What is a pituitary macroadenoma?
Pituitary macroadenomas are large, non-cancerous (benign) tumors of the pituitary gland, a pea-sized organ at the base of the brain responsible for producing hormones that influence many organ systems in the body. These tumors are typically greater than ½ inch in diameter, although there is some variation in the definition of a macroadenoma depending on the animal’s skull size. Pituitary macroadenomas are more common in dogs than in cats.
There are two types of pituitary macroadenoma:
- Functional tumors secrete hormones. Their effects on the body vary depending on the type of hormone the tumor produces.
- Non-functional tumors do not secrete hormones. These tumors affect the body through the physical pressure they exert on surrounding brain structures as they grow.
In most cases, pituitary macroadenomas are functional tumors. The most common functional pituitary macroadenoma in dogs releases a hormone called adrenocorticotropic hormone (ACTH). ACTH triggers the adrenal glands to release cortisol, which can lead to a condition called Cushing’s disease – a common metabolic disease in dogs.
What are the clinical signs of a pituitary macroadenoma?
In dogs with a functional pituitary macroadenoma causing Cushing’s disease, signs include:
- Increased appetite
- Increased thirst and urination
- Alopecia (hair loss)
- Dry skin
- Blackheads
- A pendulous (sagging) belly
- Recurrent or chronic infections due to a weakened immune system
Dogs with non-functional pituitary macroadenomas may show signs related to the tumor’s compression of nearby brain structures. Several eye and vision-related structures are located near the pituitary gland, so vision problems are common in animals with pituitary macroadenomas.
In addition, non-functional pituitary macroadenomas can interfere with hormones that help concentrate urine. If this happens, dogs can develop increased thirst and urination, and a condition known as diabetes insipidus. In many cases, initial signs may be vague, such as lethargy and decreased appetite. These symptoms worsen over time to the point where an affected dog may walk in circles or have seizures.
How is a pituitary macroadenoma diagnosed?
A pituitary macroadenoma is diagnosed through 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?
Surgical removal is the most effective treatment for pituitary macroadenoma, but it is uncommon in animals due to the cost and difficult nature of the surgery. If surgery is performed, clinical signs resolve quickly and completely.
Radiation therapy is more commonly used to shrink pituitary tumors. Radiation is administered several times per week for four to six weeks. Radiation therapy typically does not 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 be used to manage the hormonal effects of functional pituitary macroadenomas, although it does not address the tumor itself, so the tumor will continue to gradually increase in size. Dogs with Cushing’s disease can be treated with medications such as trilostane (Vetoryl®) to decrease cortisol production.
