Diagnosing canine hyperadrenocorticism (Proceedings)

Pathophysiology

Hyperadrenocorticism (HAC) is a common endocrine disease of the dog. The disease and its clinical findings are either the result of excess cortisol production by the adrenal glands or iatrogenic due to administration of glucocorticoids. The normal pathophysiology of cortisol production involves the hypothalamus, pituitary gland and adrenal glands. The hypothalamus produces corticotropin releasing hormone (CRH) as a result of physical, emotional, chemical and other stressors. The CRH stimulates the pituitary gland to produce adrenocorticotropin hormone (ACTH). The ACTH then stimulates the adrenal glands to produce cortisol. Cortisol provides negative feedback to the hypothalamus and pituitary gland. ACTH also provides negative feedback to the pituitary. There are two forms of naturally occurring hyperadrenocorticism in the dog. The most common form is pituitary hyperadrenocorticism (PDH) and affects 80 to 85% of dogs with hyperadrenocorticism. Adrenal dependent or adrenocortical hyperadrenocorticism (ACH) is less common affecting the remaining 15 to 20% of dogs with HAC. Very rarely a dog will have both pituitary and adrenal dependent HAC.

Pituitary dependent hyperadrenocorticism most commonly occurs as the result of a pituitary microadenoma (80 to 90%). Microadenomas are classified as such due to their size being less than 1 cm. Pituitary macroadenomas are less common (10 to 20%) and defined as tumors greater than 1 cm in size. This is clinically relevant because it is the macroadenomas that can result in neurologic signs in addition to other signs of hyperadrenocorticism. Adenocarcinomas of the pituitary gland have also been reported but are extremely rare. Adenomas and adenocarcinomas result in increased ACTH production from the pituitary gland. Chronic increases in ACTH result in hyperplasia of the adrenal glands and increased cortisol secretion. A less common variant of bilateral hyperplasia is nodular hyperplasia. The nodules are believed to be secondary to PDH but can be confused with adrenal tumors.

Adrenocortical hyperadrenocorticism results from adenomas or adenocarcinomas affecting one (common) or both (rare) adrenal glands. When one adrenal is affected the other becomes atrophied and non-functional. Carcinomas and adenomas of the adrenal glands occur with about equal frequency. These tumors autonomously secrete excess amounts of cortisol independent of the influence of ACTH and, in fact, the excess cortisol causes decreases in CRH and ACTH.

Atypical hyperadrenocorticism occurs as a result of increased steroid hormones, or precursors to steroid hormones, which result in clinical signs consistent with hyperadrenocorticism. Tumors of the pituitary or adrenal gland may be involved. These hormones may be androstenedione, estradiol, progesterone, 17-OH progesterone, dehydroepiandrosterone sulfate and estradiol.

Signalment

This is typically a disease of middle-aged to older dogs but it can rarely occur in young dogs. Any breed can be affected but poodles, dachshunds, terriers and beagles have an increased incidence. Interestingly, approximately 75% of dogs with PDH are less than 20 kg. In dogs greater than 20 kg, there is equal frequency of PDH and ACH.

Clinical Findings

It is important to remember that the diagnosis of HAC is dependent on clinical findings. Dogs with HAC may present with one or more of the following signs. The most common findings are polydipsia, polyuria, polyphagia, abdominal distention, panting, muscle wasting, cutaneous lesions and lethargy. Additional findings include lameness, hypertension, proteinuria, anestrus and testicular atrophy. In dogs with macroadenomas neurologic signs may be seen.

The exact cause of polyuria is unknown but may be due to interference of cortisol with antidiuretic hormone in the collecting tubules of the kidney, increased glomerular filtration and central diabetes insipidus. Polydipsia is compensatory. Approximately 5% of dogs with HAC develop diabetes mellitus and this may also contribute to polydipsia and polyuria. Some dogs may also have a urinary tract infection or pyelonephritis that could contribute to polyuria.

Polyphagia is believed to be due to direct effects of cortisol on the appetite center.

Abdominal distention is likely due to combination of muscle weakness, redistribution of fat into the abdomen and hepatomegaly. Muscle weakness occurs as a result of the catabolic effects of cortisol. The exact mechanism of redistribution of fat is unknown. Hepatomegaly is due to glycogen accumulation in hepatocytes.

Lethargy is likely a result of muscle wasting that occurs due to the catabolic effects of cortisol on muscle.

Panting may be due to weakness of the respiratory muscles, intrathoracic fat deposition, increased pressure from abdominal contents (liver, fat), pulmonary mineralization and pulmonary thromboembolic disease. Mineralization of the pulmonary interstitium occurs as a result of dystrophic mineralization. The exact cause of dystrophic mineralization is unknown. Pulmonary thromboembolism is an infrequent finding but may occur as a result of loss of antithrombin III, increased production of clotting factors and increased fibrinogen. Other predisposing factors include obesity, hypertension and lethargy.

Cutaneous lesions observed include alopecia, pyoderma, seborrhea, comedones, hyperpigmentation, thin skin, bruising, calcinosis cutis and demodicosis. The cutaneous lesions are the result of follicular atrophy (alopecia), excess follicular keratin (comedones), a suppressed immune system (pyoderma, demodicosis), increased melanocytes in the dermis (hyperpigmentation) and interference with collagen formation (bruising, thin skin, poor wound healing). Calcinosis cutis is an uncommon manifestation of HAC and the exact cause is unknown but the result is deposition of calcium/phosphate in the dermis.

Occasionally dogs can develop cranial cruciate ligament rupture or patellar luxation. This may be accounted for by cortisol effects on connective tissue.

Hypertension (systolic blood pressure > 160 mm Hg) is common with HAC. Excess cortisol leads to fluid accumulation. Hypertension may or may not resolve with successful management of the disease. Hypertension and hypervolemia could lead to cardiac changes and congestive heart failure in a dog with underlying cardiac disease.

Proteinuria is also common. Proteinuria may be the result of hypertension, urinary tract infection and/or glomerular disease. Hypertension was discussed above. Urinary tract infections are also common with HAC. Dilute urine, incomplete bladder voiding and a depressed immune response could all contribute to infection. Infections may be in the lower urinary tract or involve the kidneys.

Excess cortisol also provides a negative feedback to pituitary secretion of FSH and LH. This results in testicular atrophy and decreased libido in males. Females often fail to cycle.

Macroadenomas can compress the hypothalamus resulting in neurologic signs. These include lethargy, dullness, confusion, pacing, ataxia, circling, blindness and seizures.

Diagnostic Tests

Laboratory Tests

A minimum database is performed on all dogs suspected of HAC to identify findings supportive of HAC as well as to identify other potential causes of the clinical signs or concurrent disease.

The CBC may be normal. A stress leukogram, increased red cells and thrombocytosis may also be noted.

Liver enzymes are elevated in the majority of dogs. Alkaline phosphatase (ALP) increases as a result of glycogen accumulation and hepatocellular vacuolation. There are several isoenzymes including a corticosteroid induced isoenzyme. Unfortunately this isoenzyme is not specific for excess endogenous or exogenous cortisol. Alanine transferase also increases secondary to damaged hepatocytes. The increase is typically not as marked as the ALP. Cholesterol is also commonly elevated due to the stimulation of lipolysis by cortisol. Lipemia is also often present and suggestive of hypertriglyceridemia. Blood urea nitrogen and creatinine are commonly decreased secondary to polyuria. As mentioned before, some dogs with HAC also have diabetes mellitus so hyperglycemia might be noted. Typically these dogs present and are diagnosed with diabetes mellitus and it is only after establishing they have some form of insulin resistance that HAC is considered. Remember the clinical presentation and laboratory findings of diabetes mellitus are very similar to HAC.

Urine may be variably concentrated. Dogs with HAC can concentrate their urine and if water is inadvertently withheld, or hospitalized dogs fail to drink, a normal urine specific gravity may be obtained. Glucose and protein can be found in the urine for reasons stated above. A urine protein creatinine ratio is indicted to quantify significant proteinuria. Bacteria and white blood cells may or may not be noted in the presence of infection. This, in addition to the high incidence of infection in dogs with HAC, is reason to culture all newly diagnosed dogs.

Hyperadrenocorticism can cause decreased levels of thyroid stimulating hormone secondary to hypercortisolism. This can result in a secondary but reversible hypothyroidism.

Arterial blood gas analysis can be performed in dogs suspected of pulmonary thromboembolism. Hypoxemia, an increased alveolar gradient and hypocapnia may be noted. Hypoxemia may be due to ventilation/perfusion mismatch and intrapulmonary shunting. Hypocapnia is secondary to hyperventilation.

Diagnostic Imaging

Thoracic radiographs might reveal dystrophic mineralization. Mineralization may be seen as a generalized interstitial pattern, bronchial mineralization or tracheal mineralization. Dystrophic mineralization is a non-specific finding that can be seen in animals without HAC. Lesions consistent with pulmonary thromboembolism might be seen. This includes normal radiographs, aerated pulmonary parenchyma with decreased vasculature, alveolar disease, blunted pulmonary arteries, an enlarged main pulmonary artery, cardiomegaly and pleural effusion.

Abdominal radiographs will reveal hepatomegaly. Adrenal tumors can sometimes be seen and may be mineralized. A distended or incompletely emptied urinary bladder may be noted. Calcium-containing urinary calculi may be noted and are rare but occur with increased frequency in HAC.

Abdominal ultrasound is preferred for evaluation of the abdominal cavity. Ultrasound is much more sensitive for detecting adrenal masses. Normal adrenal glands are less than 6.0 mm. Ultrasound can be used to help differentiate PDH from ACH but is not itself a differentiating test because adrenal masses may not be functional cortisol-secreting tumors. In a dog with the appropriate clinical findings, two normal adrenals or bilateral adrenomegaly is suggestive of PDH whereas unilateral adrenomegaly, especially in the face of a small or undetectable contra lateral adrenal, is suggestive of ACH. Larger, irregular adrenal masses that invade surrounding structures are more suggestive of malignant adrenal tumors. The liver can also be assessed for evidence of metastasis in the face of an adrenal tumor.

An echocardiogram is a non-invasive method of evaluating dogs with HAC suspected of having pulmonary thromboembolism. On echocardiogram right ventricular enlargement and a high velocity jet through the tricuspid valve might be noted in dogs with PTE. Ventilation perfusion scans, pulmonary angiography and MRI are also methods of diagnosing PTE.

Screening Tests for Hyperadrenocorticism

There is no one perfect test for diagnosing hyperadrenocorticism in dogs. No single test is 100% sensitive (picks up all dogs with the disease in question) and specific (does not test positive individuals without the disease). It is important to combine clinical findings with diagnostic tests for this reason. These tests should be performed on animals suspected of having HAC based on historical, physical and initial laboratory diagnostics. The screening tests for hyperadrenocorticism are the ACTH stimulation test, low dose dexamethasone suppression (LDDS test) and urine cortisol creatinine ratio (UC:Cr). Basal cortisol levels are not reliable because of variation in cortisol levels at any given time.

ACTH Stimulation Test

The premise of the ACTH stimulation test is that administration of ACTH to a normal dog will stimulate the adrenals in a normal fashion but not excessively. ACTH gel (1 – 2.2 U/kg IM) or synthetic ACTH (5 mcg/kg IV) is given. Heparinized plasma or serum is collected prior to and 2 hours after ACTH gel administration or prior to and 1 hour after administration of synthetic ACTH. In dogs with PDH the adrenals are hyperplastic and have an exaggerated response to additional ACTH. With ACH the autonomous cortisol-secreting cells of the adrenal tumor produce an exaggerated response as well. Because both forms of HAC produce an exaggerated response, this test can not be used to differentiate between pituitary and adrenal dependent hyperadrenocorticism. It is important to evaluate the absolute numbers, particularly the post stimulation value, and not the increase. Unfortunately the ACTH stimulation test might miss up to 60% of animals with HAC and these dogs will have a normal or borderline response to adrenal stimulation by ACTH so these tests are considered non-diagnostic. If HAC is suspected in a dog with a normal response a LDDS test should be performed. If the results are suggestive of HAC then a differentiating test can be performed. Approximately 15% of dogs with positive tests do not have HAC. The ACTH stimulation test is the only test available for monitoring therapy for HAC. This test is also able to identify dogs with iatrogenic HAC. These dogs have the clinical signs and little or no response to exogenous ACTH.

Low Dose Dexamethasone Suppression Test

The premise of the LDDS test is that when exogenous cortisol, even a small amount, is given to a normal dog the hypothalamus and pituitary will be suppressed so minimal ACTH and subsequent cortisol is produced. Cortisol will be suppressed within 3 hours and for up to 48 hours in normal dogs. Interestingly enough, in dogs with HAC circulating cortisol loses its effectiveness in 3 to 6 hours. Dexamethasone is given at 0.01 mg/kg IV and samples taken prior to, at 4 and 8 hours after administration. With HAC large amounts of cortisol are already being produced so the addition of a small amount of cortisol is ineffective in suppressing pituitary function. The 8 hour cortisol should be above the reference range with both forms of HAC. PDH can have several variations. With PDH the cortisol at 4 and 8 hours might be greater than baseline, same as baseline or has suppressed but remains greater than 50% of baseline. In another scenario the 4 hour cortisol might be less than the reference range or decrease to less than 50% of baseline but the 8 hour should again be above the reference range. It is also possible with PDH that the 8 hour cortisol is less than 50% of baseline but remains above the reference range. In approximately 40% of dogs with PDH and all dogs with ACH, there is no suppression of cortisol levels at any time and the 8 hour should be above the reference range. The sensitivity of this test for HAC approaches 99% thus almost all dogs with HAC will test positive. Specificity is more variable. Dogs with other illnesses can test positive greatly diminishing specificity. If the dogs tested are truly candidates for HAC then the specificity can approach 99% as well.

Urine Cortisol:Creatinine

The premise of the urine cortisol creatinine ratio is that animals that chronically produce large amounts of cortisol will have increased amounts in their urine. Urine is collected at home to avoid stress that might affect cortisol levels. Normal dogs should have cortisol values within the reference range. Dogs with HAC should have levels above the reference range. This test is highly sensitive for HAC but up to 80% of dogs with non adrenal illness test positive. This test is used primarily to rule out HAC because a dog with a negative test is very unlikely to have HAC.

Steroid Hormone Profiles

Steroid hormone profiles are typically run in dogs suspected of HAC based on clinical signs that fail to exhibit increased cortisol levels on more traditional tests for HAC. It is possible that these animals are unable to produce excess cortisol due to a lack of enzymes necessary for its synthesis but some precursors might be found in excess. The protocol for the ACTH stimulation test is used but in addition to cortisol, 17-OH progesterone, estradiol, androstenedione, progesterone and aldosterone are measured. Dogs with atypical HAC will have increases in one or more of these 'atypical' steroid hormones. It is important to note that some of these other steroid hormones are affected by concurrent disease.

Differentiating Tests for Hyperadrenocorticism

Low Dose Dexamethasone Suppression Test

The LDDS test can also be used as a differentiating test. The protocol is as stated before. In the normal dog, dexamethasone persists for up to 48 hours but in dogs with PDH dexamethasone activity is only 3 to 6 hours. Approximately 60% of dogs with PDH will exhibit suppression at 4 hours but not at 8 hours. Since virtually no dogs with ACH suppress, this test can be used as a differentiating test if the patterns above are noted. If the dog fails to suppress then PDH or ACH may be present.

High Dose Dexamethasone Suppression Test

The high dose dexamethasone suppression test (HDDDS test) differs from the LDDS test by the dose of dexamethasone. As discussed with the LDDS test, normal dogs suppress at 4 and 8 hours with dexamethasone. Higher doses of dexamethasone should suppress a pituitary tumor but not an autonomously secreting adrenal tumor. Dexamethasone is given at 0.1 mg/kg IV and a pre, 4 hour and 8 hour cortisol level is measured. Suppression occurs if the 8 hour is less than or equal to a 50% decrease from baseline or the 8 hour cortisol is within or below the reference range. The utility of the 4 hour cortisol is questionable since it is the 8 hour cortisol level that is used for interpretation. Dogs with PDH should adapt at least one of the criteria stated above. Unfortunately 25% of dogs with PDH fail to suppress as stated above. Almost all dogs with ACH fail to suppress.

Endogenous ACTH

Adrenocorticotropin hormone can be measured to help differentiate PDH from ADH. Blood is collected in an EDTA tube, the plasma spun off immediately and frozen until analysis. Endogenous ACTH should be low in dogs with ACH because large amounts of cortisol suppress ACTH release form the pituitary. With PDH endogenous ACTH would be increased since the pituitary tumor is responsible for excess ACTH production. Unfortunately many ACH and a few of the PDH have ACTH levels within the reference range and are considered non-diagnostic. The test could be performed at another date or other discriminatory tests done.

Cross-sectional Imaging

Cross-sectional imaging can be used as a differentiating test as well. CT and MRI allow you to visualize the pituitary gland and measure the size of the mass. This can be important for treatment recommendations because irradiation of the pituitary might be recommended for larger tumors. Approximately 50% of dogs with PDH have masses visible with CT or MRI. CT or MRI can also be done with ADH to evaluate the adrenal for size, local invasion and metastasis.

References

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