Page 381 - Feline Cardiology

Chapter 26: Endocrine Diseases 399

tion that releases goitrogenic substances into the food thyroid cats, successful treatment leading to prolonged
(specifically, the plastic lining of pop-top cans). euthyroidism is not accompanied by complete normal-
Other possible causes, such as exposure to cigarette ization of left ventricular wall dimensions (Weichselbaum
smoke, pesticides, herbicides, insecticides, and lawn fer- et al. 2005). These two observations, together with the
tilizers, have been variously implicated and then dis- recognition of the high prevalence of hypertrophic car-
counted (Kass et al. 1999; Scarlett et al. 1988). diomyopathy in the cat population (16% of overtly
healthy cats in one echocardiographic study) (Paige et
Cardiovascular Pathologic Findings of al. 2009), have led to the understanding that some cats
Hyperthyroidism may be afflicted with thyrotoxic heart disease and hyper-
The characteristic gross pathologic finding of thyrotoxic trophic cardiomyopathy simultaneously (Liu et al. 1984).
heart disease is diffuse left ventricular concentric hyper- Such an observation is supported histologically: in a
trophy/thickening. Such thickening may be marked in study of 23 cats with thyrotoxic heart disease, 20 had
severe cases of hyperthyroidism, but the left ventricle of symmetrical left ventricular thickening and 3 had dis-
the average hyperthyroid cat is significantly less thick proportionate septal hypertrophy. These 3 cats also had
than the left ventricle of the average cat with hypertro- myocardial fiber disarray on microscopic examination,
phic cardiomyopathy. For example, a comparative gross the hallmark finding of idiopathic hypertrophic cardio-
pathologic study of cats with different forms of myocar- myopathy. Therefore, the identification of an asymmet-
dial disease identified mean septal and free wall thick- rically thickened left ventricle, or of ventricular
nesses, respectively, of 7.0 ± 0.4 mm and 7.1 ± 0.3 mm in thickening that has not resolved despite an otherwise
hyperthyroidism, 8.3 ± 0.2 mm and 8.7 ± 0.2 mm in effective antithyroid treatment, should lead to the clini-
symmetric hypertrophic cardiomyopathy, and 9.8 ± 0.3 cal suspicion of concurrent hypertrophic cardiomyopa-
and 7.7 ± 0.2 mm in asymmetric hypertrophic cardio- thy. Such suspicion forms the clinical diagnosis of the 2
myopathy affecting the interventricular septum more conditions occurring simultaneously, since confirma-
than the free wall (Liu et al. 1984). These postmortem tion requires histologic evaluation of the myocardium.
dimensions should not be compared to diastolic mea- An echocardiographically apparent finding that can
surements obtained echocardiographically, since the occur with hypertrophic cardiomyopathy but that is
heart remains in end-systole after death. uncommonly seen with thyrotoxic heart disease is sys-
The evolution of hyperthyroidism as a clinical entity tolic anterior motion of the mitral valve, and this feature
over time has led to less dramatic clinical expression of may further strengthen the suspicion of concurrent
the disease, likely because it is recognized routinely now hyperthyroidism and (idiopathic) hypertrophic cardio-
in the early stages of the disease as opposed to in its latest myopathy in the same cat.
stages when the disease was still new (Broussard et al.
1995; Scarlett et al. 1988; Edinboro et al. 2004). Signalment
Cardiovascular implications are that the sometimes dra-
matic gross pathologic lesions, and general extent of Hyperthyroidism affects adult cats of any breed and
both sexes. The mean age is approximately 13 years
thyrotoxic cardiovascular lesions, are less frequent and
milder now compared to those reported in the 1980s, for (range: 4–22 years), with fewer than 5% of cases occur-
ring in cats less than 8 years old (Feldman and Nelson
example. This observation helps to explain the lower Endocrine Diseases
prevalence of marked left ventricular thickening, con- 2004). Males and females are affected in similar propor-
tions, which stands in contrast to the prevalence of
gestive heart failure (12% in 1979–1983 versus 2% in
1993 [Broussard et al. 1995]), and systemic hypertension hypertrophic cardiomyopathy and its 3 : 1 or 4 : 1 predi-
lection for male cats (Rush et al. 2002). No breed pre-
described above.
disposition is known to exist; Siamese and Himalayan
Concurrent Thyrotoxic Heart Disease and cats are less likely to be affected (odds ratios: 0.44 [0.26–
Hypertrophic Cardiomyopathy 0.74] and 0.29 [0.093–0.89], respectively) (Kass et al.
1999; Scarlett et al. 1988). Case series and epidemiologic
Because thyroid hormones (principally triiodothyro-
nine, T3) exert their effects throughout the myocardium, studies have not reported the specific cardiovascular
effects of hyperthyroidism to be of greater or lesser
left ventricular concentric hypertrophy in hyperthyroid-
ism should be a diffuse process. Yet certain cases of importance in any subgroup.
hyperthyroidism are characterized by thickening that is
asymmetrical, such as those where the interventricular History and Chief Complaint
septum is thicker than the left ventricular free wall (Liu The presenting signs caused by hyperthyroidism reflect
et al. 1984). It is also worth noting that in some hyper- an inappropriate increase in metabolic activity. Weight