What is the appropriate workup and treatment for a patient with hyponatremia (low sodium levels)?

Hyponatremia Workup Based on Serum Osmolality

Begin by measuring serum osmolality to classify the hyponatremia, as this immediately narrows the differential and guides subsequent testing. 1

Initial Laboratory Assessment

Obtain the following tests immediately:

• Serum osmolality to exclude pseudohyponatremia and classify the disorder 1, 2
• Serum glucose to identify hyperglycemia-induced pseudohyponatremia (add 1.6 mEq/L to sodium for each 100 mg/dL glucose >100 mg/dL) 1
• Urine osmolality and urine sodium concentration to determine water excretion capacity and differentiate causes 1, 3
• Serum creatinine and BUN to assess renal function 1
• Thyroid-stimulating hormone (TSH) to rule out hypothyroidism 1
• Serum uric acid (<4 mg/dL has 73-100% positive predictive value for SIADH) 1

Volume Status Assessment

Physical examination alone has poor accuracy (sensitivity 41.1%, specificity 80%) for volume assessment, so supplement with laboratory findings. 1

Hypovolemic Signs

• Orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins 1
• Urine sodium <30 mmol/L suggests extrarenal losses (GI losses, burns, dehydration) with 71-100% positive predictive value for saline responsiveness 1, 3
• Urine sodium >20 mmol/L suggests renal losses (diuretics, salt-wasting nephropathy) 1

Euvolemic 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 supports SIADH 1, 2

Hypervolemic Signs

• Peripheral edema, ascites, jugular venous distention, pulmonary congestion 1
• Suggests heart failure, cirrhosis, or renal disease 4

Osmolality-Based Classification Algorithm

High Plasma Osmolality (>295 mOsm/kg)

• Hyperglycemia is the primary cause 1, 4
• Correct measured sodium using the formula above 1

Normal Plasma Osmolality (275-290 mOsm/kg)

• Pseudohyponatremia from hyperlipidemia or hyperproteinemia 1, 4
• Post-TURP syndrome 4

Low Plasma Osmolality (<275 mOsm/kg)

This is true hypotonic hyponatremia requiring further workup based on urine studies. 1, 5

Urine Osmolality <100 mOsm/kg

• Indicates appropriate ADH suppression 1
• Suggests primary polydipsia or reset osmostat 1

Urine Osmolality >100 mOsm/kg

• Indicates impaired water excretion 1
• Proceed to urine sodium measurement 1

Special Considerations for Neurosurgical Patients

In patients with CNS pathology, distinguish SIADH from cerebral salt wasting (CSW) as treatments are opposite. 1

SIADH Characteristics

• Normal to slightly elevated central venous pressure 1
• Urine sodium >20-40 mmol/L 1
• Urine osmolality >500 mOsm/kg 1
• Treatment: fluid restriction 1

Cerebral Salt Wasting Characteristics

• Low central venous pressure (<6 cm H₂O) 1
• Urine sodium >20 mmol/L despite volume depletion 1
• Clinical signs of hypovolemia 1
• Treatment: volume and sodium replacement 1

Common Diagnostic Pitfalls

• Failing to assess volume status accurately is the most common error in hyponatremia diagnosis 1
• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant, when it increases fall risk (21% vs 5%) and mortality (60-fold increase with sodium <130 mmol/L) 1, 2
• Not obtaining ADH and natriuretic peptide levels is appropriate, as these are not supported by evidence and should not delay treatment 1
• Misdiagnosing volume status in heart failure patients with hyponatremia can lead to inappropriate treatment 1

Medication Review

Review all medications, particularly:

• SSRIs, carbamazepine, NSAIDs, opioids 1
• Chemotherapy agents (platinum-based, vinca alkaloids) 1
• Diuretics (thiazides more commonly than loop diuretics) 1