Page 111 - Feline Cardiology

110 Section D: Cardiomyopathies

people: approximately 1/3 of RCM cases are familial and GROSS PATHOLOGY
caused by mutations in cardiac troponin I, cardiac tro-
ponin T, and α-cardiac actin genes (Kaski et al. 2008).
Another study found that six of nine cases of “idio- Key Points
pathic” RCM were actually caused by mutations in
cardiac troponin I (Mogensen et al. 2003). Familial RCM • Left ventricular concentric hypertrophy (i.e., increased
is a collection of heterogeneous disorders with a spec- ventricular wall thickness) is seen on gross pathology
trum of cardiac phenotypes, including HCM or DCM and leads to increased heart weight of 29–37 grams
in family members. Histopathologic abnormalities in compared to normal of <20 grams. A heart weight to
idiopathic RCM often resemble HCM because greater body weight ratio can also be used, with normal defined
as 4.8 ± 0.1 g/kg compared to 6.3 ± 0.1 g/kg in cats with
Cardiomyopathies fibrosis, and hypertrophy identical to histopathologic • Hallmark histopathologic abnormalities include
than 40% of cases have myocyte disarray, interstitial
HCM.
abnormalities seen in HCM (Kaski et al. 2008). Although
cardiomyocyte hypertrophy, myofiber disarray,
RCM, HCM, and DCM have distinct clinical differences,
interstitial and replacement fibrosis, and small coronary
the causative mutations in familial cases often involve
arteriosclerosis.
the same sarcomeric mutations. In vitro, cardiac tropo-
nin T mutations causing RCM lead to greater calcium
sensitivity and more severe diastolic impairment than Postmortem evaluation to identify whether HCM was
the mutations causing familial HCM, likely leading to present and the cause of death should include obtaining
the restrictive phenotype (Gomes et al. 2005; Yumoto the heart weight, subjective assessment of left atrial dila-
et al. 2005). It is likely that variations in the location of tion, evaluation of intracardiac thrombus, and evaluation
the mutation within the specific functional domain of for congestive heart failure based on presence of pleural
the sarcomeric disease genes can result in different clini- effusion or pulmonary edema (Box 11.1). Postmortem
cal phenotypes of dilated, restrictive, or hypertrophic wall thickness measurements reflect an end-systolic time
cardiomyopathy in people. The importance of differen- frame (and reflect the echocardiographic measurement
tiating between the cardiomyopathies lies in the poten- of end-systolic wall thickness) due to the rigor process,
tial of identifying different clinical features that relate to and do not reflect the end-diastolic ventricular wall thick-
prognosis, treatment recommendations, and expecta- ness that is used to determine presence of concentric
tions for clinical response, breeding recommendations, ventricular hypertrophy by echocardiography. Sudden
and may also define future diagnostic approaches tar- death due to a malignant ventricular arrhythmia may be
geted at the specific mechanisms of the disease. a possible cause of death in cats with HCM with patho-
Recognition that left ventricular hypertrophy can be logic evidence of increased heart weight and concentric
primary (hypertrophic cardiomyopathy) or can be sec- hypertrophy without heart failure or a massive intracar-
ondary to other diseases such as systemic hypertension diac thrombus. Cats with HCM have gross (subjective)
or hyperthyroidism is essential for patient workup and evidence of concentric LV hypertrophy, which may be
management. Before recognition of primary and sec- diffuse or asymmetrical (Figure 11.3). Asymmetrical
ondary causes of left ventricular hypertrophy, cats with hypertrophy can involve either the interventricular
thyrotoxic heart disease were often diagnosed with “car- septum or the LV free wall. Papillary muscle hypertrophy
diomyopathy due to hyperthyroidism” or “hypertrophic is a consistent finding in Maine coon cats with HCM,
cardiomyopathy due to hyperthyroidism,” which we and usually is the first region of the LV to become hyper-
now understand are incorrect terms. Distinguishing trophied (Kittleson et al. 1999). Absolute and relative
between hypertrophic cardiomyopathy (a primary heart heart weights are increased in cats with HCM that die of
muscle disease) and reversible, secondary compensatory their disease compared to normal cats (29–37 g [HCM]
left ventricular hypertrophy due to hyperthyroidism or versus <20 g [normal]; ≥6.4 ± 0.1 gram heart weight/kg
systemic hypertension is essential for both treatment of body weight [HCM] versus ≤4.8 ± 0.1 g/kg [normal])
and prognostic implications, because successful treat- (Kittleson et al. 1999; Fox 2003a; Liu 1983; Liu et al. 1993).
ment of secondary causes of hypertrophy leads to regres- Cats with severe HCM often have gross evidence of left
sion of the hypertrophy. In contrast, patients with atrial enlargement and congestive heart failure (CHF).
“primary” hypertrophic cardiomyopathy may progress Edematous lungs are heavy, red, and sink when placed in
over time to develop heart failure, sudden death, or arte- water or formalin. Pulmonary edema may be identified
rial thromboembolism and require continued monitor- as frothy fluid exuding from the cut surface of the lung
ing and possibly lifelong medical therapy for the heart or filling the trachea or smaller airways. Pleural effusion
disease. may be readily identified upon opening the thorax, but it