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
• Generally accepted recommendations are to avoid correction of more than 12 mmol/L/day (0.5mmol/L/hr) to avoid central pontine myelinolysis. Faster correction (1-2mmol/L/hr) is acceptable with 3% hypertonic saline if the patient is seizing.
• Often described in terms of tonicity and volume status of the patient with the main types by tonicity being:
  • Hypertonic Hyponatremia
  • Isotonic (pseudo) hyponatremia
  • Hypotonic Hyponatremia

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)

Differential Diagnosis

Types by Tonicity

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

Hypotonic Types by Volume

Hypovolemic

Renal Causes

• Thiazide diuretic use
• Na-wasting nephroathy (RTA, CRF)
• Osmotic diuresis (glucose, urea)
• Aldosterone deficiency

Extra-renal Causes

• GI loss
• 3rd space loss
• Burns
• Pancreatitis
• Peritonitis

Hypervolemic

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

Euvolemic

• SIADH
  • urine sodium is greater than 20-40 mEq/L
• Pain, stress, nausea
• Psychogenic polydipsia
• Hypothyroidism
• Drugs
  • NSAIDs, sulfonylureas
• H₂0 intoxication
• Glucocorticoid deficiency

Pseudohyponatremia

• Hyperglycemia
  • Na+ drops 1.6 mEq/L for every 100 mg/dL increase in glucose over 100
• Displaced sodium in lab specimen
  • Hyperlipidemia
  • Hyperproteinemia

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 by Patient Status

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% sodium chloride (to avoid volume overload) divide above rate by 3.33

Treatment by Type of Hyponatremia

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 raising the serum sodium more than 12 mmol/L/day (0.5mmol/L/hr) and 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

Disposition

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

See Also

• Electrolyte Abnormalities (Main)