Physiology of Hyponatremia

1
Physiology of Hyponatremia

• Hyponatremia results from either the excessive
  intake or inability to excrete free water.
• Water intake?dilutional fall in plasma
  osmolality? suppression of ADH.
• Plasma osmolality is elevated OR effective
  circulating volume is low?ADH secretion
• Normals can excrete over 10 L of free water a day.

2
Differential Diagnosis

• Conditions in which ADH levels are elevated
• 1. Volume depletion (vomiting, diarrhea,
  hemorrhage, profuse sweating)
• 2. CHF, cirrhosis (effective volume depletion)
• 3. Diuretic use
• 4. Adrenal insufficiency
• 5. SIADH

3
Differential Diagnosis

• Conditions in which ADH levels are suppressed
• 1. Advanced renal failure
• 2. Primary polydipsia
• 3. Low dietary solute intake
• 4. Beer potomania
• Pseudohyponatremiarare
• 1. High Sosms hyperglycemia, mannitol
• 2. Normal Sosms hyperlipidemia,
  hyperproteinemia

4
Workup

• History and PE are focused on volume status
• Later on, pattern recognition helps
• The elderly lady on a tea and toast diet who is
  also taking a diuretic
• The cirrhotic with worsening ascites, edema
• Early on, be complete with the evaluation send
  UA (specific gravity is the poor mans Sosm)
  serum osmolality, urine osmolality, urine
  electrolytes
• Urine electrolytes give another assessment of
  volume status (Na should be lt10 in volume
  depletion, unless on diuretic), K handling

5
Workup

• A normal or elevated Uosm (above 100) in the
  setting of low Posms makes the diagnosis of
  SIADH.
• --usually Na is gt40
• --usually Creatinine is normal
• --normal acid-base, K handling
• Low Uosm (less than 100) seen in primary
  polydipsia, poor solute intake

6
Workup

• Distinguishing hypovolemia/diuretic induced from
  SIADH (or presence of both)
• --Urine Na usually low
• --PE reveals signs dehydration
• --Creatinine may be elevated
• --hold diuretic and hydrate if the Urine Na
  rises to above 40 and the Uosms remain above 100
  the patient also has SIADH.

7
Treatment

• Goal to prevent cerebral edema and
  encephalopathy (lethargy, obtundation, seizures,
  coma, arrest) while preventing central pontine
  myelinolysis (correcting sodium too rapidly.)
• More rapid therapy (1.5 to 2 mEq per hour) can be
  used in the first few hours if there is serious
  cerebral dysfunction. Otherwise, maximum rate of
  elevation 10-12 mEq during the first 24 hours
  and 18 mEq/L in the first two days.

8
Ways to Raise Serum Sodium

• Water restrictionchronic primary therapy for
  hyponatremia in edematous states (CHF, cirrhosis)
  and in SIADH
• SIADH in the acute setting must be treated with
  IVF that exceeds the Uosm (NS or hypertonic
  saline)
• In volume depletionuse normal saline

9
Calculation of the Sodium Deficit

• TBW times the net sodium gain
• 0.5 times the weight in kilos for women, 0.6 for
  men
• Example of 50 kg female, sodium of 122
• So, if we wish her sodium to be raised by 12 mEq
  in the first 24 hours
• --0.5 x 50 kg x 12 300 mEq
• Hypertonic saline contains roughly 1 mEq of
  sodium per 2 cc, so 600 cc should be given over
  24 hours (25 cc/hour)

10
Ddx of SIADH

• CNS head trauma, neurosurgery, psychiatric
  illness, stroke, infection
• Tumor ectopic ADH due to small cell Ca of the
  lung (most common), also seen in bronchogenic Ca,
  pancreatic Ca, neuroblastoma (rarer)
• Drugs Carbamazepine, chlorpropamide, highdose
  IV cyclophosphamide, SSRIs
• Major surgery
• Pulmonary disease (particularly pneumonia)