1. Approach to Hyponatremia
Core Topic
UCI Internal Medicine Residency, 2012

2. Clinical Scenario
74-year-old man p/w recent gastroenteritis characterized by n/v/d x 5 days, in addition to fatigue and headache.
CT head (-) in ED. No focal neurologic deficits found. He looks dry on physical exam, with no evidence of fluid overload.
BMP significant for Na+ of 118, baseline unknown. Serum osmolality is 266. Urine osmolality is 377. Urine sodium is 8.
How would you approach this patient’s hyponatremia?
How would your approach be different if this patient presented with new-onset seizures?

3. Lecture Objectives Hyponatremia Clinical manifestations
Diagnostic approach
Clinical Scenario discussed

4. Hyponatremia Defined Definition: Serum Na+ <135 meq/L
Generally associated with decreased osmolality to <275
Most common electrolyte abnormality in the US
Caused by retention of water
Usually a drop in osmolality will suppress ADH to allow excretion of the excess water via dilute urine
Most forms of hyponatremia are associated with elevated ADH (whether appropriate or inappropriate), which concentrates urine

5. Signs & Symptoms
More profound when the decrease in sodium is very large or occurs rapidly (i.e. over hours)
Generally asymptomatic if Na+ level >125
Symptoms include:
Headache
Nausea, vomiting
Muscle cramps
Disorientation, depressed reflexes, lethargy, restlessness
Seizure, coma, permanent brain damage, respiratory arrest, brainstem herniation & death
Serious complications are more commonly seen in primary polydipsia, after surgery, and in menstruating women

6. Approach to Hyponatremia
1st assess volume status
Is the patient volume overloaded, depleted, or euvolemic?
2nd assess osmolality (hyper, iso, or hypo)
Is the blood concentrated? For hypotonic hyponatremia, continue to 3rd step:
3rd assess urinary sodium excretion and FeNa %
Is the urine concentrated?
*Remember VOU – volume status, osmolality, and urine studies

7. STEP 1 – (V) Volume Status
1st assess volume status (extracellular fluid volume)
Hypotonic hyponatremia has 3 main etiologies:
Hypovolemic – both H2O and Na decreased (H20 < Na)
Consider obvious losses from diarrhea, vomiting, dehydration, malnutrition, etc
Euvolemic – H20 increased and Na stable
Consider siADH, thyroid disease, primary polydipsia
Hypervolemic – H20 increased and Na increased (H2O > Na)
Consider obvious CHF, cirrhosis, renal failure

8. STEP 2 - (O) Osmolality 2nd assess osmolality hyper, iso, or hypo
Hypotonic hyponatremia = warrants further workup, especially when there is no obvious fluid overload or depletion
Serum Osmolality: lab value or calculation – in mosm/kg
=(2 x Na+) + (glucose/18) + (BUN/2.8) + (ethanol)/4.6
Hypertonic - >295
hyperglycemia, mannitol, glycerol
Isotonic
pseudo-hyponatremia from elevated lipids or protein
Hypotonic - <280
excess fluid intake, low solute intake, renal disease, siADH, hypothyroidism, adrenal insufficiency, CHF, cirrhosis, etc.

9. STEP 3 – (U) Urine Studies
For euvolemic hyponatremia, check urine osmolality
Urine osmolality <100 - excess water intake
Primary polydipsia, tap water enemas, post-TURP
Urine osmolality >100 - impaired renal concentration
siADH, hypothyroidism, cortisol deficiency
Check urine sodium & calculate FeNa %
A low urine sodium (<10) and low FeNa (<1%) implies the kidneys are appropriately reabsorbing sodium
A high urine sodium (>20) and high FeNa (>1%) implies the kidneys are not functioning properly

10. Hyponatremia Flow Sheet
Hypotonic Hyponatremia
Hypovolemic
Urine Na >20
FeNa >1%
Renal losses, mineralocorticoid deficiency, Addison’s disease
Urine Na <10
FeNa <1%
Extrarenal losses (diarrhea, emesis, burns)
Euvolemic – use patient history
Uosm >100
SiADH (urine osm usually much higher)
Hypothyroidism
Cortisol deficiency
Uosm <100
Primary polydipsia or low solute intake
Uosm variable
Reset osmostat (ie malnutrition, pregnancy)
Hypervolemic
CHF
Cirrhosis
Nephrosis
Renal failure

11. Treatment of Hyponatremia
Be CAUTIOUS with correction:
0.5 meq/L increase per every hour initially
Do not increase Na more than 10 meq/L in 24 hrs or 18 meq/L in 48 hrs
Treatment varies greatly by etiology of hyponatremia, and it is important to look-up via online or other resources.

12. Clinical Scenario - Conclusion
74-year-old man p/w recent gastroenteritis characterized by n/v/d x 5 days, in addition to fatigue and headache.
BMP significant for Na+ of 118, baseline unknown. Serum osmolality is 266. Urine osmolality is 377.
How would you approach this patient’s hyponatremia? The steps:
1) Serum osmolality – 266, decreased (hypotonic)
2) Urine osmolality –377, increased (>100)
3) Volume status - hypovolemic
4) Urine Na, FeNa – urine Na 8, appropriately reabsorbing, likely volume depleted 2/2 N/V
5) Treatment: Mild symptoms, correct slowly w/ isotonic saline
How would your approach be different if this patient presented with new-onset seizures?
For symptomatic, severe hyponatremia, more rapid correction using 3% normal saline

13. TAKE HOME POINTS Symptoms: Usually Na <125 or rapid decline
N/V, headache, lethargy, AMS, seizures, coma
WORK-UP in 3 important steps (V-O-U):
1) Assess volume status
2) Assess serum osmolality
3) Check urine sodium, osmolarity, & calculate FeNa
Treatment varies by etiology, but cautious correction of sodium important to prevent demyelination as fluid leaves the brain