Hyponatremia

Background

Algorithm for Hyponatremia

• Defined as sodium concentration <135meq/L
• Patients often not symptomatic until <120meq/L although this level varies by patients and may be higher if the change occurred abruptly

Clinical Features

• Nausea and Vomiting
• Anorexia
• Muscle cramps
• AMS
• Seizure (esp if Na < 113)
• Coma
• Rapid correction can cause CHF & CPM (AMS, dysphagia, dysarthria, paresis)

Types and Causes

• Often described in terms of tonicity and volume status of the patient

Hypertonic Hyponatremia

• Defined as osmolarity > 295mmol/L with the following causes:

1. Hyperglycemia
   • Traditional teaching: [Na+] decreases by 1.6-1.8mEq/L for each 100mg/dL increase in glucose over 100mg/dL
   • 2.4mEq/L may be a more accurate correction factor (Hillier 1999)
2. Mannitol excess

Isotonic (pseudo) hyponatremia

• Defined as osmolarity > 275-295mmol/L. Often referred to as pseudo hyponatremia because the elevated lipids or proteins interfere with the laboratory sodium reading. The following are common causes:

1. Hyperlipidemia
2. Hyperproteinemia

Hypotonic Hyponatremia

• Defined as an osmolarity < 275 mmol/L and categorized as hypovolemic, hypervolemic or euvolemic

Hypovolemic

1. Renal Causes
   1. Thiazide diuretic use
   2. Na-wasting nephroathy (RTA, CRF)
   3. Osmotic diuresis (glucose, urea)
   4. Aldosterone deficiency
2. Extra-renal Causes
   1. GI loss
   2. 3rd space loss
      • Burns
      • Pancreatitis
      • Peritonitis

Hypervolemic

1. Urinary Na >20
   • Renal failure
2. Urinary Na <20
   • Nephrotic syndrome
   • Cirrhosis
   • CHF

Euvolemic

1. SIADH
   • urine sodium is greater than 20-40 mEq/L
2. Pain, stress, nausea
3. Hypothyroidism
4. Drugs
   • NSAIDs, sulfonylureas
5. H₂0 intoxication
6. Glucocorticoid deficiency

Diagnosis

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

Work-Up

Prior to giving treatment

• Urine
  • UA
  • Urine electrolytes
  • Urine urea
  • urine uric acid
  • urine osmolality
  • urine creatinine
• Serum
  • Chemistry
  • Serum osmolality
  • Uric acid
  • TSH
  • Cortisol

Treatment

Hypertonic hyponatremia

• Correct underlying disorder which is often hyperglycemia
• Often volume depleted due to osmotic diuresis and normal saline provides adequate volume repletion

Isotonic (pseudo) hyponatremia

• No treatment needed

Hypotonic hyponatremia

1. Hypovolemic
   • Give NS but be cautious of total daily repletion of Na so you avoid causing central pontine demylinosis
2. Euvolemic
   • Water restrict
   • Treat underlying cause
3. Hypervolemic
   • Water restriction
   • Diuresis
   • Treat underlying cause

Na Therapy

• Max correction 10mEq/L in 24hr (avoids central pontine demylinosis)

Sodium Containing fluid Concentrations

Fluid type	Sodium Concentration
1/2 Normal Saline	77 mEq/L
Normal Saline	154 mEq/L
Lactated Ringers	130 mEq/L
3% Saline	513 mEq/L

Symptomatic

• Fluid restrict

Asymptomatic

• Step 1: Calculate total body water
  • TBW(kg) = Wt(kg) x 0.6 = [Wt(lb) x 0.45] x 0.6 = Wt(lb) x 0.27
• 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

Disposition

• Admit if symptomatic or if Na <125mEq/L

See Also

• Electrolyte Abnormalities (Main)