Lecturer: Rakesh Gulati
Lecture: 7
1
Hyponatremia
A. DEFINITIONS/TYPES OF HYPONATREMIA
* HYPONATREMIA = SERUM Na CONCENTRATION < 135
mEq
* Far more common than Hypernatremia.
* Hyponatremia refers to the tonicity of the ECF, and has
nothing to do with the ECF volume which is a function
of sodium CONTENT.
o Remember that sodium Concentration is largely regulated by
Water handling, while sodium content is largely regulated
by renal excretion of sodium.
I. Isotonic Hyponatremia: “Iso-osmolar hyponatremia”
* Normal water for amount of sodium.
* Plasma Tonicity = 290 mOsm/L
o Caused by Pseudohyponatremia: which is a laboratory artifact.
o Amount of sodium is actually appropriate for the amount of water but presence of excess lipid or protein
results in an artificially high plasma volume resulting in an artificially low Sodium concentration.
o Rare with new measuring technology.
II. Hypertonic Hyponatremia: “hyperosmolar hyponatremia”
* Two much water for the amount of sodium, but serum is hypertonic from another solute.
* Plasma Tonicity >300 mOsm/L
o Excessive glucose (or other effective solute) creates an osmotic gradient pulling water out of the cells
and into the EC space = decreased concentration of sodium.
o Serum sodium concentration decreases by 1.6mEq/L per 100mg/dL increase in serum glucose.
o Symptoms are d/t hypertonicity, not hyponatremia- treat the hypertonicity!!
III. Hypotonic Hyponatremia: “Hypo-osmolar Hyponatremia”
* Far more common than the other two- most clinically significant.
* Too much water relative to the amount of Sodium.
* Plasma Tonicity <280 mOsm/L
B. HYPOTONIC HYPONATREMIA
I. General:
o Plasma Tonicity < 280 mOsm/L = Excessive water relative to sodium (and other solutes, but sodium is
most important)
o **Low tonicity refers to too much water, not necessarily too little sodium!!
* There must be ingestion of, or administration of water coupled with a failure to excrete water for this to
develop!!!
 Thus, although the water may be ingested with normal living, there is almost always some defect in
renal water excretion = USUALLY OCCURS D/T ANTI-DIURETIC HORMONE (ADH).
* NOTE: The “test” for ADH presence or absence is based on urine osmolality.
 Uosm > 100 = ADH must be present in at least some amount to allow concentration of
urine.
 If Serum Osm is low, having a Uosm < Sosm is NOT enough! A low Sosm SHOULD result in
complete inhibition of ADH production and thus, very dilute urine (<20 mEq/L)
 **UNLESS: the source of sodium loss is the kidney itself such as in chronic or acute kidney disease!!
1. CLASSIFICATION:
o Based on Total body SODIUM CONTENT (i.e. Extracellular fluid volume)
II. Hypervolemic Hyponatremia
1. General
o Total sodium content is elevated, with elevated total body water, however most of the water is usually in the
interstitium and 3rd spaces!!  body “thinks” it is volume depleted because it has a low effective vascular
volume  increased ADH secretion  Uosm inappropriately high (>100 mOsm) + low urine sodium
concentration.
* Excessive total body sodium is accompanied by an even greater excessive total body water!!
Dave Reilly
Lecturer: Rakesh Gulati
Lecture: 7
2
2. Causes
o CHF, cirrhosis and nephrotic syndrome
 Causing: Edema, Ascites, pulmonary edema, and other signs of Extracellular volume expansion.
3. Signs and symptoms
o Classic signs and symptoms of volume overload such as Crackles, dyspnea, S3 gallop, lateral PMI, etc…
depending on etiology.
III. Hypovolemic Hyponatremia
1. General
o Low total sodium content (Low ECF volume) = increased ADH secretion = Uosm inappropriately High
(>100 mOsm) + low Urine sodium concentration.
2. Causes
 Renal Loss
o Thiazide diuretics, Adrenal failure (no aldosterone- Na takes water with it), Renal failure
 Extrarenal Loss
o Vomiting, Diarrhea, Sweating, Third Spacing.
3. Signs and symptoms
* All of the signs/symptoms of low body water: see lecture on physical exam.
* Remember: Hypovolemia and Euvolemia are the two most difficult to separate
 Use all available clinical findings: orthostatic hypotension, tachycardia, tenting, flat neck veins, etc..
IV. Euvolemic Hyponatremia
1. General
o Excess of “free water” only. Total Body sodium Content is normal (thus technically euvolemic) however there is
an excess of water relative to the sodium!!
o May present with Elevated or depressed ADH which should give a good idea of the cause.
2. Causes
 Depressed ADH
o Water intoxication, Psychogenic polydipsia, “beer potomania” (low osmotic load hyponatremia), Reset
Osmostat, “you fixed it” hyponatremia overshoot (fix Hypovolemia  euvolemia = no ADH) but hyponatremia
has not yet normalized)
o Uosm <100 mOsm.
 Elevated ADH
o Endocrine Conditions: Cortisol Deficiency, hypothyroidism, reset osmostat
* SIADH (Syndrome of inappropriate ADH secretion)
 ADH release is NOT sensitive to serum tonicity, volume status or hormones.
 **Urine Osm > 100mOsm
 Many causes. Most common is DRUGS- NSAIDs, Caffeine, Narcotics, PPIs, antidepressants,
chemotherapy.
a.Many other causes (details see page 34 of syllabus).
C. CLINICAL APPROACH TO HYPONATREMIA
I. Measure Serum Osmolality
o Rule out Non-hypotonic causes of hyponatremia.
o If low: proceed to next step.
II. Determine if ADH is being secreted
o Measure urine osmolality
 >100 = yes  proceed to next step.
 <100 = no  likely d/t water toxicity.
III. Evaluate Volume Status
o Hypervolemia should be easy to diagnose
o Hypo- and Eu- volemia will depend a great deal on history
and physical exam!
 Urine [Na] < 10mEq/L = hypovolemia (unless
diuretics, kidney disease, aldosterone def,
cerebral salt wasting…)
 Elevated serum uric acid = hypovolemia.
 Decreased serum uric acid = SIADH
o If volume depleted: Hypovolemic Hyponatremia  look for causes and proceed to next step.
o If not volume depleted: Euvolemic Hyponatremia  likely SIADH.
Dave Reilly
Lecturer: Rakesh Gulati
IV. Determine Severity
Lecture: 7
3
o Is the patient symptomatic?
 Headache, Nausea, Vomiting, cramps, confusion, twitching.
 Coma and seizures usually being with Serum [Na] < 115
o How rapidly did the sodium drop?
 The more rapid the more dangerous.
o Is this an emergency or no?
 >120 = no emergency  evaluate for treatment.
 <120 = emergency  send to ICU and proceed with many Na measurements/treatment.
D. TREATMENT OF HYPOTONIC HYPONATREMIA
I. General
* NO MATTER WHAT, DO NOT RETURN THE SODIUM TO NORMAL TOO QUICKLY!!!!
 NO MORE THAN 12mEq/day!!!!
o Correcting too quickly will cause Central Pontine Myelinolysis
 More common in pre-menopausal females!
II. Specific Treatment plans based on Type:
1. Symptomatic Hyponatremia (of any kind)
o Hypertonic Saline- administered SLOWLY and only to get out of the Danger zone!!
 Get levels back to around 120 or wait for the symptoms to resolve then ease off the hypertonic saline!
o Rule of thumb for Hypertonic saline:
 1cc/Kg/hr hypertonic saline = 1 mEq change in Serum [Na] / hour.
o Administer with Loop diuretics to prevent sodium loss by the kidneys.
2. Hypovolemic Hyponatremia
o Normal Saline to replete volume  once patient is Euvolemic the kidney will begin excreting excess
water as needed.
3. Hypervolemic Hyponatremia
o Water restriction + loop diuretics
4. Euvolemic Hyponatremia (SIADH)
o Water Restriction** may be all that is needed.
o If severe: Water Restriction + Hypertonic Saline (slowly) + Loop Diuretics.
o Demeclocycline: drug that antagonizes ADH action.
III. Pros and Cons of using Hypertonic vs. Isotonic Saline
1. Hypertonic
 Pros
o Will correct Hyponatremia regardless of cause
o Can increase Na independent of large volumes.
 Cons
o Risk of over-correction can kill your patient
o Will delay volume restoration in pt with undetected hypovolemia
 Use if:
o Emergency (<120), symptomatic, or you are not sure if it is hypo- or Eu- volemic but you are afraid of Na
dropping further.
2. Isotonic
 Pros
o Less risk of over correcting
o Easier to restore volume
 Cons
o Can worsen hyponatremia in SIADH if Urine Electrolyte content is > Saline content you are simply adding more
volume to the problem.
o Over correction is still possible (especially in hypovolemic hyponatremia when volume is restored resulting in
drop in ADH but hyponatremia is not yet normal  rapid water loss with no ADH  rapid increase in plasma
sodium concentration = effectively the same as too quickly over correcting!!!)
 Classic example of “you fixed it” over-correction.
 Use if:
o Dangerous hypovolemia.
o mild-moderate hyponatremia that you are sure is d/t volume depletion (hypovolemic) or SIADH (with a urine
electrolyte < saline… note, you can use loop diuretics to make this happen artificially!)
Dave Reilly