What are the symptoms of Syndrome of Inappropriate Antidiuretic Hormone (SIADH)?

Symptoms of SIADH

The primary symptoms of SIADH are neurological and gastrointestinal manifestations directly related to the severity and rapidity of hyponatremia, ranging from subtle cognitive changes in mild cases to life-threatening seizures and coma in severe cases. 1

Neurological Symptoms

The neurological manifestations are the hallmark of SIADH and correlate with both the absolute serum sodium level and the rate of decline:

• Confusion and altered mental status are common presenting symptoms, particularly when serum sodium falls below 125 mEq/L 1, 2
• Headache occurs frequently as an early symptom of developing hyponatremia 1
• Seizures represent a severe manifestation, typically occurring with sodium levels ≤125 mEq/L or with rapid decline (>0.5 mmol/L/h) 3, 2
• Coma and death can occur in untreated severe cases with profound hyponatremia 1, 3
• Lethargy and weakness are common, particularly as hyponatremia progresses 4, 2

Gastrointestinal Symptoms

• Nausea and vomiting are frequent early symptoms that often precede more severe neurological manifestations 1, 3
• Anorexia commonly accompanies the syndrome 3

Musculoskeletal Symptoms

• Muscle cramps and general weakness are reported in patients with hyponatremia from SIADH 1

Symptom Severity Based on Sodium Levels

The clinical presentation varies significantly based on the degree of hyponatremia:

• Mild to moderate hyponatremia (Na 126-135 mEq/L): Patients may be completely asymptomatic or experience only subtle symptoms such as mild confusion, attention deficits, or increased fall risk 1
• Severe hyponatremia (Na ≤125 mEq/L): Symptoms generally become pronounced and include the full spectrum of neurological and gastrointestinal manifestations 3, 5

Special Populations at Higher Risk

• Children are at particularly high risk of developing symptomatic hyponatremia due to their larger brain-to-skull size ratio, making them more vulnerable to cerebral edema 1
• Patients with underlying neurological conditions may develop symptoms at higher sodium levels than otherwise healthy individuals 1
• Pediatric patients may present with fussiness as a nonspecific symptom 1

Critical Diagnostic Context

The symptoms occur because SIADH causes cerebral edema through water retention and dilutional hyponatremia 4. The rate of sodium decline is as important as the absolute value—rapid drops (>0.5 mmol/L/h) produce more severe symptoms even at higher sodium levels 2.

Common Diagnostic Pitfalls

• Symptoms are often nonspecific and easily attributed to other conditions, leading to delayed diagnosis 1
• Even mild hyponatremia may cause neurocognitive problems, including falls and attention deficits, which are frequently overlooked 1
• Failure to recognize SIADH in cancer patients, particularly those with small cell lung cancer, where it commonly occurs as a paraneoplastic syndrome 1
• Not distinguishing between SIADH and cerebral salt wasting, which present similarly but require opposite treatments 1