64 yo F with recent head trauma presents with hyponatremia

Original case by Skand Bhatt, MD. Adapted by Yilin Zhang, MD.


OBJECTIVES:

  • Inpatient work-up and initial evaluation of a patient with hyponatremia
  • Differentiation between cerebral salt wasting and SIADH

CASE:

A 64 yo F presents to the ED complaining of lower extremity weakness and gait instability, poor PO intake, polyuria, and orthostatic lightheadedness.

She was discharged one week ago after prolonged hospitalization, including rehab stay, following a closed head injury after bike vs. car accident. She sustained a small parietal IPH that was managed medically, multiple rib fractures, and clavicular fracture. She was briefly intubated, transitioned to inpatient rehab x 2 weeks, then to home under the care of husband. At baseline, she is very fit and biking several hundred miles a month.

Her PMH is notable for hypothyroidism, HLD, HTN, depression and insomnia. Her medications include citalopram (new within the past few weeks), trazodone (new within the past few weeks), HCTZ, levothryoxine and simvastatin. She has no substance use.

On exam

VS: 36.1, 87, 18, 144/81, 98RA
Neuro: A&O x 3, CN intact, nml reflex and sensation exam, UE strength intact. 3/5 LE strength requiring 1-person assist to stand and unsteady gait observed.
HEENT: Dry mucous membranes
Cardiopulmonary exam: nml
GI: normal

Labs

BMP: Na 122, K 2.9, Cl 92, HCO3 18, BUN 8, Cr 0.85, glucose 87
Ca 8.6, Mg 1.7
CBC normal
LFTs normal
BNP normal

When evaluating a pt with hyponatremia, what are some initial important questions to ask?

After ruling-out severe symptomatic hyponatremia, what is your diagnostic approach to determining etiology of hyponatremia?  


What are some possible causes of hyponatremia in this patient?


CASE CONTINUED:

How do this additional information change your differential?


What are common causes of hypovolemic hyponatremia?

What fluids would you use to correct his hyponatremia? 

How quickly should one correct a patient with hyponatremia? 


What are common causes of SIADH? 

How can one discern cerebral salt wasting from SIADH? Why is this important in terms of treatment? 


OUTCOME:  


TAKE HOME POINTS:

  • Determination of time course of hyponatremia and presence of severe symptomatic hyponatremia should be made upon on initial evaluation.
  • Work-up hyponatremia with serum Osm, urine Osm and urine Na.
  • Cerebral salt wasting and SIADH have identical laboratory findings and are only differentiated by volume status.
  • Err on the side of slowly correcting serum Na in asymptomatic or minimally symptomatic patients to avoid risk of osmotic demyelinating syndrome.

ADDITIONAL LEARNING:

What are risk factors for osmotic demyelinating syndrome? 

How do CHF and cirrhosis result in marked hyponatremia? What drugs are FDA approved for treatment of hyponatremia in this setting?