Approach to the Patient with Hyponatremia - Case 1
Mr. P is a 66-year-old man who comes to the emergency department with a chief complaint of an inability to urinate. Shortly after arrival he has a generalized seizure. Initial labs reveal a serum sodium concentration of 122 mEq/L.
What are the symptoms of hyponatremia? What is the differential diagnosis of hyponatremia? How would you frame the differential?
CONSTRUCTING A DIFFERENTIAL DIAGNOSIS
Step 1: Look for highly suggestive diagnostic clues.
As noted in Chapter 1, the first task when evaluating patients is to identify their problem(s). Mr. P’s problems clearly include seizure, marked hyponatremia, and inability to urinate. While other causes of seizures must be considered, the hyponatremia clearly requires evaluation because it is severe, potentially life-threatening, and likely to have caused the seizure.
Hyponatremia is the most common electrolyte abnormality in hospitalized patients and associated with an increase in mortality that is profoundly modified by the underlying cause of hyponatremia. It is defined as a serum sodium concentration < 135 mEq/L and is classified as mild (130–135 mmol/L), moderate (125–129 mmol/L), or profound (< 125 mmol/L).
Due to Mr. P.’s seizure and subsequent postictal state, Mr. P cannot give a medical history. His chart is requested. Physical exam reveals a man in jogging attire, appearing his stated age. His vital signs are BP, 140/95 mm Hg; pulse, 90 bpm; temperature, 36.0°C; RR, 18 breaths per minute. His neck veins are flat. His lungs are clear to auscultation. Cardiac exam reveals a regular rate and rhythm. There is no jugular venous distention (JVD), S3 gallop, or murmur. His abdomen is obese with no clear mass. No ascites is appreciated. Extremity exam reveals no edema.
At this point, what is the leading hypothesis, what are the active alternatives and is there a must not miss diagnosis?
RANKING THE DIFFERENTIAL DIAGNOSIS
Mr. P’s differential is extensive, but as noted above the first step in evaluating patients with hyponatremia is to review their history and laboratory findings to search for highly specific results that suggest a particular diagnosis. This includes the serum creatinine, glucose, potassium, urine and serum osmolality (Figure 24-1).
Mr. P’s laboratory studies reveal a glucose of 118 mg/dL; K+, 3.9 mEq/L; BUN, 14 mg/dL; creatinine, 0.8 mg/dL; and a serum osmolality of 254 mOsm/L. Urine osmolality is 80 mOsm/L.
Given these laboratory results what is the leading hypothesis, what are the active alternatives, and is there a must not miss diagnosis? Given this differential diagnosis, what tests should be ordered?
Mr. P.’s normal serum glucose and creatinine rules out hyponatremia from marked hyperglycemia, and kidney failure, respectively. His normal potassium does ...