Restrictive Cardiomyopathy - Feline Cardiomyopathies

Overview

Restrictive cardiomyopathy (RCM) is a series of acquired pathologies of the feline myocardium. The common element is marked diastolic dysfunction without major alteration of systolic function. In RCM, the compliance and distensibility of the ventricular walls is reduced (ventricles are stiff), such that filling of the ventricles in diastole is compromised. This ultimately reduces the amount of blood that the left ventricle (LV) can accept. During the rapid filling phase of diastole, the pressure within the LV rises abruptly, preventing the entry of additional blood from atrial contraction in late diastole. This consequently increases left atrial pressure and the diameter of the left atrium or both atria. Grossly, RCM is characterised by a normal or near-normal appearance of the ventricles with dilation of the left atrium or both atria. The echocardiographic appearance of the cardiac chambers 2D images of cats with early RCM may be normal; however severe diastolic dysfunction may be evident from M-mode and Doppler imaging.

Aetiology

In humans, RCM is usually secondary to systemic infiltrative pathologies (e.g. amyloidosis, sarcoidosis) or radiation exposure; with pathology localized at the myocardial or endomyocardial level. Cats may also have myocardial and endomyocardial forms of RCM, but this seems to occur as a primary cardiomyopathy in cats. However, endomyocardial fibrosis has been associated with viral, hypereosinophilic and immune-mediated pathologies in rare cases.

Pathophysiology

The ventricular chambers appear normal or nearly normal, but are restricted in accepting diastolic filling due to the reduced compliance and rigidity of the ventricular walls.

Under normal physiological conditions, diastole can be divided into four phases:

1. Isovolumetric relaxation

2. Rapid ventricular filling

3. Slow ventricular filling (diastasis)

4. Atrial contraction

Ventricular filling is influenced by ventricular relaxation, ventricular compliance, atrial contraction and the pressure gradient between the left and right ventricles. The intraventricular pressure gradient is important as it causes an 'untwisting' at the cardiac apex in early diastole; which has a suction effect contributing to ventricular filling.

In RCM, reduced ventricular compliance and distensibility increases the final diastolic pressure, resulting in enlargement of the left atrium. Increased left atrial pressure has a knock-on effect of increased pressure in the pulmonary veins; thus leading to left-sided congestive heart failure (CHF). Tachycardia, associated with icreased sympathetic drive in cardiac dysfunction, contributes to disease progression. Coronary blood flow to the myocardium usually occurs during diastole, tachycardia results in a shorter period of diastole; therefore coronary blood flow is reduced. Reduced myocardial perfusion stimulates myocardial fibrosis, which contributes to rigidity and poor compliance of the ventricles. Finally, the increased atrial pressure and reduced atrial function results in slow blood flow within the atria. This may result in thrombus formation and, in some cases, embolisation of the clot to cause arterial thromboembolism (ATE).

Clinical Signs

The clinical presentation of cats with RCM, as with other cardiomyopathies, is extremely variable. There may be a long preclinical phase during which the cat remains asymptomatic. Early signs of cardiac dysfunction, such as exercise intolerance, often goes undetected in cats due to their sedentery nature. Cats with RCM are rarely identified in the asymptomatic phase. Cats with RCM frequently develop congestive heart failure (CHF), arterial thromboembolism (ATE) and are predisposed to arrhythmias.

Physical Examination

• May be completely normal
• Low intensity systolic murmur, sternal or left parasternal
• Gallop rhythm
• Tachyarrythmia
• Signs of congestive heart failure
• Arterial thromboembolism (ATE)

Left-sided Congestive Heart Failure

• Dyspnoea, tachypnoea, crackles (pulmonary oedema)
• Dyspnoea, restrictive pattern (fast,shallow breathing), muffled heart and ventral lung sounds (pleural effusion)

Right-sided Congestive Heart Failure

• Jugular pulse
• Hepatojugular reflux
• Occasionally ascites

Arterial Thromboembolism

The enlarged left atrium, stasis of blood within the left atrium, and reduced atrial function predispose to thrombus formation, and emboli may result. Typically these cases present with paresis or paralysis of one or both rear limbs due to occlusion at the aorta-iliac trifurcation. In some cases, emboli can involve other areas and can cause complex neurological manifestations, forelimb paralysis or acute renal ischemia.

Diagnosis

Radiography

Radiographs are useful for the diagnosis of congestive heart failure, however they are not able to distinguish the underlying cardiomyopathy. Radiographs may reveal left atrial dilation. On the dorso-ventral projection, a classic 'Valentine heart' shape may be evident due to the marked left atrial/biatrial dilation. Pulmonary venous conjestion, interstitial/alveolar pattern or the presence of a pleural effusion may be identified. These findings are not specific for RCM.

Echocardiography

This is the diagnostic test of choice for identification of RCM and excluding other cardiomyopathies.

Two-dimensional (2D) and M-mode imaging may reveal pronounced dilation of one or both atria and normal diameter and thickness of the ventricular chambers. Within the left atrium it is often possible to identify areas of spontaneous echo-contrast, reflecting blood stasis, or thrombi. The left atrium and the left auricular appendage should be carefully examined for this reason. It is useful to measure left auricular appendage flow velocity. Even in the absence of thrombi, evidence suggests that if velocity is <0.2 m/s on pulsed wave Doppler, the cat is at risk of thromboembolic complications.

Endomyocardial form of Restrictive Cardiomyopathy (eRCM)

This form of RCM is characterised by severe endomyocardial fibrosis with a 'bridging scar' crossing the left ventricle. This endomyocardial bridging scar may be associated with left ventricular obstruction during systole. In early forms, focal left ventricular hypertrophy may be evident, which over time progresses to the formation of a 'bridging scar'.

Myocardial form of Restrictive Cardiomyopathy (mRCM)

In this form of RCM, the left ventricle appears normal, but there is concurrent left atrial or biatrial enlargement.