Hyponatremia¶
Lauren Chan
Background¶
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Definition:
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Mild: Na+ 130-134
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Moderate: Na+ 125-129
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Severe: Na+ \<125
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Hyponatremia occurs when free water reabsorption (i.e. ADH is on) or intake exceeds free water excretion
Presentation¶
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Mild to moderate symptoms: lethargy, N/V, dizziness, confusion, fatigue, cramping
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Severe symptoms: obtundation, coma, respiratory arrest, seizure
Evaluation and Management¶
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Step 1: Serum osm
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>295: Hyper-osmolar, presence of other molecules that contribute to serum osmolarity
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Glucose, mannitol, iodinated contrast
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If hyperglycemic, corrected serum Na+ = measured Na+ + 1.6*[(glucose – 100)/100]
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If corrected Na+ is normal, treat hyperglycemia; not a water balance problem
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If corrected Na+ is low, there is hypotonic hyponatremia + coexisting hyperglycemia
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Renal failure (urea) and ethanol: Ineffective osmoles that can freely diffuse across cells and do NOT lead to hyponatremia
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275-295: Iso-osmolar
- Pseudohyponatremia 2/2 hypertriglyceridemia, paraproteinemia, or lipoprotein X: Serum Na not actually low, due to how the lab is calculated
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\<275: Hypo-osmolar à Step 2
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Step 2: Urine Osm
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Surrogate for ADH activity
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Uosm \<100 or Uosm \< Sosm correlates with low ADH
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Primary polydipsia: Free water intake>output
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Tea and toast: Lack solute to effectively concentrate urine
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Beer drinkers’ potomania: Mixture of the two above
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Uosm >100 or Uosm > Sosm correlates with high ADH Step 3
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Step 3: Urine Na
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Is ADH on in the setting of decreased effective arterial bloodvolume (EABV) or decreased mean arterial pressure (i.e. appropriate ADH)?
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UNa \<20: Low EABV à RAAS upregulation w/ Na avidity-> appropriate ADH release
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If true volume depletion, then trial 500cc-1L NS bolus and monitor serum Na. IVF bolus->Increase EABV à ↓ ADH release à ↑ free water excretion
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If edematous state (e.g. heart failure or cirrhosis), then decongestion with diuretics may improve serum Na
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UNa >40: Euvolemic with no stimulus for ADH-> SIADH
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SIADH from: n/v, malignancy, meds, surgery, pulmonary disease, hormones, pain, bladder distension: ↑ ADH out of proportion to stimulus
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Treat with water restriction. Can add NaCl or urea tabs if fluid restriction is severe
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Water restriction (L/day) = 600 / uosm (600 mEq Na in American diet/day)
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Salt wasting: diuretics, cerebral salt wasting (aka hypovolemic SIADH), SSRIs
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Other: Hypothyroidism, adrenal insufficiency
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If still stumped, can check a FeNa and measure a serum uric acid
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FeNa \<0.5 % suggests appropriate ADH activity.
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High uric acid suggests some degree of volume depletion and appropriate ADH activity.
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Rate of correction¶
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Acute (\<48 hrs)
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If symptomatic, give 150 cc bolus 3% NaCl up to two times.
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Monitor Na+ q1-2 hr
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Goal is an initial rapid 4-6 mEq/L correction and then hold
- May require Hypertonic Saline infusion with DDAVP clamp if at risk of over-correcting
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Chronic (>48 hrs or unknown, higher risk for osmotic demyelination if corrected too quickly):
- Goal Na+ correction rate 4-6 mEq/L over 24 hrs (Max 8mEq/L)
When to call Nephrology¶
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If you are worried about rapid over-correction:
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High risk patients are those with rapidly reversible causes
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Low solute states (Beer drinker’s potomania, psychogenic polydipsia, tea-toast)- as soon as they decrease their excess free water intake, they will rapidly clear free water
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Volume depletion- as volume is replaced and the stimulus for ADH release is switched off, then they will rapidly clear the excess free water if they have normal underlying kidney function
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High risk for ODS includes: chronic liver disease, Na \<105 meq/dL, alcoholism, and malnutrition.
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Consideration of DDAVP clamp