Small Animal Pituitary Surgery

Pituitary Surgery (Hypophysectomy)

Sep 19, 2019

What is Hypophysectomy?Transsphenoidal Hypophysectomy

Hypophysectomy is a surgery performed in people, dogs and cats for the treatment of clinical 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. Hypophysectomy is technically challenging and post-operative recovery requires extremely close monitoring. 

Cat surgical candidates:

The most common disease caused by a pituitary tumor in cats is acromegaly or gigantism, an over production of growth hormone by the pituitary gland.  This can then cause diabetes mellitus which often leads to insulin resistance which requires increasingly higher doses of insulin to treat the diabetes. A study in the United Kingdom reported that 25% of diabetic cats presented to their clinic were actually suffering from an overproduction of growth hormone.  This is a much higher incidence than was previously thought. Cats can also suffer from Cushing’s disease (pituitary dependent hyperadrenocorticism, PDH) a condition in which the pituitary gland overproduces a hormone that tells the body to produce an excess amount of steroid.  Another type of pituitary tumor seen in cats is one that is “silent” does not overproduce a hormone but does cause neurologic signs attributable to a growing space occupying mass.  These signs typically include lethargy, inappetence, head and neck pain and mild ataxia.  

The United Kingdom reported on a group of approximately 60 cats suffering from acromegaly that were treated surgically by hypophysectomy.  Ninety percent of cats survived surgery with approximately 70% of the surviving cats experiencing remission of their diabetes mellitus and the majority of the remaining cats displaying a decrease in their insulin requirement.  Ninety-two percent of cats achieved diabetic remission long term.

 

Dog surgical candidates:

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 unregulated secretion of steroids by the adrenal glands in response to over production of adrenocorticotrophic hormone (ACTH) from the pituitary tumor (most commonly an adenoma). This increase in circulating steroids cause bodily changes such as excessive thirst and urination, hair loss, weight gain and may lead to high blood pressure, diabetes, osteoporosis and behavioral disturbances. PDH is progressive and may be fatal if left 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, Terriers, Boxers, Labrador Retrievers, Australian Shepherds, Maltese and Cocker Spaniels.  Dogs may also suffer from “silent” pituitary tumors that cause neurologic signs without hormonal changes.  

Treatment Options

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

Medical Management (Cats): 

Medical management of cats with acromegaly, and frequently cats with PDH, usually consists of insulin therapy targeted at controlling the diabetes mellitus. Unfortunately, insulin resistance is a common issue that requires frequent escalation of the insulin dose and therefore frequent veterinary monitoring and added costs. There are medications that are available to treat cats with pituitary tumors that cause either PDH or acromegaly.  Administration of medications is met with some success but long-term control of the disease process is uncertain at this time.  The goal of medical management is to control clinical signs but generally does not achieve a cure at this time.  

Medical Management (Dogs):

Several medications 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 the synthesis of steroid hormones. While these medications 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 can be seen.  

Radiation Therapy:

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 require a reduced insulin dose. Cats with neurologic signs secondary to the pituitary tumor experience either partial or complete remission of their neurologic signs.

Radiation therapy is also a treatment option for dogs diagnosed with PDH.  Radiation generally shrinks the tumor by approximately 25-50 percent and therefore may not control the neurologic signs associated with a large space-occupying tumor. Smaller tumors 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.
Clinical signs secondary to overproduction of hormones by the pituitary tumor typically still need to be treated medically during and after radiation therapy.  In other words, radiation treatment may help control the growth of the tumor but is not as helpful in controlling the hormonal signs.

Surgical Therapy:

Hypophysectomy has been successfully used to treat dogs since 1997, with good long-term remission rates.  Hypophysectomy is the only treatment option that offers decompression, rapid resolution of both hormonal and neurologic signs, a definitive diagnosis, and intent to cure.These surgeries have not routinely been performed in the United States due to lack of expertise. Recently, publications have documented a better outcome with smaller pituitary tumors. Long-term survival is better, recurrence is less and surgical mortality is lower in patients with small tumors. The successful removal of larger tumors in dogs with PDH has recently been documented in the literature however with higher risk.  It is important to have documentation of pituitary size via brain imaging early in the course of treatment, as the outcome with smaller tumors is more favorable.  

In people, hypophysectomy is the most successful long-term therapy to remove clinical pituitary tumors.  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 have a similar long-term remission rate.  Procedural mortality rates associated with hypophysectomy is approximately 8-20 percent with a lower rate associated with smaller tumors.

Cats undergoing surgery for pituitary tumors secondary to PDH or acromegaly have a good outcome. In a case series of 60 cats with acromegaly, 70 percent went into diabetic remission, 30 percent had a reduction in insulin needs and there was an 8% mortality rate associated with surgery.  Hypophysectomy in cats with PDH resulted in clinical remission in 70% of the cases, with a recurrence rate of 20%.

 

Who does the surgery?

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.  A comprehensive and collaborative team increases our ability to offer high quality care to our patients and compassionate, personal attention to our clients.  

Washington State University