"A Forgetful Lung Cancer Patient With Hyponatremia"

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
      
      

      Hyperglycemia Hypertriglyceridemia Paraproteinemia

   2. Exogenous (iatrogenic)
      
      

      IV mannitol 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)