Initial Assessment for Hyponatremia (Sodium 128 mEq/L)
For a patient with a sodium level of 128 mEq/L, begin with immediate assessment of symptom severity and volume status, followed by serum and urine osmolality, urine sodium, and uric acid to determine the underlying cause. 1
Immediate Clinical Assessment
Symptom Severity Evaluation
- Assess for severe symptoms immediately: confusion, seizures, coma, somnolence, obtundation, or cardiorespiratory distress—these constitute a medical emergency requiring hypertonic saline 1, 2
- Look for moderate symptoms: nausea, vomiting, headache, weakness, lack of concentration, forgetfulness, apathy, or loss of balance 3, 4
- Even if asymptomatic, recognize that sodium 128 mEq/L increases fall risk (21% vs 5% in normonatremic patients) and is associated with cognitive impairment and gait disturbances 1, 2
Volume Status Assessment
- Examine for hypovolemia: orthostatic hypotension, tachycardia, dry mucous membranes, decreased skin turgor, sunken eyes, furrowed tongue, confusion, non-fluent speech, extremity weakness 1, 5
- Check for hypervolemia: jugular venous distention, peripheral edema, ascites, orthopnea, dyspnea 1, 5
- Euvolemia indicators: absence of both hypovolemic and hypervolemic signs, normal skin turgor, moist mucous membranes 1
Essential Laboratory Workup
Initial Blood Tests
- Serum osmolality to confirm hypotonic hyponatremia (most common) versus isotonic or hypertonic causes 1, 5
- Serum creatinine and BUN to assess renal function and volume status (elevated in hypovolemia) 1
- Serum uric acid: levels <4 mg/dL suggest SIADH with 73-100% positive predictive value 1
- Thyroid-stimulating hormone (TSH) to exclude hypothyroidism 1
- Cortisol level if adrenal insufficiency suspected 1
- Liver function tests and albumin if cirrhosis suspected 1
Critical Urine Studies
- Urine osmolality: >100 mOsm/kg suggests inappropriate ADH activity (SIADH or other causes); <100 mOsm/kg suggests primary polydipsia or reset osmostat 1, 5
- Urine sodium concentration:
- Spot urine sodium/potassium ratio: >1 correlates with 24-hour sodium excretion >78 mmol/day with ~90% accuracy 1
Determine Acuity and Duration
- Establish timing: acute (<48 hours) versus chronic (>48 hours) hyponatremia—this critically affects correction rate safety 1, 2
- Acute hyponatremia causes more severe symptoms and can be corrected more rapidly, while chronic hyponatremia requires slower correction to prevent osmotic demyelination syndrome 1, 4
Identify High-Risk Features
- Screen for osmotic demyelination risk factors: advanced liver disease, alcoholism, malnutrition, severe hyponatremia, hypophosphatemia, hypokalemia, prior encephalopathy—these patients require maximum correction of only 4-6 mmol/L per day 1
- In neurosurgical patients: distinguish between SIADH and cerebral salt wasting (CSW), as CSW is more common in this population and requires fundamentally different treatment (volume replacement vs. fluid restriction) 1
Common Diagnostic Pitfalls to Avoid
- Do not ignore mild hyponatremia (130-135 mmol/L) as clinically insignificant—it increases mortality 60-fold (11.2% vs 0.19%) and substantially increases fall risk 1
- Do not obtain ADH or natriuretic peptide levels—these are not supported by evidence and delay treatment 1
- Do not assume volume status without proper assessment—misdiagnosing volume status leads to inappropriate treatment (e.g., giving saline for SIADH worsens hyponatremia) 1
- In cirrhotic patients, recognize that hyponatremia <130 mmol/L increases risk of spontaneous bacterial peritonitis (OR 3.40), hepatorenal syndrome (OR 3.45), and hepatic encephalopathy (OR 2.36) 1