Page 382 - Feline Cardiology

400 Section N: Endocrine Diseases Affecting the Heart

loss (87%), polyphagia (49%), vomiting (44%), hypertrophy, heightened by the greater audibility of
polydipsia/polyuria (36%), and increased activity/ heart sounds through a thin body wall if the patient has
restlessness (31%) are some of the most common his- a thin body condition.
torical signs identified in the medical histories of hyper- A gallop sound is commonly heard, although such a
thyroid cats (Broussard et al. 1995). Many of these signs finding is not necessarily as strongly suggestive of
occur less dramatically, and less frequently, than the clas- congestive heart failure in the cat as it is in other species.
sically described initial reports of feline hyperthyroid- By definition, a gallop sound/“gallop rhythm” is a
ism, and some authors have theorized that routine low-frequency third heart sound, usually S 4 rather
recognition of and testing for hyperthyroidism in cats than S 3 , although this distinction is difficult or impos-
has led to earlier recognition of the disorder in a larger sible to make when ausculting a tachycardic cat (see
number of more mildly affected cats. A small proportion Chapter 1).
of affected cats present with so-called “apathetic hyper- The pulse of hyperthyroid cats is typically normal to
thyroidism” characterized by weakness and anorexia. It increased in strength, the latter due to a greater pulse
appears that many or most of these cats have concurrent pressure caused by increased left ventricular systolic
nonthyroidal illness, with chronic kidney disease, function and normal diastolic arterial pressure.
primary heart disease, and neoplasia most commonly A variety of cardiac arrhythmias is possible with
implicated (Feldman and Nelson 2004). hyperthyroidism, and these may be apparent on physical
Overt signs attributable to cardiovascular dysfunction examination. Tachycardia is often present (42% of cases
are uncommon as chief complaints in cats with hyper- [Broussard et al. 1995]), although such a finding does
thyroidism. Rather, cardiovascular effects often are iden- not separate the hyperthyroid cat population from the
tified incidentally during subsequent diagnostic healthy cat population in a hospital setting (Côté 2004).
evaluation of the hyperthyroid patient. Congestive heart Premature beats may be noted during the physical
failure, manifesting with dyspnea and anorexia, is a well- examination. Typically, the postextrasystolic pause that
recognized complication of hyperthyroidism in cats, but follows the premature beat is most striking, such that on
it occurs in a small proportion of hyperthyroid cats (as physical exam in cats, premature beats stand out by this
high as 4/23 [17%] in one early, small case series, but pause and are often misinterpreted as atrioventricular
subjectively far fewer cats in current practice) (Jacobs et block or other causes of “dropped beats.” An electrocar-
al. 1992; Liu et al. 1984). Clinical signs of aortic throm- diogram provides the correct diagnosis and is indicated
boembolism are rarely reported, and this syndrome in any hyperthyroid patient with an arrhythmia noted
appears to be less common with thyrotoxic cardiovascu- on examination.
lar disease than with other forms of heart disease in the
cat, possibly due to the hyperdynamic state and arterial
dilation associated with this endocrinopathy. Differential Diagnosis and Disease Variants
Differential diagnosis
The systemic signs of hyperthyroidism classically may
Endocrine Diseases General physical findings in hyperthyroid cats can matory bowel disease, chronic kidney disease, and
Physical Examination
be confused for diabetes mellitus, infiltrative or inflam-
include palpable thyroid gland enlargement (83–90% of
various forms of neoplastic disease. From a cardiovas-
cases, with bilateral involvement palpable in about 2/3
cular standpoint, the main differential diagnoses revolve
around left ventricular concentric hypertrophy and sys-
of these), a thin body condition (65% of cases), nervous,
hyperactive, or aggressive behavior, an unkempt hair
coat, and ocular lesions (Feldman and Nelson 2004; temic hypertension. Disorders that cause left ventricular
concentric hypertrophy include hypertrophic cardiomy-
Broussard et al. 1995). opathy, systemic hypertension, valvular aortic stenosis,
Cardiovascular abnormalities are noted frequently on acromegaly, and hyperviscosity. History, physical exami-
physical examination, often as incidental findings. Heart nation, laboratory blood and urine tests, blood pressure
murmurs were present in 54% of 202 hyperthyroid cats measurement, and echocardiography help to differenti-
in 1 study and 53% of 131 cats in another study at the ate hyperthyroidism from these conditions. Disorders
same institution (Broussard et al. 1995). The source of that cause systemic hypertension include chronic kidney
these murmurs may include a combination of hyperdy- disease, pheochromocytoma, and other uncommon
namic inotropic/systolic and lusitropic/diastolic func- entities.
tion with dynamic right ventricular outflow obstruction Cases of equivocal or mild hyperthyroidism may be
(Rishniw and Thomas 2002) or left ventricular outflow confusing diagnostically if the level of circulating thy-
tract obstruction due to left ventricular concentric roxine seems inconsistent with the severity of clinical