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: Diffusely, the zona fasciculata and zona reticularis of the adrenal cortex 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 (nodular cortical hyperplasia). 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 occasional arterioles within the myocardium are mildly expanded by acellular, eosinophilic, waxy material (hyalinosis). Focally within the subendocardial myocardium there is an area in which cardiomyocytes have swollen, vacuolated sarcoplasm (degenerate) or are lost and replaced by adipose tissue.

MORPHOLOGIC DIAGNOSIS:

Adrenal gland, cortex: Pancortical atrophy, diffuse, severe, German shepherd dog, canine.
Heart: Fibromyxomatous valvular degeneration (endocardiosis), chronic, focally extensive, moderate, with focal mild subendocardial cardiomyocyte degeneration.

ETIOLOGY: Immune-mediated

ETIOLOGIC DIAGNOSIS: Idiopathic adrenocortical atrophy

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

GENERAL DISCUSSION:

Normal adrenal gland

Adrenal Cortex:

The adrenal gland has two distinct parts:
1. Adrenal cortex, composed of coelomic epithelial cells of mesodermal origin (related to steroid-producing cells of the gonads)
2. Adrenal medulla, composed of neural crest cells of ectodermal origin
The cortex to medulla ratio is normally 1: 1-2: 1
The adrenal cortex has three distinct layers:
1. Zona glomerulosa (outermost):
  - 15% of the cortex, produces mineralocorticoids (e.g., aldosterone)
  - 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
  - Increased aldosterone -> Increased sodium and chloride retention and increased potassium excretion by renal tubular epithelium (especially distal convoluted tubules via Na/K cation exchange)
  - Aldosterone release is controlled by the renin-angiotensin-aldosterone system (see below)
  - 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
  - Glucocorticoids antagonize the effects of insulin: They promote gluconeogenesis and glycogenesis and decrease glucose uptake by insulin-sensitive tissues -> Net increase in serum glucose concentration, increased tissue catabolism
  - Glucocorticoids cause decreased lipogenesis and increased lipolysis
  - 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 (in response to hyponatremia or renal hypotension, feedback inhibition from angiotensin II and aldosterone) -> Renin proteolytically cleaves angiotensinogen (a circulating constitutive protein) into angiotensin I -> 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 -> Stimulates production of aldosterone -> 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 -> CRH stimulates rapid release of ACTH from pre-formed granules in corticotrophs in the pituitary gland adenohypophysis (pars intermedia and pars distalis) -> ACTH stimulates adrenocortical cells via the melanocortin 2 receptor -> Increased cAMP -> Activation of certain protein kinases -> 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 -> Adrenocortical trophic atrophy (iatrogenic associated with exogenous corticosteroid administration), and vice versa
Progestins also can suppress the hypothalamic-pituitary-adrenocortical axis

Hypoadrenocorticism (HA):

Hypoadrenocorticism is a functional disorder that results from insufficient adrenocortical production of mineralocorticoid and/or glucocorticoid hormones
May be primary, due adrenal gland insufficiency (more common), or secondary, due to failure of ACTH secretion by the pituitary gland
Idiopathic adrenocortical atrophy (“Addison’s disease”) (primary HA)
1. Bilateral idiopathic adrenocortical atrophy affecting all 3 cortical layers resulting in decreased mineralocorticoids (primarily aldosterone) and glucocorticoids
2. Most common in dogs and most commonly due to immune-mediated adrenal cortical destruction (lymphoplasmacytic adrenalitis – E-M05)
Iatrogenic adrenocortical atrophy
1. Due to administration of exogenous glucocorticoids or compounds causing lysis of the adrenal cortex (e.g. o,p’-DDD or mitotane - see Pathogenesis)
Summary:
1. Primary hypoadrenocorticism (“adrenal-driven”) affects all 3 cortical layers
2. Secondary hypoadrenocorticism (“Pituitary/ACTH-driven”) affects only the zonae fasciculata and reticularis (zona glomerulosa unaffected (i.e., mineralocorticoid levels normal)
“Atypical Addison’s”: Similar to Addison’s, except the Na:K ratio is not <27:1
1. ACTH levels are normal but inner two layers of adrenal gland are not functioning
2. 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
1. Decreased mineralocorticoids (aldosterone)
  - Marked alterations of serum potassium, sodium, and chloride levels
  - Decreased renal potassium excretion -> Increased serum potassium -> Severe hyperkalemia -> marked cardiovascular disturbances
    - Dehydration and loss of IV fluid volume
  - Due to increased renal sodium (and water) loss excretion of water by the kidney (Renin-Angiotensin-Aldosterone system (RAAS) no longer fully functioning)
  - Hemoconcentration due to dehydration and loss of IV fluid volume
  - Metabolic acidosis; decreased renal excretion of H⁺
2. Decreased glucocorticoids
  - Failure of gluconeogenesis and increased insulin sensitivity -> Moderate hypoglycemia (in some dogs)
  - Decreased negative feedback on pituitary gland -> Increased release of ACTH -> Binding to melanocyte-stimulating hormone (MSH) receptors on skin melanocytes -> Skin hyperpigmentation
Idiopathic (primary) adrenocortical atrophy
1. Pathogenesis unknown, but most commonly immune-mediated destruction of adrenal cortical cells
2. Genetic breed predisposition: Portuguese water dogs, bearded collies, and standard poodles
Other uncommon causes of (primary) adrenocortical insufficiency: Granulomatous inflammation (e.g. tuberculosis); adrenal vessel thrombosis (leading to cortical infarct), metastatic neoplasm, amyloidosis
Iatrogenic adrenocortical atrophy: Due to excess administration of exogenous glucocorticoids or compounds causing lysis of the adrenal cortex
1. Excess glucocorticoids (e.g., endogenous from contralateral adrenal neoplasm or exogenously administered)
  - Increased circulating glucocorticoids -> Increased negative feedback (hypothalamic-pituitary-adrenocortical axis) -> Decreased ACTH -> Adrenocortical trophic atrophy of the internal two layers (zonae fasciculata and reticularis)
2. o,p’-dichlorodiphenyldichloroethane (o,p’-DDD or mitotane (Lysodren)) is a selective toxin for the zonae fasciculata and reticularis and is used for treatment of pituitary—dependent hyperadrenocorticism in dogs
  - Decreased ACTH -> Adrenocortical trophic atrophy of the internal two layers (zonae fasciculata and reticularis)
Trophic adrenocortical atrophy (pituitary-dependent):
1. Destructive pituitary lesion (e.g., intracranial neoplasm, infection, trauma -> Decreased ACTH -> Adrenocortical trophic atrophy of the internal two layers (zonae fasciculata and reticularis) (zona glomerulosa not affected by ACTH; mineralocorticoid levels normal)

TYPICAL CLINICAL FINDINGS:

Most commonly young to middle-aged female dogs
Insidious, waxing/waning signs: Lethargy, recurrent gastroenteritis (emesis, diarrhea, and anorexia), weight loss/loss of body condition, failure to respond to stressful situations (e.g., illnesses, surgery)
1. Other signs: Muscle weakness (due to hyperkalemia); possible megaesophagus
May present with severe dehydration with acute circulatory collapse, evidence renal failure, (“Addisonian crisis”) or sudden death
Clinicopathologic abnormalities
1. CBC: +/- lymphocytosis, eosinophilia, mild normocytic normochromic (nonregenerative) anemia (may be masked by hemoconcentration)
2. Chem:
  - Progressive severe hyperkalemia; also hyponatremia, hypochloremia (with concomitant hypernatriuria and hyperchloruria)
  - It’s suggested that a Na/K ratio <27:1 (or 25 or 22) is diagnostic of HA in dogs; however a low ratio is not unique to HA
  - Hypercalcemia (unknown mechanism, normalizes with corticosteroid replacement therapy, ionized calcium level remains within normal limits)
  - Moderate hypoglycemia (in some dogs)
Laboratory evaluation of the adrenal cortex:
1. Serum cortisol: Baseline within normal limits or decreased; exogenous steroids except dexamethasone cross-react in laboratory assays
2. 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)
3. Low-dose and high-dose dexamethasone suppression test: Not recommended for animals with HA, these are tests for hyperadrenocorticism
4. 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:
1. Adrenal gland:
  - Lymphoplasmacytic adrenalitis
  - All three cortical zones are reduced in thickness due to lysis of the cortical cells
    - +/- lipofuscin-laden macrophages
    - The cortex to medulla ratio well below normal 1: 1-2: 1
  - Later in the disease, the adrenal cortex may be undetectable and inflammation may be gone
2. Pituitary gland: May see compensatory hyperplasia of corticotrophs
Iatrogenic adrenocortical atrophy:
1. 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) -> Mineralocorticoid deficiency symptoms (e.g. hyperkalemia, hyponatremia)
In fetuses, adenohypophysis aplasia can result in adrenocortical atrophy, and subsequent prolonging of gestation
Prolonged gestation: Fetal adrenal hypoplasia leads to no fetal signal to induce parturition -> prolonged gestation; reported in cattle and sheep due to inherited disorders (Holstein and Ayrshire: Primary adrenal hypoplasia; Guernsey, Jersey, and Swedish Red and White: Adenohypophyseal aplasia) or toxin ingestion (e.g. pregnant ewes ingest Veratrum californicum gestation day 9-14 -> CNS/adenohypophyseal abnormalities)
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

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