Case Studies In Small Animal

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Case 5

Hypertrophic Cardiomyopathy Chapter from "Small Animal Cardiovascular Medicine" On-Line

Text from "Small Animal Cardiovascular Medicine"

Hypertrophic cardiomyopathy

DIAGNOSIS

Echocardiography: The diagnosis of HCM is most commonly and preferably made using echocardiography. Angiocardiography can be used to make the diagnosis but it is invasive and is associated with a much higher complication rate. On echocardiography, cats with severe HCM usually have markedly thickened left ventricular walls (8-10 mm), papillary muscle hypertrophy, and an enlarged left atrium. The hypertrophy can be global, affecting all areas of the left ventricular wall or can be more regional or segmental. Segmental forms can have the entire or a region of the interventricular septum or free wall primarily affected, the apex primarily affected, or the papillary muscles (and often the adjacent free wall) primarily affected. Because of these forms, HCM is a diagnosis that should be made by examining several different two-dimensional echocardiographic views and measuring wall thicknesses in diastole from the region or regions of thickening on the two-dimensional images. M-mode echocardiography may miss regional thickening unless it is guided by the two-dimensional view. An M-mode echocardiogram taken from immediately below the mitral valve leaflets (the standard view for measuring left ventricular wall thicknesses and chamber diameters) may miss regional wall thickening. Dogs with HCM have similar echocardiographic findings and usually also have SAM.

Systolic Anterior Motion of the Mitral Valve: Color flow Doppler echocardiography can be used to demonstrate the hemodynamic abnormalities associated with SAM. With this modality one observes two turbulent jets originating from the same region - one regurgitating back into the left atrium and the other projecting into the aorta. Mitral valve SAM can be identified definitively on an M-mode echocardiogram or on a two-dimensional echocardiogram. Spectral Doppler can be used to determine the pressure gradient across the region of subaortic stenosis produced by the SAM. The pressure gradient roughly correlates with the severity of the SAM. Consequently, the pressure gradient can be used to document the success or failure of therapy to decrease SAM. Care must be taken not to record the high velocity mitral regurgitation jet that is positioned close to the dynamic subaortic stenosis jet. Usually, the dynamic subaortic stenosis jet is narrower than the mitral regurgitation jet, often becoming even more narrow in mid - to late systole. Systolic anterior motion is not present in all cats with HCM. Some cats develop SAM before they have any evidence of wall thickening while others only have SAM as the manifestation of their disease. Within our colony of Maine coon cats with HCM, some cats with severe HCM have SAM, some cats with severe HCM have no SAM, and some cats with echocardiographic evidence of mild wall thickening have SAM. The latter cats usually have evidence of papillary muscle hypertrophy and often develop more wall thickening over time.

Mild to Moderate HCM: The diagnosis of HCM is much more difficult and controversial in cats with lesser wall thickening or only regional thickening on an echocardiogram. In human families with HCM associated with specific gene abnormalities, varying degrees of severity have been noted within the affected family members. Echocardiographic findings in family members with the mutation range from no abnormalities to severe HCM. This may also be true in cats so that milder forms of the disease do exist. However, distinguishing mild disease from normal or distinguishing mild to moderate HCM from hypertrophy secondary to other abnormalities is not easy in individual cases. In a clinical situation where one is examining an older cat with what appears to be mild hypertrophy, one must first decide whether hypertrophy is present or not. Then one must determine if another disease is causing the hypertrophy before one can really make the diagnosis of feline HCM. The upper limit for normal left ventricular wall thickness is 5 - 5.5 mm in the cat. We consider concentric hypertrophy in the cat to be anything equal to or greater than 6 mm.

Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.