardium occurs as with other organs affected by this disease. Sarcoid granulomas can show a localized distribution within the myocardium. The granulomas particularly affect the conduction system of the heart, left ventricular free wall, septum, papillary muscles, and, infrequently, heart valves. Fibrosis and thinning of the myocardium occurs as a result of the infiltrative process affecting the normal function of the myocardium.
Diagnosis involves finding noncaseating granulomas from cardiac biopsy or other tissues. Often, patients present with conduction disturbances or ventricular arrhythmias. In fact, in patients with normal left ventricular function, these conduction disturbances may be the primary clinical feature.
Treatment of cardiac sarcoidosis with low-dose steroids may be beneficial, especially in patients with progressive disease, conduction defects, or ventricular arrhythmias. The true benefit is unknown because of the lack of placebo-controlled studies. This also holds true for the use of other immunosuppressive agents (eg, chloroquine, hydroxychloroquine, methotrexate) in the treatment of cardiac sarcoidosis.
Carnitine deficiency
A carnitine transporter defect is characterized by severely reduced transport of carnitine into skeletal muscle, fibroblasts, and renal tubules. All children with dilated cardiomyopathy or hypoglycemia and coma should be evaluated for this transporter defect because it is readily amenable to therapy, which results in prolonged prevention of cardiac failure. The prognosis for long-term survival in pediatric dilated cardiomyopathy is poor.
Tachycardia-induced cardiomyopathy
Generally, this type of cardiomyopathy is reversible once treatment of the tachycardia is successful. Persistent tachycardia is known to lead to myocyte dysfunction and cardiomyopathy. The exact mechanisms by which tachycardia affects cell function are poorly understood. The following are possible mechanisms by which myocyte dysfunction arises from tachycardia:
- Depletion of energy stores
- Abnormal calcium channel activity
- Abnormal subendocardial oxygen delivery secondary to abnormalities in blood flow
- Reduced responsiveness to beta-adrenergic stimulation
Epidemiology
The true incidence of cardiomyopathies is unknown. As with other diseases, authorities depend on reported cases (at necropsy or as a part of clinical disease coding) to define the prevalence and incidence rates. The inconsistency in nomenclature and disease coding classifications for cardiomyopathies has led to collected data that only partially reflect the true incidence of these diseases. Whether secondary to improved recognition or other factors, the incidence and prevalence of cardiomyopathy appear to be increasing. The reported incidence is 400,000-550,000 cases per year, with a prevalence of 4-5 million people.
Cardiomyopathy is a complex disease process that can affect the heart of a person of any age, and clinical manifestations appear most commonly in the third or fourth decade.
Prognosis
Although some cases of dilated cardiomyopathy reverse with treatment of the underlying disease, many progress inexorably to heart failure. With continued decompensation, heart transplantation may be necessary. The prognosis for patients with heart failure depends on several factors, with the etiology of disease being the primary factor. Other factors play important roles in determining prognosis; for example, higher mortality rates are associated with increased age, male sex, and severe CHF. Prognostic indices include the New York Heart Association functional classification.
The Framingham Heart Study found that approximately 50% of patients diagnosed with CHF died within 5 years. Patients with severe heart failure have more than a 50% yearly mortality rate. Patients with mild heart failure have significantly better prognoses, especially with optimal medical therapy.
sources : http://emedicine.medscape.com