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)

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

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. Renal Failure
   • Urinary Na < 20
2. CHF
   • Nephrotic Syndrome
3. Cirrhosis

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

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

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

Symptomatic

• Fluid restrict

Disposition

• Admit if Na <125

See Also

• Electrolyte Abnormalities (Main)

Sources

1. Hillier TA, Abbott RD, Barrett EJ. Hyponatremia: evaluating the correction

factor for hyperglycemia. Am J Med. 1999 Apr;106(4):399-403. PubMed PMID: 10225241.