Symptoms of Syndrome of Inappropriate Antidiuretic Hormone (SIADH) Secretion
The primary symptoms of SIADH are related to hyponatremia and include neurological manifestations such as confusion, headache, nausea, vomiting, seizures, and in severe cases, coma and death. 1, 2
Clinical Manifestations Based on Severity
Mild to Moderate Hyponatremia (Na 126-135 mEq/L)
- Patients may be asymptomatic or experience subtle symptoms 3
- Early symptoms include headache, irritability, and difficulty concentrating 2
- Nausea and vomiting may occur as sodium levels decrease 2, 4
- Muscle cramps and general weakness 5
- Fussiness (especially in pediatric patients) 5
Severe Hyponatremia (Na <125 mEq/L)
- Confusion and altered mental status 1, 6
- Lethargy progressing to drowsiness 5, 2
- Seizures (particularly when sodium drops rapidly or severely) 1, 7
- Coma in extreme cases 2, 6
- Respiratory arrest in life-threatening situations 6
Timing and Progression of Symptoms
- Acute hyponatremia (<48 hours) typically produces more severe neurological symptoms than chronic hyponatremia 3, 1
- Symptoms are generally more pronounced when serum sodium falls below 125 mEq/L 2, 4
- Rapid development of hyponatremia (>0.5 mmol/L/hour) increases risk of severe neurological manifestations 6
- Hospital-acquired hyponatremia often develops acutely in less than 48 hours, leaving little time for the brain to adapt 5
Risk Factors for Severe Manifestations
- Children are at particularly high risk of developing symptomatic hyponatremia due to their larger brain/skull size ratio 5
- Patients with underlying neurological conditions may develop symptoms at higher sodium levels 1
- First cycle of certain chemotherapies (like cyclophosphamide/doxorubicin) can precipitate severe symptomatic SIADH 7
- Post-operative patients receiving hypotonic fluids are at increased risk 6
Associated Clinical Findings
- Patients with SIADH appear clinically euvolemic (no edema, no signs of dehydration) 1, 4
- Normal skin turgor and moist mucous membranes despite hyponatremia 3
- Absence of orthostatic hypotension 3, 1
- Normal jugular venous pressure 3
- Continued urinary excretion of sodium despite hyponatremia 4, 8
Diagnostic Clues in SIADH
- Hyponatremia (serum sodium <135 mEq/L) with hypoosmolality (plasma osmolality <275 mosm/kg) 1, 4
- Inappropriately high urine osmolality (>500 mosm/kg) relative to plasma osmolality 1, 6
- High urinary sodium concentration (>20 mEq/L) despite hyponatremia 1, 4
- Serum uric acid <4 mg/dL has a positive predictive value of 73-100% for SIADH 3
Common Pitfalls in Recognizing SIADH Symptoms
- Symptoms can be nonspecific and easily attributed to other conditions, delaying diagnosis 5
- Failing to distinguish between SIADH and cerebral salt wasting (CSW), which require different treatments 3, 1
- Not recognizing that even mild hyponatremia may cause neurocognitive problems including falls and attention deficits 3
- Overlooking SIADH in patients with cancer, particularly small cell lung cancer where it commonly occurs as a paraneoplastic syndrome 5, 1