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
• Seizure (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
      1. [Na+] decreases by 1.6-1.8mEq/L for each 100mg/dL increase in glucose
   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

• Max correction 10mEq/L in 24hr (avoids CPM)
• NS = 154 meq/L
• 3% NS = 513 meq/L
• each 100 ml will raise sodium by ~2 mmol/l

Asymptomatic

• Step 1: Calculate total body water
  • TBW = Wt(kg) x 0.6
• Step 2: Calculate mEq deficit
  • (Desired Na - Measured Na) ~ must be ≤ 10
• Step 3: Calculate NS rate to be given over 24hr
  • NS rate (cc/hr) = TBW x mEq deficit x 0.27
    • If using 3% NS (to avoid volume overload) divide above rate by 3.33

Symptomatic

• 3% NS 100cc bolus over 10min; repeat after 10min x1 if no improvement
  • Each 100 ml will raise sodium by ~2 mmol/l
• Fluid restrict

Disposition

• Admit if Na <125

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)