What is the appropriate management for a 57‑year‑old man with an acute influenza‑like illness and moderate hyponatraemia (serum sodium 122 mmol/L)?

Management of Acute Influenza with Moderate Hyponatremia

For a 57-year-old man with influenza and serum sodium 122 mmol/L, immediately assess symptom severity and volume status to determine if this represents SIADH (the most likely diagnosis with influenza), then initiate fluid restriction to 1 L/day for mild symptoms or 3% hypertonic saline for severe neurological symptoms, while never exceeding 8 mmol/L correction in 24 hours.

Immediate Assessment Required

Determine symptom severity first – this dictates urgency of intervention 1:

• Severe symptoms (confusion, seizures, altered consciousness, coma) require immediate 3% hypertonic saline 1, 2
• Mild symptoms (nausea, vomiting, headache, weakness) can be managed with fluid restriction 1, 3
• Asymptomatic patients still require treatment given sodium 122 mmol/L represents moderate-to-severe hyponatremia 1

Assess volume status clinically 1:

• Look for orthostatic hypotension, dry mucous membranes, decreased skin turgor (hypovolemic)
• Check for peripheral edema, ascites, jugular venous distention (hypervolemic)
• Absence of both suggests euvolemic state (most consistent with SIADH)

Influenza-Associated SIADH: The Most Likely Diagnosis

Influenza is a recognized cause of SIADH, though less commonly reported than other infections 4. The presentation of euvolemic hyponatremia in the setting of acute influenza strongly suggests SIADH as the underlying mechanism 4, 5.

Confirm SIADH with these findings 1, 5:

• Serum osmolality <275 mOsm/kg (hypotonic)
• Urine osmolality >100 mOsm/kg (inappropriately concentrated)
• Urine sodium >20-40 mmol/L despite hyponatremia
• Normal thyroid and adrenal function
• Euvolemic clinical status

Treatment Algorithm Based on Symptom Severity

For Severe Symptomatic Hyponatremia (Seizures, Coma, Altered Mental Status)

Administer 3% hypertonic saline immediately 1, 2:

• Give 100 mL boluses over 10 minutes, can repeat up to 3 times 1
• Target correction of 6 mmol/L over first 6 hours or until symptoms resolve 1, 2
• Critical safety limit: never exceed 8 mmol/L correction in 24 hours 1, 2
• Check serum sodium every 2 hours during initial correction 1

For Mild Symptomatic or Asymptomatic Hyponatremia

Implement fluid restriction as first-line therapy 1, 2:

• Restrict fluids to 1 L/day (or <800 mL/day for refractory cases) 1
• If no response after 24-48 hours, add oral sodium chloride 100 mEq three times daily 1
• Monitor serum sodium every 24 hours initially 1

Consider urea as an alternative pharmacological option 1, 2:

• Urea 15-30 grams daily can be effective for SIADH 2
• Better palatability concerns compared to salt tablets 2

Vaptans (tolvaptan) are an option for resistant cases 2:

• Start tolvaptan 15 mg once daily, can titrate to 30-60 mg 1
• Risk of overly rapid correction requires close monitoring 2
• More expensive than fluid restriction or urea 2

Critical Correction Rate Guidelines

The single most important safety principle: never exceed 8 mmol/L correction in any 24-hour period 1, 2. Exceeding this limit risks osmotic demyelination syndrome, which manifests 2-7 days after overcorrection with dysarthria, dysphagia, oculomotor dysfunction, and quadriparesis 1.

Target correction rates 1, 3:

• Severe symptomatic: 6 mmol/L over first 6 hours, then slow to stay within 8 mmol/L/24h limit 1
• Mild symptomatic: 4-6 mmol/L per day 1
• Maximum ceiling: 8 mmol/L in 24 hours for all patients 1, 2

Monitoring Protocol

During active correction 1:

• Severe symptoms: check sodium every 2 hours
• Mild symptoms: check sodium every 4-6 hours initially
• After stabilization: check sodium every 24 hours

Watch for signs of osmotic demyelination syndrome 1:

• Typically occurs 2-7 days after rapid correction
• Presents with dysarthria, dysphagia, ocular movement abnormalities, quadriparesis
• If overcorrection occurs, immediately give D5W or desmopressin to relower sodium 1

Common Pitfalls to Avoid

Do not delay treatment while pursuing diagnostic workup 3, 5. Begin therapy based on symptom severity and volume status assessment while confirming the diagnosis.

Do not use normal saline for euvolemic SIADH 1. This can paradoxically worsen hyponatremia because the kidneys will excrete the sodium while retaining free water 1.

Do not aim for normonatremia acutely 2. The goal is 125-130 mmol/L, not normal range, to avoid overcorrection 1.

Do not ignore mild hyponatremia 2. Even sodium 130-135 mmol/L is associated with increased falls, fractures, cognitive impairment, and mortality 2.

Treating the Underlying Influenza

Manage influenza concurrently 4:

• Consider oseltamivir if within 48 hours of symptom onset
• Supportive care with antipyretics and rest
• SIADH typically resolves as the acute illness improves 4

Hyponatremia should improve as influenza resolves 4, but continue monitoring sodium levels and maintain fluid restriction until sodium normalizes 1.