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SIADH is a disorder of impaired water excretion caused by excessive antidiuretic hormone (ADH) activity in the absence of appropriate physiologic stimuli. It leads to water retention, dilutional hyponatremia, and low serum osmolality, despite euvolemia.

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SIADH (Syndrome of Inappropriate ADH secretion) - mechanism, pathophysiology, treatment

https://www.youtube.com/watch?v=0NHT8ERUBo0&t=2s

https://www.youtube.com/watch?v=zyeMxsbi7uM

https://youtu.be/IX2hvCJ6Qrw

Etiopathogenesis

Etiology

Category	Examples
Malignancy (Ectopic ADH Secretion)	- Small-cell lung carcinoma (SCLC) – most common

Pancreatic carcinoma
Head and neck tumors (e.g., nasopharyngeal, oropharyngeal) | | Pulmonary Disorders | - Pneumonia (bacterial/viral)
Tuberculosis
Chronic obstructive pulmonary disease (COPD)
Positive pressure ventilation | | Central Nervous System (CNS) Disorders | - Stroke
Meningitis/encephalitis
Traumatic brain injury
Brain tumors
Subarachnoid hemorrhage | | Medications | - SSRIs, tricyclic antidepressants
Carbamazepine, oxcarbazepine
Vincristine, cyclophosphamide
NSAIDs, MDMA (ecstasy) | | Other Causes | - Postoperative stress (especially neurosurgery)
Pain, nausea
HIV/AIDS
Hereditary SIADH (rare) |

Pathophysiology:

Normal ADH Action	In SIADH
ADH secreted in response to ↑ plasma osmolality or ↓ blood volume	ADH secreted inappropriately despite low osmolality
ADH binds to V2 receptors → ↑ aquaporin channels → water reabsorption	Results in free water retention, hyponatremia, and concentrated urine

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Clinical Features

Severity of Hyponatremia	Symptoms
Mild (Na 130–135)	Often asymptomatic
Moderate (Na 125–129)	Nausea, headache, lethargy
Severe (<125 or rapid onset)	Confusion, seizures, coma, respiratory arrest

Diagnosis

Bartter and Schwartz Diagnostic Criteria:

To diagnose SIADH, the following must be met: