Hyponatremia

Background

• Urine Na only useful before beginning tx
• Low = <135meq/L
• Symptomatic = <120meq/L (may be higher if occurs abruptly)

Clinical Features

• N/V
• Anorexia
• Muscle cramps
• AMS
• Sz (esp if Na < 113)
• Coma
• Rapid correction can cause CHF & CPM (AMS, dysphagia, dysarthria, paresis)

Diagnosis

• Must determine volume status and calculated osm
  • In true hyponatremia the osm is reduced

Work-Up

Prior to giving treatment

1. Urine
   1. UA
   2. Urine electrolytes
   3. Urine urea
   4. urine uric acid
   5. urine osmolality
   6. urine creatinine
2. Serum
   1. Chemistry
   2. Serum osmolality
   3. Uric acid
   4. TSH
   5. Cortisol

DDX

1. Hypertonic hyponatremia (osm > 295)
   1. Hyperglycemia
   2. Mannitol excess
2. Isotonic (pseudo) hyponatremia (osm 275-295)
   1. Hyperlipidemia
   2. Hyperproteinemia
3. Hypotonic hyponatremia (osm < 275)
   1. Hypovolemic
      1. Renal
         1. Thiazide diuretic use
         2. Na-wasting nephroathy (RTA, CRF)
         3. Osmotic diuresis (glucose, urea)
         4. Aldosterone deficiency
      2. Extra-renal
         1. GI loss
         2. 3rd space loss
            1. Burns
            2. Pancreatitis
            3. Peritonitis
   2. Hypervolemic
      1. Urinary Na > 20
         1. Renal failure
      2. Urinary Na < 20
         1. CHF
         2. Nephrotic syndrome
         3. Cirrhosis
   3. Euvolemic (urine Na usually > 20)
      1. SIADH
         1. Pain, stress, nausea
      2. Hypothyroidism
      3. Drugs
         1. NSAIDs, sulfonylureas
      4. H20 intoxication
      5. Glucocorticoid deficiency

Treatment

• 1. Hypertonic hyponatremia
  • Correct underlying disorder
  • Often volume depleted (give NS)
• 2. Isotonic (pseudo) hyponatremia
  • No tx needed
• 3. Hypotonic hyponatremia
  • A. Hypovolemic
    • Give NS (see below)
  • B. Euvolemic
    • Water restrict
    • Treat underlying cause
  • C. Hypervolemic
    • Water restriction
    • Diuresis
    • Treat underlying cause

Na Therapy

Background

• Rule of 6's:
  • Sodium correction should be no more than 6 meq/L in initial 24hr
  • Sodium correction should be no more than 6 meq/L in initial 6hr for CNS symptoms

(although max correction 10mEq/L in 24hrs and/or 18mEq/L in 48hrs)

• NS = 154 meq/L
• 3% NS = 513 meq/L
  • 100cc raises Na by ~2 meq/L
• Na Deficit (meq) = wt x 0.6 x (140 - Na)
• Na Administered = (Desired Na - Measured Na)(0.6)(wt)
  • Example
    • Measured Na = 100
    • Desired Na = 120
      • (120-100)(.6)(70kg)= 840 meq
        • if 513meq in 1L 3% then 840 meq in 1.6L
          • Correct over 24hr so 68cc hypertonic Na /hr for 24 hr

Treatment

• Asymptomatic
  • Replete with NS
• Severe (<120) or CNS dysfunction
  • 3% NS bolus
    • 100cc over 10min; repeat after 10min x1 if no improvement (consider adding benzos)
  • Then, foley, fluid restrict and do nothing more (avoids risk of osmotic myelinolysis)
    • If pt hypotensive from hypovolemia (IVC collapse on US) consider 1L NS
  • CT brain if no improvement
  • Hypokalemia
    • Be careful when correcting as Na may increase quickly d/t Na/K exchange, q1hr chem
• Over-correction of sodium (concern for CPM)
  • Consider DDAVP/desmopressin
    • Turns off dilute urine production, more water retained = Na stabilizes or slight drop
    • 1-2 mcg IV or SC x 1

Disposition

• Na <125
  • Admit

Source

• Tintinalli
• Pontine and extrapontine myelinoslysis: a neurologic disorder following rapid correction of hyponatremia Medicine/ 1993;72(6):359-373
• emcrit.org (http://emcrit.org/podcasts/hyponatremia/)
• Review by Schrier (Curr Opin Crit Care 2008;14:627)
• Review of Drug-Induced Hyponatremia (Am J Kidney Dis 2008;52:144)
• Understanding Lab Testing for Hyponatremia (Clin J Am Soc Nephrol 2008;3:1175)
• The hyponatremia formulas do not work so well (Clin J Am Soc Nephrol 2007;2:1110 and Nephrol Dial Transplant 2006;21:1564)