10.2 Adrenal Cortex & Medulla: Cortisol, ACTH, Cushing, Addison & Catecholamines

10.2 Adrenal Cortex & Medulla: Cortisol, ACTH, Cushing, Addison & Catecholamines

[!NOTE] Clinical Chemistry Core Principle: The adrenal gland integrates steroid endocrine signaling in its three-layered cortex with neuroendocrine catecholamine secretion in its chromaffin medulla. Cortisol synthesis follows a strict diurnal circadian rhythm, peaking shortly after waking (06:00 - 08:00 AM) and declining to a nadir near midnight (23:00 - 00:00). Diagnostic evaluation of adrenocortical pathology requires understanding this circadian cycle, utilizing dynamic suppression tests (dexamethasone) for suspected hyperfunction, and provocative stimulation tests (cosyntropin) for suspected hypofunction.

Adrenal Cortex Functional Anatomy and Steroid Biosynthesis

The adrenal cortex comprises three histologically and biochemically distinct concentric zones, each expressing unique steroidogenic enzymes that direct cholesterol into specific hormonal end-products.

+-----------------------------------------------------------------------------------------+
|                 Adrenal Cortical Zoning, Regulation, and Hormonal Output                |
+-----------------------------------------------------------------------------------------+
|                                                                                         |
|  CAPSULE                                                                                |
|  ─────────────────────────────────────────────────────────────────────────────────────  |
|  ZONA GLOMERULOSA (~15% of cortex)                                                      |
|    - Primary Product: ALDOSTERONE (Mineralocorticoid)                                   |
|    - Primary Regulators: RAAS (Angiotensin II) and Extracellular Potassium [K+]        |
|    - Key Enzyme: Aldosterone Synthase (CYP11B2); lacks 17-alpha-hydroxylase (CYP17A1)  |
|  ─────────────────────────────────────────────────────────────────────────────────────  |
|  ZONA FASCICULATA (~75% of cortex)                                                      |
|    - Primary Product: CORTISOL (Glucocorticoid)                                         |
|    - Primary Regulators: Hypothalamic CRH -> Pituitary ACTH (Diurnal Rhythm)            |
|    - Key Enzymes: 17-alpha-hydroxylase (CYP17A1), 21-hydroxylase (CYP21A2),             |
|                   11-beta-hydroxylase (CYP11B1)                                         |
|  ─────────────────────────────────────────────────────────────────────────────────────  |
|  ZONA RETICULARIS (~10% of cortex)                                                      |
|    - Primary Product: ADRENAL ANDROGENS (DHEA, DHEA-S, Androstenedione)                 |
|    - Primary Regulator: Pituitary ACTH                                                  |
|    - Key Enzymes: 17,20-lyase (CYP17A1), Sulfotransferase (SULT2A1)                     |
|  ─────────────────────────────────────────────────────────────────────────────────────  |
|  ADRENAL MEDULLA (Chromaffin Cells)                                                     |
|    - Primary Products: EPINEPHRINE (~80%) and NOREPINEPHRINE (~20%)                     |
|    - Primary Regulator: Sympathetic preganglionic cholinergic fibers (Acetylcholine)    |
|    - Key Enzyme: Phenylethanolamine N-methyltransferase (PNMT, cortisol-inducible)     |
+-----------------------------------------------------------------------------------------+
|  Clinical Mnemonic: "GFR - Salt, Sugar, Sex: The deeper you go, the sweeter it gets."    |
+-----------------------------------------------------------------------------------------+

Steroid Biosynthetic Cascade

All steroid hormones derive from cholesterol (supplied by circulating LDL or synthesized de novo from acetate). The rate-limiting step in steroidogenesis is the transport of hydrophobic cholesterol across the inner mitochondrial membrane, catalyzed by the Steroidogenic Acute Regulatory (StAR) protein.

1. Cleavage to Pregnenolone: Mitochondrial CYP11A1 (cholesterol side-chain cleavage enzyme / desmolase) converts cholesterol ($C_{27}$) into pregnenolone ($C_{21}$).
2. Mineralocorticoid Pathway (Glomerulosa): $\text{Pregnenolone} \xrightarrow{3\beta\text{-HSD}} \text{Progesterone} \xrightarrow{\text{CYP21A2}} \text{11-Deoxycorticosterone (DOC)} \xrightarrow{\text{CYP11B2}} \text{Corticosterone} \xrightarrow{\text{CYP11B2}} \text{Aldosterone}$
3. Glucocorticoid Pathway (Fasciculata): $\text{Pregnenolone} \xrightarrow{\text{CYP17A1}} \text{17}\alpha\text{-OH-Pregnenolone} \xrightarrow{3\beta\text{-HSD}} \text{17}\alpha\text{-OH-Progesterone} \xrightarrow{\text{CYP21A2}} \text{11-Deoxycortisol} \xrightarrow{\text{CYP11B1}} \text{Cortisol}$
4. Androgen Pathway (Reticularis): $\text{17}\alpha\text{-OH-Pregnenolone} \xrightarrow{\text{17,20-lyase}} \text{DHEA} \xrightarrow{\text{SULT2A1}} \text{DHEA-Sulfate (DHEA-S)}$ $\text{DHEA} \xrightarrow{3\beta\text{-HSD}} \text{Androstenedione} \xrightarrow{\text{Peripheral Tissues}} \text{Testosterone}$

Circulating Transport of Cortisol

• Corticosteroid-Binding Globulin (CBG / Transcortin): Binds ~90% of circulating cortisol with high affinity. CBG is synthesized in the liver; its concentration rises with estrogen (pregnancy, oral contraceptives) and declines in cirrhosis or nephrotic syndrome.
• Albumin: Binds ~7% of circulating cortisol with low affinity.
• Free Cortisol: Approximately 3% to 5% of circulating cortisol is unbound and biologically active. Only free cortisol is filtered across the renal glomerulus into urine, where it is measured as Urinary Free Cortisol (UFC).

Hypercortisolemia / Cushing Syndrome

Cushing Syndrome describes the constellation of clinical and biochemical abnormalities resulting from chronic, autonomous glucocorticoid excess.

• Clinical Signs & Symptoms: Progressive central truncal obesity, rounded facial fullness ("moon facies"), dorsocervical adipose deposition ("buffalo hump"), supraclavicular fat pads, thin fragile skin with easy bruising, wide violaceous cutaneous striae (>1 cm width on abdomen and thighs), proximal muscle wasting, osteopenia/osteoporosis, secondary diabetes mellitus (cortisol promotes gluconeogenesis and peripheral insulin resistance), hypertension, and hypokalemic metabolic alkalosis (excess cortisol saturates renal 11$\beta$-HSD2 enzymes and binds mineralocorticoid receptors).

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|                 Stepwise Diagnostic Evaluation of Cushing Syndrome                      |
+-----------------------------------------------------------------------------------------+
|                                                                                         |
|  STEP 1: RULE OUT EXOGENOUS GLUCOCORTICOIDS (Most common cause overall)                 |
|  ─────────────────────────────────────────────────────────────────────────────────────  |
|  STEP 2: CONFIRM HYPERCORTISOLEMIA (Require at least TWO concordant positive tests)     |
|    1. 24-Hour Urinary Free Cortisol (UFC) [> 3x upper limit of normal]                  |
|    2. Late-Night Salivary Cortisol (11:00 PM) [Loss of normal circadian nadir]          |
|    3. Overnight Low-Dose (1 mg) Dexamethasone Suppression Test [Morning Cortisol >= 1.8 ug/dL]|
|  ─────────────────────────────────────────────────────────────────────────────────────  |
|  STEP 3: MEASURE PLASMA ACTH TO ESTABLISH ETIOLOGICAL MECHANISM                         |
|                                                                                         |
|                ┌───────────────────────────────┴───────────────────────────────┐        |
|                ▼                                                               ▼        |
|       [ ACTH < 5 pg/mL ]                                             [ ACTH > 15-20 pg/mL ]     |
|    ACTH-INDEPENDENT CUSHING                                           ACTH-DEPENDENT CUSHING    |
|    - Adrenal Adenoma (~10%)                                           - Cushing Disease (~70%)  |
|    - Adrenal Carcinoma (~5%)                                          - Ectopic ACTH (~15%)     |
|    - Proceed to Adrenal CT/MRI                                                 │        |
|                                                                                ▼        |
|                                                                  STEP 4: DIFFERENTIATE  |
|                                                                  PITUITARY VS ECTOPIC   |
|                                                                  - High-Dose (8 mg) DST |
|                                                                  - CRH Stimulation Test |
|                                                                  - Bilateral IPSS       |
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Step 1: Initial Screening Tests for Cushing Syndrome

To establish pathological hypercortisolemia, the laboratory must demonstrate excessive cortisol secretion or loss of normal feedback/diurnal rhythm:

1. 24-Hour Urinary Free Cortisol (UFC):
   • Patient collects an accurate 24-hour urine specimen (creatinine excretion measured to verify completeness: 15-25 mg/kg/day in males, 10-20 mg/kg/day in females).
   • Measures integrated unbound free cortisol filtered by glomeruli. High diagnostic sensitivity (>95%). Levels greater than 3 to 4 times the upper reference limit (>150-200 mcg/24 hr; normal reference <50 mcg/24 hr) confirm hypercortisolemia.
2. Late-Night Salivary Cortisol (11:00 PM / Midnight):
   • Cortisol enters saliva via passive intracellular diffusion, independent of salivary flow rate. Salivary cortisol reflects circulating free cortisol.
   • In healthy individuals, midnight cortisol drops to a profound nadir (<0.10 - 0.18 mcg/dL or <2.8 - 4.0 nmol/L). Patients with Cushing syndrome lose this circadian rhythm, displaying elevated midnight salivary cortisol.
3. Overnight Low-Dose (1 mg) Dexamethasone Suppression Test (DST):
   • Patient takes 1.0 mg dexamethasone orally at 23:00; fasting total serum cortisol is measured at 08:00 the following morning.
   • Dexamethasone is a potent synthetic glucocorticoid (30 times more potent than cortisol) that binds pituitary glucocorticoid receptors, suppressing endogenous ACTH and morning cortisol in healthy individuals.
   • Diagnostic Cutoff: In normal subjects, morning serum cortisol suppresses to $<1.8\text{ mcg/dL}$ ($<50\text{ nmol/L}$). Cortisol levels $\ge 1.8\text{ mcg/dL}$ indicate failure of suppression, signaling Cushing syndrome.

Step 2: Etiological Differential via Plasma ACTH

Once endogenous hypercortisolemia is established, EDTA plasma is collected on ice to measure baseline ACTH via sandwich immunoassay:

• Suppressed ACTH ($<5\text{ pg/mL}$): ACTH-Independent Cushing Syndrome. Autonomous cortisol hypersecretion by an adrenocortical neoplasm (unilateral adrenal adenoma or carcinoma) exerts intense negative feedback on pituitary corticotrophs, shutting down ACTH synthesis. Proceed to thin-section unenhanced adrenal CT.
• Normal or Elevated ACTH ($>15 - 20\text{ pg/mL}$): ACTH-Dependent Cushing Syndrome. Bilateral adrenocortical hyperplasia is driven by excess circulating ACTH, originating either from a pituitary corticotroph adenoma (Cushing Disease, ~70% of endogenous cases) or an Ectopic ACTH-secreting tumor (~10-15% of cases; e.g., small cell lung carcinoma, bronchial carcinoid, thymic carcinoid, pancreatic neuroendocrine tumor).

Step 3: Differentiating Pituitary Cushing Disease from Ectopic ACTH

1. High-Dose (8 mg) Dexamethasone Suppression Test:
   • Pituitary corticotroph microadenomas retain partial, high-threshold negative feedback sensitivity. Non-pituitary neuroendocrine tumors express aberrant promoters uncoupled from glucocorticoid feedback.
   • Interpretation: A $>50%$ reduction in morning serum cortisol relative to baseline confirms Pituitary Cushing Disease. Ectopic ACTH tumors fail to suppress ($<50%$ reduction).
2. CRH Stimulation Test:
   • Intravenous administration of 1 mcg/kg ovine or human CRH.
   • Pituitary adenomas express functional CRH receptors, mounting a brisk $\ge 35-50%$ surge in plasma ACTH and a $\ge 20%$ increase in cortisol. Ectopic ACTH tumors do not express CRH receptors and fail to respond.
3. Bilateral Inferior Petrosal Sinus Sampling (IPSS):
   • The diagnostic gold standard when pituitary MRI fails to show a definitive microadenoma ($<6\text{ mm}$).
   • Interventional radiologists cannulate the femoral veins, advancing microcatheters bilaterally into the inferior petrosal sinuses (IPS) (draining pituitary venous blood) and a peripheral vein. Blood is drawn simultaneously for ACTH before and at 2, 5, and 10 minutes following peripheral IV administration of 100 mcg ovine CRH.
   • Central-to-Peripheral (IPS/P) ACTH Ratio:
     • Baseline IPS/P Ratio $>2.0$ or Post-CRH IPS/P Ratio $>3.0$ definitively confirms Pituitary Cushing Disease.
     • Baseline IPS/P Ratio $<2.0$ and Post-CRH IPS/P Ratio $<3.0$ confirms Ectopic ACTH Secretion.
     • An inter-sinus ratio $>1.4$ accurately lateralizes the adenoma within the right or left pituitary wing, directing transsphenoidal microresection.

Adrenocortical Insufficiency

Adrenocortical insufficiency manifests as inadequate glucocorticoid and/or mineralocorticoid production, presenting acutely as life-threatening adrenal crisis (hypotension, circulatory collapse) or chronically as insidious fatigue, weight loss, and electrolyte disturbances.

Pathophysiological Distinction: Why Hyperpigmentation and Hyperkalemia Occur ONLY in Addison Disease

1. Mechanism of Cutaneous Hyperpigmentation: In primary Addison disease, loss of cortisol negative feedback drives massive pituitary transcription of the Pro-opiomelanocortin (POMC) precursor gene. Post-translational enzymatic cleavage of POMC yields both ACTH and alpha-Melanocyte-Stimulating Hormone ($\alpha$-MSH). Both molecules contain the identical heptapeptide core sequence that binds with high affinity to the Melanocortin 1 Receptor (MC1R) on basal epidermal melanocytes, stimulating eumelanin synthesis. Hyperpigmentation is prominent in gingival/buccal mucosa, palmar creases, extensor knuckles, vermilion lip borders, and recent scars. In secondary/tertiary adrenal insufficiency, ACTH and POMC synthesis is deficient; the skin is pale.
2. Preservation of the Renin-Aldosterone Axis: In secondary adrenal insufficiency, anterior pituitary ACTH loss deprives only the cortisol-secreting zona fasciculata and androgen-secreting zona reticularis of trophic support. The zona glomerulosa remains intact because its primary regulatory inputs are circulating angiotensin II and potassium. Thus, aldosterone secretion is preserved, preventing renal potassium retention and preventing the severe hyperkalemia characteristic of Addison disease.

Provocative Adrenal Testing: Cosyntropin (Synthetic ACTH 1-24) Stimulation Protocol

• Biochemical Rationale: Cortisol exhibits pulsatile secretion; a single random morning cortisol between 3 and 15 mcg/dL is indeterminate. The Cosyntropin Stimulation Test (Short Synacthen Test) evaluates adrenocortical functional reserve directly.
• Protocol: Collect baseline serum cortisol at 0 minutes. Administer 250 mcg of cosyntropin (synthetic N-terminal ACTH amino acids 1-24, possessing full biological receptor-binding activity) intravenously or intramuscularly. Draw repeat serum cortisol specimens at 30 and 60 minutes post-injection.
• Diagnostic Interpretation:
  • Normal Response: Peak serum cortisol rises to $\ge 18.0\text{ to } 20.0\text{ mcg/dL}$ ($>500 - 550\text{ nmol/L}$) at 30 or 60 minutes, ruling out primary adrenocortical insufficiency.
  • Primary Adrenal Insufficiency: The damaged adrenal cortex cannot respond to exogenous trophic stimulation; peak cortisol remains subnormal ($<18.0\text{ mcg/dL}$, typically flat $<10\text{ mcg/dL}$). Baseline ACTH will be markedly elevated.

Mineralocorticoid Axis & Secondary Endocrine Hypertension

Primary Hyperaldosteronism (Conn Syndrome)

• Etiology: Autonomous, non-renin-dependent hypersecretion of aldosterone by a unilateral Aldosterone-Producing Adrenal Adenoma (APA) (~35%) or Bilateral Idiopathic Adrenal Hyperplasia (IAH) (~65%).
• Pathophysiology: Aldosterone binds mineralocorticoid receptors in the principal cells of the renal cortical collecting duct, up-regulating the apical Epithelial Sodium Channel (ENaC) and basolateral $Na^+/K^+$-ATPase. This promotes excessive sodium reabsorption in exchange for urinary potassium ($K^+$) and proton ($H^+$) wasting via intercalated cell $H^+$-ATPase pumps.
• Clinical Manifestations: Severe, drug-resistant hypertension, muscle cramps, weakness, hypokalemia, and metabolic alkalosis. Intravascular volume expansion elevates renal perfusion pressure, suppressing juxtaglomerular renin release.
• Screening: Aldosterone-to-Renin Ratio (ARR):
  • Patient must have hypokalemia corrected prior to testing (hypokalemia directly inhibits aldosterone synthesis, yielding false-negative ratios). Centrally acting antihypertensives (spironolactone, eplerenone) must be discontinued for 4-6 weeks.
  • Collect morning upright Plasma Aldosterone Concentration (PAC, ng/dL) and Plasma Renin Activity (PRA, ng/mL/hr) or Direct Renin Concentration (DRC, mIU/L).
  • Diagnostic Criterion: PAC $>15\text{ ng/dL}$ ($>415\text{ pmol/L}$) with suppressed PRA $<1.0\text{ ng/mL/hr}$, yielding an ARR $>20\text{ to } 30$.
  • Secondary Hyperaldosteronism Comparison: In renovascular hypertension (renal artery stenosis), compromised renal perfusion stimulates excessive juxtaglomerular renin secretion. Both Renin and Aldosterone are high, producing an ARR $<10$.
• Confirmatory Saline Infusion Test: Administer 2.0 liters of 0.9% normal saline IV over 4 hours. Volume expansion expands extracellular fluid and suppresses PAC in normal individuals ($<5\text{ ng/dL}$). In Conn syndrome, autonomous aldosterone fails to suppress (post-infusion PAC $>10\text{ ng/dL}$).

Congenital Adrenal Hyperplasia (CAH): 21-Hydroxylase Deficiency

• Genetics & Molecular Lesion: Autosomal recessive inborn error of steroidogenesis caused by mutations or gene deletions in the CYP21A2 gene (chromosome 6p21 within the HLA complex), accounting for $>90%$ of all CAH cases.
• Biochemical Block: 21-Hydroxylase converts progesterone to 11-deoxycorticosterone (mineralocorticoid pathway) and 17-hydroxyprogesterone to 11-deoxycortisol (glucocorticoid pathway).

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|                 Biochemical Shunt in 21-Hydroxylase Deficiency CAH                      |
+-----------------------------------------------------------------------------------------+
|                                                                                         |
|  [ Cholesterol ] ──> [ Pregnenolone ] ──> [ 17-OH-Pregnenolone ] ──> [ DHEA ]          |
|                             │                     │                     │               |
|                             ▼                     ▼                     ▼               |
|                      [ Progesterone ]     [ 17-OH-Progesterone ]  [ Androstenedione ]   |
|                             │                     │                     │               |
|                             X 21-HYDROXYLASE      X 21-HYDROXYLASE      ▼               |
|                             X BLOCK               X BLOCK         [ Testosterone ]      |
|                             ▼                     ▼                     │               |
|                      11-Deoxycorticosterone 11-Deoxycortisol            ▼               |
|                             │                     │             VIRILIZATION /          |
|                             ▼                     ▼             AMBIGUOUS GENITALIA     |
|                      [ Aldosterone ]        [ Cortisol ]        (Females 46,XX)         |
|                        DEFICIENCY            DEFICIENCY                 │               |
|                             │                     │                     │               |
|                             ▼                     ▼                     │               |
|                      SALT-WASTING CRISIS   Loss of Pituitary            │               |
|                      - Hyponatremia        Feedback -> MASSIVE          │               |
|                      - Hyperkalemia        ACTH HYPERSECRETION          │               |
|                      - Hypotension / Shock        │                     │               |
|                                                   └─────────────────────┘               |
|                                          ACTH drives substrate accumulation behind      |
|                                          block; massive shunting into androgens!        |
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• Clinical Subtypes:
  1. Classic Salt-Wasting CAH (Severe Complete Deficiency): Complete loss of enzyme function. Inability to synthesize aldosterone produces life-threatening neonatal adrenal crisis at 1 to 3 weeks of life (severe hyponatremia, hyperkalemia, hypovolemia, shock). Excessive prenatal androgens cause virilization in 46,XX female infants (ambiguous genitalia: clitoromegaly, labioscrotal fusion, urogenital sinus); 46,XY males have normal external genitalia but develop salt-wasting crisis.
  2. Classic Simple Virilizing CAH: Partial enzyme activity (1-2%). Sufficient aldosterone prevents neonatal salt wasting, but prenatal androgen excess virilizes female genitalia and causes precocious pseudopuberty in males.
  3. Non-Classic CAH (Late-Onset): Mild enzyme deficiency (20-50%). Presents in adolescence or adulthood with hirsutism, severe acne, oligomenorrhea, and polycystic ovarian morphology, mimicking PCOS.
• Laboratory Diagnosis: Diagnostic gold standard is a markedly elevated basal serum 17-hydroxyprogesterone (17-OHP) ($>1,000\text{ ng/dL}$ or $>30\text{ nmol/L}$; normal neonatal <100-200 ng/dL), or dramatic accumulation following cosyntropin stimulation. Mandatory in universal newborn dried blood spot screening using tandem mass spectrometry (LC-MS/MS).

Adrenal Medulla and Catecholamines

The adrenal medulla constitutes the neuroendocrine core of the adrenal gland, derived embryologically from ectodermal neural crest cells that differentiate into modified post-ganglionic sympathetic chromaffin cells.

Catecholamine Biosynthetic Pathway

Chromaffin cells synthesize catecholamines from the amino acid L-tyrosine: $\text{L-Tyrosine} \xrightarrow{\text{Tyrosine Hydroxylase (Rate-Limiting)}} \text{L-DOPA} \xrightarrow{\text{Aromatic L-Amino Acid Decarboxylase}} \text{Dopamine} \xrightarrow{\text{Dopamine }\beta\text{-Hydroxylase}} \text{Norepinephrine} \xrightarrow{\text{PNMT}} \text{Epinephrine}$

• Phenylethanolamine N-Methyltransferase (PNMT): Catalyzes the N-methylation of norepinephrine to epinephrine. PNMT expression is induced specifically by high local concentrations of cortisol delivered directly from the adrenal cortex via the intra-adrenal portal vascular system. Consequently, the adrenal medulla synthesizes predominantly epinephrine (~80%) and norepinephrine (~20%).

Catecholamine Catabolism and Metabolites

Free circulating catecholamines have brief plasma half-lives (1 to 2 minutes), undergoing sequential enzymatic degradation by Catechol-O-Methyltransferase (COMT) (expressed in vascular endothelium, liver, and chromaffin granules) and Monoamine Oxidase (MAO) (mitochondrial enzyme in liver and kidneys):

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|                        Catecholamine Metabolic Degradation Pathways                     |
+-----------------------------------------------------------------------------------------+
|                                                                                         |
|    [ Epinephrine ] ────────────> [ Metanephrine ] ─────────────> [ Vanillylmandelic ]   |
|          │            (COMT)            │             (MAO)       [   Acid (VMA)   ]   |
|          │ (MAO)                        │ (Sulfation / Glucuronidation) │               |
|          ▼                              ▼                               ▼               |
|   [ Dihydroxymandelic Acid ] ──> [ Excreted Metanephrines ]     [ Urinary VMA ]         |
|          ▲                              ▲                               ▲               |
|          │ (MAO)                        │                               │               |
|          │            (COMT)            │             (MAO)             │               |
|   [ Norepinephrine ] ──────────> [ Normetanephrine ] ───────────────────┘               |
|                                                                                         |
|    [ Dopamine ] ───────────────> [ Methoxytyramine ] ──────────> [ Homovanillic  ]      |
|                 (COMT / MAO)                     (MAO / COMT)    [   Acid (HVA)  ]      |
+-----------------------------------------------------------------------------------------+

Pheochromocytoma

• Definition: A catecholamine-secreting tumor of adrenal medullary chromaffin cells (10% of cases arise from extra-adrenal sympathetic paraganglia, termed paragangliomas). Classically follows the "Rule of 10s": 10% bilateral, 10% extra-adrenal, 10% malignant, 10% familial (associated with MEN 2A/2B, Von Hippel-Lindau [VHL], and Neurofibromatosis type 1 [NF1]).
• Clinical Presentation: Severe paroxysmal or sustained hypertension associated with the classic triad of: severe episodic headache, profuse generalized diaphoresis, and tachycardia / palpitations.
• Diagnostic Laboratory Testing:
  • Plasma Free Fractionated Metanephrines: Blood drawn resting in a supine position for 20-30 minutes. Diagnostic sensitivity exceeds 97-99%. Chromaffin tumor cells contain high levels of membrane-bound COMT that continuously metabolize catecholamines into metanephrine and normetanephrine inside the tumor, independent of paroxysmal exocytic catecholamine release. Thus, free metanephrines are continuously elevated even when patient is normotensive.
  • 24-Hour Urinary Fractionated Metanephrines: High clinical specificity (95-98%) and sensitivity (95%). Measured via liquid chromatography-tandem mass spectrometry (LC-MS/MS).
  • Pitfall of Parent Catecholamines and VMA: Parent epinephrine and norepinephrine have episodic bursts, yielding false-negative rates up to 20-30%. Urinary VMA has lower sensitivity (~65-75%) and is prone to dietary interferences (vanilla, coffee, tea, chocolate).
• Clonidine Suppression Test: Clonidine is a centrally acting $\alpha_2$-adrenergic agonist that reduces sympathetic outflow in essential hypertension. In normal individuals, plasma normetanephrine suppresses by $>50%$; autonomous pheochromocytomas fail to suppress.

Pediatric Neuroblastoma

• Pathophysiology: Malignant embryonal neoplasm of primitive neural crest neuroblasts occurring predominantly in infants and children younger than 5 years. Arises in the adrenal medulla (40%) or paraspinal retroperitoneal sympathetic chain.
• Biochemical Biomarkers: Neuroblasts lack mature dopamine $\beta$-hydroxylase and PNMT; consequently, tumors overproduce dopamine and norepinephrine metabolites.
• Diagnostic Gold Standard: Quantitative measurement of 24-Hour Urinary Homovanillic Acid (HVA) (dopamine metabolite) and Vanillylmandelic Acid (VMA) (norepinephrine metabolite) using LC-MS/MS or HPLC with electrochemical detection (HPLC-ECD). Urinary HVA and VMA are elevated in $>90-95%$ of neuroblastoma patients. A higher HVA/VMA ratio indicates immature, undifferentiated tumor histology and poorer clinical prognosis.