Page 77 - Clinical Small Animal Internal Medicine

6 Feline Acromegaly 45

Periosteal reaction, osteophyte production, soft tissue are commonly used in human medicine for the treat­
VetBooks.ir swelling, and collapse of joint spaces are signs associated ment of acromegaly and have efficacy rates of 50–60%.
The somatostatin analog octreotide has been evaluated
with the degenerative arthropathy linked to feline acro­
megaly. Thoracic radiographs may reveal cardiomegaly
success. One study in four cats found no change in GH
(hypertrophic cardiomyopathy) and/or congestive heart in a small number of feline acromegalics with limited
failure. Nonspecific signs such as abdominal organo­ following treatment. Another study measured the short‐
megaly (hepatic, renal, and adrenal) may be revealed by term effects of octreotide in five feline acromegalics and
abdominal ultrasound. found a decrease in growth hormone concentrations for
Advanced imaging is needed to document the presence up to 90 minutes. A small study evaluating an octreotide
of a pituitary macroadenoma. Computed tomography analog (Sandostatin® LAR) showed no benefit in cats
(CT) and magnetic resonance imaging (MRI) are both treated for 3–6 months.
useful for identifying pituitary masses. However, one study However, a recent study examined the long‐term
found MRI to be the more sensitive imaging modality. The medical management of acromegaly in cats using pasire­
presence of a pituitary tumor alone is not diagnostic for otide, a novel somatostatin analog, which had been
feline acromegaly as other functional tumors of the pitui­ shown to decrease serum IGF‐1 and improve insulin
tary may also result in insulin‐resistant diabetes, such as sensitivity in cats with acromegaly when administered as
adrenocorticotropic hormone (ACTH)‐producing tumors a short‐acting preparation. The study used once‐monthly
in patients with Cushing disease. Conversely, the absence administration of long‐acting pasireotide (pasireotide
of a pituitary mass does not rule out acromegaly as there LAR) for treatment of cats with acromegaly. Fourteen
have been reported cases where a patient had a negative cats with acromegaly, diagnosed based on the presence
MRI but a pituitary mass was identified at necropsy and of diabetes mellitus, pituitary enlargement, and serum
histopathology confirmed a GH‐secreting adenoma. IGF‐1 >1000 ng/mL, were enrolled. Cats received pasire­
otide LAR (6–8 mg/kg SC) once monthly for six months.
Histopathology Fructosamine and IGF‐1 concentrations, and 12‐hour
blood glucose curves (BGCs) were assessed at baseline
Histopathology is needed for definitive diagnosis which and then monthly. Product of fructosamine concentra­
makes antemortem diagnosis challenging. However, with tion and insulin dose was calculated as an indicator of
advancements in surgical procedures such as transsphe­ insulin resistance (Insulin Resistance Index). Eight cats
noidal hypophysectomy, surgical excisional biopsy is pos­ completed the trial. Three cats entered diabetic remission.
sible. The main histopathologic change associated with Median IGF‐1 (baseline 1962 ng/mL [range 1051–
acromegaly is acidophil proliferation in pituitary tumors. 2000 ng/mL]; month 6 1253 ng/mL [524–1987 ng/mL];
P < 0.001) and median Insulin Resistance Index (baseline
Adrenocortical Function Testing 812 μmolU/L kg [173–3565 μmolU/L kg]; month 6

There is no single test for the diagnosis of feline acro­ 135 μmolU/L kg [0–443 μmolU/L kg]; P = 0.001)
megaly. Clinical suspicion based on a thorough history decreased significantly. No significant change was found
and physical exam is essential. As stated earlier, the most in mean fructosamine (baseline 494 ± 127 μmol/L; month
common presenting complaint for patients with acro­ 6 319 ± 113.3 μmol/L; P = 0.07) or mean blood glucose
megaly is insulin resistance with weight gain. The two (baseline 347.7 ± 111.0 mg/dL; month 6 319.5 ± 113.3 mg/
most common causes of insulin resistance in cats are dL; P = 0.11), despite a significant decrease in median
hyperadrenocorticism and acromegaly. Both of these insulin dose (baseline 1.5 [0.4–5.2] U/kg; month 6 0.3
diseases can be associated with a pituitary mass and [0.0–1.4] U/kg; P < 0.001). Adverse events included diar­
bilateral adrenomegaly. As such, all suspected acromeg­ rhea (n = 11), hypoglycemia (n = 5), and worsening poly­
alics should undergo adrenocortical testing via the phagia (n = 2). This initial study indicates that pasireotide
ACTH stimulation test and/or low‐dose dexamethasone LAR has potential as a long‐term management option
suppression test. Normal results on these tests would for cats with acromegaly.
then be an indication to screen for acromegaly. Conflicting results with various somatostatin analogs
may be related to difference in somatostatin receptor
subtypes. Future studies are required to identify the
Treatment somatostatin receptor subtypes in GH‐secreting feline
pituitary tumors to determine if they are similar to the
ones found in humans.
Medical Treatment
Dopamine agonists and more recently GH receptor
Somatostatin is a hypothalamic hormone that acts on antagonists are also used in human medicine for the treat­
the pituitary to inhibit GH release. Somatostatin analogs ment of acromegaly. The use of GH receptor antagonists