Ann Della Maggiore
Ann Della Maggiore, DVM, DACVIM, serves as an assistant professor of clinical internal medicine at the University of California, Davis School of Veterinary Medicine, with a specific interest in small animal endocrinology. She received her veterinary degree from UC Davis, then completed a residency in small animal internal medicine at the same institution.Read Articles Written by Ann Della Maggiore
Sugar, a 12-year-old spayed female Maine Coon cat, presented for poorly controlled diabetes and diabetic neuropathy.
Sugar was diagnosed with diabetes mellitus 2 years prior to presentation. Initially, her diabetes was moderately controlled on 5 to 6 units of recombinant human protamine zinc insulin (PZI) (40 U/mL; ProZinc, bi-vetmedica.com), but over the year prior to presentation the insulin dose had been progressively increased with no improvement in glycemic control.
Upon presentation, Sugar was receiving 14 units of recombinant human PZI. The owner was performing blood glucose curves at home, but struggling to maintain Sugar’s blood glucose below 300 mg/dL.
In addition to diabetes mellitus, Sugar had concurrent hypertrophic cardiomyopathy and chronic rhinitis, and persistent polyuria, polydipsia, polyphagia, and weakness.
Key Points: Feline Diabetes Mellitus
- Diabetes is a disease of insulin deficiency.
- Diabetes in cats is most commonly classified as type 2-like diabetes—a disease process in which insufficient insulin production from beta cells takes place in the setting of insulin resistance.
- Insulin requirements can be altered by obesity, inflammation, or concurrent endocrine disease, such as hypersomatotropism (acromegaly) or hyperadrenocorticism.
- Some refer to a subclass of diabetic cats with secondary diabetes—patients in which diabetes occurs subsequent to (1) another endocrine disease (eg, acromegaly, hyperadrenocorticism) or (2) administration of diabetogenic drugs (eg, glucocorticoids).1
Physical examination revealed a symmetrically muscled cat, weighing 7.7 kg, with a body condition score of 6/9. Sugar had an unkempt hair coat, mild prognathia inferior, and a broad head (Figure 1), and walked with a plantigrade stance (Figure 2). No organomegaly was appreciated, and the cat appeared to have normal mentation.
FIGURE 1. Although subtle, note Sugar’s broad facial features, clubbed paws, and prognathia inferior (A and B). Courtesy Dr. Erica Queen
Consider These Questions…
- What are the possible causes of insulin resistance in this patient?
- Which additional diagnostic tests would you choose to determine a diagnosis?
- Based on your suspected diagnosis, what therapeutic options are available for Sugar?
Routine complete blood count (CBC), serum biochemical profile, and urinalysis were performed. The primary abnormalities noted on the serum biochemical profile were (Table 1):
- Mild increase in alanine aminotransferase
- Elevated blood urea nitrogen
- High normal creatinine.
Urinalysis was unremarkable except for glucosuria (> 1000 mg/dL; reference range, negative); urine culture revealed no growth. A blood glucose curve was performed that showed no response to insulin, and all blood glucose values were between 350 to 400 mg/dL.
Consider 2 main areas of focus for the poorly controlled feline diabetic patient (Table 2).
When approaching these patients, carefully:
- Evaluate case management for inconsistencies and owner handling/techniques
- Evaluate insulin administered and dose
- Consider causes of insulin resistance to determine the individual patient’s diagnostic workup.
Make sure to take a thorough history and perform a complete physical examination, CBC, serum biochemical profile, fructosamine, urinalysis, and urine culture. In addition, also review insulin administration and handling with the owner.
Perform one or multiple blood glucose curves at home or in hospital to assess response to insulin, insulin dose, and duration of effect. Single blood glucose measurements collected in hospital can help identify hypoglycemia, but should not be used to assess control.
Additional testing to consider, depending on the physical examination and initial diagnostic findings in each individual patient, includes:
- Serum feline pancreatic lipase immunoreactivity
- Total thyroxine
- Low-dose dexamethasone suppression test if physical examination supports a diagnosis of hyperadrenocorticism
- Insulin-like growth factor-1 (IGF-1)
- Abdominal ultrasound
- Thoracic radiographs.
Unusual Concurrent Endocrine Diseases
Evaluate the patient for clinical signs or examination findings consistent with hyperadrenocorticism, such as thin/fragile skin (Figure 3) or pot belly. If hyperadrenocorticism is suspected, the test of choice is a low-dose dexamethasone suppression test, using dexamethasone, 0.1 mg/kg IV.1
Acromegaly is an uncommon cause of insulin resistance in cats. In a poorly regulated diabetic cat, lack of a typical acromegalic appearance (prognathism, enlarged feet) does not negate the importance of testing for this condition by measuring serum levels of IGF-1 and performing advanced imaging of the brain.2
Consider This Disease: Acromegaly
Acromegaly, also known as hypersomatotropism, is a disease characterized by excess GH, which stimulates increased production of IGF-1 in the liver and other tissues. GH and IGF-1 cause a variety of changes in the body, including organ enlargement and increased proliferation of bone that can be seen visually as jaw enlargement (prognathism) or large paws.2,4,5
Acromegaly and excess GH are typically caused by a pituitary adenoma. GH has anti-insulin effects, causing increased glucose production in the liver and reducing insulin sensitivity in the body by a variety of mechanisms. When insulin resistance and glucose production cause elevation of serum blood glucose, the animal becomes diabetic.
Prevalence of acromegaly in diabetic cats is unknown, but is suspected to be as high as 30% of difficult-to-regulate diabetic cats.4 It is typically seen in elderly male cats.
In practice, diagnosis of acromegaly is based on:
- Common clinical signs, including polyuria, polydipsia, and polyphagia—clinical signs associated with diabetes mellitus—as well as weight gain. Neurologic signs may be seen.
- Physical examination findings, including an enlarged liver and kidneys on abdominal palpation. In more advanced disease, prognathia inferior and enlargement of the head and paws may be noted.
- IGF-1 levels—considered a sensitive but not specific test.
- If values are > 1000 ng/mL (131 nmol/L), acromegaly should be suspected, and additional testing, specifically pituitary imaging, is indicated.
- The “gray” zone is considered 800 to 1000 ng/mL.
- If acromegaly is suspected, but IGF-1 levels are within reference range (208–443 ng/mL), perform repeat testing in 6 to 8 weeks.
- IGF-1 levels can be falsely normal if a cat is in an insulin deficient state.2,4,5
- Intracranial imaging, either by CT or magnetic resonance, which documents a pituitary mass and confirms the diagnosis (because IGF-1 levels are not 100% reliable).
Additional biomarkers are being evaluated, but limited data are available. GH assays are not readily available—blood sampling has to be very specific to prevent degradation of the hormone, and the assay lacks sensitivity and specificity.
Pituitary-dependent hyperadrenocorticism can also cause severe insulin resistance and a pituitary mass—diagnostic results also seen in diabetic cats with acromegaly. However, clinical presentation of these diseases is often very different:
- Cats with hyperadrenocorticism typically present with atrophy of the epidermis; fragile, easily torn skin; and progressive weight loss.
- Cats with acromegaly typically gain weight over time and do not develop problems with their skin/hair coats, other than what is expected in a poorly controlled diabetic cat (eg, unkempt hair coat).
This Case: Diagnosis
In Sugar’s case, acromegaly was considered a primary differential due to:
- History of poorly controlled diabetes mellitus, with associated polyuria, polydipsia, and polyphagia
- Physical examination findings of mild prognathia inferior and a broad head
- Elevated IGF-1 levels: 2313 ng/mL (303 nmol/L); reference range, 208 to 443 ng/mL.3
A computed tomography (CT) scan was performed to evaluate whether a mass was present in the pituitary gland. The CT scan demonstrated a homogenously contrast enhancing mass of the pituitary gland. The pituitary mass was slightly irregular, measuring 0.7 cm in diameter (Figure 4).
These findings confirmed a diagnosis of acromegaly—the cause of the severe insulin resistance and poorly controlled diabetes mellitus.
Ideal treatment for acromegaly has not yet been determined. Current treatment options include radiation therapy, surgery (hypophysectomy), or medical management.
Radiotherapy is the current treatment of choice for pituitary tumors.2,4 Many cats experience improvement or resolution of diabetes mellitus, although other acromegalic signs, such as prognathism and enlarged feet, often persist. However, this treatment modality is expensive and outcomes are not always successful.
Hypophysectomy is the treatment of choice in humans, and may become the treatment of choice in cats. At this time, transsphenoidal hypophysectomy has been performed in acromegalic cats, but is only offered in a few, very specialized veterinary institutions. Although it is a procedure with significant risks, good outcomes have been reported.
Medical management using somatostatin analogues, which inhibit growth hormone (GH) secretion, has been used with some success in humans. However, this medical approach has been evaluated in acromegalic cats, demonstrating little to no effect.2,4
Regardless of treatment choice, close diabetic monitoring—on a regular basis—is necessary to prevent life-threatening hypoglycemia as IGF-1 levels fall and insulin resistance decreases.
This Case: Treatment and Follow-Up
Sugar was treated with a definitive radiation protocol: a total dose of 50 Gy was delivered in 20 treatments using a 2-field technique at 2.5 Gy per fraction.
During hospitalization for radiation treatment, blood glucose was monitored closely:
- Sugar’s insulin dose was decreased based on blood glucose monitoring.
- When Sugar was discharged, she was receiving 5 units SC Q 12 H of recombinant human PZI.
- The owner continued to monitor blood glucose at home and, over the next 6 to 9 months, the PZI dose was reduced to 2 units SC Q 12 H.
One year later, Sugar was re-evaluated:
- Physical examination was unremarkable, except for persistent prognathia inferior.
- CBC, serum biochemical profile, and urinalysis were unremarkable other than mild hyperglycemia (175 mg/dL) but no glucosuria.
- Fructosamine was normal at 315 mcmol/L (reference range, 0–375 mcmol/L).
- IGF-1 was still elevated at 1262 ng/mL (168 nmol/L), but was significantly reduced from original value.
- Contrast CT was repeated and the pituitary mass was decreased in size, measuring 0.3 cm in diameter (Figure 5).
Based on these findings, Sugar’s radiation therapy was determined to be effective. In addition, due to the appearance that her diabetes was controlled, the PZI dose was reduced to 1 unit SC Q 12 H.
Over the next 2 years, Sugar’s PZI insulin dose was further reduced to 0.5 units SC Q 12 H. Her IGF-1 levels were also followed, and continued to decrease to 855 ng/mL (112 nmol/L) and then 672 ng/mL (88 nmol/L).
Prognosis for cats with uncontrolled acromegaly and poorly controlled diabetes mellitus is poor to guarded; without treatment most cats are euthanized within a few months of diagnosis. However, in cats, such as Sugar, in which the concurrent disease causing insulin resistance is recognized and treated appropriately, a favorable outcome is possible since glycemic control often improves.
This Case: Outcome
Sugar died of hypertrophic cardiomyopathy 3 years following diagnosis and radiation treatment for acromegaly. A complete necropsy was performed; pertinent findings included pituitary gland multifocal adenomas (Figure 6) and significant pancreatic islet amyloidosis (Figure 7), which is characteristic of feline type 2 diabetes. Prognathism inferior was also noted.
Key to Terms
CBC = complete blood count; CT = computed tomography; GH = growth hormone; IGF-1 = insulin-like growth factor-1; PZI = protamine zinc insulin
- Feldman EC, Nelson RW, Reusch CE, et al. Feline diabetes mellitus. Canine and Feline Endocrinology and Reproduction, 4th ed. St. Louis: Saunders, 2015, pp 258-314.
- Niessen SJM. Feline acromegaly: An essential differential diagnosis for the difficult diabetic. J Fel Med Surg 2010; 12:15-23.
- Niessen SJ, Petrie G, Gaudiano F, et al. Feline acromegaly: An underdiagnosed endocrinopathy? J Vet Intern Med 2007; 21(5):899-905.
- Feldman EC, Nelson RW, Reusch CE, et al. Disorders in growth hormone. Canine and Feline Endocrinology and Reproduction, 4th ed. St. Louis: Saunders, 2015, pp 55-76.
- Niessen SJM, Church DB, Forcada Y. Hypersomatotropism, acromegaly, and hyperadrenocorticism and feline diabetes mellitus. Vet Clin Small Anim 2013; 43:319-350.