- 1 Incidence
- 2 Cause of Dilated Cardiomyopathy
- 3 Pathophysiology of Dilated Cardiomyopathy
- 4 Pathology of Dilated Cardiomyopathy
- 5 Natural History and Prognosis
- 6 Dilated Cardiomyopathy: Clinical Manifestations
- 7 Differential Diagnoses
- 8 Dilated Cardiomyopathy: Acute Therapy
- 9 Dilated Cardiomyopathy: Chronic Therapy
Myocardial contractility and myocardial contraction are not the same. Myacardial contractility is the inherent capability of the myocardium to contract without any forces acting on it. Myocardial contraction is how much the myocardium moves and thickens from end-diastole to end-systole (the amount of wall motion and thickening usually observed on an echocardiogram) and is determined by myocardial contractility, preload, afterload, and other variables. It is commonly calculated as the shortening fraction. Myocardial failure is defined as a decrease in myocardial contractility and is most commonly manifested as an increase in end-systolic diameter on an echocardiogram. dilated cardiomyopathy is the name given to diseases in which myocardial failure is present due to primary myocardial disease. Myocardial failure can occur secondary to nonprimary myocardial disease processes.
Dilated cardiomyopathy is characterized on echocardiography by a primary increase in left ventricular (LV) end-systolic diameter and volume (and usually by a compensatory increase in left ventricular end-diastolic diameter and volume). Because end-diastolic diameter and volume do not need to increase as much as end-systolic diameter and volume to maintain a normal stroke volume, shortening fraction (one measure of the amount of wall motion) decreases.
Although most of the cats that are diagnosed with dilated cardiomyopathy have clinical signs, the diagnosis of dilated cardiomyopathy may be made in the absence of heart failure, arrhythmia, or sudden death. dilated cardiomyopathy has a long subclinical phase during which myocardial function, along with cardiac compensatory mechanisms, are adequate to maintain normal hemodynamics. This phase of the disease, during which echocardiographic or electrocardiographic evidence (or both) of the disease is present but clinical signs of the disease are not present, is often termed occult in the literature. A patient with dilated cardiomyopathy may spend most of its life in a subclinical phase and only a short time during which it has clinical signs.
Because of the discovery of taurine deficiency as the major cause of feline dilated cardiomyopathy and the subsequent increase in taurine content in all cat foods, the current incidence of dilated cardiomyopathy is rare. From 1993 to 2003, only 24 cases of feline dilated cardiomyopathy were diagnosed at the University of California, Davis Veterinary Medical Teaching Hospital (UCD-VMTH). Eight had dilated cardiomyopathy due to taurine deficiency, whereas 13 were idiopathic. The other three died before a plasma sample could be obtained.
Cause of Dilated Cardiomyopathy
Taurine deficiency was identified as the primary cause of dilated cardiomyopathy in cats in 1986 to 1987. Taurine is a sulfur-containing amino acid. One of its functions is to conjugate bile acids. Although some species can also conjugate bile acids with glycine, cats lack’this ability and therefore have an obligate loss of taurine through the bile. In addition, they have very limited ability to synthesize taurine in the liver because of the lack of key enzymes. Taurine’s definitive role in cellular function is not known, but it is concentrated in the cytoplasm of excitable cells up to 250 times that in plasma.fi In the heart, taurine has been postulated to help modulate intracellular osmolality, calcium concentration, and transmembrane ion fluxes.
The cause of taurine deficiency in cats is primarily nutritional, although a genetic factor may also be involved. Prior to 1987, dry cat foods contained too little taurine, whereas the taurine in canned foods was not biologically available in adequate amounts, probably because of bacteria] overgrowth in the intestinal tract.
Feline dilated cardiomyopathy due to taurine deficiency has not totally disappeared. It can still occur with the exclusive feeding of one canned food diet. More frequendy, taurine deficiency is caused by feeding the cat a home-cooked diet, most commonly one including chicken. One study has documented that cooking meat reduces the taurine content, especially when the meat is constantly surrounded by water during the cooking process. The classic method of producing taurine deficiency is to feed dog food to a cat. Because no taurine is found in vegetables, vegetarian diets also produce taurine deficiency.
Idiopathic feline dilated cardiomyopathy is now the predominant form of dilated cardiomyopathy seen in cats. In humans, it is estimated that 30% to 40% of dilated cardiomyopathy is hereditary, and numerous mutations in a relatively large number of genes have now been identified as causing dilated cardiomyopathy in people (these include mutations in the genes that encode for cardiac dystrophin, 6-sarcoglycan, actin, ct-tropomyosin, B-myosin heavy chain, troponin T, titin, adhalin, lamin A/C, tafazzin, and emerin). It is likely that some cats with idiopathic dilated cardiomyopathy have a mutational cause for their disease, either because they inherited a mutation from a parent or because they developed a de novo mutation in utero. The latter may be more likely because no cat breeds are known to have feline dilated cardiomyopathy as a problem.
Pathophysiology of Dilated Cardiomyopathy
Dilated cardiomyopathy is caused by a myocardial disease that results in a progressive decrease in myocardial contractility, either because of global myocyte dysfunction or because of myocyte death (necrosis or apoptosis). Contractility is mildly decreased initially, which may progress over months to years to a severe decrease in contractility. 12 The decrease in contractility results in an increase in end-systolic diameter and end-systolic volume, which results in a decrease in stroke volume. Chronically the heart compensates for this decreased function by growing larger ventricular chambers through eccentric (volume overload) hypertrophy. This growth is also commonly referred to as ventricular remodeling and occurs primarily due to renal sodium and water retention leading to an increase in blood volume that leads to an increase in venous return to the heart. This places stretch on the myocardium that stimulates the myocytes to grow longer and the chambers to grow larger (increased end-diastolic diameter and end-diastolic volume). The increased chamber size allows stroke volume to return to normal when the disease is mild to moderate. At some stage the myocardial failure becomes so severe that the ability of the cardiovascular system to compensate for the disease is overwhelmed. At this stage, left ventricular end-diastolic pressure increases as renal sodium and water retention continues to increase blood volume, resulting in congestive heart failure (CHF).
Myocardial diseases that cause dilated cardiomyopathy may also cause concurrent diastolic dysfunction, primarily reduced ventricular compliance. A stiffer or less compliant left ventricular results in a higher diastolic pressure for any given diastolic volume, causing pulmonary edema, pleural effusion, or both. Decreased left ventricular compliance has not been documented in cats with dilated cardiomyopathy but has been in dogs.
Secondary mitral regurgitation is commonly present in dilated cardiomyopathy. In dilated cardiomyopathy the atrioventricular (AV) annulus dilates as the left ventricular enlarges, and the papillary muscles are displaced laterally and apically as the ventricular chamber enlarges. Both result in the leaflets failing to close completely during systole (incomplete valve closure). The regurgitation is usually mild, however, and so its contribution to the production of heart failure is probably minimal.
Pathology of Dilated Cardiomyopathy
In cats with severe dilated cardiomyopathy, the most striking gross pathologic finding is moderate to marked enlargement of all four cardiac chambers. Often the left heart appears more affected. The walls of the left ventricular may appear thin but when measured may be normal or thin. The papillary muscles often appear flattened. Heart weight is greater than normal, indicating the presence of hypertrophy. On cut-section, the left ventricular myocardium often appears remarkably normal, especially in cats with dilated cardiomyopathy due to taurine deficiency. In other cases, pale regions may be noted. In dilated cardiomyopathy due to taurine deficiency, histologic findings primarily reveal hypertrophy. In cats with idiopathic dilated cardiomyopathy, histologically there may be evidence of myocytolysis, myofibril fragmentation, abnormal mitochondria, vacuolization, and fibrosis.
Natural History and Prognosis
Only about 30% of cats that are taurine deficient develop echocardiographic evidence of myocardial failure. Of the cats with myocardial failure, about 30% develop severe myocardial failure (shortening fraction < 20%). Some cats have severe myocardial failure for months to years prior to heart failure, yet many cats appear to develop heart failure suddenly.
The short-term prognosis for survival in cats with dilated cardiomyopathy due to taurine deficiency is guarded. They usually have severe congestive heart failure (often with low-output heart failure) and require intensive therapy. Taurine supplementation does not help clinically during diis phase. Almost all cats that live longer than 2 to 3 weeks survive. Within 2 to 4 weeks they appear to improve clinically, although no apparent change occurs echocardiographically. Their myocardial function then normalizes over the ensuing 3 to 5 months. Consequently, their long-term prognosis is excellent. In cats with dilated cardiomyopathy that is not due to taurine deficiency, the long-term prognosis is usually grave (die median survival time [MST] is 2 weeks).
Rarely hypertrophic cardiomyopathy will progress to severe myocardial failure. Some cases of regional myocardial failure appear to be due to myocardial infarction. Myocardial failure can occur secondary to chronic, severe volume overload, such as primary mitral regurgitation. Sustained tachycardia can also produce myocardial failure, although the rate to produce this phenomenon is not known in cats.
Cats with dilated cardiomyopathy commonly have pleural effusion. This effusion may be a modified transudate, pseudochylous, or chylous and may change from a modified transudate to a chylous effusion over time. Heart failure is the most common cause of chylous pleural effusion in cats.