Approach to
Hyponatremia
Core Topic
UCI Internal Medicine Residency, 2012
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?
Lecture Objectives
Hyponatremia
 Clinical manifestations
 Diagnostic approach
 Clinical Scenario discussed
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
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:
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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
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
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
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 - 280-295
 pseudo-hyponatremia from elevated lipids or protein
 Hypotonic - <280
 excess fluid intake, low solute intake, renal disease, siADH,
hypothyroidism, adrenal insufficiency, CHF, cirrhosis, etc.
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
Hyponatremia Flow Sheet
Hypotonic
Hyponatremia
Euvolemic – use
patient history
Hypovolemic
Urine Na >20
Urine Na <10
FeNa >1%
FeNa <1%
Renal losses,
mineralocorticoid
deficiency, Addison’s
disease
Extrarenal losses
(diarrhea, emesis,
burns)
Uosm >100
SiADH (urine osm
usually much higher)
Hypothyroidism
Cortisol deficiency
Hypervolemic
Uosm <100
Uosm variable
Primary polydipsia or
low solute intake
Reset osmostat (ie
malnutrition,
pregnancy)
Urine Na <10
Urine Na >20
FeNa <1%
FeNa >1%
CHF
Cirrhosis
Nephrosis
Renal failure
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.
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:
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
1) Serum osmolality – 266, decreased (hypotonic)
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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?
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For symptomatic, severe hyponatremia, more rapid correction using 3% normal saline
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