JPC SYSTEMIC PATHOLOGY
ENDOCRINE SYSTEM
January 2022
E-M04 (NP)

Slide A:  Signalment (JPC #2028971):  12-year-old German shepherd dog

HISTORY:  This dog was intermittently treated with prednisolone for skin allergies.

HISTOPATHOLOGIC DESCRIPTION:  Adrenal gland, adrenal cortex:  Diffusely, the zona fasciculata and zona reticularis are severely atrophied, reduced to approximately one-third normal thickness.  Multifocally, cortical cells near the corticomedullary junction have abundant vacuolated cytoplasm (degeneration) that contains small amounts of yellow-gold, granular pigment (lipofuscin).  The capsule and connective tissue septa are diffusely moderately expanded by fibrous connective tissue and there are multifocal intracapsular nests of cortical cells.  Vessels are mildly congested.

MORPHOLOGIC DIAGNOSIS:  Adrenal gland, zonae fasciculata and reticularis:  Atrophy, diffuse, moderate, German shepherd dog, canine.

ETIOLOGY:  Exogenous corticosteroid administration

ETIOLOGIC DIAGNOSIS:  Iatrogenic adrenocortical atrophy

Slide B:  SIGNALMENT (JPC #1669572):  10-year-old, male German shepherd dog

HISTORY:  Incidental finding

HISTOPATHOLOGIC DESCRIPTION:  Adrenal gland:  Diffusely all three cortical zones (zonae glomerulosa, fasciculata, and reticularis) are severely atrophic with loss of cortical architecture (stromal collapse). The few remaining cortical cells frequently contain abundant vacuolated cytoplasm (degeneration) with small amounts of yellow-gold, granular pigment (lipofuscin).  Multifocally within the cortex, medulla, and capsule, there are low to moderate numbers of infiltrating plasma cells and lymphocytes.

Heart, ventricle with AV valve: The free margin of the valve is mildly thickened by clusters of spindle cells on a loosely arranged myxomatous matrix (fibromyxomatous degeneration).  Multifocally, the walls of several arterioles within the myocardium are mildly expanded by acellular, eosinophilic, waxy material (hyalinosis).

MORPHOLOGIC DIAGNOSIS:  1. Adrenal gland, cortex:  Pancortical atrophy, diffuse, severe, German shepherd dog, canine.

2. Heart, valve: Fibromyxomatous degeneration (valvular endocardiosis).

ETIOLOGY:  Immune-mediated

ETIOLOGIC DIAGNOSIS:  Idiopathic adrenocortical atrophy

CONDITION SYNONYMS:  Hypoadrenocorticism (HA), adrenocortical insufficiency, Addison’s disease

GENERAL DISCUSSION:

Adrenal Cortex:

• The adrenal gland has two distinct parts: 1) Medulla composed of neural crest cells of ectodermal origin, and 2) Cortex composed of coelomic epithelial cells of mesodermal origin
  • Cortex to medulla ratio is normally 2-3:1
  • The cortex has three distinct layers that develop from stem cells in the adrenal capsule, migrate inward through each of the three layers, then undergo apoptosis at the corticomedullary junction:

1. Zona glomerulosa (outermost): 15% of the cortex, produces mineralocorticoids (e.g. aldosterone)
   1. Principal effects of mineralocorticoids are on ion transport by epithelial cells (especially the renal tubular epithelium) à maintenance of sodium and potassium concentrations and extracellular fluid volume
   2. Increased aldosterone à increased sodium and chloride retention and increased potassium excretion by renal tubular epithelium (especially distal convoluted tubules via Na/K cation exchange)
   3. Amount of aldosterone released is controlled by the renin-angiotensin-aldosterone system (see below)
   4. In the ABSENCE of aldosterone, sodium and chloride are lost and potassium is retained (hyponatremia, hypochloremia, hyperkalemia); may result in lethal hyperkalemia in canines
2. Zona fasciculata (middle layer): 70% of the cortex; primary site of glucocorticoid production:  Cortisol, corticosterone, and cortisone
   1. Glucocorticoids antagonize the effects of insulin: they promote gluconeogenesis and glycogenesis and decrease glucose uptake by insulin-sensitive tissues à net increase in serum glucose concentration, increased tissue catabolism
   2. Glucocorticoids cause decreased lipogenesis and increased lipolysis
   3. Glucocorticoids suppress inflammatory and immunologic responses and wound healing
3. Zona reticularis (inner layer): 15% of the cortex; produces sex steroids (progesterone, estrogens, and androgens) in minute amounts normally, and some glucocorticoids

• Renin-angiotensin-aldosterone system (RAAS):
  • RAAS is the major regulator of aldosterone production by the zona glomerulosa
  • Renin secreted by renal juxtaglomerular cells (secreted in response to hyponatremia or renal hypotension, feedback inhibition from angiotensin II and aldosterone) à renin proteolytically cleaves angiotensinogen (a circulating constitutive protein) into angiotensin I à angiotensin I hydrolyzed by angiotensin converting enzyme (ACE) in the lungs into angiotensin II, a potent vasoconstrictor that is also trophic for the zona glomerulosa à stimulates production of aldosterone à angiotensin II is quickly inactivated by angiotensinases
• Hypothalamic-pituitary-adrenocortical axis:
  • Corticotropin releasing hormone (CRH) is released from neurons of the hypothalamus into capillaries of the hypothalamic-pituitary portal system in response to alterations in homeostasis à CRH stimulates rapid release of ACTH from pre-formed granules in corticotrophs in the pituitary gland adenohypophysis (pars intermedia and pars distalis) à ACTH stimulates adrenocortical cells via the melanocortin 2 receptor à increased cAMP à activation of certain protein kinases à production of glucocorticoids (zona fasciculata) and sex hormones (zona reticularis)
  • Circulating cortisol (e.g. exogenous administration or cortisol-producing adrenal tumor) exerts direct negative feedback on CRH and ACTH release à adrenocortical trophic atrophy (iatrogenic associated with exogenous corticosteroid administration), and vice versa
  • Progestins also can suppress the hypothalamic-pituitary-adrenocortical axis

Hypoadrenocorticism (HA):

• Usually the result of adrenal gland insufficiency (primary HA) rather than failure of ACTH secretion by the pituitary gland (secondary HA)
• Idiopathic adrenocortical atrophy (Addison’s disease): most common in dogs; bilateral idiopathic adrenocortical atrophy affecting all 3 cortical layers
• Iatrogenic adrenocortical atrophy: Due to excess administration of exogenous glucocorticoids, or administration of compounds causing lysis of the adrenal cortex (e.g. o,p’-dichlorodiphenyldichloroethane, also known as o,p’-DDD or mitotane/Lysodren, used for treatment of hyperadrenocorticism); affects only the inner 2 cortical layers (i.e. mineralocorticoid levels are not affected)
• Lysodren administration, and rarely mitotane, can also rarely result in mineralocorticoid deficiency
• “Atypical Addison’s”: Similar to Addison’s, except the Na:K ratio is not <27:1
  • ACTH levels are normal but inner two layers of adrenal gland are not functioning
  • Disease may start “atypical” but progress to involve the zona glomerulosa also over time

PATHOGENESIS:

• Adrenocortical insufficiency: at least 90% of the adrenal cortex must be nonfunctional before clinical signs are observed; the cortex is nearly absent in dogs that die from untreated HA
  • Decreased mineralocorticoids (aldosterone) à hyperkalemia à marked cardiovascular disturbances
  • Decreased glucocorticoids à failure of gluconeogenesis and increased insulin sensitivity à moderate hypoglycemia
  • Decreased glucocorticoids à decreased negative feedback on pituitary gland à increased release of ACTH à binding to melanocyte-stimulating hormone (MSH) receptors on skin melanocytes à skin hyperpigmentation
• Idiopathic (primary) adrenocortical atrophy
  • Destruction/atrophy of all 3 cortical layers à deficient production in all classes of corticosteroids (mineralocorticoids, glucocorticoids, and adrenal sex steroids)
  • Pathogenesis is unknown: believed to be an inherited disorder (there is an association between MHC II alleles and disease), and thought to be a result of immune-mediated inflammation; studies have shown a predominance of T rather than B lymphocytes in lesions, and a recent study (Friedenberg et al, Vet Pathol. 2018) demonstrated these are primarily CD4+ rather than CD8+ T-lymphocytes
• Trophic adrenocortical atrophy = decreased ACTH
  • Excess glucocorticoids (endogenous from e.g. contralateral adrenal neoplasm, or exogenously administered) à negative feedback on ACTH release à adrenocortical trophic atrophy of the internal two layers (zona glomerulosa is not affected by ACTH)
• Other uncommon causes of adrenocortical insufficiency:
  • Granulomatous inflammation (e.g. tuberculosis)
  • Thrombosis of adrenal vessels à cortical infarction
  • Metastatic neoplasm à destruction of adrenal cortex
  • Amyloid deposition along adrenal sinusoids
  • Overtreatment with o,p'-DDD (Mitotane/Lysodren)
  • Ketoconazole à decreased cortisol due to inhibition of steroid biosynthesis
• Secondary HA (pituitary-dependent) is less common than adrenal-dependent; pituitary disease (e.g. intracranial neoplasms, infection, trauma) à inadequate ACTH secretion à lack of glucocorticoid secretion only (mineralocorticoid production is not changed) (this is different from atypical primary Addison’s disease in that the ACTH concentration is low)

TYPICAL CLINICAL FINDINGS:

• Insidious development, non-specific, often waxing and waning course, lethargy, depression, vomiting, diarrhea, anorexia, weakness, weight loss (often severe), bradycardia, hypovolemia, hyperpigmentation, syncope
• Rarely, sudden onset shock-like coma with no previous history of illness
• Addisonian crisis: acute circulatory collapse (hypovolemia, +/- hemoconcentration) and evidence of renal failure, severe dehydration
• HA can occur in dogs of any breed, sex, or age, but idiopathic adrenocortical insufficiency occurs more often in young adult dogs
• Mineralocorticoid insufficiency:
  • Progressive severe hyperkalemia
  • Hyponatremia, hypochloremia with concomitant hypernatriuria and hyperchloruria; a Na/K ratio < 23:1 is highly suggestive of HA, and < 26:1 may also be suggestive, although Na and K values may be normal
  • Hemoconcentration due to dehydration and loss of IV fluid volume
  • Metabolic acidosis; decreased renal excretion of H⁺
• Glucocorticoid insufficiency:
  • Moderate hypoglycemia (hyperglycemia may be observed if there is concurrent diabetes mellitus)
  • Complete blood count: +/- lymphocytosis, eosinophilia, mild normocytic normochromic anemia (may be masked by hemoconcentration)
  • Hypercalcemia (unknown mechanism, normalizes with corticosteroid replacement therapy, ionized calcium level remains within normal limits)
• Laboratory evaluation of the adrenal cortex:
  • Serum cortisol: Baseline within normal limits or decreased; exogenous steroids except dexamethasone cross-react in laboratory assays
  • ACTH stimulation test: evaluates the ability of the adrenal cortex to increase plasma cortisol concentration following administration of synthetic ACTH (Cosyntropin); dogs with naturally occurring HA will have little or no response often with ACTH <2ug/dL at both pre-and post-stimulation; this test does not differentiate between HA pathogeneses (pituitary, adrenal, or iatrogenic)
  • Low-dose and high-dose dexamethasone suppression test: Not recommended for animals with HA, these are tests for hyperadrenocorticism
  • Endogenous ACTH: decreased in dogs with pituitary-dependent (secondary) HA, increased in dogs with adrenal-dependent (primary) HA

TYPICAL GROSS FINDINGS:

• Adrenal gland: Marked reduction in adrenal cortical thickness, often <10% of original thickness consisting mostly of capsule, with relatively prominent medulla
• Skin: +/- Hyperpigmentation with long-standing adrenocortical insufficiency
• Lymph nodes: Moderate peripheral lymphadenopathy due to lymphoid hyperplasia

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Idiopathic adrenocortical atrophy:
  • Adrenal gland: All three cortical zones are reduced in thickness; multiple foci of lymphocytes and plasma cells interspersed between sinusoids and fibroblasts early in the disease
  • Pituitary gland: Compensatory hyperplasia of corticotrophs
• Iatrogenic atrophy: Severe atrophy of the two inner cortical zones with retention of the zona glomerulosa
• Lymph nodes: Lymphoid hyperplasia with prominent germinal centers, +/-high numbers of eosinophils infiltrating sinuses

ADDITIONAL DIAGNOSTIC TESTS:

• Definitive diagnosis requires demonstration of inadequate adrenal mass

DIFFERENTIAL DIAGNOSIS:

• Serum chemistry abnormalities (azotemia, hyperphosphatemia, hyperkalemia, hyponatremia, and hypercalcemia): HA, renal failure, vitamin D toxicity
• Decreased adrenal cortex thickness: Hypoplasia (e.g. animals with maldevelopment of the hypophysis, anencephaly, and cyclopia), cortisol-producing (functional) adenoma/carcinoma of the contralateral adrenal gland

COMPARATIVE PATHOLOGY:

• Pigs: Chronic or high exposure to carbadox and related compounds olaquindox and cytadox (antibiotics/growth promotants used in young pigs) has been associated with zona glomerulosa lesions (disorganization, hydropic degeneration, atrophy, fibrosis, mononuclear inflammation, cells with PAS-positive cytoplasmic granules) à mineralocorticoid deficiency symptoms (e.g. hyperkalemia, hyponatremia)
• Humans: In autoimmune HA (Addison’s disease), the major antigenic target of autoantibodies is the enzyme 21-hydroxylase
• Hypoadrenocorticism described in a Hoffmann’s two-toed sloth and giant anteater; clinical signs included respiratory distress, dehydration, dysphagia, and hypotension; decreased serum Na:K, low baseline cortisol, decreased ACTH stimulation assay
• In birds, corticosterone is the main circulating glucocorticoid (versus cortisol in the majority of other species)

REFERENCES:

1. Agnew D, Nofs S, Delaney MA, Rothenburger JL. Xenartha, erinacoemorpha, some afrotheria, andphloidota. In: Terio K, McAloose D, Leger J, eds. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018:549.
2. Ferguson DC, Hoenig M. Endocrine system. In: Latimer KS, ed. Duncan & Prasse’s Veterinary Laboratory Medicine: Clinical Pathology. 5^th Ames, IA: Wiley-Blackwell; 2011:317-323.
3. Friedenberg SG, Brown DL, Meurs KM, Law JM. Lymphocyte subsets in the adrenal glands of dogs with primary hypoadrenocorticism.  Vet Pathol.  2018;55(1):177-181.
4. Miller MA. Endocrine system. In: Zachary JF, McGavin MD, ed. Pathologic Basis of Veterinary Disease. 6^th St. Louis, MO: Elsevier; 2017:708.
5. Rosol TJ, Gröne Endocrine glands. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. 6^th ed. St. Louis, MO: Elsevier; 2016:336-338, 341-343.
6. Stockham SL, Scott MA. Adrenocortical function. In: Stockham SL, Scott MA, eds. Fundamentals of Veterinary Clinical Pathology. 2^nd Ames, IA: Blackwell Publishing: 2008:166, 505-6, 515, 519, 601-602, 811.

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