What are the symptoms and management of Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) in a patient experiencing hyponatremia and a sweet taste when drinking water?

SIADH Symptoms and Clinical Presentation

SIADH presents with symptoms ranging from subtle cognitive changes to life-threatening neurological emergencies, with severity directly correlating to the rapidity and degree of hyponatremia. 1

Clinical Manifestations by Severity

Mild to Moderate Hyponatremia (125-134 mEq/L)

• Nonspecific symptoms including weakness, nausea, anorexia, and headache are common initial presentations 2, 3
• Cognitive impairment with attention deficits and confusion, even in chronic mild cases 1, 4
• Gait disturbances with increased fall risk—21% of hyponatremic patients report falls versus 5% of normonatremic patients 3
• Increased fracture risk over time (23.3% vs 17.3% in normonatremic controls) due to secondary osteoporosis 3

Severe Symptomatic Hyponatremia (<120-125 mEq/L)

• Altered mental status progressing from lethargy to obtundation 1, 5
• Seizures representing hyponatremic encephalopathy 1, 3
• Coma in advanced cases 2, 5
• Cardiorespiratory distress in life-threatening presentations 3
• Cerebral edema as the underlying pathophysiological mechanism 5

Diagnostic Criteria

SIADH is confirmed by demonstrating hypoosmolar hyponatremia with inappropriately concentrated urine in a euvolemic patient. 1

Essential Laboratory Findings

• Serum sodium <134 mEq/L with plasma osmolality <275 mosm/kg 1
• Urine osmolality >500 mosm/kg despite low plasma osmolality 1, 2
• Urine sodium >20 mEq/L indicating inappropriate natriuresis 1
• Serum uric acid <4 mg/dL has 73-100% positive predictive value for SIADH 1

Clinical Assessment

• Euvolemic state is critical—no edema, no orthostatic hypotension, normal skin turgor, moist mucous membranes 1
• Exclusion criteria: hypothyroidism, adrenal insufficiency, and volume depletion must be ruled out 1

Management Approach

Severe Symptomatic Hyponatremia (Medical Emergency)

For patients with seizures, coma, or altered mental status, immediate administration of 3% hypertonic saline is mandatory. 1, 3

• Initial correction target: 6 mmol/L increase over 6 hours or until severe symptoms resolve 1
• Critical safety limit: Total correction must not exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 3
• ICU monitoring with serum sodium checks every 2 hours initially 1
• Hypertonic saline administration: 100 mL boluses of 3% NaCl over 10 minutes, repeatable up to 3 times at 10-minute intervals 6

Mild to Moderate Symptomatic or Asymptomatic SIADH

Fluid restriction to 1 L/day is the cornerstone of chronic SIADH management. 1, 2, 4

First-Line Treatment

• Fluid restriction to 1000 mL/day for patients with sodium <120 mEq/L or mild symptoms 1
• Monitor serum sodium every 24 hours initially, adjusting frequency based on response 1

Second-Line Pharmacological Options

• Demeclocycline can be used when fluid restriction is ineffective or poorly tolerated 1, 2
• Oral sodium chloride 100 mEq three times daily if no response to fluid restriction alone 1
• Urea is considered very effective and safe in recent literature 1, 3

Vaptan Therapy (Tolvaptan)

Tolvaptan 15 mg once daily is FDA-approved for clinically significant euvolemic hyponatremia, with careful monitoring to prevent overcorrection. 1, 7

• Starting dose: 15 mg once daily, titrate to 30 mg after 24 hours, maximum 60 mg daily 1
• Efficacy: Achieves 3.0 mEq/L/day correction rate, equivalent to hypertonic saline 1
• Critical monitoring: Check serum sodium at 0,6,24, and 48 hours to prevent overly rapid correction 4
• Common side effects: Thirst, polydipsia, polyuria 7, 4
• Contraindications: Use with extreme caution in cirrhosis due to increased gastrointestinal bleeding risk (10% vs 2% placebo) 7

Special Considerations and Pitfalls

High-Risk Populations Requiring Slower Correction

Patients with malnutrition, alcoholism, or advanced liver disease require more cautious correction rates of 4-6 mmol/L per day. 1

Common Diagnostic Pitfalls

• Failing to distinguish SIADH from cerebral salt wasting in neurosurgical patients—these require opposite treatments (fluid restriction vs. volume replacement) 1
• Using fluid restriction in subarachnoid hemorrhage patients at risk for vasospasm should be avoided 1
• Ignoring mild chronic hyponatremia as clinically insignificant—even mild cases increase mortality and morbidity 3

Treatment Complications

• Osmotic demyelination syndrome results from overly rapid correction (>8 mmol/L in 24 hours), causing parkinsonism, quadriparesis, or death 3, 4
• Hyponatremic relapse may occur after discontinuing vaptan therapy >5-6 days without tapering or fluid restriction 4

Underlying Causes Requiring Treatment

Common etiologies include malignancy (especially SCLC), CNS disorders, pulmonary pathology, and medications (SSRIs, carbamazepine, cyclophosphamide, vincristine). 1, 2

• Medication-induced SIADH: Discontinue offending agents immediately if symptomatic hyponatremia present 1
• Paraneoplastic SIADH: Treatment of underlying malignancy is essential alongside hyponatremia management 1