ABIM Exam Review Practice Question of the Week: Hyponatremia in Pt with Lung Cancer

Here’s a practice question directly from Knowmedge’s Internal Medicine Board (ABIM) Exam QVault.

60-year-old male presents to the clinic to review labs. He has a history of hypertension, diabetes, and lung cancer for which he is undergoing treatment. He voices no complaints at this time. He underwent a colonoscopy two years ago that did not identify any lesions or polyps. He is up to date with his eye exams. His only medications are metformin and lisinopril. His physical exam does not reveal any jugular venous distention (JVD), lower extremity edema, or other abnormality. Other results include:

Sitting blood pressure 124/82mmHg
Standing blood pressure 120/80mmHg
HgbA1c is 6.4%
CBC – unremarkable
Lipid panel shows an LDL 58mg/dL
CMP is normal except for a sodium level of 123mg/dL
Serum osmolarity 250mOsm/L

He says his appetite has been great and denies any weight loss. He also denies vomiting or diarrhea. Which of the following is the best solution for his hyponatremia at this time?

A. Furosemide

B. 3% normal saline

C. 0.9% normal saline

D. Fluid restriction and possibly demecocycline

E. 5% dextrose in water (D5W solution)

Explanation

This patient has a low sodium level with a low serum osmolarity. Next step is to assess fluid volume status. He does not have jugular venous distention (JVD), lower extremity edema or any other signs of fluid overload to suggest hypervolemia. This patient denies any vomiting, diarrhea, blood loss and when vital signs were checked, he did not become orthostatic, which indicates that he is not volume depleted or hypovolemic.

Thus, this patient has hypotonic, euvolemic hyponatremia, which can result from ectopic production of anti-diuretic hormone (ADH) from his lung cancer.

Let’s go through the answer choices to determine the proper treatment for his low sodium level:

Choice A (Furosemide) is incorrect since the patient is not fluid overloaded and does not require a diuretic.
Choice B (3% normal saline) could be considered if the patient had hyponatremia with mental status changes. This solution can be used initially to bring up the sodium to around 125mg/dL.
Choice C (0.9% normal saline) would be used if the patient was volume depleted. He has many clues that he is not volume-depleted as his renal function is normal, he is not orthostatic, and denies vomiting and diarrhea.
Choice D (Fluid restriction and possibly demecocycline) is the correct answer. When a patient has SIADH, the best action to take is to restrict fluids to around 800-1000cc/day and consider demecocycline for long-term management.
Choice E (D5W solution) is not required since he is not hypoglycemic and giving free water will decrease his sodium even more.

You can see all the previous ABIM Exam Review Questions of the Week at the Knowmedge Blog. You can also find additional topics and questions directly from the Knowmedge Internal Medicine ABIM Board Exam Review Questions QVault.