A patient with serum sodium 119 mmol/L, chloride 86 mmol/L, white blood cell count 19,800/µL presenting with nocturnal polyuria and normal urinalysis who now has an episode of vomiting—what is the most likely diagnosis, which further investigations are indicated, and what is the appropriate treatment?

Diagnosis: Severe Hyponatremia with Leukocytosis – Likely SIADH vs. Cerebral Salt Wasting

This patient with Na 119 mmol/L, nocturia, vomiting, and leukocytosis (WBC 19,800) most likely has severe symptomatic hyponatremia requiring urgent evaluation and treatment with 3% hypertonic saline if neurological symptoms develop, while simultaneously investigating the underlying cause.

Immediate Diagnostic Workup

Obtain the following tests immediately to determine volume status and etiology:

• Serum osmolality to confirm hypotonic hyponatremia and exclude pseudohyponatremia 1
• Urine osmolality and urine sodium concentration – if urine osmolality >100 mOsm/kg with urine Na >20-40 mmol/L, this suggests SIADH 1
• Serum creatinine and BUN to assess renal function and calculate BUN:creatinine ratio (>20:1 suggests hypovolemia) 1
• Thyroid-stimulating hormone (TSH) to exclude hypothyroidism 1
• Morning cortisol level to rule out adrenal insufficiency 1
• Serum uric acid – if <4 mg/dL, this has 73-100% positive predictive value for SIADH 1
• Complete blood count to evaluate the leukocytosis (WBC 19,800) for infection or other causes 1

Assess extracellular fluid volume status through physical examination:

• Look for hypovolemic signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins 1
• Look for hypervolemic signs: peripheral edema, ascites, jugular venous distention 1
• Look for euvolemic state: absence of both hypovolemic and hypervolemic signs 1

Most Likely Diagnosis

The combination of severe hyponatremia (Na 119), nocturia, normal urinalysis, and vomiting suggests SIADH as the most likely diagnosis 1. The leukocytosis (19,800) raises concern for an underlying infection or malignancy triggering SIADH 2.

Alternative consideration: If the patient shows signs of true hypovolemia (orthostatic hypotension, tachycardia, CVP <6 cm H₂O), cerebral salt wasting must be considered, especially if there is any CNS pathology 1.

Treatment Algorithm Based on Symptom Severity

If Severe Symptoms Present (Seizures, Altered Mental Status, Coma)

Administer 3% hypertonic saline immediately:

• Give 100 mL boluses of 3% NaCl IV over 10 minutes, repeating up to three times at 10-minute intervals 1
• Target correction of 6 mmol/L over first 6 hours or until severe symptoms resolve 1
• Never exceed 8 mmol/L correction in any 24-hour period to prevent osmotic demyelination syndrome 1
• Check serum sodium every 2 hours during initial correction 1
• Admit to ICU for close monitoring 1

If Mild-to-Moderate Symptoms (Nausea, Vomiting, Headache)

Treatment depends on volume status:

For Euvolemic Hyponatremia (SIADH – Most Likely)

• Fluid restriction to 1 L/day is the cornerstone of treatment 1
• If no response to fluid restriction, add oral sodium chloride 100 mEq three times daily 1
• Avoid hypertonic saline unless severe symptoms develop 1
• Monitor serum sodium every 24 hours initially 1

For Hypovolemic Hyponatremia (If Volume Depletion Present)

• Administer isotonic saline (0.9% NaCl) for volume repletion at 15-20 mL/kg/h initially, then 4-14 mL/kg/h based on response 1
• Discontinue any diuretics 1
• Target correction rate of 4-8 mmol/L per day, maximum 8 mmol/L in 24 hours 1

For Hypervolemic Hyponatremia (If Edema/Ascites Present)

• Fluid restriction to 1-1.5 L/day for Na <125 mmol/L 1
• Discontinue diuretics temporarily if Na <125 mmol/L 1
• Avoid hypertonic saline unless life-threatening symptoms 1

Investigating the Leukocytosis (WBC 19,800)

The elevated white blood cell count requires urgent evaluation:

• Chest X-ray to identify pulmonary causes of SIADH (pneumonia, malignancy) 1
• Blood cultures if fever or signs of infection 1
• CT chest/abdomen/pelvis if malignancy suspected (small cell lung cancer is a common cause of SIADH) 1
• Review all medications for SIADH-inducing drugs (SSRIs, carbamazepine, NSAIDs, opioids, chemotherapy) 1

Critical Safety Considerations

Never correct chronic hyponatremia faster than 8 mmol/L in 24 hours – this causes osmotic demyelination syndrome characterized by dysarthria, dysphagia, oculomotor dysfunction, and quadriparesis 2-7 days after rapid correction 1, 2.

If overcorrection occurs:

• Immediately discontinue current fluids and switch to D5W (5% dextrose in water) 1
• Consider administering desmopressin to slow or reverse the rapid rise 1
• Target bringing total 24-hour correction back to ≤8 mmol/L from baseline 1

Common Pitfalls to Avoid

• Do not rely on physical examination alone for volume assessment – sensitivity is only 41.1% and specificity 80% 1
• Do not order ADH or natriuretic peptide levels – these add no clinical value and delay diagnosis 1
• Do not use fluid restriction in cerebral salt wasting – this worsens outcomes and can be fatal 1
• Do not ignore mild hyponatremia – even Na 130-135 mmol/L increases fall risk and mortality 1
• Do not apply fluid restriction before confirming euvolemia – hypovolemic patients need volume repletion first 1

Monitoring Protocol

• Serum sodium every 2 hours during initial correction of severe symptoms 1
• Serum sodium every 4-6 hours after symptom resolution 1
• Daily weights and strict intake-output monitoring 1
• Watch for signs of osmotic demyelination syndrome (dysarthria, dysphagia, oculomotor dysfunction, quadriparesis) typically 2-7 days after correction 1