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Hyponatremia

Lauren Chan

Background

  • Definition:

    • Mild: Na+ 130-134

    • Moderate: Na+ 125-129

    • Severe: Na+ \<125

  • Hyponatremia occurs when free water reabsorption (i.e. ADH is on) or intake exceeds free water excretion

Presentation

  • Mild to moderate symptoms: lethargy, N/V, dizziness, confusion, fatigue, cramping

  • Severe symptoms: obtundation, coma, respiratory arrest, seizure

Evaluation and Management

  • Step 1: Serum osm 

    • >295: Hyper-osmolar, presence of other molecules that contribute to serum osmolarity 

      • Glucose, mannitol, iodinated contrast

      • If hyperglycemic, corrected serum Na+ = measured Na+ + 1.6*[(glucose – 100)/100]  

        • If corrected Na+ is normal, treat hyperglycemia; not a water balance problem 

        • If corrected Na+ is low, there is hypotonic hyponatremia + coexisting hyperglycemia 

      • Renal failure (urea) and ethanol: Ineffective osmoles that can freely diffuse across cells and do NOT lead to hyponatremia

    • 275-295: Iso-osmolar 

      • Pseudohyponatremia 2/2 hypertriglyceridemia, paraproteinemia, or lipoprotein X: Serum Na not actually low, due to how the lab is calculated 

    • \<275: Hypo-osmolar à Step 2 

  • Step 2: Urine Osm

    • Surrogate for ADH activity

    • Uosm \<100 or Uosm \< Sosm correlates with low ADH

      • Primary polydipsia: Free water intake>output  

      • Tea and toast: Lack solute to effectively concentrate urine 

      • Beer drinkers’ potomania: Mixture of the two above

    • Uosm >100 or Uosm > Sosm correlates with high ADH Step 3 

  • Step 3: Urine Na

    • Is ADH on in the setting of decreased effective arterial bloodvolume (EABV) or decreased mean arterial pressure (i.e. appropriate ADH)?

    • UNa \<20: Low EABV à RAAS upregulation w/ Na avidity-> appropriate ADH release

      • 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

      • If edematous state (e.g. heart failure or cirrhosis), then decongestion with diuretics may improve serum Na

    • UNa >40: Euvolemic with no stimulus for ADH-> SIADH

      • SIADH from: n/v, malignancy, meds, surgery, pulmonary disease, hormones, pain, bladder distension: ↑ ADH out of proportion to stimulus  

        • Treat with water restriction. Can add NaCl or urea tabs if fluid restriction is severe 

        • Water restriction (L/day) = 600 / uosm (600 mEq Na in American diet/day)

        • Salt wasting: diuretics, cerebral salt wasting (aka hypovolemic SIADH), SSRIs

        • Other: Hypothyroidism, adrenal insufficiency 

  • If still stumped, can check a FeNa and measure a serum uric acid

    • FeNa \<0.5 % suggests appropriate ADH activity.

    • High uric acid suggests some degree of volume depletion and appropriate ADH activity.

Rate of correction

  • Acute (\<48 hrs)

    • If symptomatic, give 150 cc bolus 3% NaCl up to two times.

    • Monitor Na+ q1-2 hr

    • 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

  • 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

  • If you are worried about rapid over-correction:

    • High risk patients are those with rapidly reversible causes

      • 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

      • 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

    • High risk for ODS includes: chronic liver disease, Na \<105 meq/dL, alcoholism, and malnutrition.

  • Consideration of DDAVP clamp