Hyperadrenocorticism (Cushing's disease)
While less commonly seen than in their canine counterparts, cats can suffer from an excessive production of cortisol by the
adrenal glands. 85% of the cases of feline hyperadrenocorticism are due to a pituitary tumor whereas 15% stem from an adrenal
tumor. Progesterone-secreting adrenal tumors have been documented a handful of cats.
Hyperadrenocorticism (HAC) is a disease of middle-aged to older cats. There is no breed predilection but females appear to
be over-represented (70% of cases). 80% of cats with hyperadrenocorticism have concurrent diabetes mellitus. Cortisol often
causes insulin resistance in cats so frequently hypercortisolemia from hyperadrenocorticism leads to overt diabetes mellitus.
It was long believed that the typical signs of polyuria and polydipsia seen in cats with HAC were actually manifestations
of the secondary diabetes mellitus; however, cats with only HAC who had not yet progressed to diabetes mellitus have been
described as displaying these same clinical signs. Hyperadrenocorticism still is most frequently diagnosed in cats that have
poorly controlled diabetes mellitus. Other clinical signs frequently seen in cats include generalized muscle wasting, pendulous
abdomen, thin, inelastic skin, lethargy, and unkempt hair coats.
Hyperglycemia is typically found in feline HAC. Other changes that may be noted on a biochemical panel include hypercholesterolemia
and an increase in alanine transaminase (ALT). Because cats do not have the steroid-induced isoenzyme of alkaline phosphatase
(ALP), elevations in this enzyme are less commonly seen and, when present, are due to concurrent diabetes mellitus and hepatic
lipidosis most likely.
Plain radiographs are of little value in the diagnosis of feline HAC. Abdominal ultrasound may be of benefit in examining
the adrenal glands. In published reports of naturally occurring HAC in cats, ultrasound reveals bilaterally enlarged adrenal
glands in approximately ⅔ of cases. Unilateral enlargement of an adrenal gland is suggestive of an adrenal tumor. MRI has
been proven useful in documenting pituitary tumors in cats.
Specific testing of the pituitary-adrenal axis is necessary to diagnose HAC. Unfortunately, there is no one perfect
test. Clinicians usually rely on a combination of appropriate clinical signs in conjunction with multiple supporting test
results before establishing a diagnosis of feline HAC. There are two tiers to testing; first it is necessary to confirm the
diagnosis of HAC, secondly it must be determined whether the HAC is pituitary-dependent (PDH) or due to an adrenal tumor (AT).