Q & A: Acromegaly in an Uncontrolled Diabetic Cat?

Enlargement of head in acromegalic cat

Zack is a 9.5-year-old, M/N cat weighing 13 pounds and is uncontrolled on 13 units of human NPH BID. He has been diabetic for about a year. Previously he was on glargine insulin with no response. 

His weight is relatively stable and his physical exam is normal. He has had a full workup. No adrenal tumors observed on ultrasound. His fructosamines have all been high, reflecting his poor glucose control. His fPLI (feline pancreatic-like immunoreactivitiy) level was high, consistent with pancreatitis. He is currently on metronidazole (62.5 mg/day), prednisolone (2.5 mg/day), and buprenorphine to treat his pancreatitis.

To rule out acromegaly, we ran an insulin-like growth factor 1 (IGF-1) level which was mildly high at 132 nmol/L (reference range <100 nmol/L).  We also did a serum GH level and that was also mildly elevated (17.1 ng/ml; normal <10 ng/ml).

So my questions to you are:

1. Does this cat definitely have acromegaly?
2. Should we do a MRI to look for a pituitary tumor?
3. Anything medical treatments for acromegaly?
4. Is surgery a good option if tumor is found?

I read some of your papers and chapters showing that treatment with 150 units insulin or higher may be needed in some of these cats. So should I start to increase the insulin dosage rapidly? Should I increase 10 units at a time?

My Response:

I'm not that impressed with the IGF-1 or GH results.  Most acromegalic cats have much higher values, especially by this stage of their disease (2-9). Not all cats with have physical changes consistent with acromegaly (large head and paws, weight gain, etc), the absence of such characteristic signs make that diagnosis less likely (5). You may want to recheck the GH/IGF-1 values in a few weeks. Remember, like with almost all endocrine tests, false positive results can occur, especially with the IGF-1 assay (1,10).

Pituitary macrotumor in acromegalic cat

If you still suspect acromegaly, a CT or MRI would really be helpful (2-5,7-9). Virtually all of these cats have a GH-secreting adenoma, so if a pituitary tumor was identified that would greatly aid in diagnosis (See figure on left which demonstrated large pituitary mass). If a pituitary adenoma was documented, that would also help in determining prognosis and predicting future insulin needs.

As far as definitive treatments for feline acromegaly, we really don't have any good ones. Medical treatments used in human patients don't appear to work at all in cats. Hypophysectomy has been tried in cats (4), but most cats have macroadenomas by the time of diagnosis so surgery is not effective. External radiation therapy offers the best chance for long-term control or cure in cats with acromegaly, at least at this time (3,6,8).

Human NPH insulin doesn't work well in many cats. If glargine didn't work, I'd consider switching to PZI insulin (ProZinc, Boehringer Ingelheim). Without the pituitary MR or CT, I certainly wouldn't increase by more that 0.5-2 U/day weekly or so. And then measuring glucose concentrations either at home or in the hospital.

References:

1. Alt N, Kley S, Tschuor F, et al. Evaluation of IGF-1 levels in cats with transient and permanent diabetes mellitus. Res Vet Sci 2007;83:331-335.
2. Berg RI, Nelson RW, Feldman EC, et al. Serum insulin-like growth factor-I concentration in cats with diabetes mellitus and acromegaly. J Vet Intern Med 2007;21:892-898.
3. Littler RM, Polton GA, Brearley MJ. Resolution of diabetes mellitus but not acromegaly in a cat with a pituitary macroadenoma treated with hypofractionated radiation. J Small Anim Pract 2006;47:392-395.
4. Meij BP, Auriemma E, Grinwis G, et al. Successful treatment of acromegaly in a diabetic cat with transsphenoidal hypophysectomy. J Feline Med Surg 2010;12:406-10.
5. Niessen SJ. Feline acromegaly: an essential differential diagnosis for the difficult diabetic. J Feline Med Surg 2010;12:15-23.
6. Niessen SJ, Khalid M, Petrie G, et al. Validation and application of a radioimmunoassay for ovine growth hormone in the diagnosis of acromegaly in cats. Vet Rec 2007;160:902-907.
7. Niessen SJ, Petrie G, Gaudiano F, et al. Feline acromegaly: an underdiagnosed endocrinopathy? J Vet Intern Med 2007;21:899-905.
8. Peterson ME. Acromegaly in cats: are we only diagnosing the tip of the iceberg? J Vet Intern Med 2007;21:889-891.
9. Peterson ME, Taylor RS, Greco DS, et al. Acromegaly in 14 cats. J Vet Intern Med 1990;4:192-201.
10. Starkey SR, Tan K, Church DB. Investigation of serum IGF-I levels amongst diabetic and non-diabetic cats. J Feline Med Surg 2004;6:149-155.