Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

Patient presentation
K.G., a 54 year-old male with a pack-a-day, 30-year history of smoking and small cell lung cancer (staging: extensive) presents to the oncology outpatient clinic for lab work and physical following chemotherapy two weeks earlier.  Patient presents with 12 lb. weight gain since the week before, increased weakness/fatigue, and reports muscle cramps with occasional nausea, vomiting, and diarrhea.  Pt is lethargic but AAOx3, with occasional headaches. No edema noted. Patient’s wife reports patient has “been urinating less frequently and even accidentally peed the bed once yesterday. This is so unlike him.”  Patient is admitted to oncology unit for a full work-up.

Differential list
Differential diagnoses include: dehydration, fluid retention, cardiac disease, hepatic dysfunction, adrenal insufficiency, renal disorders, thyroid disease, SIADH and/or poorly managed chemotherapy side effects.

Diagnosis
Lab results/Tests: CBC, CMP, plasma osmolality, UA, urine sodium and urine osmolality, blood cultures. Significant lab results: Serum Na+= 123 mEq/L. Serum osmolality= 276 mOsm/kg. Urine osmolality= 559 mOsm/kg. Urine sodium= 24 mEq/L.  (4 key diagnostic lab values: hyponatremia, decreased serum osmolality, increased urine osmolality and increased urine sodium). Electrolytes, BUN, creatinine, albumin and uric acid are also elevated. Chest X-ray to r/o pulmonary infection. CT scan was later done and revealed cerebral edema (and ruled out brain tumor/herniation).

Potential causes: There is a fluid imbalance as the ADH hormone is directing the kidney to conserve water and concentrate urine, leading to water intoxication and build-up of fluid in the intracellular space. It was determined that his SCLC tumor (similar to other malignant neoplasms) is secreting inappropriate and excessive amounts of ADH that interfere with fluid balance. The patient’s current medications are reviewed to ensure that SIADH is not medication induced (including opioids, antidepressants, non-steroidal anti-inflammatory drugs, cytotoxic chemotherapy).  Other nonmalignant causes were ruled out, such as CNS disorders (infection, hemorrhage, trauma), pulmonary disorders (infection, TB, abscess) and/or pain, stress, and/or nicotine.  After ruling out other causes, the diagnosis was SIADH secondary to SCLC.

Treatment
For K.G., chemotherapy was given as scheduled in an attempt to eliminate tumor cells causing the SIADH. Corticosteroids were given to help with cerebral swelling and decrease neurological implications.  A fluid restriction (800-1,000 ml/day) was implemented and the 4 key lab values were monitored daily. Lasix® (furosemide) was given to increase urinary water excretion. If patient is nonresponsive to fluid restriction, and Lasix®, demeclocycline (a medication that inhibits effect of ADH on renal tubules) can be given. Although K.G. did not require it, hypertonic 3% saline infusions, are indicated if hyponatremia becomes life threatening.