Learning Objectives - Hyponatremia

Question 1

Define hyponatremia.

Answer 1

Plasma sodium concentration <135 mmol/L

Question 2

Symptoms of hyponatremia usually begin once Na levels fall to ___. What happens?

Answer 2

<120;
There is too much water relative to sodium. This causes osmotic water shifts, leading to increased ICF, brain swelling, and cerebral edema

Question 3

Signs and symptoms of hyponatremia?

Answer 3

Headache, increased ICP, seizure, HTN
Delirium, coma
Irritability
Muscle twitching/cramps
Weakness
Hypoactive DTRs
N/V, ileus, watery diarrhea
Increased salivation/lacrimation
Oliguria -> anuria

Question 4

First step in evaluating hyponatremia?

Answer 4

Unlike hypernatremia, which is always hypertonic, hyponatremia may be hypo/iso/hypertonic. The first step is to determine serum osmolality.

Question 5

Effective osmolality?

Answer 5

Measured osmolality - BUN/2.8 (corrects for urea, which is an ineffective osmole)

Question 6

What is pseudohyponatremia?

Answer 6

If the assay measures sodium indirectly, elevated levels of serum lipids or proteins may cause the level to be falsely lowered

Proteins and lipids expand the non-aqueous portion of plasma

Question 7

What happens in hypertonic hyponatremia?

Answer 7

Osmotically active solutes like mannitol or glucose draw water out of the cells and into the serum, therefore lowering the sodium concentration of the serum (dilutional hyponatremia). For every 100 mg/dL increase in glucose, the serum sodium decreases by 1.6-2.4 mmol/L

NO CHANGE in the actual sodium content of the ECF

Question 8

What happens in hypotonic hyponatremia?

Answer 8

Patients may be hypovolemic, euvolemic, or hypervolemic.

Question 9

Very common causes of hypotonic hyponatremia?

Answer 9

Diuretic use (especially thiazides)
SIADH

Question 10

After determining the serum osmolality, ruling out pseudohyponatremia or hypertonic hyponatremia, what is the next step?

Answer 10

Assess volume status

Question 11

History and physical supporting hypovolemia?

Answer 11

Vomiting, diarrhea, diuretic use, bleeding, postural dizziness

Low JVP, orthostatics, decreased skin turgor

Question 12

Urine sodium concentration to determine volume status?

Answer 12

If urine sodium is <25, hypovolemia

If >40, euvolemic

Exceptions: renal salt-wasting due to diurietcs, Addison’s, or cerebral salt-wasting (urine sodium may be elevated, but patient is hypovolemic)

Question 13

History and physical supporting hypervolemia?

Answer 13

History of CHF, renal failure, cirrhosis

Anasarca, pulmonary edema

Question 14

What is the third step after assessing volume status?

Answer 14

Assess urine osmolality to determine the presence of ADH

ADH present: UOsm >150 (ADH concentrating urine)

UOsm <100 (primary polydipsia - ADH secretion shut down, kidneys respond by maximally diluting urine), malnutrition, beer potomania (solute intake and stores are low, decreased solute excretion), reset “osmostat”-> serum sodium level at which ADH release occurs is lowered

Question 15

Low serum osmolality + high urine osmolality?

Answer 15

Inappropriate presence of ADH (ADH should not be present if serum osmolality is low)

Question 16

DDx - hypotonic hyponatremia associated with impaired renal water excretion - hypovolemic

Answer 16

Diuresis (medication or osmotically induced)
Thiazides
Adrenal insufficiency
Salt-wasting nephropathy
Bicarbonaturia
Ketonuria
Diarrrhea/votmiing/blood loss/excess sweating/fluid sequestration (third spacing)

High urine sodium (>20) suggests renal salt loss
Low urine sodium (<10) suggests extrarenal losses

Question 17

DDx - hypotonic hyponatremia associated with impaired renal water excretion - euvolemic

Answer 17

SIADH
Thiazides
Hypothyroidism
Adrenal insufficiency
Decreased solute intake (beer potomania, tea and toast)
Oxytocin use
Drugs (haloperidol, cyclophosphomide, anti-neoplastics)
Post-operative

Question 18

DDx - hypotonic hyponatremia associated with impaired renal water excretion - hypervolemic

Answer 18

CHF
Cirrhosis
Renal failure
Nephrotic syndrome
Pregnancy

Question 19

DDx - hypotonic hyponatremia associated with impaired water intake

Answer 19

Primary polydipsia
Irrigation of the bladder or uterus with sodium-free solutions during hysteroscopy, cystoscopy, or transurethral resection of the prostate

Question 20

Most common causes of SIADH?

Answer 20

1. Disorders of the lung (small-cell carcinoma, infections, PPV, acute respiratory failure)
2. Disorders of the CNS (mass lesions, trauma, inflammatory or demyelinating disorders, stroke, hemorrhage, acute psychosis)

Question 21

Management of hyponatremia?

Answer 21

IV fluids (if SIAD is a consideration, restrict oral intake of free water to 1 L daily)

Correct sodium NO FASTER than 8-12 mEq/L over 24 hours

If chronic hyponatremia (higher risk for demyelination), serum sodium should be raised by 0.5-1 mEq/L per hour.

If severe symptoms/severe hyponatremia known to have developed within 48 hours, rapid correction - 3% saline to raise serum sodium level by 1-2 mEq/L per hour, but no more than 8-10 in the first 24 hours

Question 22

___ may be caused by severe hyponatremia. They are not caused by rapid correction, but they are an indication for rapid initial correction.

Answer 22

Seizures

Question 23

How should patients with alcohol use disorder be given IV solutions?

Answer 23

Thiamine BEFORE dextrose IV solutions

Question 24

Presentation of osmotic demyelination or central pontine myelinolysis?

Answer 24

Confusion
Quadriplegia
Pseudobulbar palsy
Catatonia
Locked-in syndrome
Parkinsonism
Mutism
Dystonia

Several days after hyponatremia correction

Question 25

Pathogenesis of central pontine myelinolysis?

Answer 25

When the patient is hyponatremia, fluid enters brain cells along the osmotic gradient, causing them to swell. During the first 24 hours, the cells compensate by shifting sodium and potassium out of the cells and into the CSF. By 48 hours, cells have also shifted organic osmolytes into the CSF. After 48 hour of hyponatremia, patients are at higher risk of developing ostmoic demyelination. If the rapid rise in serum tonicity exceeds the rate at which organic osmolytes can be synthesized and/or transported back into brain cells, the cells may shrink and die

Question 26

Estimate change in serum sodium?

Answer 26

= [(Infusate sodium + infusate potassium) - serum Na]/(TBW + 1)

Infusate sodium content:
3% NS = 513 mmol/L
0.9% saline = 154 mmol/L
0.45% saline = 77 mmol/L
Dextrose in water = 0

Question 27

Total Body Water = ?

Answer 27

Weight (kg) x % Body Weight that is water

Elderly men + non-elderly women = 0.5
Non-elderly men = 0.6
Elderly women = 0.45

Question 28

DDx - hypertonic hyponatremia?

Answer 28

Hyperglycemia
Mannitol, sorbitol, glycerol, maltose
Radiocontraste

Question 29

What is free water clearance?

Answer 29

Volume of blood plasma cleared of solute free water per unit time

Question 30

Positive free water clearance?

Answer 30

Producing urine more dilute than the plasma

Question 31

Negative free water clearance?

Answer 31

Urine more concentrated than the plasma (free water being extracted)

Question 32

General diagnostic strategy?

Answer 32

See paper

Question 33

Treat isotonic and hypertonic hyponatremias?

Answer 33

Diagnose and treat the underlying disorder

Question 34

Treat hypotonic hyponatremia?

Answer 34

Mild (120-130) - withhold free water, allow spontaneous equilibration

Severe (<110 or symptomatic) - hypertonic saline to increase serum sodium by 1-2 mEq/L/hr during the first 24 hours

Question 35

Treat hypotonic hypervolemic hyponatremia?

Answer 35

Fluid restrict

Question 36

Treat hypotonic euvolemic hyponatremia in the setting of SIADH?

Answer 36

NEVER give NS

Water restrict, use an ADH receptor antagonist, treat the underlying condition

Question 37

Risk of too rapid therapy for hyponatremia?

Answer 37

From low to high, the pons will die (central pontine demyelination)

Question 38

Risk of too slow therapy for hyponatremia?

Answer 38

From high to low your brown will blow (increased ICP)