SIADH due to Lung Cancer and Aspiration Pneumonia

Author: V. Dimov, M.D.

A 56 yo CF who is treated at a SNF for aspiration pneumonia and metastatic lung SCC to brain is admitted to the hospital with increasing SOB. CXR and CT chest show a lung mass and increasing infiltrates.

Physical examination:
Emaciated patient, visibly SOB
37.2-110-20-140/70-SpO 92% on 2 L/min NC
Chest: (B) crackles
CVS: Clear S1S2
Abdomen: cachectic, +BS, NT, ND
Extremities: no c/c/e

Laboratory results:

Typical laboratory findings in SIADH

What is the reason for hyponatremia?
Most likely it is SIADH

What labwork would you order to diagnose SIADH?
BMP
Plasma osmolality
Plasma uric acid
Urine sodium
Urine osmolality

What happened?
ABG showed hypoxemia. The patient was placed on higher flow O2. ABG 30 minutes later showed resolution of hypoxemia. Blood cultures were drawn. Vancomycin was added. The patient is at risk for Pseudomonas due to structural lung disease (lung cancer) and MRSA (hospital stay).

Laboratory results confirmed typical SIADH.

Diagnostic criteria for SIADH include the following:
Hyponatremia (serum sodium less than 135 mEq/L)
Hypotonicity (plasma osmolality less than 280 mOsm/kg)
Inappropriately concentrated urine (more than 100 mOsm/kg water)
Elevated urine sodium concentration (more than 20 mEq/L), except during sodium restriction
Clinical euvolemia
Normal renal, adrenal, and thyroid function

Final diagnosis:
SIADH due to Lung Cancer and Aspiration Pneumonia

References:
Syndrome of Inappropriate Antidiuretic Hormone Secretion. eMedicine.
Hyponatremia. NEJM 2000.
Hypernatremia. NEJM 2000.
Recurrent Aspiration Pneumonia. NEJM Images in Clinical Medicine, 11/2008.

Published: 05/28/2007
Updated: 11/22/2008