Hyponatremia with Hypotension — Differential Diagnosis

Step 1 — Exclude Pseudohyponatremia First

Step 2 — Confirm Hypo-osmolality (Serum Osm < 275 mOsm/kg)

Step 3 — Volume Status + Urine Indices

Key rule: UNa < 30 mEq/L argues for low effective arterial blood volume; UNa > 30 mEq/L (outside diuretic use/CKD) suggests euvolemia. — Barash Clinical Anesthesiology, p. 1197; Harrison's 22e

Category A — Hypovolemic Hyponatremia (Most direct cause of hypotension)

Extrarenal Losses (UNa < 10 mEq/L)

Renal Losses (UNa > 20 mEq/L)

Adrenal insufficiency is critical not to miss: primary (Addison's) gives hyponatremia + hyperkalemia + hypotension + hyperpigmentation. Secondary (pituitary failure) gives hyponatremia without hyperkalemia (mineralocorticoid intact), and without hyperpigmentation. Both can present with profound hypotension. Check morning cortisol and ACTH stimulation test. — Harrison's 22e; Morgan & Mikhail Clinical Anesthesiology 7e

Category B — Hypervolemic Hyponatremia (Hypotension from reduced effective arterial volume, not true volume depletion)

Category C — Euvolemic Hyponatremia (Hypotension usually absent or mild; but relevant to exclude)

Key Diagnostic Labs

High-Yield Summary: Hyponatremia + Hypotension

Hyponatremia + Hypotension
        │
        ├─► Hyperkalemia? → Primary Adrenal Insufficiency (Addison's)
        │
        ├─► Edema? → Hypervolemic (CHF, Cirrhosis, Nephrosis, CKD)
        │       U_Na usually < 10 in decompensated states
        │
        ├─► No edema, U_Na < 10 → Extrarenal losses
        │       (vomiting, diarrhea, third-spacing)
        │
        ├─► No edema, U_Na > 20 → Renal losses
        │       Diuretics, Adrenal Insufficiency, Salt-losing nephropathy,
        │       Cerebral Salt Wasting (CNS injury context),
        │       RTA, Osmotic diuresis
        │
        └─► Appears euvolemic, U_Na > 20 → Adrenal insufficiency (2°),
                Severe hypothyroidism, or Severe SIADH with vasomotor instability

Hyponatremia + Hypotension — Differential Diagnosis

Framework: Volume Status + Urine Sodium

Priority Diagnoses Not to Miss

1. Primary Adrenal Insufficiency (Addison's Disease)

• Mechanism: ↓ aldosterone → renal Na wasting + volume depletion; ↓ cortisol → vasodilation
• Key triad: hyponatremia + hyperkalemia + hypotension
• Also: hyperpigmentation, eosinophilia, hypoglycemia, nausea/vomiting, fatigue
• Labs: ↑ ACTH, ↓ morning cortisol, failed ACTH stimulation test
• Treat: IV hydrocortisone + normal saline immediately if suspected

2. Secondary Adrenal Insufficiency (Pituitary/Hypothalamic)

• Mechanism: ↓ cortisol only (aldosterone preserved) → no hyperkalemia
• Presentation: pale, loss of axillary/pubic hair, ↓ libido, hypotension without hyperpigmentation
• Hyponatremia can mimic SIADH — always exclude before diagnosing SIADH

3. Cerebral Salt Wasting (CSW)

• Setting: subarachnoid hemorrhage, TBI, intracranial surgery (first 10 days)
• Mechanism: renal Na wasting → ECF volume depletion → ADH release → hyponatremia
• Hypovolemic (key distinction from SIADH which is euvolemic)
• Labs: high UNa, high UOsm, elevated hematocrit, hypotension
• Treat: isotonic or hypertonic saline + fludrocortisone (not fluid restriction)

Hypovolemic Causes (True Volume Depletion)

• GI: vomiting, diarrhea, GI suction
• Third spacing: pancreatitis, bowel obstruction, burns, rhabdomyolysis
• Skin: excessive sweating

• Thiazide diuretics (most common cause of severe hyponatremia overall)
• Mineralocorticoid deficiency / Addison's
• Salt-losing nephropathy (interstitial nephritis, polycystic kidneys)
• Osmotic diuresis (glucose, mannitol)
• Renal tubular acidosis
• Cerebral salt wasting

Hypervolemic Causes (Low Effective Arterial Volume)

Euvolemic Causes with Vasomotor Instability

• Secondary adrenal insufficiency — glucocorticoid deficiency; no hyperkalemia
• Myxedema / severe hypothyroidism — bradycardia, hypothermia, dry skin, slow reflexes; confirm with TSH/fT4
• Severe SIADH (Na < 115 mEq/L) — hyponatremia itself causes widened QRS, ventricular ectopy, and vasomotor instability at extreme levels

Diagnostic Workup at a Glance

Decision Algorithm

Hyponatremia + Hypotension
        │
        ├─ Hyperkalemia? ──────────────────► Primary Adrenal Insufficiency
        │
        ├─ Edema present? ─────────────────► Hypervolemic (CHF / Cirrhosis / Nephrosis / CKD)
        │                                     U_Na < 10 (decompensated)
        │
        ├─ No edema, U_Na < 10 ────────────► Extrarenal losses (GI, burns, third-spacing)
        │
        ├─ No edema, U_Na > 20 ────────────► Renal losses:
        │                                     Thiazides | Adrenal insufficiency
        │                                     CSW (CNS setting) | Salt-losing nephropathy
        │                                     Osmotic diuresis | RTA
        │
        └─ Appears euvolemic, U_Na > 20 ───► 2° Adrenal insufficiency
                                             Hypothyroidism
                                             Severe SIADH (Na < 115)

Clinical pearl: Adrenal insufficiency is the single most dangerous diagnosis to miss. It can present with UNa > 20 and euvolemic appearance (secondary type) or overt hypovolemia with hyperkalemia (primary type). When in doubt, draw cortisol and give empirical hydrocortisone 100 mg IV — it is safe and potentially life-saving.