Diagnosis

The definitive diagnostic test for feline HCM is echocardiography with the demonstration of thickening of the interventricular septum and/or left ventricular free wall with normal to reduced left ventricular chamber size and normal to enhanced contractility.

a) Auscultation: may find

• Systolic heart murmur of mitral valve insufficiency and/or left ventricular outflow tract obstruction
• Gallop (typically an S4 gallop)
• Dysrhythmia
• Muffled heart sounds (usually due to pleural effusion)

b) Radiology:

• The appearance of the heart on the D/V or V/D view has been described as “valentine-shaped” with mild cardiomegaly
• However, at least 50% of the cases have more generalized cardiomegaly typical of feline dilated cardiomyopathy. Furthermore, many cases of feline dilated cardiomyopathy have cardiac silhouettes that look somewhat valentine shaped. In conclusion, radiographic appearance of the cardiac silhouette on the V/D or D/V view is unreliable
• Pleural effusion is common which obscures the cardiac silhouette
• Other findings typical of congestive heart failure may be found (See the Thoracic Radiographic Tutorial)

c) ECG:

• May be normal
• May have evidence of a left shift of the MEA in the frontal plane (-30 to -60 degrees)
• May have a left ventricular enlargement pattern (tall R wave)
• A dysrhythmia may be present (ventricular premature contractions are most common)
• Microscopic intramural myocardial infarction (MIMI) may be present (notch in the QRS complex)

d) Blood work:

• As per congestive heart failure in Clinical Evauluation of Heart Disease
• Elevated BUN or creatinine, or abnormal fundic examination may suggest systemic arterial hypertension
• Elevated T4 (if HCM is due to concurrent hyperthyroidism)
• NT-proBNP levels show great promise in the diagnosis of occult (preclinical) HCM and in the differentiation of CHF from respiratory disease in dyspneic cats. See Clinical Evaluation of Heart Disease for more info.
• DNA tests for the myosin binding protein-C mutations are available for Maine Coons and Ragdolls through Dr. Kate Meur’s cardiac genetics lab at Washington State University

e) Echocardiography:

• Left ventricular concentric hypertrophy (thickening of the interventricular septum and/or left ventricular free wall (>6mm) with a normal to reduced left ventricular cavity). This increased LV wall thickness may be diffuse or focal. If focal it may affect only the interventricular septum, apex or free wall.
• Feline HCM can be divided into obstructive and non-obstructive HCM.
  • Obstructive HCM is a common variant wherein the left ventricular outflow tract (LVOT) is obstructed by the abnormal anatomy of the interventricular septum (IVS) that encroaches into the LVOT (dynamic outflow tract obstruction) with or without an abnormal action of the anterior leaflet of the mitral valve which may also encroach on the LVOT during systole. The obstructive form of HCM is characterized by turbulence and increased velocity of blood flow across the LVOT.
  • Non-obstructive HCM is a variant that does not involve obstruction of the LVOT. In this form, the left ventricular wall (LVFW) and IVS may be symmetrically thickened or there may be only focal thickening of the walls of the left ventricle involving only the IVS, LVFW or apex of the LV. There is no increased velocity of blood flow across the LVOT.
• Left atrial enlargement and mitral valve insufficiency are common.
• Pleural effusion or pericardial effusion may occur.

Comment: As cardiomyopathy is frequently an incidental finding in otherwise symptom-free cats, we recommend that all cats with heart murmurs or a gallop rhythm undergo an echocardiographic examination.