Week 2: Hyponatremia

Question 1

Define Hyponatremia.

Answer 1

Serum [Na+]<135 mEq/L

-pathogenesis: water intake is greater than water excretion

Question 2

How do you categorize hyponatremia based on serum osmolality?

Answer 2

1. High >290 mOsm/kg
   - hyperosmolar hyponatremia
   - increased glucose, mannitol
2. Normal osmolality
   - pseudohyponatremia
   - high lipids and protein causes measurement of S[Na+] to be low
3. Low >275 mOsm/kg
   - hypoosmolar hyponatremia
   - increased water intake

Question 3

How do you correct serum [Na+] when there is an increase in serum glucose in hyperosmolar hyponatremia?

Answer 3

For every 100mg/dL increase in serum glucose, correct serum [Na+] by adding 1.6mEq/L

Question 4

What are causes of hypoosmolar hyponatremia? (lecture)

Answer 4

1. water intake>renal excretion
   - psychogenic polydipsia
2. Non osmotic ADH secretion+ water intake
   - Decreased effective circulating volume: CHF, cirrhosis
   - true hypovolemia (bleeding, diarrhea, vomiting, diuresis)
   - medications like narcotics
   - pain, nausea, vomiting
   - CNS or lung lesions increase ADH
3. Impaired urinary diluting ability+ water intake
   - thiazides
   - chronic renal failure
4. Multifactorial+ water intake
   - hypothyroidism, adrenal insufficiency
   - hypopituitarism

Question 5

What are non-osmotic causes of excess ADH?

Answer 5

1. Drugs
   - chlorproamide
   - cyclophosphamide
   - vincristine
   - psychoactive drugs
2. postoperative patient: pain, nausea, narcotics
3. Ectopic production by tumors
   - small cell carcinoma of the lung
4. pulmonary diseases (uncommon)
   - pneumonia and TB
5. Neuropsychiatric disorder
   - meningoencephalitis, CVA, neoplasm, psychosis

Question 6

Causes of hyponatremia with low serum osmolality? (up to date)

Answer 6

Two most common causes, both associated with persistent ADH release w/ water intake
1. effective arterial blood volume depletion
-True volume depletion: GI losses- diarrhea/vomiting
-diuretic induced (thiazide)
-Heart failure and cirrhosis
2. Syndrome of inappropriate antidiuretic hormone (ADH) secretion
-CNS disease, malignancy, drugs, recent surgery
Less common
3. Endocrine disorders: pregnancy, adrenal insufficiency, hypothyroidism
4. Ectopic ANP
5. Exercise associated
6. Hyponatremia despite appropriate suppression of ADH:
-advanced renal failure
-primary polydipsia
-lower dietary solute intake

Question 7

What are the clinical symptoms of hyponatremia?

Answer 7

-symptoms only when serum Na falls to <120-125 mEq/L
-Lethargy, weakness
-seizures
-coma
-death
Due to ECF to ICF shift of water leading to cerebral edema

Question 8

How do you diagnose hypoosmolal hyponatremia based on volume status?

Answer 8

1. Hypovolemic:
   - diarrhea, hemorrhage (U[Na]100: hypothyroid, hypo adrenal, hypopituatry, stress, meds, pulmonary or CNS lesions
   - U[Na]<20
   - renal failure

Question 9

Describe principles of treating hyponatremia.

Answer 9

-isotonic: none
-hypertonic: treat cause, such s hyperglycemia
-hypovolemic: give volume, .9% saline, blood, albumin
-hypervolemic: Na and water restriction and furosemide
-euvolemic: treat underlying cause and restrict free water
RATE of correction
-If acute: 1-2mEq/L per hour
-if Chronic: increase by 0.5mEq/L per hour

Question 10

What is osmotic demyelination syndrome?

Answer 10

• Central and extrapontine myelinolysis
• occurs due to rapid correction of chronic hyponatremia. Adaptations of brain by decreasing intracellular organic osmoles as well as rapid increase in serum Osm due to treatment causes osmotic shift of water from ICF to ECF
• –> brain shrinkage and demyelination
• presents with dysphagia, quadriparesis, locked in syndrome

Question 11

What are characteristics of pre-renal conditions?

Answer 11

• low urine Na concentration (100mOsm) because of increased ADH
• increased water intake due to thirst which may relate to increased ADH and/or RAAS