Hyponatremia

Question 1

Inappropriate ADH secretion vs DI

Answer 1

• Syndrome of inap. ADH secretion (SI ADH) cause serum osm to be low and Uosm to be high (reabsorbs water, concentrates urine)
• DI is opposite, the serum osm is high (or normal) and the Uosm is low (can’t reabsorb water- either central or nephrogenic deficit)
• Hyponatremia in part due to excess free water intake
• Hypernatremia in part due to no free water intake

Question 2

Calculating serum osmolality

Answer 2

• Sosm: (2Na) + (BUN/3) + (glc/20)

- If it goes above 285 the osmoreceptors will stimulate ADH secretion

Question 3

Hyponatremia (Sna < 135 mEq/L)

Answer 3

• 3 different types depending on Sosm
• Hyperosmolar (>290 Sosm)
• Normal Sosm (pseudohyponatremia)
• Hypoosmolar (<275 Sosm)

Question 4

Pseudohyponatremia

Answer 4

• Due to serum dilution, Na is not actually low, but there is less of it since the % of water in the serum is low
• This occurs when there is hyperproteinemia or hyperlipidemia, which increases the protein or lipid phase of plasma and reduces the amount of water in plasma
• Since there is less water there is less Na dissolved in a given volume and the lab reads hyponatremic
• But within the portion of water present the concentration of Na is normal

Question 5

Hyperosmolar hyponatremia 1

Answer 5

• Sosm is high due to presence of osmotically active solutes (glc, mannitol, ionic contrast) which shifts water from ICF to ECF and dilutes out the Na
• With hyperglycemia, water is pulled out of the cell into the ECF and thus serum Na shows a low value
• However, the Na amount is not actually low, it is just diluted (still hyponatremia since concentration is low)

Question 6

Hyperosmolar hyponatremia 2

Answer 6

-We correct for this by calculating a corrected Sna value (the value that Sna would be if the glc were corrected and the water went back into the cell)
This is done by adding 1.6mEq/L to the Sna for every 100mg glc over 100mg/dL

Question 7

ADH response to hyperosmolar hyponatremia

Answer 7

-ADH levels will be high, since the body is trying to reabsorb water and bring down the Sosm

Question 8

Hypoosmolar hyponatremia 1

Answer 8

• Water intake is greater than water excretion
• Normally this will decrease ADH levels to near zero, but some pts will have abnormally elevated ADH levels, which are contributing to the low Sosm
• Psychogenic polydipsia can cause hypoosmolar hyponatremia b/c there is excess water intake
• There is low ADH since the body has so much water it doesn’t want to reabsorb anymore (appropriately dilute urine)

Question 9

Hypoosmolar hyponatremia 2

Answer 9

• Can also be due to inability to excrete free water (inappropriate water retention)
• Inappropriately high ADH levels due to:
• Decreased effective circulating volume (arterial blood) from CHF or liver failure
• Hypovolemia (bleeding, diuresis, vomiting, diarrhea)
• Drugs (narcotics)
• Pain, nausea, and vomiting

Question 10

Hypoosmolar hyponatremia 3

Answer 10

• SIADH (Dx of exclusion) often caused by lung and CNS lesions/tumors can lead to euvolemic hypoosmolar hyponatremia
• Impaired urinary diluting ability (renal failure) and nephrotic syndrome
• Multifactorial, including hormonal mechanisms

Question 11

Hypoosmolar hyponatremia based on volume state

Answer 11

• 3 different subset of hypoosmolat hyponatremia: hypovolemic, euvolemic, and hypervolemic
• Hypovolemic pts will have dry mucous membranes, skin turgor, other hypovolemia findings
• Euvolemic pts will have no findings (no edema, dryness)
• Hypervolemic pts will have edema
• Within each one there are various causes, but for hypovolemia they can be either extra renal or renal

Question 12

Hypovolemic hypoosmolar hyponatremia

Answer 12

• There will be elevated ADH levels since there is low ECFV (pt is dehydrated)
• Can be due to sweating, vomiting, blood loss, renal causes
• If Una is low (kidneys reabsorbing Na like they should be) the problem is extra renal
• If Una is high (kidneys excreting Na) the problem is renal, as in diuretics

Question 13

Euvolemic hypoosmolar hyponatremia

Answer 13

• Can be due to a number of things:
• Polydipsia (will have low ADH, low Uosm)
• Hormone problems: pituitary, thryroid, or adrenal insufficiency (leads to high Una, high Uosm)
• Or SIADH (requires euvolemic hypoosmolar hyponatremia plus high Una plus high Uosm)
• Important note: main factor in determining Uosm is not Na, but is urea

Question 14

Causes of SIADH

Answer 14

• Durgs
• Post op: pain, nausea, narcotics
• Ectopic production by tumors (small cell CA of lung)
• Pulm disease (pneumonia or TB)
• Neuropsychiatric d/o: CVA, neoplasm, psychosis

Question 15

Hypervolemic hypoosmolar hyponatremia

Answer 15

• There is Na and H2O retention, but more H2O retention than Na retention
• This causes dilution of Na so much so to cause hyponatremia
• Can either be due to CHF, liver failure, or nephrotic syndrome (proteins being filtered by glom)
• These causes will result in a low Una (kidney is reabsorbing Na since it thinks it has low volume due to the underlying disease)
• The other cause is renal failure, which presents as a high Una, since the kidney cannot reabsorb Na

Question 16

Clinical manifestations of hyponatremia

Answer 16

• Severe Sx when Sna falls below 120-125
• Lethargy
• Seizures
• Coma/death
• Due to decrease in Sosm thus shift of H2O from ECF to ICF and resulting cerebral edema

Question 17

Calculating free water excess

Answer 17

• Exact same as calculating free water deficit (just switch the numerator)
• TBW x (140-Sna/140)

Question 18

Rx of hyponatremia

Answer 18

• Rx underlying causes
• Water restriction
• Hypertonic saline for serious CNS Sx
• Furosemide: inhibits urinary concentrating ability, and add NaCl
• More specific Rx: based on isotonic, hypertonic, or hypotonic

Question 19

Specific Rxs

Answer 19

• Isotonic hyponatremia: no Rx
• Hypertonic: Rx cause such as hyperglycemia
• Hypovolemic hypoosmolar hyponatremia: give volume (saline, blood)
• Hypervolemic hypoosmolar
  hyponatremia: Rx the liver failure/CHF and limit Na/H2O intake and use furosemide
• Euvolemic hyponatremia: Rx underlying cause, restrict free water, and furosemide
• No survival benefit for V2 receptor blockers

Question 20

Rate correction of hyponatremia

Answer 20

• Acute and Sx hyponatremia: do not exceed 10mEq/L increase in 1st day to avoid brain shrinkage (H2O moves from inside to outside the cells when serum Na is replenished)
• This can lead to osmotic demyelination syndrome
• Chronic and ASx: increase by .5 mEq/L per hour
• Acuity more important than severity

Question 21

Osmotic demyelination syndrome

Answer 21

• Central and extrapontine myelinolysis
• Risk factors: excessive rate of correction of serum Na after being hyponatremic
• Also: malnutrition, and alcoholism, severe liver disease, hypoxia
• Presentation: dysphagia, quadriparesis, locked in syndrome
• Can be permanent or fatal

Question 22

Key points

Answer 22

• Hyponatremia is caused by water excess, likely due to increase in ADH and/or water intake
• Rate of decrease in serum Na best predictor of Sx
• Rapid correction of hyponatremia may induce osmotic demyelination