Although listed as a separate entity in this post, in most situations restrictive cardiomyopathy is a subclassification of unclassified cardiomyopathy. Feline restrictive cardiomyopathy is a diverse group of myocardial conditions characterized by abnormal diastolic function, normal to mildly increased left ventricular wall thickness, and normal to mildly reduced systolic function. restrictive cardiomyopathy occurs when ventricular diastolic compliance is impaired (i.e., stiffness is increased) by infiltration of the endocardium, subendocardium, or myocardium by fibrous tissue or another component. In contrast to human medicine where specific causes, such as amyloidosis and eosinophilic infiltration are causes of restrictive cardiomyopathy, specific causes for restrictive cardiomyopathy have not been clearly denned in the cat. Without the use of invasive diagnostic procedures to direcdy measure left ventricular diastolic function, DTI, other indirect measures of diastolic function, or necropsy examination, it is often impossible to distinguish this disorder from the form or forms of unclassified cardiomyopathy that are idiopathic.
Feline Restrictive Cardiomyopathy: Incidence
The exact incidence of restrictive cardiomyopathy is unknown. Unclassified cardiomyopathy is the second most common feline cardiomyopathy, of which restrictive cardiomyopathy is a component.
Cause of Feline Restrictive Cardiomyopathy
The precise cause of feline restrictive cardiomyopathy is unknown. Some evidence indicates that it may be inflammatory in nature. Endomyocarditis has been recognized in cats at postmortem examination for over 25 years. Endomyocarditis in cats is characterized by focal or diffuse infiltration of the endocardium by lymphocytes,
plasma cells, histiocytes, and a lesser number of neutrophils. The inflammatory component of endomyocarditis may be infectious, immune-mediated, or toxic. An interstitial pneumonia may also be common in this group. Within the past 10 years a study documented endomyocarditis in 37 cats at necropsy over a 7-year period. Over this same 7-year span, 25 cats with endocardial fibrosis were identified at the same institution. Four of these 62 cats had features of both diseases. This study and previous findings from Dr. Sam Liu have lead to the speculation that chronic endomyocarditis leads to endocardial fibrosis, a pathologic disease that leads to the physiologic entity termed restrictive cardiomyopathy. However, unless an agent that causes endomyocarditis in cats is identified, the theory that endomyocarditis leads to endocardial fibrosis will remain unproven.
In addition to the previous findings and speculation, one study has identified a transmissible myocarditis-diaphragmitis in young cats. The disease is generally self-limiting and causes a transient fever and depression. The investigators were unable to isolate a causative agent in these cats.
Eosinophilic endocardial inflammation is common in humans, more commonly in tropical climates. restrictive cardiomyopathy has been reported in several cats with hypereosinophilic syndrome, although it is impossible to determine if the hypereosinophilia in this small number of cats caused restrictive cardiomyopathy or the two diseases happened to occur together.
Pathophysioiogy of Feline Restrictive Cardiomyopathy
In its classic form, endocardial, subendocardial, or myocardial fibrosis impede ventricular diastolic filling and so impair diastolic function. These disorders are generally and primarily characterized by decreased compliance leading to an elevated IV diastolic pressure with a normal left ventricular filling volume. The elevation in diastolic ventricular pressure results in atrial enlargement and the formation of pulmonary edema and pleural effusion. The lesions are generally confined to the left ventricular in humans and left congestive heart failure predominates the clinical presentation. In cats, right atrial enlargement is also common. In some cats the left ventricular is perfectly normal in appearance, whereas in odiers the left ventricular may be misshapen and may have additional false tendons traversing the ventricle.
Pathology of Feline Restrictive Cardiomyopathy
The postmortem changes are unique to this form of cardiomyopathy and may be used to differentiate it from other disorders. Patchy or diffuse endocardial, subendocardial, or myocardial depositions of fibrous tissue are characteristic necropsy findings. The endocardium may appear whitish-gray, opaque, and thickened when endocardial or endomyocardial fibrosis is present. Fibrous adhesions between papillary muscles and the myocardium with distortion and fusion of the chordae tendineae and mitral valve leaflets may also be noted in restrictive cardiomyopathy. In extreme cases a portion of the left ventricular cavity, most commonly the mid-LV, may be obliterated. As with most cardiomyopathies, the left ventricular appears to be most severely affected, although other cardiac chambers may exhibit similar pathologic findings. Extreme left atrial and auricular enlargement is common. Other cats demonstrate apparendy earlier stages of the disease in which microscopic evidence of myocarditis is evident without gross pathologic abnormalities. The lesions suggest an inflammatory response; however causative factors have not been identified. Systemic thromboembolism is prevalent.
Histologic features of endocardial fibrosis include extreme endocardial thickening by hyaline, fibrous, and granulation tissue. Chondroid metaplasia is occasionally exhibited by the surface layer of hyaline tissue. A layer of loose fibrous tissue lies beneath this layer with a layer of granulation tissue adjacent to the myocardium. These changes are similar but not identical to those seen in humans with restrictive cardiomyopathy.
Natural History and Prognosis
As with other forms of cardiomyopathy, prognosis is difficult to predict for individual cases, especially prior to observing the initial response to therapy, A high incidence of serious arrhythmias, systemic thromboembolism, and refractory congestive heart failure is often present. In the author’s experience, cats with restrictive cardiomyopathy, on average, have a poor long-term prognosis. Although an initial response to standard therapy is often possible, progressive and refractory heart failure develops in the majority of cases.
History Signalment is difficult to accurately report, because little agreement exist among veterinary cardiologists as to which cases fall within this classification. From a series of pathologic studies in cats, Liu reported an age range of 8 months to 19 years. No breed predisposition has been reported. There may be a male predominance, or it may be that males develop more severe disease, as in hypertrophic cardiomyopathy. Most cats are middle-aged or older. Presenting complaints and clinical signs are similar to other forms of myocardial disease and include dyspnea and tachypnea, poor general condition, weakness, lethargy or rarely exercise intolerance, and anorexia. In one report, 45% had evidence of systemic thromboembolism at necropsy.
Physical Examination A heart murmur heard best on the sternum, just medial to the left apex beat, is common. An arrhythmia may be ausculted. Many cats are dyspneic.
Radiographs No specific findings exist for unclassified or restrictive cardiomyopathy. Pulmonary edema and pleural effusion are common.
Echocardiography The echocardiographic findings in restrictive cardiomyopathy are variable. Left atrial dilation is the common feature, and the left ventricular internal dimensions are typically normal but may be mildly to moderately reduced or mildly increased. Most commonly the left ventricular looks normal. However, two-dimensional echocardiography may demonstrate loss of normal left ventricular symmetry, distorted or fused papillary muscles, mild left ventricular concentric hypertrophy, mild wall thickening, a mild reduction in shortening fraction, and a mild increase in chamber diameter. Some cats may have evidence of more pronounced endocardial scarring. In these cases the endocardium may be notably thickened and irregular with an increased echogenicity. Others have evidence of cavity obliteration. Mild mitral regurgitation may be detectable with color flow Doppler. A large color flow Doppler jet suggests that the primary problem is primary mitral valve disease rather than unclassified cardiomyopathy. A left atrial thrombus may be identified.
Doppler echocardiography may be abnormal in cats with restrictive cardiomyopathy. However, this remains to be proven. As noted previously, the early diastolic waveform on DTI is reduced in cats with restrictive cardiomyopathy, whereas it is normal in cats with the idiopathic form of unclassified cardiomyopathy.
Electrocardiography No specific findings appear on electrocardiogram (ECG). Supraventricular and ventricular arrhythmias may be present, but sinus tachycardia is most commonly present in the clinic. Shifts in the mean electrical axis and evidence of chamber enlargement may occur.
Clinical Pathology No specific clinical pathologic findings exist. Azotemia due to reduced cardiac output, either from congestive heart failure or dehydration caused by diuretic therapy, is the most common abnormality. Elevated liver enzymes may be encountered.
Differential Diagnosis The primary differential diagnoses for restrictive cardiomyopathy are unclassified cardiomyopathy not due to restrictive cardiomyopathy, mitral regurgitation due to primary mitral valve disease, and myocardial infarction. Chronic myocardial infarction usually results in a region of akinetic or hypokinetic myocardium. Often the region of affected myocardium is thinner than normaJ. With severe mitral regurgitation due to primary mitral valve disease, a large jet is seen with color flow Doppler (although it may take a lower-frequency transducer and various views, including the left apical four-chamber view, to identify it). The IVS is commonly hyperdynamic, whereas the left ventricular free wall commonly moves less than normal. The left ventricular chamber is usually larger than normal in diastole.
Therapy of Feline Restrictive Cardiomyopathy
Therapy of restrictive and unclassified cardiomyopathy is the same. It is palliative and consists primarily of the administration of furosemide and an angiotensin-converting enzyme inhibitor as outlined in the two previous posts of this site. Diltiazem is not indicated because a calcium channel blocker is not able to help to relax scar (i.e., fibrous) tissue. A recent study suggests that atenolol may decrease the survival time in patients with diastolic dysfunction. However, if the cat is persistendy tachycardic at home, diltiazem or atenolol may be indicated in an attempt to slow the heart rate.