Page 76 - Clinical Small Animal Internal Medicine

44 Section 2 Endocrine Disease

sequence variants were identified (AIP:c.481C > T and liver enzymes (ALT, ALP), hypercholesterolemia, hyper­
VetBooks.ir AIP:c.826C > T). This was the first molecular study to phosphatemia, hyperglobulinemia, azotemia, glucosuria,
ketonuria, proteinuria, and isosthenuria.
investigate a potential genetic cause of feline acromegaly
Growth hormone concentration is a common diagnos­
and identified a nonsynonymous AIP SNP in 20% of the
acromegalic cat population evaluated, as well as in one of tic for acromegaly in humans, but assays specifically for
the sibling pairs evaluated. feline growth hormone are not widely available. An assay
using ovine GH as the antigen has been validated for use
in cats, but is only available in Europe. However, even if
History and Clinical Signs an assay were available, growth hormone concentrations
alone may not be a reliable diagnostic for acromegaly.
Feline acromegaly most commonly affects middle‐aged to Growth hormone production is cyclic and levels may
older, male castrated cats. In one study, 13 of 14 cats with vary throughout the day. A single high value may not
acromegaly were males with an average age of 10.2 years. necessarily be diagnostic for acromegaly. Additionally, it
This association may be biased, however, as most cats has been shown that growth hormone may be elevated in
that are diagnosed with acromegaly present for insulin‐ nonacromegalic diabetic cats. This may be due to the
resistant diabetes mellitus, which is also more common in fact that portal insulin is required for the liver to produce
older, male castrated cats. Based on available data, there is IGF‐1. In diabetics being treated with insulin subcutane­
no known breed association for acromegaly. ously, portal insulin concentrations will remain low,
Most patients with acromegaly present for insulin‐ resulting in decreased IGF‐1 production and theoreti­
resistant diabetes mellitus (insulin doses greater than cally decreased inhibition of GH release. In addition, GH
1.5–2.2 units/kg BID) with concurrent weight gain levels may also not be elevated early in the course of the
rather than weight loss. Growth hormone has effects disease, but later typically increase significantly.
on all the tissues in the body and therefore the disease Insulin‐like growth factor‐1 is the endocrine assay most
has a range of clinical signs. Physical characteristics of commonly used to diagnose feline acromegaly and is
acromegaly include increased body weight, a broad­ widely available through the Michigan State University
ened face, enlarged feet, protrusion of the mandible Diagnostic Center for Population and Animal Health
(prognathia inferior), increased interdental spacing, (www.animalhealth.msu.edu/Forms/F.ADM.7.pdf ).
stertorous breathing, organomegaly, and a poor hair­ Unlike GH, IGF‐1 concentrations are less likely to fluctu­
coat. Cardiovascular signs include the presence of a ate over the course of the day as the majority of IGF‐1
heart murmur, hypertension, arrhythmia, associated is protein bound, giving it a longer half‐life in the body. In
with hypertrophic cardiomyopathy. Neurologic dis­ addition, IGF‐1 increases in response to chronically ele­
ease associated with feline acromegaly is uncommon vated GH concentrations and is thought to be a reflection
but can occur with a pituitary macroadenoma. of GH levels over the previous 24 hours. However, just as
Neurologic signs that have been observed with acro­ with GH, elevations in IGF‐1 concentration alone may not
megaly include dullness, lethargy, abnormal behavior, be diagnostic for acromegaly. One study found that IGF‐1
circling, and blindness. Glomerulopathy and second­ levels in nonacromegalic cats on long‐term insulin treat­
ary renal failure have also been associated with feline ment (>14 months) had higher levels of IGF‐1 than non­
acromegaly. Histopathologic evaluation of the kidneys diabetics. It was proposed that insulin treatment allowed
from acromegalic cats has revealed thickening of the for beta cell regeneration and increased portal insulin,
glomerular basement membrane and Bowman’s cap­ leading to elevations in IGF‐1. A subsequent study evalu­
sule, periglomerular fibrosis, and degeneration of the ating IGF‐1 levels in confirmed acromegalics, diabetics,
renal tubules. Arthropathy and peripheral (diabetic) diabetics, and healthy cats found that acromegalic diabet­
neuropathy have been shown to cause lameness in ics had significantly higher levels of IGF‐1 than diabetics
acromegalic cats. and nondiabetics. This study concluded that IGF‐1 was
84% sensitive and 92% specific for diagnosing feline acro­
megaly. No correlation between long‐term insulin use and
elevations in IGF‐1 concentrations was found in this study.
Diagnosis

Diagnosis of feline acromegaly starts with clinical suspi­ Diagnostic Imaging
cion, using a thorough history, signalment, and clinical Radiographic findings associated with feline acromegaly
signs. Many of the abnormalities in the minimum data­ are related to the hypertrophic effects of excessive GH.
base of affected cats reflect concurrent diabetes mellitus Hyperostosis of the calvarium, spondylosis of the spine,
and include erythrocytosis, hyperglycemia, increased and protrusion of the mandible are common findings.