9.6 Hyponatremia, Hypernatremia & Water Balance

Fluid, electrolyte, acid-base disorders, and nephrolithiasis represent some of the most analytically rigorous topics on the ABIM Internal Medicine board examination. Clinical competence requires a systematic, formula-driven approach to deciphering mixed acid-base disorders, navigating strict correction speed limits in dysnatremias to prevent catastrophic neurologic injury, managing hyperkalemic emergencies, and formulating evidence-based stone prevention regimens.

1. Stepwise Diagnostic Approach to Hyponatremia

Hyponatremia (Serum Sodium < 135 mEq/L) represents a relative excess of water in relation to sodium. Diagnostic evaluation must follow a strict three-step sequence:

Step 1: Determine Serum Osmolality

• Hypertonic Hyponatremia (Serum Osm > 295 mOsm/kg): Caused by unmeasured effective osmoles drawing intracellular water into the extracellular space. Most commonly severe hyperglycemia or IV mannitol. $\text{Corrected Sodium} = \text{Measured Na} + 1.6 \times \left(\frac{\text{Serum Glucose} - 100}{100}\right)$ (Note: For glucose >400 mg/dL, use a correction factor of 2.0 or 2.4).
• Isotonic Hyponatremia / Pseudohyponatremia (Serum Osm 275–295 mOsm/kg): Laboratory artifact caused by excessive lipids or proteins displacing the aqueous phase of plasma when measured by indirect ion-selective electrodes. Seen in severe hypertriglyceridemia (triglycerides >1500 mg/dL) or paraproteinemia / multiple myeloma.
• Hypotonic Hyponatremia (Serum Osm < 275 mOsm/kg): True hypotonic hyponatremia; proceed to Step 2.

Step 2: Assess Clinical Extracellular Fluid (ECF) Volume Status

1. Hypovolemic Hypotonic Hyponatremia (Total Body Na Loss > Total Body Water Loss):

   • Physical Exam: Orthostatic hypotension, tachycardia, dry mucous membranes, decreased skin turgor, flat neck veins.
   • Urine Sodium < 20 mEq/L (FeNa < 1%): Extrarenal sodium losses (vomiting, diarrhea, third-spacing, burns).
   • Urine Sodium > 20 mEq/L (FeNa > 1%): Renal sodium losses (thiazide diuretics, cerebral salt wasting, mineralocorticoid deficiency / primary adrenal insufficiency [Addison disease with high K+ and low cortisol]).
   • Treatment: Volume resuscitation with 0.9% Isotonic Saline.

2. Euvolemic Hypotonic Hyponatremia (Normal Total Body Na, Moderately Increased Water):

   • Physical Exam: No peripheral edema, no ascites, normal JVP, normal BP.
   • Syndrome of Inappropriate ADH Secretion (SIADH):
     • Diagnostic Criteria: Hypotonic hyponatremia, high urine osmolality (Urine Osm > 100 mOsm/kg, typically >300), high urine sodium (Urine Na > 40 mEq/L), low serum uric acid (<4.0 mg/dL), low BUN (<10 mg/dL), and normal thyroid/adrenal/renal function.
     • Etiologies: Malignancies (Small Cell Lung Cancer [SCLC], head/neck tumors), CNS pathology (stroke, subdural hematoma, meningitis), Pulmonary disease (pneumonia, TB), Drugs (SSRIs, SNRIs, Carbamazepine, Oxcarbazepine, Cyclophosphamide, NSAIDs).
     • Treatment: Fluid restriction (<800–1000 mL/day) is first-line; oral salt tablets + low-dose loop diuretic (Furosemide); oral urea; or Vasopressin V2 Receptor Antagonists (Vaptans: Tolvaptan 15–30 mg daily) for refractory euvolemic hyponatremia (avoid fluid restriction during vaptan therapy; do not exceed 30 days due to hepatotoxicity risk).
   • Low Solute Intake Syndromes (Urine Osm < 100 mOsm/kg): Psychogenic Polydipsia (massive water intake overwhelming normal renal diluting capacity), Beer Potomania, "Tea-and-Toast" diet.
   • Endocrine: Severe hypothyroidism, Secondary adrenal insufficiency (ACTH deficiency -> cortisol deficiency impairs free water excretion without mineralocorticoid loss).

3. Hypervolemic Hypotonic Hyponatremia (Total Body Na Excess < Total Body Water Excess):

   • Physical Exam: Peripheral pitting edema, ascites, elevated JVP, pulmonary crackles.
   • Urine Sodium < 20 mEq/L: Congestive Heart Failure, Cirrhosis, Nephrotic Syndrome (decreased effective arterial blood volume triggers non-osmotic ADH release and RAAS activation).
   • Urine Sodium > 20 mEq/L: Advanced Acute Kidney Injury or ESKD.
   • Treatment: Dietary sodium and fluid restriction, loop diuretics, neurohormonal blockade.

2. Emergency Management of Dysnatremias & Osmotic Demyelination Syndrome

Critical Correction Speed Limits in Hyponatremia

• Acute Symptomatic Hyponatremia (Duration < 48 hours with seizures, coma, respiratory arrest, herniation):
  • Administer 3% Hypertonic Saline (513 mEq/L) 100–150 mL IV bolus over 10 to 20 minutes.
  • May repeat up to 2 times PRN to rapidly elevate serum sodium by 4 to 6 mEq/L within 1 to 2 hours, which rapidly reduces cerebral edema and halts active herniation.
• Chronic Hyponatremia (Duration > 48 hours or unknown):
  • In chronic hyponatremia, brain cells extrude organic osmolytes (myo-inositol, taurine, glutamate) to adapt to hypotonicity. Rapid sodium correction causes sudden hypertonic shrinkage of brain parenchyma.
  • STRICT MAXIMUM LIMIT: <= 8 mEq/L total increase within 24 hours (and <= 14–16 mEq/L in 48 hours).
  • High-Risk Patients: In patients with advanced cirrhosis, chronic alcoholism, malnutrition, hypokalemia, or baseline sodium < 105 mEq/L, target 4 to 6 mEq/L per 24 hours (never exceed 8 mEq/L).

Osmotic Demyelination Syndrome (ODS / Central Pontine Myelinolysis)

• Pathophysiology: Overly rapid correction (>8–10 mEq/L/24h) strips water from oligodendrocytes, triggering apoptosis and demyelination of the central pontine white matter tracts and extrapontine basal ganglia.
• Clinical Presentation: Classically biphasic. The patient initial improves as sodium rises, but 2 to 6 days later develops progressive dysarthria, dysphagia, horizontal gaze paralysis, spastic quadriparesis, encephalopathy, and irreversible "Locked-in" Syndrome.
• Rescue Therapy for Overcorrection: If the sodium rises by >8 mEq/L in 24 hours, immediately stop all saline infusions, administer IV Dextrose 5% in Water (D5W) at 3 mL/kg/h, and administer Subcutaneous Desmopressin (DDAVP) 1 to 2 µg every 6 to 8 hours to arrest free water diuresis and safely re-lower serum sodium back within target limits.