Acromegaly in cats results from excess growth hormone (GH)
secreted from an adenoma within the pituitary pars distalis. The anabolic
effects of GH are mediated by insulin-like growth factor (IGF)-l (somatomedin
C) and include an increase in body size, enlargement of the abdomen
and head, weight gain and organomegaly of the heart, kidney, liver and tongue.
Insulin antagonism occurs due to a GH-induced post-receptor defect in glucose
transport leading to hyperinsulinism and a down regulation of insulin receptors.
Acromegaly typically is seen in male DSHs, over 10 years old, with poorly
controlled diabetes mellitus, or less commonly, neurological signs. Conformational
alterations are insidious in onset and often not noted by owners. Occasionally
weight loss is seen. Later clinical signs include degenerative arthropathies,
congestive heart failure and renal failure. Pituitary tumours may cause
local neurological damage resulting in stupor, somnolence, poor appetite,
adipsia, temperature deregulation, circling, seizures, and changes in behaviour.
Most of the laboratory test abnormalities in acromegaly are due to the diabetes:
hyperglycaemia, glycosuria, hypercholesterolemia, and mild increases in
ALT and ALP. Ketonuria is an infrequent finding. Persistent hyperphosphataemia
without concurrent azotemia, mild erythrocytosis and a persistently raised
total protein (up to 95 g/L) may also be found. Associated renal
failure may result in azotemia, isosthenuria, and proteinuria. A tentative
diagnosis of agromegaly may be made in a cat with insulin-resistant diabetes
(requiring > 2 iu/kg insulin) and consistent conformational changes.
Ideally other causes of insulin resistance are ruled out, a pituitary CT
or MR scan is performed and an increased serum IGF-1 concentration is found
(GH measurement is not readily available commercially). Some cases
of acromegaly have IGF-I values in the high normal range.
Successful therapy for acromegaly has not been established
in the cat. Potential treatments include radiation therapy, medical therapy
(Octreotide), and hypophysectomy. The prognosis is guarded to poor
with survival times of 4-42 months (median, 21 months). The tumour
usually grows slowly-neurologic signs are uncommon until late in the course
of the disorder, diabetes mellitus can usually be somewhat controlled with
large doses of insulin given twice daily (because insulin resistance often
fluctuates, to avoid hypoglycaemia, insulin doses should not exceed 12 to
15 units per injection) but most cats die from heart or renal failure. |