What is the initial step in managing a patient with hyponatremia?

Hyponatremia Workup

Initial Diagnostic Assessment

The first step in managing hyponatremia is to confirm true hypotonic hyponatremia and assess volume status through clinical examination, serum osmolality, and urine studies. 1

Step 1: Confirm True Hypotonic Hyponatremia

• Measure serum osmolality to exclude pseudohyponatremia (normal osmolality) or hyperglycemia-induced hyponatremia (high osmolality) 1, 2
• Correct sodium for hyperglycemia: add 1.6 mEq/L to measured sodium for each 100 mg/dL glucose above 100 mg/dL 1
• True hypotonic hyponatremia is defined as serum sodium <135 mEq/L with low serum osmolality (<275 mOsm/kg) 3, 4

Step 2: Assess Symptom Severity

Determine if the patient has severe symptoms requiring immediate intervention, as this dictates treatment urgency. 1

• Severe symptoms (requiring immediate 3% hypertonic saline): seizures, coma, somnolence, obtundation, cardiorespiratory distress, or altered mental status 1, 3, 2
• Mild symptoms: nausea, vomiting, headache, confusion, weakness 1, 4
• Asymptomatic: no neurological symptoms but may have chronic complications (falls, cognitive impairment, gait disturbances) 1, 3

Step 3: Measure Urine Osmolality

• Urine osmolality <100 mOsm/kg: indicates appropriate ADH suppression (primary polydipsia, reset osmostat) 1
• Urine osmolality >100 mOsm/kg: indicates impaired free water excretion due to elevated ADH or decreased solute intake 1, 4

Step 4: Assess Volume Status and Urine Sodium

Physical examination for volume status has limited accuracy (sensitivity 41%, specificity 80%), so combine clinical findings with urine sodium measurement. 1

Hypovolemic Hyponatremia (ECF depletion)

• Clinical signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins 1
• Urine sodium <30 mmol/L: extrarenal losses (vomiting, diarrhea, burns, third-spacing) 1, 2
• Urine sodium >20 mmol/L: renal losses (diuretics, salt-wasting nephropathy, adrenal insufficiency) 1, 2

Euvolemic Hyponatremia (normal ECF)

• Clinical signs: no edema, no orthostatic hypotension, normal skin turgor, moist mucous membranes 1
• Urine sodium >20-40 mmol/L with urine osmolality >300 mOsm/kg: SIADH 1, 4
• Serum uric acid <4 mg/dL: 73-100% positive predictive value for SIADH 1

Hypervolemic Hyponatremia (ECF expansion)

• Clinical signs: peripheral edema, ascites, jugular venous distention, pulmonary congestion 1
• Common causes: heart failure, cirrhosis, nephrotic syndrome 1, 5
• Urine sodium <30 mmol/L: typically seen in heart failure and cirrhosis 1

Step 5: Rule Out Endocrine Disorders and Renal Failure

• Thyroid-stimulating hormone (TSH): exclude hypothyroidism 1, 2
• Morning cortisol or ACTH stimulation test: exclude adrenal insufficiency 1, 2
• Serum creatinine and BUN: assess renal function 1, 2

Critical Diagnostic Pitfalls to Avoid

• Do not ignore mild hyponatremia (130-135 mmol/L): associated with increased falls (23.8% vs 16.4%), fractures, cognitive impairment, and 60-fold increase in mortality when <130 mmol/L 1, 3
• Do not rely solely on physical examination for volume assessment: supplement with urine sodium and osmolality 1
• In neurosurgical patients, distinguish SIADH from cerebral salt wasting (CSW): CSW shows true hypovolemia with CVP <6 cm H₂O and requires opposite treatment (volume replacement vs. fluid restriction) 1
• Do not obtain ADH or natriuretic peptide levels: not supported by evidence and delays treatment 1

Additional Useful Tests

• 24-hour urine sodium collection: helps assess total sodium excretion in cirrhotic patients with ascites 1
• Spot urine sodium/potassium ratio >1: correlates with 24-hour sodium excretion >78 mmol/day with 90% accuracy 1
• Complete blood count, liver function tests, lipid profile: part of comprehensive evaluation 1