Page 106 - Feline Cardiology

Chapter 11: Hypertrophic Cardiomyopathy 105

veterinary literature in the 1980s, when it was recog- workup at a referral institute were diagnosed with HCM
nized that there are other forms of cardiomyopathies in (Riesen et al. 2007). When excluding other forms of
cats in addition to HCM, and that there are secondary heart disease and only including cats with cardiomyopa-
causes of concentric hypertrophy of the left ventricle thy, 58% of cats were diagnosed with HCM, followed by
that should not be labeled as HCM. HCM is a primary restrictive cardiomyopathy (21%), dilated cardiomyopa-
and likely heritable myocardial disorder that is not sec- thy (10%), and unclassified cardiomyopathy (10%)
ondary to diseases such as hyperthyroidism or systemic (Ferasin et al. 2003). Echocardiographic evidence of
hypertension, which cause a secondary, compensatory concentric hypertrophy was present in 10% of asymp-
increase in ventricular muscle thickness (i.e., concentric tomatic breeding Swedish Maine coon cats, but hyper-
hypertrophy). Unlike HCM, successful treatment of sec- tension, hyperthyroidism, or other secondary causes of
ondary causes of left ventricular concentric hypertrophy concentric hypertrophy were not ruled out, and the
leads to regression of the ventricular hypertrophy. The genetic status of the cats is unknown since they were not
term “hypertrophic cardiomyopathy secondary to screened for the mutation of myosin binding protein C Cardiomyopathies
hyperthyroidism” is therefore, inappropriate because the gene (MYBPC3) (Gundler et al. 2008). A mutation in
changes are not primary but compensatory, and the the myosin binding protein C gene (MYBPC3) has
correct term is “thyrotoxic heart disease” or “concentric been found to cause HCM in some Maine coon cats, and
ventricular hypertrophy secondary to hyperthyroidism.” this mutation is highly prevalent (identified in 34% of
In the 1960s, dynamic obstruction of the left ventricu- Maine coon cats worldwide (Fries et al. 2008) Similar
lar outflow tract (later termed systolic anterior motion of to Maine coon cats, echocardiographic screening of
the mitral valve) was identified in people with left ven- healthy British shorthair cats revealed a prevalence of
tricular hypertrophy and had several names, including 8.2% for HCM, but this was based on an increased left
muscular subaortic stenosis, idiopathic hypertrophic ventricular wall thickness of ≥5.5 mm rather than the
subaortic stenosis, or dynamic subaortic stenosis. Today, usual criteria of ≥6 mm, which will overestimate the
hypertrophic obstructive cardiomyopathy (HOCM) is the prevalence (Granstrom 2010).
term used to describe presence of systolic anterior
motion (SAM) of the mitral valve accompanying HCM, ETIOLOGY
and for the remainder of the chapter HOCM will be
included in the category of HCM. SAM causes a dynamic
obstruction of the left ventricular outflow tract that Key Points
worsens throughout systole and will be discussed later.
• Familial HCM occurs in Maine coon cats and Ragdoll
Prevalence
cats as a dominant heritable defect with incomplete
HCM is the most common cardiac disease in cats, and penetrance.
is highly prevalent in the general feline population. • Two missense mutations in cardiac myosin binding
Prevalence rates of left ventricular hypertrophy identi- protein C gene (MYBPC3) have been identified to cause
fied on echocardiography, attributed to HCM, in clini- HCM in some families of Maine coon cats and Ragdoll
cally healthy cats range from 14.5% to 34%, although cats, and a commercial assay is available for genetic
hydration, thyroxine concentration, and systolic blood screening.
pressure were not measured in all cats in these studies • Heterozygous cats may not have phenotypic evidence of
disease (i.e., no concentric ventricular hypertrophy) but
(Paige et al. 2009; Wagner et al. 2010). However, presence may transmit the defect to progeny.
of a murmur is not diagnostic for cardiomyopathy (in • Some Maine coon cats and Ragdoll cats develop HCM
the first study), with a low sensitivity of 31% and speci- in the absence of the MYBPC3 mutation and likely
ficity of 87%, and murmurs may be benign (i.e., inno- have another sarcomeric defect yet to be identified.
cent) or caused by other heart diseases. In contradiction, Similarly, in other breeds of cats and mixed-breed cats,
the latter study found that 31–62% of cats with a a multitude of genetic mutations is likely present, which
murmur had left ventricular hypertrophy based on dif- also explains the variable expression (i.e., age of onset,
ferent hypertrophy classification schemes. This study distribution and severity of ventricular hypertrophy, and
found that a murmur grade of 3 or higher was moder- rate of progression) of HCM in cats.
ately specific (70%) and sensitive (61%) for left ven- • A sarcomere is the smallest individual contractile unit
tricular hypertrophy. The absence of a murmur does not within a muscle cell. The initial sarcomeric defect is
rule out HCM, because 69% of cats with asymptomatic hypothesized to cause sarcomeric dysfunction, which
increases cell stress and activates signaling cascades
HCM in one study did not have a murmur (Paige et al. Continued
2009). Approximately 68% of cats referred for a cardiac