Small Animal Pituitary Surgery

Transsphenoidal Hypophysectomy


Transsphenoidal HypophysectomyWhat is Transsphenoidal Hypophysectomy?

Transsphenoidal hypophysectomy (TSHS) is a common surgery in people for the treatment of pituitary tumors and other masses within the region of the pituitary gland. The surgical approach in people is through the nose using a cannula and endoscope for visualization. Dogs and cats, of course, have a different skull structure, so the surgical approach is open mouth through the soft palate to gain access to the base of the skull and the area of the pituitary gland called the pituitary fossa. TSHS is technically challenging and post-operative recovery requires extremely close monitoring.


What dogs need this surgery?

The most common pituitary tumor in dogs is a functional pituitary mass causing Cushing’s disease or pituitary dependent hyperadrenocorticism (PDH). PDH is caused by an unregulated and excess secretion of cortisol by the adrenal glands in response to excess adrenocorticotrophic hormone (ACTH) production from pituitary tumors (most commonly adenomas). This increase in circulating cortisol causes bodily changes such as excessive thirst and urination, hair loss, weight gain, high blood pressure, diabetes, osteoporosis and behavioral disturbances. PDH is progressive and may be fatal if untreated. PDH is quite common in dogs and it is estimated that up to 100,000 dogs yearly in the United States develop pituitary tumors. Although any breed can develop PDH, the most commonly affected breeds are: Poodles, Dachshunds and Terriers, Boxers, Labrador Retrievers, Australian Shepherds, Maltese and Cocker Spaniels.


What cats need this surgery?

Cats can also get functional and non-functional masses of the pituitary gland. Cats can develop Cushing’s disease secondary to an ACTH secreting pituitary tumor. Cats can also develop a condition called acromegaly in which the pituitary gland produces an excess amount of growth hormone.

Treatment Options

Treatment options for pituitary tumors include medical management, radiation therapy and surgical management via TSHS.

Medical Management (Dogs): Several medical treatments are available for treatment of PDH. Treatment options include drugs that chemically destroy the adrenal glands, inhibit the release of ACTH from the pituitary gland, and inhibit synthesis of steroid hormones. While these treatments can improve the clinical signs in 40-80 percent of patients they need to be chronically administered, necessitate frequent monitoring, and do not cure or address the primary cause of the disease (the pituitary tumor). Disadvantages associated with these medications include vomiting and diarrhea either due to a drug reaction or secondary to adrenocortical insufficiency and relapse rates as high as 50 percent.  

Medical Management (Cats): There are medications that are available to treat cats with pituitary tumors that cause either Cushing’s disease or acromegaly. Administration is met with some success but long-term control of the disease process is uncertain at this time.

Radiation Therapy: Radiation therapy is also a treatment option for dogs diagnosed with PDH. Radiation may decrease the size of the tumor but does not completely address the size of the mass. Radiation generally shrinks the tumor by approximately 25-50 percent and therefore may not control the neurologic signs associated with a large mass occupying tumor. Mean survival with radiation treatment is 22-46 months. Smaller macroadenomas treated early are those that will likely respond more favorably to radiation treatment. Radiation side effects can include partial or complete blindness, conjunctivitis, hair loss, whitening of hair, and recurrence.

Cats with pituitary tumors have successfully been treated with radiation therapy. Approximately 60 percent of cats treated have improved clinical signs and between 50-60 percent either no longer need insulin treatment or have an improved insulin response. Cats with neurologic signs secondary to the pituitary tumor experience either partial or complete remission of their neurologic signs.

Dr. Annie Chen-Allen testing a dog

WSU neurologist Dr. Annie Chen-Allen testing a dog’s vision before surgery.

Transsphenoidal hypophysectomy (TSHS) has been successfully used to treat dogs since 1997, with good long-term remission rates. These surgeries have not often been performed in the United States due to lack of expertise and the relative cost of surgery compared with medical therapy. Recently, publications have documented the successful removal of larger macroadenomas in dogs with PDH that had previously been regarded as an increased risk for hypophysectomy.

In people, TSHS to remove the tumor is the most successful long-term therapy. Surgical cure rates for people are reported to be in the range of 65-85 percent with a recurrence rate of 25 percent within five years. Hypophysectomy in dogs resulted in similar long-term remission rates. Procedural mortality rate associated with TSHS is approximately 8-20 percent and incomplete hypophysectomy occurs in about 6 percent of the cases. Two primary limiting factors to widespread use of TSHS and subsequent successful surgery are: (1) difficulty in accurately defining the bony borders of the sella turcica based on anatomic landmarks alone; and (2) difficulty in visualizing the surgical site because of poor illumination and magnification. A high-definition telescope system (VITOMTM, Karl Storz Endoscopy, Tuttlingen, Germany) has been used successfully and repeatedly by human surgeons during microneurosurgeries such as craniotomies and spinal surgery. The VITOMTM functions like a microscope and has improved surgical outcome by increasing magnification and visualization. An initial study of dogs undergoing TSHS to remove functional pituitary macroadenomas, conducted by Dr. Owen and colleagues using the VITOMTM, reports improved safety and efficacy and an improved initial overall remission rate of 81 percent (21/26) with a sustained remission rate of 95 percent (20/21) and a relapse rate of 5 percent at one year with 7 of 21 dogs in remission at two years post-surgery. Mortality rate associated with TSHS ranges from 8 - 20 percent, which is partially related to pituitary tumor size.

Cats undergoing surgery for pituitary masses secondary to PDH or acromegaly have a good outcome. In a case series of 12 cats with acromegaly, 70 percent went into diabetic remission within 20 days post-surgery and the remaining 30 percent had a reduction in insulin needs. An 8-20 percent mortality rate was associated with surgery. TSHS in cats with PDH resulted in clinical and biochemical remission. Two of the seven cats died within four weeks of causes unrelated to surgery. One cat or 20 percent had recurrence of the tumor.

 

Who does the surgery?

Dr. Owen has been performing transsphenoidal hypophysectomy for the last five years. The initial work was done at VCA West Los Angeles Animal Hospital in collaboration with Adam Mamelak, MD from Cedars Sinai Medical Center, a renowned neurosurgeon and expert on transsphenoidal hypophysectomy. Dr. Owen is now a part of the surgery faculty at the WSU College of Veterinary Medicine and is currently accepting dogs and cats for TSHS.

The WSU College of Veterinary Medicine has assembled a cohesive team of veterinary specialists who work together to manage patients requiring this type of surgery. The veterinarians on our pituitary team include: Annie Chen-Allen, DVM, DACVS (neurology), Linda Martin, DVM, DACVECC (emergency and critical care) and Tina Owen, DVM, DACVS (surgery). Our team also has strong support from our medicine, radiology and anesthesia departments and a well trained staff of veterinary technicians and veterinary students.

Washington State University