Pituitary Macroadenoma in Cats

What is a pituitary macroadenoma?

Pituitary macroadenomas are large, non-cancerous (benign) tumors of the pituitary gland. These tumors are typically greater than ½ inch in diameter, though there is some subjective variation in the definition of macroadenoma depending on the animal’s skull size. Pituitary macroadenomas are more common in dogs, although they can also occur in cats.

There are two types of pituitary macroadenomas, and their effects depend on the type of tumor.

1) Functional tumors secrete hormones. The effects of functional macroadenomas depend on the type of hormones being produced by the tumor.

2) Nonfunctional tumors do not secrete any hormones. The effects of nonfunctional pituitary macroadenomas are directly related to the physical pressure that the growing tumor places on surrounding brain structures.

In most cases, pituitary macroadenomas are functional tumors (they secrete hormones). The most common functional pituitary macroadenomas in cats is associated with increased production of growth hormone, or acromegaly. Acromegaly can lead to abnormally rapid growth rates, as well as a number of other medical conditions, including diabetes mellitus, kidney disease, liver disease, heart disease, and degenerative joint disease.

What are the clinical signs of a pituitary macroadenoma?

1) Cats with a functional pituitary macroadenoma may show a variety of clinical signs, depending on the organ systems being affected at different stages of acromegaly. Initial clinical signs are often related to diabetes, including increased appetite, weight loss (despite an increased appetite), and increased thirst/urination. Over time, cats will often begin to demonstrate abnormal growth of bone and cartilage, including enlarged facial features and a protruding jaw.

2) Cats with nonfunctional pituitary macroadenomas may show signs related to the tumor’s compression of nearby brain structures. Several optic (eye) structures are located in the area of the pituitary gland; therefore, visual defects are often seen with pituitary macroadenomas. Additionally, these tumors can interfere with the production of hormones that help to concentrate urine. If this happens, cats can develop increased thirst and urination, and a condition known as diabetes insipidus. In many cases; however, initial signs include vague signs such as lethargy and decreased appetite. These symptoms progress over time to the cat walking in circles or having seizures.

How is a pituitary macroadenoma diagnosed?

Definitive diagnosis of a pituitary macroadenoma requires brain imaging with advanced techniques such as CT or MRI. This imaging is performed under anesthesia, at a specialty hospital or university.

How is a pituitary macroadenoma treated?

The most effective treatment for pituitary macroadenoma is surgical removal. Although pituitary tumors in humans are often treated surgically, this surgery is uncommon in animals due to the cost and the difficult nature of the surgery. If surgery is performed, resolution of clinical signs is rapid and complete.

Radiation therapy is a more commonly used method to shrink pituitary tumors. Radiation is administered several times per week over a period of four to six weeks, with the goal of decreasing the size of the tumor, and decreasing clinical signs. Radiation therapy typically does not completely eliminate the tumor, but will often decrease the tumor’s size enough to alleviate clinical signs. Side effects of radiation include damage to the skin, as well as the risks associated with repeated anesthetic procedures.

Medications can also be used to manage the hormonal effects of functional pituitary macroadenomas. While this treatment does not address the actual tumor, and therefore the tumor will continue to gradually increase in size, medication can be used to counteract the effects of the hormones produced by these tumors. Cats with growth hormone secreting tumors can be treated with insulin to control diabetes, medications to manage heart disease, and other drugs to control symptoms as needed.