What are the steps for diagnosing and managing hyponatremia (low sodium levels)?

Diagnosis and Management of Hyponatremia

The diagnosis of hyponatremia requires assessment of volume status, measurement of serum and urine osmolality, and urine sodium concentration to determine the underlying cause and guide appropriate treatment. 1

Initial Diagnostic Steps

1. Define Severity of Hyponatremia

• Mild: 130-134 mEq/L
• Moderate: 125-129 mEq/L
• Severe: <125 mEq/L 2

2. Assess Symptoms

• Mild symptoms: Weakness, confusion, headache, nausea
• Severe symptoms: Delirium, obtundation, coma, seizures, cardiorespiratory distress 1

3. Determine Volume Status

Categorize patients into one of three volume states:

Hypovolemic Hyponatremia

• Clinical signs: Orthostatic hypotension, tachycardia, dry mucous membranes
• Laboratory findings: Urine Na <20 mEq/L (unless renal salt wasting)
• Common causes: Diuretics, vomiting, diarrhea, third-spacing

Euvolemic Hyponatremia

• Clinical signs: No edema, no orthostasis
• Laboratory findings: Urine Na >20 mEq/L, urine osmolality >100 mOsm/kg
• Common causes: SIADH, medications, hypothyroidism, adrenal insufficiency

Hypervolemic Hyponatremia

• Clinical signs: Edema, ascites
• Laboratory findings: Urine Na <20 mEq/L (except renal failure)
• Common causes: Heart failure, cirrhosis, nephrotic syndrome 2, 3

4. Laboratory Evaluation

• Serum sodium
• Serum osmolality
• Urine osmolality
• Urine sodium concentration
• Thyroid function tests
• Adrenal function assessment
• Fractional excretion of urate (can improve diagnostic accuracy for SIADH to 95%) 1

Specific Diagnostic Criteria for SIADH

SIADH diagnosis requires all of the following:

• Hyponatremia (serum sodium <134 mEq/L)
• Hypoosmolality (plasma osmolality <275 mOsm/kg)
• Inappropriately high urine osmolality (>500 mOsm/kg)
• Inappropriately high urinary sodium concentration (>20 mEq/L)
• Absence of hypothyroidism, adrenal insufficiency, or volume depletion 1

Additional findings supporting SIADH:

• Urine osmolality >300 mOsm/kg
• Urinary sodium >40 mEq/L
• Serum uric acid <4 mg/dL 1

Management Algorithm

For Severely Symptomatic Hyponatremia (Medical Emergency)

1. Administer 3% hypertonic saline IV
2. Target increase: 4-6 mEq/L within 1-2 hours
3. Do not exceed correction of 10 mEq/L in first 24 hours 2
4. Monitor serum sodium frequently (every 2-4 hours)

For Hypovolemic Hyponatremia

1. Discontinue diuretics if applicable
2. Administer normal saline (0.9% NaCl) 3
3. Treat underlying cause (e.g., vomiting, diarrhea)

For Euvolemic Hyponatremia (including SIADH)

1. Fluid restriction (<1 L/day) as first-line treatment 1
2. Consider salt tablets or urea in persistent cases 2
3. For refractory cases, consider vasopressin receptor antagonists (vaptans) 1

For Hypervolemic Hyponatremia

1. Fluid restriction (1-1.5 L/day) 1
2. Sodium restriction (5-6.5 g/day) 1
3. Diuretic therapy:
   • For cirrhosis: Spironolactone (starting 100 mg, max 400 mg) with or without furosemide (starting 40 mg, max 160 mg) 1
4. For severe symptomatic hyponatremia: Hypertonic saline (3%) with caution 1

Special Considerations

Rate of Correction

• Acute hyponatremia (<48 hours): Can correct more rapidly
• Chronic hyponatremia: Limit correction to <10 mEq/L in first 24 hours to prevent osmotic demyelination syndrome 1, 2

Hyponatremia in Cirrhosis

• Indicates poor prognosis and increased mortality
• Associated with increased risk of hepatic encephalopathy, hepatorenal syndrome, and spontaneous bacterial peritonitis 1
• Treatment approach:
  1. Fluid restriction to 1,000 mL/day for moderate hyponatremia (120-125 mEq/L)
  2. More severe fluid restriction plus albumin infusion for severe hyponatremia (<120 mEq/L) 1

Pitfalls to Avoid

1. Overly rapid correction of chronic hyponatremia can lead to osmotic demyelination syndrome, which can cause parkinsonism, quadriparesis, or death 2
2. Failure to recognize pseudohyponatremia caused by hyperlipidemia or hyperproteinemia 4
3. Misdiagnosis of volume status, which can lead to inappropriate treatment
4. Overlooking medication-induced hyponatremia (e.g., SSRIs, carbamazepine) 1

By following this systematic approach to diagnosis and management, clinicians can effectively identify the cause of hyponatremia and implement appropriate treatment strategies to improve outcomes and reduce mortality.