Definition
Pituitary tumor: an abnormal growth in the pituitary gland, the part of the brain that regulates the body’s balance of hormones.
Clinical Signs
May see any combination of the following:
Mechanical disturbances
- Gradual weakness and lack of coordination (often more noticeable in the forelimbs).
- Stiffness or inability to flex forelimbs, that gradually progresses to the hind limbs as the tumor continues to grow.
- One eye protruding more than the other does. This can be due to asymmetrical growth of the tumor pressing on structures behind the eye, and is referred to as exophthalmia.
- Unequal pupil size in one eye compared to the other (may appear as blown/dilated pupil due to pressure or bleeding behind the eye).
- Difficulty holding food when eating.
- Circling, head tilt, seizures, hydrocephalus and sudden death.
- Head pressing/Head bumping. This sign may be present when there is increased intracranial pressure (rat may bump head up when petted, or extend head pressing against a fixed surface).
Hormonal disturbances
- Lactation (ejection of milk) in an aged non-pregnant female
- Decreased fertility
- Thinning skin
- Weight loss
- Excessive thirst (need to refill water bottles more than usual)
- Excessive excretion of very dilute urine
- Dehydration
Etiology
The pituitary gland, also known as the hypophysis or formerly known as the master gland, is a small endocrine gland that stimulates and controls hormone production of other endocrine glands. These glands, in turn, regulate a variety of body processes. Endocrine glands expel their secretions directly into the blood, as opposed to exocrine glands such as sweat glands.
In rats the pituitary gland is divided into the adenohypophysis (anterior lobe or pars distalis), the intermediate lobe (pars intermedia), and the neurohypophysis (posterior lobe or pars nervosa). It is situated in a bony cavity at the base of, and below, the brain, and is connected to the hypothalamus by the pituitary stalk, also referred to as the hypophyseal stalk. The hypothalamus is that region of the brain that, in large part, controls or modulates the activity of the pituitary gland.
Studies show that the pituitary gland growth in female rats and male rats differs at around 6-7 weeks of age, at which time the pituitary gland in the female becomes heavier and increases in size. The difference increases even more as the female ages.
In addition, normal hyperplastic (overgrowth of normal cells) changes that occur to the pituitary gland in aged rats often show no evidence of compression of surrounding tissue, unlike that which can occur with adenomas (Pathology of Laboratory Rodents and Rabbits, Second Edition, Dean H. Percy, Stephen W. Barthold).
Laboratory studies also indicate that previously bred females, as well as spayed females, tend to show a decreased rate of pituitary tumors. Spaying your female, even though it may protect her from hormonally-driven tumors, is an invasive surgery with risks that need to be considered.
Adenomas are tumors of glandular tissue. Pituitary tumors, often benign adenomas, are slow-growing tumors. Many of these tumors arise in the anterior lobe of the pituitary gland.
Pituitary adenomas can vary in size and are often differentiated as microadenomas (small nodules) or macroadenomas (sometimes ranging greater than 10 mm in diameter).
Macroadenomas can be further differentiated as non-secreting adenomas, or secreting adenomas, e.g., chromophobe adenoma (space-occupying, non-hormone secreting tumor), prolactin-secreting or lactotroph adenoma (also called a prolactinoma), basophil or corticotroph (ACTH secreting) adenoma.
Pituitary adenomas, specifically chromophobe adenomas or lactotroph adenomas are among the more common tumors found in older female rats, occurring between 13 and 24 months of age.
While pituitary tumors are not as prevalent in male rats, they can and do occur.
Even though pituitary tumors are often benign, and do not metastasize to other areas of the body, some can grow large enough to compress nearby brain tissue. This compression of tissue can produce mechanical disturbances (such as listed above in the Clinical Signs section).
Pituitary tumors can also produce signs of hormonal disturbances when they secrete excess hormones, or cause an insufficient amount of hormone to be secreted. Examples of this are:
- Lactation in an aged non-pregnant female rat with a prolactin-producing tumor.
- Cushing’s syndrome seen in tumors producing excess ACTH (adrenocorticotropic hormone), in rats, such as: decreased fertility, thinning skin, and weight loss.
- Central Diabetes Insipidus resulting from a deficiency of vasopressin/ADH (antidiuretic hormone) release from the posterior lobe.
*Note: clinical signs can reflect a combination of both mechanical and hormonal disturbances depending on the type of tumor present.
While the cause of pituitary tumors is unknown, factors that may play a role in their development in rats include:
- Age
- Genetic factors
- Diet (incidence shown to increase where high-calorie ad lib diets are given)
- Breeding history
- Infections
- Injury
The prognosis for rats suspected of having, or diagnosed as having a pituitary adenoma, is grave. Just the location of the tumor and the pituitary gland alone make them inoperable; however, a treatment regimen with prednisone and antibiotics can provide both comfort and short term extension of life.
Case Histories Of Pituitary Tumors In Rats
Photos, and video examples of neurological impairment
- Fig. 1:
Neurologic changes related to pituitary tumor in sixteen-month-old female rat
Case history, photos, and video
- Fig. 2:
Neurologic changes related to pituitary tumor in fifteen-month-old female rat
Case history, photos, and video
Diagram of the rat pituitary gland
- Fig. 1: Pituitary gland anatomy diagram, and functions
Diagnostics
Obtain history from pet owner. Observe for clinical signs that may indicate presence of tumor.
Wheelbarrow test (http://ivis.org/). May indicate central nervous system deficit if rat is unable to move forelimbs normally when hind limbs are elevated by base of tail near the rump.
Presence of head pressing/ head bumping.
Blood sampling for hormone levels may determine type of hormone-producing tumor, but may only be pertinent if normal values are known.
If x-ray of skull taken, and enlargement of the sella turcica can be determined, suspect pituitary mass.
Since pituitary adenomas, in rats, can only be confirmed upon necropsy, assess clinical signs for presence of possible pituitary tumor formation and initiate treatment for palliative care.
Histology of tissue upon necropsy.
Upon necropsy, pituitary adenomas can be found to vary in size. Margins may be well-defined and hemorrhaging may also be present.
Treatment
Recommended treatment is geared for comfort and the reduction of clinical signs.
Prednisone, prednisolone, or oral dexamethasone, and a broad-spectrum antibiotic (e.g. Baytril) can be given to help reduce tissue swelling that accompanies the tumor temporarily.
For information on the medication listed above for recommended treatment see:
Rat Medication Guide.
For information on seizures see: Seizure article in the Rat Guide Health section.
For information on hydrocephalus see: Hydrocephalus article in the Rat Guide Health section.
Euthanasia should be considered if the rat’s clinical signs preclude comfort and quality of life.
Nursing Care
Give medications as prescribed.
Provide safe environment.
If mobility is impaired, or if seizures develop, provide a one level cage with soft bedding/litter.
Prevent dehydration and weight-loss. Keep food and water within easy reach.
Provide additional food and fluid supplements if appetite is poor. If rat is unable to grasp and hold food, assist. If chewing becomes difficult, offer easy-to-chew foods.
Assist with grooming if rat is not able to do so.
Outcomes
Comfort, mobility, and quality of life helped with medications prescribed.
Emotional support for those having to consider euthanasia for their rat.
Prevention
Offering a diet that is nutritiously low in fat, calories, amines and nitrates, is recommended, (example:Harlan 2014 rat block, http://www.teklad.com/globaldiet/g2014.html).
Because some pituitary tumors, like some mammary tumors, are believed to be estrogen-induced, it is thought that spaying female rats may reduce their chance of incidence (Russo & Russo 1998, from A. Hanson, MS, Ph.D. Animal Behavior: http://ratbehavior.org). The spaying of rats remains controversial and can be a risky procedure. Discuss this option thoroughly with a veterinarian familiar with rats prior to consenting.
References
Pathobiology of the Aging Rat Volume 2, 1994 International Life Sciences Institute, editors: Donald. L. Dungworth DVM, Ulrich Mohr DVM, Charles C. Capen DVM
The Laboratory Rat, Second Edition 2006
editors: Mark A. Suckow DVM, Steven H. Weisbroth DVM, Craig L. Franklin DVM
Rat Behavior
Anne Hanson, MS, Ph.D Animal Behavior
http://www.ratbehavior.org/TumorSpaying.htm
Online journals on Pituitary gland, and tumors.
http://endo.endojournals.org/cgi/content/full/138/5/2128
http://www.ebmonline.org/cgi/content/full/227/7/492
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3194659&dopt=Abstract
http://64.233.161.104/search?q=cache:2GKGGTKIKeEJ:brainpath.medsch.ucla.edu/pdfs/1103pdf/yu.pdf+study+of+pituitary+tumors+in+rats&hl=en
http://www.answers.com/topic/pituitary-adenoma
http://www.thamburaj.com/pituitary.htm
Posted on June 25, 2003, 12:21,
Last updated on July 11, 2008, 21:05
| Metabolic / Endocrine | Neoplasia
