Page 591 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 26 Tumors of the Endocrine System 569

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A B
• Fig. 26.1 A 14-year-old male neutered domestic long-hair cat with pituitary-dependent hypercortisolism.
(A) The patient appears weak with muscle atrophy and has an unkempt hair coat. (B) The abdomen has a
potbellied appearance with thinning of the skin.

Transsphenoidal hypophysectomy was described in seven cats underdiagnosed. In an early study of 184 diabetic cats, 59 had
with PDH. Two of the cats died within 4 weeks of surgery from markedly increased serum insulin–like growth factor-1 (IGF-1)
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unrelated causes, and the remaining five cats experienced remis- concentrations, and acromegaly was confirmed in 17 of 18 cats
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sion of their disease. Two additional cats died several months after that were examined by CT, MRI, or necropsy. The same group
surgery, and one cat experienced a relapse of PDH 19 months subsequently published the results of screening for acromegaly
after treatment, likely associated with pituitary remnants after in a much larger group of 1221 diabetic cats. They found that
surgery. Potential complications of this surgery include transient 319 (26.1%) of the cats had serum insulin-like growth factor 1
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or long-term hypopituitarism, electrolyte disturbances, soft palate (IGF-1) concentrations consistent with acromegaly; 63 of these
dehiscence, and reduced tear production. Relatively few reports cats underwent pituitary imaging, and the presence of a pituitary
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have been published on the outcome of RT for cats with PDH, lesion was confirmed in 94% of this group. A study of dia-
and it is still unclear if this modality leads to robust management betic cats in Europe recently found that 36 of 202 (17.8%) cats
of endocrine disease. 66,67 SRT is an appealing option for these evaluated had serum IGF-1 concentrations consistent with acro-
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cats because it requires significantly fewer anesthetic events in megaly. Clearly the prevalence of acromegaly differs between
patients that often are clinically fragile. Surgical bilateral adrenal- these studies, and selection bias may play in role in some of the
ectomy also has been described in cats with PDH and historically data. For example, in an initial study of 184 cats, the IGF-1
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was considered the treatment of choice. However, these cats are assays were performed on blood samples submitted to a labora-
often poor surgical candidates, they have diminished healing abil- tory at a referral institution for measurement of serum fructos-
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ity, and complications are common. Laparoscopic adrenalectomy amine, which is a test that may be more likely to be performed
potentially is a better option for these patients because the inci- in cats with diabetes that is difficult to regulate. The prevalence
sions are much smaller; however, this has not yet been reported of acromegaly in the average or typical diabetic cat is likely in the
in cats with PDH. Trilostane currently appears to be the most range of 10% to 15%, but the prevalence in cats that are difficult
reasonable medical therapy for cats with PDH. 66,73,78–80 Clini- to regulate is probably closer to 30%. Regardless of the exact
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cians intending to pursue this therapy are encouraged to become numbers or differences between studies, considerable evidence
familiar with resources that provide detailed protocols for the use now suggests that acromegaly is neither rare nor uncommon.
of trilostane in cats. 66,67 Most veterinary endocrinologists agree that a cat with any of
the clinical features of acromegaly, including insulin resistance,
should be screened for this disorder. 89
Pituitary Somatotroph Tumors (Feline Acromegaly is more common in male cats, with no apparent
Acromegaly) breed predilection, and most affected cats are middle-aged or
older. The typical history is one of insulin-resistant diabetes mel-
Feline acromegaly is a disease of older cats that results from litus, with affected cats requiring 10 to 20 units of insulin per
chronic excessive GH secretion, usually from a functional somato- dose or more, often with inadequate control of the diabetes. This
troph adenoma of the pars distalis of the pituitary gland. 81–83 The insulin resistance is due to a GH-induced postreceptor defect in
genetic cause of feline acromegaly is unknown; however, the aryl- the action of insulin on target cells. Affected cats remain poly-
hydrocarbon-receptor interacting protein gene has been the sub- uric, polydipsic, and polyphagic and continue to gain weight.
ject of preliminary investigations because it is known to play a Most cats with poorly regulated diabetes mellitus lose weight;
role in the development of human familial acromegaly. The term therefore weight gain in this situation may be suggestive of feline
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hypersomatotropism refers to a condition of excessive growth hor- acromegaly.
mone, whereas acromegaly refers to the constellation of associated The physical changes of acromegaly develop slowly and often
clinical signs. 85 are not noted by the owner until they are advanced. These changes
Feline acromegaly historically was regarded as a rare con- may include enlarged feet, broadening of the face, protrusion of
dition; however, more recent findings indicate that it may be the mandible (Fig. 26.2), increased spacing between the teeth, and