11.2 Acute Adrenocortical Crisis (Addisonian Crisis) & Hypoadrenocorticism

Acute Adrenocortical Crisis (Addisonian Crisis) & Hypoadrenocorticism

VTS Critical Concept: Acute adrenocortical crisis (Addisonian crisis) is an endocrine emergency characterized by life-threatening hypovolemic shock, severe hyperkalemia, and cardiovascular collapse. The hallmark clinical paradox is a shocky, collapsed patient presenting with a normal or bradycardic heart rate due to hyperkalemic myocardial depression. Immediate resuscitation requires rapid isotonic fluid expansion, emergency cardioprotection, and Dexamethasone Sodium Phosphate, which uniquely does not invalidate the diagnostic ACTH stimulation test.

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1. Functional Adrenal Anatomy & Pathophysiology

The adrenal cortex consists of three anatomically and functionally distinct histological zones, each responsible for synthesizing specific steroid hormones from cholesterol precursors.

ADRENAL GLAND ANATOMY & STEROIDOGENESIS
┌────────────────────────────────────────────────────────────────────────┐
│ CAPSULE                                                                │
├──────────────────────────┬───────────────────────┬─────────────────────┤
│ HISTOLOGICAL ZONE        │ PRIMARY HORMONE CLASS │ PRIMARY FUNCTION    │
├──────────────────────────┼───────────────────────┼─────────────────────┤
│ 1. Zona Glomerulosa      │ Mineralocorticoids    │ Sodium retention,   │
│    (Outer 15%)           │ (Aldosterone)         │ Potassium excretion │
├──────────────────────────┼───────────────────────┼─────────────────────┤
│ 2. Zona Fasciculata      │ Glucocorticoids       │ Gluconeogenesis,    │
│    (Middle 75%)          │ (Cortisol)            │ Vascular tone, SVR  │
├──────────────────────────┼───────────────────────┼─────────────────────┤
│ 3. Zona Reticularis      │ Androgens             │ Sex steroids        │
│    (Inner 10%)           │ (DHEA / Androstened.) │ (Minor significance)│
├──────────────────────────┴───────────────────────┴─────────────────────┤
│ ADRENAL MEDULLA: Catecholamines (Epinephrine & Norepinephrine)         │
└────────────────────────────────────────────────────────────────────────┘
*Mnemonic: G-F-R corresponds to Salt (Mineral), Sugar (Glucocort), Sex (Androgens)*

Primary vs. Secondary (Atypical) Hypoadrenocorticism

• Primary Hypoadrenocorticism (Classic Addison's): Accounts for $>95%$ of cases. Results from bilateral immune-mediated destruction (idiopathic lymphocytic adrenalitis) of all three cortical zones. Causes combined mineralocorticoid (aldosterone) and glucocorticoid (cortisol) deficiency.
• Secondary Hypoadrenocorticism: Caused by pituitary failure of Adrenocorticotropic Hormone (ACTH) synthesis (e.g., neoplasia, trauma, or prolonged exogenous corticosteroid suppression). Because the zona glomerulosa is primarily regulated by the Renin-Angiotensin-Aldosterone System (RAAS) rather than pituitary ACTH, secondary Addison's causes isolated cortisol deficiency with completely normal electrolyte concentrations.
• Atypical Primary Addison's: An initial phase of primary disease where immune-mediated destruction has affected the zona fasciculata (cortisol deficiency) while temporarily sparing the zona glomerulosa (normal electrolytes). Most atypical cases eventually progress to classic mineralocorticoid deficiency within months.

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2. Pathophysiological Consequences of Hormone Deficiencies

Consequences of Aldosterone Deficiency

Aldosterone acts on the principal cells of the renal late distal convoluted tubules and cortical collecting ducts, binding to mineralocorticoid receptors to upregulate epithelial sodium channels (ENaC) on the apical membrane and $\text{Na}^+/\text{K}^+$-ATPase pumps on the basolateral membrane.

[ Loss of Aldosterone Action at Renal Collecting Ducts ]
                          │
       ┌──────────────────┴──────────────────┐
       ▼                                     ▼
[ Failure of Renal Na+ & Cl- Reabsorption ]  [ Failure of K+ & H+ Excretion ]
• Massive urinary sodium & water wasting     • Severe hyperkalemia ([K+] > 6.5-9.0 mEq/L)
• Medullary interstitial washout             • Hyperchloremic metabolic acidosis
• Profound hypovolemic / distributive shock  • Myocardial conduction depression
• Prerenal azotemia & severe hypotension     • Ventricular arrhythmias & asystole

Consequences of Cortisol Deficiency

1. Loss of Vascular Tone: Cortisol exerts a vital permissive action on vascular smooth muscle $\alpha_1$-adrenergic receptors. Cortisol deficiency induces blunted responsiveness to circulating catecholamines, causing systemic vasodilation, refractory hypotension, and distributive shock.
2. Gastrointestinal Barrier Breakdown: Glucocorticoids maintain gastric and intestinal mucosal epithelial turnover and microvascular perfusion. Cortisol deficiency precipitates severe mucosal ischemia, widespread erosions, hematemesis, and severe melena or hematochezia.
3. Impaired Gluconeogenesis: Reduced hepatic gluconeogenesis and increased peripheral insulin sensitivity cause episodic or severe hypoglycemia.
4. Impaired Stress Adaptation: Inability to respond to physiological stressors (illness, boarding, travel) triggers acute decompensation.

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3. Clinical Presentation of Acute Addisonian Crisis

Signalment & Breed Predispositions

• Typically affects young to middle-aged dogs ($2-7\text{ years}$ of age), with a strong female overrepresentation ($\sim 70%$ female).
• High-Risk Breeds: Standard Poodles, Bearded Collies, Portuguese Water Dogs, Nova Scotia Duck Tolling Retrievers, Great Danes, West Highland White Terriers, and Rottweilers.

The "Great Pretender" Clinical Signs

Addison's is known as the "Great Pretender" because chronic signs wax and wane (often temporarily improving with non-specific IV fluid therapy or corticosteroid administration). In an acute Addisonian crisis, patients present in catastrophic collapse:

• Profound Hypovolemic Shock: Weak or absent femoral pulses, prolonged capillary refill time ($>3\text{ seconds}$), pale mucous membranes, hypothermia ($<98^\circ\text{F}$), severe mental depression, stupor, or coma.
• The Cardinal Paradox: Normal to Bradycardic Heart Rate: Hypovolemic shock ordinarily triggers intense compensatory sympathetic tachycardia (HR $>160-200\text{ bpm}$). In an Addisonian crisis, severe hyperkalemia exerts a direct depressant effect on the sinoatrial node and cardiac conduction system, resulting in paradoxical bradycardia or an inappropriately normal heart rate ($60-90\text{ bpm}$) in the face of profound shock.
• Gastrointestinal Ulceration: Profuse vomiting, diarrhea, anorexia, abdominal pain mimicking acute pancreatitis or surgical peritonitis, and severe melena/hematochezia.

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4. Diagnostics & Hyperkalemic ECG Manifestations

Sodium-to-Potassium Ratio (Na:K Ratio)

$\text{Na:K Ratio} = \frac{\text{Serum Sodium (mEq/L)}}{\text{Serum Potassium (mEq/L)}}$

• Normal Canine/Feline Na:K Ratio: $27:1\text{ to }40:1$.
• Suspicious: $\text{Na:K} < 27:1$.
• Strongly Suggestive of Primary Hypoadrenocorticism: $\text{Na:K} < 20:1$ (e.g., $\text{Na}^+ = 125\text{ mEq/L}, \text{K}^+ = 7.8\text{ mEq/L} \implies \text{Na:K} = 16.0:1$).
• Differential Diagnoses for Low Na:K Ratio: Acute kidney injury (oliguric/anuric), urinary tract obstruction (feline urethral obstruction / uroabdomen), severe trichuriasis (Trichuris vulpis / whipworm infection), cavitary effusions with repeated drainage, and severe salmonellosis.

Hyperkalemic Electrocardiographic (ECG) Progression

As extracellular potassium rises, it partially depolarizes the resting membrane potential of cardiac myocytes, inactivating fast voltage-gated sodium channels and slowing myocardial conduction velocity.

[ [K+] 5.5 - 6.5 mEq/L ] ──► Tall, peaked, narrow ('tented') T waves; shortened QT interval
            │
            ▼
[ [K+] 6.5 - 7.5 mEq/L ] ──► Prolonged PR interval, decreased P-wave amplitude, widening of QRS
            │
            ▼
[ [K+] 7.5 - 8.5 mEq/L ] ──► Complete loss of P waves (Sinoventricular Rhythm); marked QRS widening
            │
            ▼
[ [K+] > 8.5 - 10.0 mEq/L ] ──► Biphasic Sinusoidal Waves ──► Ventricular Fibrillation / Asystole

Definitive Diagnosis: ACTH Stimulation Test

The ACTH Stimulation Test is the gold-standard diagnostic test for hypoadrenocorticism.

1. Pre-Sample: Draw a baseline blood sample for serum cortisol concentration.
2. ACTH Administration: Administer Synthetic ACTH (Cosyntropin / Cortrosyn) at $5\text{ mcg/kg IV}$ (or $250\text{ mcg/dog}$ for large dogs; $125\text{ mcg/dog}$ for small dogs).
3. Post-Sample: Collect a second blood sample exactly $1\text{ hour}$ post-injection.
4. Interpretation: In a normal dog, post-ACTH cortisol rises significantly ($>6-18\text{ mcg/dL}$ / $>170-500\text{ nmol/L}$). In hypoadrenocorticism, both pre- and post-ACTH cortisol concentrations are profoundly suppressed at $<1.0\text{ mcg/dL}\ (<28\text{ nmol/L})$, demonstrating zero adrenocortical reserve.

The Glucocorticoid Selection Rule (Dexamethasone vs. Others)

• Dexamethasone Sodium Phosphate ($0.1-0.2\text{ mg/kg IV}$): Does NOT cross-react with cortisol radioimmunoassays or chemiluminescent immunoassays. It can be administered immediately upon admission during emergency resuscitation without affecting the validity of an ACTH stimulation test.
• Hydrocortisone, Prednisone, Prednisolone, and Methylprednisolone: These glucocorticoids possess chemical structures identical or near-identical to endogenous cortisol and strongly cross-react with cortisol assays, producing falsely elevated cortisol results. They are strictly contraindicated until after the 1-hour post-ACTH blood sample is drawn.

5. Emergency Resuscitation & Hyperkalemia Management

Step 1: Intravenous Fluid Resuscitation

• Fluid of Choice: $0.9%\text{ NaCl}$ has historically been favored because it contains a high sodium concentration ($154\text{ mEq/L}$) and $0\text{ mEq/L}$ potassium. However, balanced isotonic crystalloids (Plasmalyte-148, Normosol-R) are equally safe and effective, and carry a lower risk of producing hyperchloremic metabolic acidosis.
• Dosing: Administer rapid isotonic shock boluses ($20-30\text{ mL/kg}$ over $15-20\text{ minutes}$ in dogs; reassess perfusion parameters, blood pressure, and lactate). Volume expansion restores effective circulating volume, improves renal glomerular filtration, and promotes rapid renal potassium excretion (kaliuresis).

Step 2: Emergency Cardioprotection with Calcium Gluconate

When hyperkalemia causes severe conduction disturbances (loss of P waves, marked QRS widening, sinusoidal waveforms, severe bradycardia):

• Medication: $10%\text{ Calcium Gluconate}$ at $0.5-1.0\text{ mL/kg IV}$ administered slowly over $10-15\text{ minutes}$ under continuous ECG monitoring.
• Mechanism: Calcium does NOT alter serum potassium concentration. Instead, extracellular calcium ions screen negative surface charges on cardiac myocyte membranes, shifting the threshold potential to a less negative level. This re-establishes the normal voltage difference between the resting membrane potential and threshold potential, immediately restoring normal myocardial excitability and conduction within $2-5\text{ minutes}$ (lasting $20-40\text{ minutes}$).
• Safety Note: Stop infusion immediately if bradycardia worsens or ventricular arrhythmias develop.

Step 3: Active Potassium Shifting Therapies

If severe hyperkalemia ($[\text{K}^+] > 7.5-8.0\text{ mEq/L}$) persists despite fluid therapy and calcium gluconate:

1. Regular Insulin & Dextrose: Administer Regular Insulin at $0.1-0.25\text{ U/kg IV}$ combined with $50%\text{ Dextrose at }1-2\text{ g (2-4 mL of }50%\text{ dextrose) per unit of insulin}$ diluted $1:1$ with saline, followed by a $2.5-5.0%\text{ dextrose}$ fluid infusion to prevent hypoglycemia. Insulin activates cell membrane $\text{Na}^+/\text{K}^+$-ATPase pumps, rapidly driving potassium into cells.
2. $\beta_2$-Adrenergic Agonists: Terbutaline ($0.01\text{ mg/kg IM/SQ}$) or Albuterol inhalation stimulates intracellular cyclic AMP, shifting potassium into skeletal muscle.
3. Sodium Bicarbonate: Administer $1-2\text{ mEq/kg IV slow over }15-20\text{ min}$ only in severe, refractory acidemia ($\text{pH} < 7.10$). $H^+$ ions leave cells in exchange for $K^+$ entering cells.

Step 4: Maintenance & Long-Term Hormone Replacement

Once the patient is stabilized, out of shock, and eating voluntarily:

• Mineralocorticoid Replacement: Desoxycorticosterone Pivalate (DOCP / Percorten-V, Zycortal) at $2.2\text{ mg/kg SQ}$ every $25-30\text{ days}$. DOCP is a pure mineralocorticoid with no glucocorticoid activity. Alternatively, oral Fludrocortisone Acetate ($0.01-0.02\text{ mg/kg PO q12-24h}$) provides dual mineralocorticoid and mild glucocorticoid activity.
• Glucocorticoid Replacement: Oral Prednisone ($0.1-0.2\text{ mg/kg PO q24h}$) at physiological maintenance doses, with instructions to double or triple the dose during periods of physiological or psychological stress.