Page 386 - Feline Cardiology

404 Section N: Endocrine Diseases Affecting the Heart

once daily was thought by some to be adequate Outcome and Prognosis
(Trepanier et al. 2003). The dosage may be titrated up Natural Evolution of Hyperthyroidism in Cats
to better control persistence of the hyperthyroid state,
or titrated down if chronic kidney disease with or Untreated hyperthyroidism in the cat leads to promi-
without systemic hypertension are unmasked by the nent externally observable clinical signs, with extracar-
euthyroid state. Methimazole treatment is generally diac signs such as weight loss and ravenous appetite
effective and safe; reported adverse effects include almost invariably preceding cardiovascular signs.
thrombocytopenia, gastrointestinal signs such as acute When treated, hyperthyroid cats survive for a period
vomiting, transient hepatopathy, and facial pruritus of time that appears to be related at least in part to treat-
causing self-mutilation. Such effects may prompt the use ment type. In a retrospective study that addressed this
of alternative drugs, such as carbimazole (not available question, 47 cats treated with methimazole survived for
in the United States) 2.5 mg PO q 12 h for 7 days, then a median of 2.0 years (interquartile range: 1–3.9 years),
5 mg PO q 12 h with further dosage adjustments made which was significantly shorter than the median survival
based on serum thyroxine levels and clinical signs time for 55 cats treated with radioiodine alone (4.0
(Feldman and Nelson 2004). years; interquartile range = 3.0–4.8 years) or the median
survival time for 65 cats treated with methimazole for a
Cardiovascular Drugs median 88 days followed by radioiodine therapy (5.3
years; interquartile range = 2.2–6.5 years).
Cardiac-specific drugs given to hyperthyroid cats aim to
offset the effects of excess thyroid hormone indirectly.
Beta blockers such as atenolol (6.25 mg PO q 12 h) are OTHER ENDOCRINOPATHIES
most widely used in this respect. Beta blockers may be
considered in hyperthyroid cats that have sinus tachy- Acromegaly
cardia with atrial or ventricular premature complexes, Acromegaly is a syndrome caused by an excess in circu-
with or without systemic hypertension. Such drugs aim lating growth hormone, usually a result of a functional
to blunt sympathetically mediated increases in heart rate tumor of the somatotropic cells of the pituitary pars
and inotropy, both of which may be detrimental or life- distalis in cats (Feldman and Nelson 2000). After hyper-
threatening in severely hyperthyroid cats (see “Thyroid thyroidism, acromegaly is the most common feline
Storm,” above). It is essential, however, that such pre- endocrinopathy that produces clinically relevant cardio-
scriptions be given only if the cat is fully cooperative and vascular effects. It is a rare disorder, however, with
willing to receive oral medications at home. An owner’s between 100 and 200 feline cases reported worldwide to
perception of the critical importance of giving heart date (Niessen et al. 2007).
medication, together with a cat’s resistance to pilling, Measurement of serum growth hormone concentra-
may trigger a struggle that initiates a thyroid storm in tions revealed a higher than normal concentration in
the home setting, and such a complication can and must one study of cats with hypertrophic cardiomyopathy,
be avoided through clear communication with the raising the possibility that a sizable subset of cats with
Endocrine Diseases 91 hyperthyroid cats with respect to echocardiographic would in fact have cardiac manifestations of acromegaly,
what was thought to be hypertrophic cardiomyopathy
owner proactively.
It is worth noting that no difference was observed in
and therefore the opportunity for reversal of heart
lesions with control of the endocrinopathy (Kittleson
dimensions when cats who received prior oral medica-
1992). Unfortunately, the serum growth hormone level
tions (methimazole, beta blocker, calcium-channel
blocker, angiotensin converting enzyme inhibitor) were
compared to normal controls (Weichselbaum et al. 2005). elevations were modest, unlike those of acromegalic cats
with pituitary tumors, and of the 8 cats who died and
Antihypertensive drugs are indicated in cats showing were examined at necropsy, none had a pituitary tumor.
target-organ damage resulting from the hypertension Therefore, in some cats with idiopathic hypertrophic
(see Chapter 21), in cats with severe hypertension (e.g., cardiomyopathy, serum growth hormone levels appear
persistent systolic arterial blood pressure >180 mm Hg to be modestly elevated, and the way in which this
in calmest environment) that has not responded to anti- finding is associated with heart disease—if there is an
thyroid medication, and in cats with a new onset of association at all—is unclear.
systemic hypertension during antithyroid treatment Clinical signs of acromegaly in cats typically are those
(Syme and Elliott 2003). The drug of choice is amlodip- of diabetes mellitus (polyuria, polydipsia, polyphagia),
ine begun at 0.625 mg per cat PO q 24 h, with dosage because of the insulin antagonizing effects of growth
adjusted based on clinical signs of hypertensive damage hormone, but the anabolic effects of growth hormone
and follow-up blood pressure measurements. may cause weight gain in contrast to the weight loss of