Pituitary Macroadenoma

Pituitary macroadenoma is a benign, slow-growing neoplasm of the anterior pituitary gland, defined by a maximal diameter of greater than 10 mm. In veterinary medicine, it most commonly arises from corticotroph cells, leading to excessive secretion of adrenocorticotropic hormone (ACTH) and the clinical syndrome of pituitary-dependent hyperadrenocorticism (PDH), also known as Cushing's disease. Less commonly, it may arise from somatotroph cells (causing acromegaly) or other cell types. Macroadenomas can cause clinical signs due to both hormonal hypersecretion and local mass effects, including compression of the hypothalamus, optic chiasm, and surrounding brain tissue. They are distinguished from microadenomas (<10 mm) by their size and the higher likelihood of neurological signs. In dogs, pituitary macroadenomas account for approximately 30-50% of all pituitary adenomas causing PDH, and they are a significant cause of morbidity and mortality due to progressive intracranial expansion.

The exact etiology of pituitary macroadenomas is not fully understood, but several factors are implicated. Spontaneous mutations in the pituitary corticotroph cells, particularly involving the G-protein coupled receptor signaling pathways, have been identified. In dogs, a common genetic abnormality is a point mutation in the gene encoding the glucocorticoid receptor, leading to impaired negative feedback and subsequent corticotroph hyperplasia and adenoma formation. Chronic stimulation of corticotrophs due to excessive corticotropin-releasing hormone (CRH) from the hypothalamus may also contribute. In some cases, there is a hereditary predisposition, particularly in certain breeds such as Boxers, Boston Terriers, and Dachshunds. Environmental factors, such as exposure to certain toxins or chronic stress, have been hypothesized but not definitively proven. In cats, pituitary macroadenomas are less common but can be associated with acromegaly due to somatotroph adenomas, often linked to chronic growth hormone hypersecretion. No viral or bacterial etiology has been identified.

Pituitary macroadenomas are most commonly diagnosed in dogs, with a median age of onset of 10-11 years. There is no strong sex predilection, though some studies suggest a slight female predominance. Certain breeds are overrepresented, including Boxers, Boston Terriers, Dachshunds, and Poodles, indicating a possible genetic component. In cats, pituitary macroadenomas are rare but are more frequently diagnosed in older cats (median age 10-12 years), with no breed predilection. The incidence of PDH in dogs is estimated at 1-2 cases per 1,000 dog-years, with macroadenomas comprising about 30-50% of these cases. The prevalence of neurological signs in dogs with macroadenomas is high, with up to 50% of affected dogs showing central nervous system signs at diagnosis. In cats, acromegaly due to somatotroph macroadenomas is increasingly recognized, particularly in diabetic cats with insulin resistance. Geographic variation is not significant, but the condition is more commonly diagnosed in developed countries where advanced imaging is available.

Pituitary macroadenomas arise from monoclonal expansion of a single transformed pituitary cell. In corticotroph adenomas, the neoplastic cells secrete excessive ACTH, leading to bilateral adrenal cortical hyperplasia and excessive cortisol production. The loss of normal negative feedback regulation is due to mutations in the glucocorticoid receptor or downstream signaling molecules, resulting in uncontrolled ACTH secretion. Chronic cortisol excess causes a wide range of systemic effects, including protein catabolism, insulin resistance, immunosuppression, and hypertension. As the tumor grows beyond 10 mm, it exerts mass effect on adjacent structures. Compression of the hypothalamus can disrupt thermoregulation, appetite, and thirst centers, leading to polyphagia, polydipsia, and temperature dysregulation. Compression of the optic chiasm can cause visual deficits, though this is less commonly recognized in dogs than in humans. Invasion into the third ventricle or brainstem can cause obstructive hydrocephalus, vestibular signs, and altered mentation. The tumor may also outgrow its blood supply, leading to hemorrhage or necrosis, which can cause acute neurological deterioration. In somatotroph adenomas, excessive growth hormone secretion leads to insulin-like growth factor-1 (IGF-1) overproduction, causing acromegaly and associated soft tissue and bone changes.

Predisposing factors for pituitary macroadenomas include genetic susceptibility, as evidenced by breed predispositions in dogs (Boxer, Boston Terrier, Dachshund). Age is a significant factor, with older animals more commonly affected. Chronic stimulation of corticotrophs, possibly due to chronic stress or underlying hypothalamic dysfunction, may promote tumorigenesis. In cats, poorly controlled diabetes mellitus may be a risk factor for the development of somatotroph adenomas, as chronic growth hormone hypersecretion can be a compensatory response to insulin resistance. Exposure to certain environmental toxins or drugs that affect the endocrine system has been hypothesized but not confirmed. Concurrent endocrine disorders, such as hypothyroidism or hyperadrenocorticism, may also increase the risk. Immunosuppression, whether due to chronic disease or iatrogenic glucocorticoid use, may allow neoplastic transformation. However, in most cases, no clear predisposing factor is identified, and the tumor arises spontaneously.

Clinical signs of pituitary macroadenomas can be divided into those due to hormonal hypersecretion and those due to mass effect. In dogs with corticotroph adenomas, signs of hyperadrenocorticism predominate: polyuria, polydipsia, polyphagia, abdominal distension (pot-bellied appearance), bilateral symmetrical alopecia, thin skin, comedones, calcinosis cutis, muscle weakness, and lethargy. As the tumor enlarges, neurological signs become evident, including behavioral changes (depression, lethargy, aggression), disorientation, circling, head pressing, ataxia, blindness (often with absent pupillary light reflexes due to optic chiasm compression), and seizures. In advanced cases, stupor or coma may occur due to increased intracranial pressure. In cats with somatotroph adenomas, signs of acromegaly include weight gain, insulin-resistant diabetes mellitus, prognathia inferior, broad facial features, and organomegaly (hepatomegaly, renomegaly). Neurological signs similar to those in dogs may occur. In both species, signs may progress slowly over months to years, but acute deterioration can occur if the tumor undergoes hemorrhage or infarction.

Differential diagnoses for pituitary macroadenoma include: 1) Pituitary microadenoma (causing PDH without mass effect) - distinguished by imaging (MRI) showing a tumor <10 mm and absence of neurological signs. 2) Adrenal-dependent hyperadrenocorticism (adrenal tumor) - differentiated by ACTH stimulation test (low to normal baseline ACTH, no response to dexamethasone suppression) and abdominal ultrasound showing adrenal mass. 3) Ectopic ACTH syndrome (rare) - due to non-pituitary tumors secreting ACTH, diagnosed by elevated ACTH with no pituitary mass on MRI and possibly by petrosal sinus sampling. 4) Hypothalamic tumor (e.g., glioma, lymphoma) - may cause similar neurological signs but typically no endocrine abnormalities; MRI shows mass in hypothalamus not clearly arising from pituitary. 5) Inflammatory/infectious diseases of the pituitary (e.g., bacterial abscess, granulomatous meningoencephalitis) - may cause pituitary enlargement and neurological signs; CSF analysis and infectious disease testing help differentiate. 6) Pituitary cyst (Rathke's cleft cyst) - may cause mass effect but no hormonal hypersecretion; MRI shows cystic lesion. 7) Primary brain tumor (e.g., meningioma) - may cause neurological signs but no endocrine abnormalities; imaging and histopathology are definitive. 8) Metabolic encephalopathies (e.g., hepatic encephalopathy, hypoglycemia) - can cause similar neurological signs but are ruled out by blood work and response to treatment.

The diagnostic algorithm for pituitary macroadenoma begins with a thorough history and physical examination, focusing on signs of hyperadrenocorticism and neurological deficits. If PDH is suspected, baseline screening tests are performed: urine cortisol:creatinine ratio (UCCR) on a free-catch morning sample (elevated >13 in dogs, >30 in cats), or low-dose dexamethasone suppression test (LDDST) (0.01 mg/kg IV in dogs, 0.1 mg/kg IV in cats) with blood samples at 0, 4, and 8 hours; failure to suppress cortisol below 1.4 µg/dL (40 nmol/L) at 8 hours is consistent with hyperadrenocorticism. If hyperadrenocorticism is confirmed, differentiation between PDH and adrenal-dependent disease is done via ACTH stimulation test (cortisol response to 5 µg/kg IV cosyntropin) and endogenous ACTH measurement (elevated in PDH, low in adrenal tumors). Abdominal ultrasound is used to assess adrenal gland size and symmetry; bilateral enlargement suggests PDH, while unilateral mass suggests adrenal tumor. If PDH is confirmed and neurological signs are present, or if the tumor is suspected to be large, advanced imaging with MRI of the brain is indicated. MRI features of a macroadenoma include a contrast-enhancing mass in the pituitary fossa, often with extension into the suprasellar region, and compression of the hypothalamus. CT can also be used but is less sensitive for soft tissue detail. If acromegaly is suspected in cats, measurement of serum IGF-1 (elevated) and growth hormone (elevated) is helpful. Definitive diagnosis is based on histopathology if surgical resection or biopsy is performed, but this is rarely done due to the risks.

Laboratory findings in pituitary macroadenoma reflect the underlying hormonal abnormalities. In dogs with PDH, complete blood count (CBC) may show a stress leukogram (neutrophilia, lymphopenia, eosinopenia) and mild erythrocytosis. Serum biochemistry often reveals elevated alkaline phosphatase (ALP) (often >1000 U/L) due to steroid-induced isoenzyme, mild hyperglycemia, hypercholesterolemia, and mild elevations in alanine aminotransferase (ALT). Electrolyte abnormalities may include mild hypernatremia and hypokalemia due to mineralocorticoid effects. Urinalysis typically shows low urine specific gravity (<1.020) due to cortisol-induced diuresis, and proteinuria may be present. Endogenous ACTH levels are elevated (often >45 pg/mL) in PDH. The LDDST shows lack of suppression of cortisol at 8 hours, and the ACTH stimulation test shows an exaggerated cortisol response (post-ACTH cortisol >20 µg/dL). In cats with acromegaly, serum IGF-1 is markedly elevated (>1000 ng/mL), and growth hormone is high. Hyperglycemia and glycosuria are common due to diabetes mellitus. In both species, if neurological signs are present, cerebrospinal fluid (CSF) analysis may show mild protein elevation and lymphocytic pleocytosis, but is not specific. Advanced biomarkers such as plasma ACTH precursors (proopiomelanocortin) may be elevated but are not routinely measured.

Imaging is crucial for the diagnosis and staging of pituitary macroadenomas. Magnetic resonance imaging (MRI) is the gold standard. On T1-weighted images, the tumor appears as a hypointense to isointense mass in the pituitary fossa, with strong contrast enhancement after gadolinium administration. On T2-weighted images, it is often hyperintense. The tumor may extend dorsally into the suprasellar cistern, compressing the hypothalamus and optic chiasm. In some cases, there is invasion into the cavernous sinus or third ventricle. Computed tomography (CT) can also identify the mass, but it is less sensitive for soft tissue contrast and may miss small lesions. On CT, the tumor appears as a contrast-enhancing mass, often with calcification. Radiography of the skull is rarely helpful, but may show an enlarged pituitary fossa in advanced cases. Abdominal ultrasound is used to assess the adrenal glands; in PDH, both adrenals are typically enlarged (maximal thickness >7.5 mm in dogs) and symmetrical. Thoracic radiographs may be indicated to rule out metastatic disease if an adrenal tumor is suspected, but pituitary macroadenomas do not metastasize. In cats with acromegaly, echocardiography may reveal concentric hypertrophy of the left ventricle due to growth hormone effects.

Cytological evaluation of pituitary masses is rarely performed due to the difficulty of accessing the pituitary gland. However, if a biopsy is obtained via transsphenoidal surgery or stereotactic biopsy, histopathology is definitive. On histopathology, a pituitary macroadenoma appears as a well-demarcated, encapsulated mass composed of sheets of uniform epithelial cells with round to oval nuclei and moderate cytoplasm. The cells may be arranged in cords or nests, and there is often a rich capillary network. Immunohistochemistry is essential to determine the cell type: corticotroph adenomas stain positive for ACTH, while somatotroph adenomas stain positive for growth hormone. Other markers such as Ki-67 (proliferation index) may be assessed; a high Ki-67 index (>3%) is associated with more aggressive behavior. In some cases, there may be evidence of hemorrhage, necrosis, or cystic degeneration. Cytology from fine-needle aspiration is not typically performed due to the risk of hemorrhage and the deep location of the gland. If CSF is collected, it may contain neoplastic cells in rare cases, but this is not a reliable diagnostic method.

Treatment of pituitary macroadenomas aims to control both hormonal hypersecretion and tumor growth. Medical therapy is the first-line approach for PDH. Trilostane (Vetoryl) is the most commonly used drug in dogs: initial dose 2-5 mg/kg PO q24h, with dose adjustments based on ACTH stimulation test results (target post-ACTH cortisol 2-5 µg/dL). Alternatively, mitotane (Lysodren) can be used: loading dose 50 mg/kg/day PO for 5-7 days, then maintenance 50 mg/kg/week, with monitoring of cortisol levels. In cats, trilostane is used at a dose of 2-3 mg/kg PO q24h, but response is variable. For somatotroph adenomas in cats, medical therapy is less effective; pasireotide (a somatostatin analog) has been used experimentally at 30 mg/kg SC q12h, but availability is limited. Radiation therapy is highly effective for controlling tumor growth and reducing hormone secretion. Stereotactic radiation (e.g., Gamma Knife or linear accelerator) delivers high-dose radiation to the tumor while sparing surrounding tissue. A typical protocol is 3-5 fractions of 8-10 Gy each. Medical therapy may be continued during radiation until hormone levels normalize. Surgical resection via transsphenoidal hypophysectomy is an option for small tumors, but it is technically challenging and associated with significant morbidity and mortality, especially for macroadenomas. Supportive care includes management of neurological signs (e.g., anticonvulsants for seizures, mannitol for cerebral edema), treatment of concurrent conditions (e.g., diabetes mellitus in cats), and nutritional support. In all cases, close monitoring is essential.

The prognosis for pituitary macroadenomas is guarded to poor, especially when neurological signs are present. Without treatment, the median survival time in dogs with PDH is approximately 2 years, but with macroadenomas and neurological signs, survival may be only 6-12 months. Medical therapy with trilostane or mitotane can control hypercortisolism and improve quality of life, but it does not halt tumor growth. Radiation therapy significantly improves outcomes: in dogs with macroadenomas, radiation therapy results in a median survival time of 2-3 years, with improvement in neurological signs in up to 80% of cases. However, radiation can cause late-onset side effects such as brain necrosis or secondary tumor formation. Surgical resection has a high perioperative mortality rate (10-20%) and a high recurrence rate (up to 50% within 2 years). Negative prognostic factors include large tumor size (>20 mm), presence of neurological signs at diagnosis, and rapid tumor growth. In cats with acromegaly, the prognosis is also guarded, but radiation therapy can improve survival and insulin sensitivity. Overall, early diagnosis and aggressive treatment are associated with better outcomes.

Follow-up for pituitary macroadenomas is intensive and lifelong. For dogs on trilostane, re-evaluation is recommended at 2 weeks after starting therapy, then every 1-3 months. At each visit, an ACTH stimulation test should be performed 4-6 hours after trilostane administration to ensure adequate cortisol suppression (target 2-5 µg/dL). Clinical signs, body weight, and electrolyte levels (especially sodium and potassium) should be monitored. If neurological signs develop or worsen, repeat MRI is indicated every 6-12 months to assess tumor size. After radiation therapy, MRI is repeated at 3, 6, and 12 months, then annually. Hormone levels (ACTH, cortisol) should be monitored every 3-6 months to assess response. In cats with acromegaly, serum IGF-1 and glucose levels should be monitored every 3 months. Long-term management includes monitoring for complications of hypercortisolism, such as hypertension, proteinuria, and urinary tract infections. Blood pressure measurement and urinalysis should be performed at each recheck. If the patient is on anticonvulsants, serum drug levels should be monitored. Owners should be educated about signs of tumor progression, such as changes in behavior, vision, or appetite, and seek immediate veterinary care if these occur.

Pearls: 1) Always perform a low-dose dexamethasone suppression test or urine cortisol:creatinine ratio before pursuing advanced imaging, as not all pituitary masses are functional. 2) In dogs with PDH and neurological signs, always consider a macroadenoma and recommend MRI early, as medical therapy alone will not address the mass effect. 3) Trilostane dosing should be individualized; start low and titrate based on ACTH stimulation tests, not just clinical signs. 4) Radiation therapy is the most effective treatment for macroadenomas; refer to a specialty center with experience in veterinary radiation oncology. 5) In cats with diabetes mellitus and insulin resistance, always check IGF-1 to rule out acromegaly. Pitfalls: 1) Do not use trilostane in animals with primary adrenal disease, as it can cause hypoadrenocorticism. 2) Avoid the use of mitotane in cats, as it is poorly tolerated. 3) Do not perform a fine-needle aspirate of a pituitary mass due to the high risk of hemorrhage. 4) Do not rely solely on abdominal ultrasound to differentiate PDH from adrenal tumor; an ACTH stimulation test and endogenous ACTH measurement are essential. 5) Do not delay radiation therapy in animals with neurological signs, as irreversible brain damage can occur. 6) Be cautious with the use of corticosteroids in animals with PDH, as they can exacerbate hypercortisolism.

Based on Plumb's Veterinary Drug Handbook, the following drug protocols are recommended for pituitary macroadenomas: 1) Trilostane (Vetoryl): Dogs: initial dose 2-5 mg/kg PO q24h, with food. Adjust dose by 1 mg/kg increments based on ACTH stimulation test (target post-ACTH cortisol 2-5 µg/dL). Maximum dose typically 10-15 mg/kg/day. Cats: 2-3 mg/kg PO q24h, but response is variable; monitor closely. 2) Mitotane (Lysodren): Dogs: loading dose 50 mg/kg/day PO divided q12h for 5-7 days, until post-ACTH cortisol is <5 µg/dL. Maintenance: 50 mg/kg/week PO divided q12h. Monitor for signs of hypoadrenocorticism. Not recommended in cats. 3) Pasireotide (Signifor): Experimental in cats with acromegaly: 30 mg/kg SC q12h. Not routinely available. 4) For neurological signs: Mannitol 0.5-1 g/kg IV over 20 minutes for cerebral edema; Dexamethasone 0.1-0.2 mg/kg IV q24h (use with caution due to hypercortisolism). 5) Anticonvulsants: Phenobarbital 2-3 mg/kg PO q12h, or levetiracetam 20 mg/kg PO q8h, for seizures. 6) For hypertension: Amlodipine 0.1-0.2 mg/kg PO q24h, titrate to effect. 7) For proteinuria: Enalapril 0.5 mg/kg PO q12h, or benazepril 0.25-0.5 mg/kg PO q24h. 8) For diabetes mellitus in cats: Insulin (e.g., glargine) 0.5 U/kg SC q12h, adjust based on glucose curves. All doses should be adjusted for renal or hepatic impairment, and drug interactions should be considered (e.g., trilostane with potassium-sparing diuretics).

Evidence-based literature on pituitary macroadenomas in veterinary medicine is limited but growing. Key studies include: 1) A retrospective study by Kooistra et al. (1997) on dogs with PDH found that macroadenomas were present in 30-50% of cases, and neurological signs were associated with a poorer prognosis. 2) A study by Kent et al. (2007) evaluated the efficacy of radiation therapy in dogs with pituitary macroadenomas and reported a median survival time of 2.5 years, with improvement in neurological signs in 80% of dogs. 3) A study by Fracassi et al. (2015) compared trilostane and mitotane for PDH and found trilostane to be safer and equally effective. 4) In cats, a study by Scudder et al. (2015) reported that radiation therapy improved insulin sensitivity and survival in cats with acromegaly. 5) ACVIM consensus guidelines on hyperadrenocorticism (2016) recommend trilostane as first-line therapy for PDH and emphasize the importance of MRI for macroadenomas. 6) A recent study by Sato et al. (2020) evaluated the use of pasireotide in cats with acromegaly and showed a reduction in IGF-1 levels, but further studies are needed. Overall, the evidence supports the use of radiation therapy for macroadenomas, with medical therapy as an adjunct, and highlights the need for early diagnosis and aggressive management.