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Solution to "A Forgetful Lung Cancer Patient With Hyponatremia"

This is the solution to a case we presented recently for readers to solve. First, here is a quick review of the case:

A 70-year-old man with non-small-cell lung cancer presented to the clinic with fatigue and dizziness for 1 month. He complained of weight loss (5-10 pounds) and anorexia, and his wife said he seems forgetful. His blood pressure was 95/47 without orthostasis or tachycardia. He did not have peripheral edema, and the remainder of the exam was unremarkable.

These were his lab values: sodium 115 mEq/L; potassium 6.1 mEq/L; chloride 83 mEq/L; bicarbonate 20 mEq/L; blood urea nitrogen (BUN) 28; creatinine 1.4 mg/dL; glucose 72 mg/dL; Serum osm 243 mOsm/kg; urine osm 343 mOsm/kg.

Solution and Discussion

I have a standard method for evaluating serum sodium. I will present a stepwise system that I encourage you to use. Internal medicine requires some compulsivity, and electrolyte disorders require you to work through the possibilities carefully.

  1. Look for pseudohyponatremia. The easiest test here is a serum osmolality, as true hyponatremic patients have decreased serum osms.

    1. Causes of pseudohyponatremia include 3 endogenous and 2 exogenous factors:

    1. Endogenous

      1. Hyperglycemia

      2. Hypertriglyceridemia

      3. Paraproteinemia

    2. Exogenous (iatrogenic)

      1. IV mannitol

      2. Glycine used in transurethral prostatectomy

  2. If the patient has true hyponatremia, check volume status

    1. Volume contraction -- Volume contraction stimulates antidiuretic hormone (ADH), so the patient retains free water, making the patient susceptible to hyponatremia. Volume contraction-related hyponatremia responds to volume expansion.

    2. Edematous states -- This occurs mostly with heart failure and cirrhosis. These patients have effective intravascular volume depletion; again, ADH is stimulated, and they do not excrete free water. These conditions respond to treatment of the underlying cause.

    3. Euvolemic patients -- see below.

  3. The highest energy level with hyponatremia occurs when the patient is euvolemic.

    1. First, check urine osms. If the patient has dilute urine (urine osms < 100), then the problem is likely:

      1. Psychogenic polydipsia

      2. Beer drinker's potomania

      3. Tea and toast diet

    2. If the urine osms are elevated, first check for 4 conditions:

      1. Chronic kidney disease (CKD) -- elevated serum creatinine (probably 2 mg/dL or greater). CKD patients have an impaired ability to dilute their urine.

      2. Addison's disease

      3. Hypothyroidism

      4. Use of thiazide diuretics (which impair urinary dilution)

    3. If the urine osms are elevated and the above 4 conditions are excluded:

      1. Consider physiologic stimulants of ADH: nausea, vomiting, pain, narcotics

      2. Consider transient causes of increased ADH: pulmonary processes, intracranial processes

      3. Consider drugs; the most common class I have seen in 2006 is the SSRIs

      4. If all of these are absent, consider the possibility of a condition causing long-term SIADH (syndrome of inappropriate antidiuretic hormone secretion)

Now we can begin to draw some conclusions:

  1. The decreased serum osms confirm true hyponatremia. (Our first step is to exclude pseudohyponatremia.)

  2. The next step is to assess volume status. The low blood pressure in our patient suggests volume contraction; however, the lack of orthostasis and tachycardia weigh against this.

  3. The patient has no edema; thus, we either have a volume contraction-associated hyponatremia or a euvolemic hyponatremia. Assuming the latter, one should first consider hypothyroidism, Addison's disease, and chronic kidney disease.

  4. The hyperkalemia is the second clue. It is accompanied by a decreased bicarbonate level, suggestive of a type IV renal tubular acidosis (RTA).

  5. Type IV RTA is caused either by hypoaldosteronism or by a drug that interferes with aldosterone.

  6. While lung cancer often causes SIADH, one should suspect adrenal metastases in this case. The patient has hyponatremia and hyperkalemia plus signs and symptoms of corticosteroid insufficiency.

  7. For completeness, we obtained a TSH (thyroid-stimulating hormone) level, which was normal. We did an ACTH (adrenocorticotropic hormone) stimulation test, finding a low cortisol level (3.1), which did not increase with ACTH stimulation.

  8. An abdominal CT scan revealed bilateral adrenal metastases.