Syndrome of Inappropriate Antidiuretic Hormone (SIADH)
The answer is (b) euvolemia, as euvolemia is a defining characteristic of SIADH, while the other options represent findings inconsistent with this syndrome.
Diagnostic Criteria for SIADH
SIADH is characterized by the following key features:
- Hyponatremia (serum sodium <134 mEq/L) 1
- Plasma hypoosmolality (<275 mOsm/kg) 1
- Euvolemia (normal extracellular fluid volume) 1
- Inappropriately high urine osmolality (>500 mOsm/kg) relative to plasma osmolality 1
- Elevated urinary sodium concentration (>20 mEq/L) 1
- Normal thyroid and adrenal function 1
Why the Other Options are Incorrect
Hypovolemia (option a): Hypovolemia is inconsistent with SIADH. Patients with SIADH are clinically euvolemic, not hypovolemic 2. Hypovolemia is actually a characteristic of Cerebral Salt Wasting (CSW), which is an important differential diagnosis for SIADH 1.
Hypernatremia (option c): SIADH is defined by hyponatremia (low sodium), not hypernatremia. The excessive ADH secretion causes water retention, which dilutes serum sodium leading to hyponatremia 1, 3.
Plasma hyperosmolarity (option d): SIADH is characterized by plasma hypoosmolality (<275 mOsm/kg), not hyperosmolarity 1. The excessive water retention dilutes plasma solutes, resulting in decreased osmolarity.
Excessive diuresis (option e): SIADH typically presents with normal or decreased urine output due to water retention, not excessive diuresis 3. Excessive diuresis would be more consistent with diabetes insipidus, which is characterized by ADH deficiency (the opposite of SIADH).
Clinical Significance of SIADH
SIADH is the most common cause of euvolemic hyponatremia 4. The clinical features are primarily neurological and depend on both the absolute serum sodium concentration and its rate of fall 3. Symptoms may include:
- Mild: Headache, irritability, poor concentration
- Moderate: Nausea, vomiting, confusion
- Severe: Seizures, coma, respiratory arrest
Distinguishing SIADH from Cerebral Salt Wasting
The key difference between SIADH and Cerebral Salt Wasting (CSW) is volume status:
| Feature | SIADH | CSW |
|---|---|---|
| Volume status | Euvolemic | Hypovolemic |
| Treatment | Fluid restriction | Fluid replacement |
| Urine output | Normal/low | High |
| Response to saline | Poor | Good |
| Central venous pressure | 6-10 cm H₂O | <6 cm H₂O |
Management Considerations
For patients with SIADH:
- Fluid restriction (1,000-1,500 mL/day) is the mainstay of treatment 1
- Avoid fluid restriction in subarachnoid hemorrhage patients at risk of vasospasm 2
- Correction of serum sodium should not exceed 8-10 mmol/L per day to avoid osmotic demyelination syndrome 1
- Pharmacologic options include urea, demeclocycline, and vasopressin receptor antagonists for chronic cases 1, 5
Remember that accurate diagnosis is crucial, as misdiagnosing CSW as SIADH can worsen outcomes since the treatment approaches are opposite 1.