Hyponatremia

T/F: Disorder of sodium (hyponatremia, hypernatremia) are truly disorders of water homeostasis

True

What is hyponatremia?

a condition where there is low sodium (Na⁺) levels in the blood

Mild hyponatremia indication

Sodium levels between 125–135 mEq/L.

moderate hyponatremia indication

Sodium levels between 120–125 mEq/L

Severe hyponatremia indication

Sodium levels below 120 mEq/L or when neurologic complications are present.

What are hyponatremia symptoms based on?

Symptoms depend on both the absolute sodium level and the rate at which sodium levels drop.

• this is why for chronic and/or mild hyponatremia they are often asymptomatic because the body gradually adapts to low sodium levels over time.

What are the neurological changes that indicate hyponatremia?

Symptoms range from mild (headache) to severe (confusion, lethargy, seizures, coma) as sodium levels drop further and faster.

Categorizing types of hyponatremia (low sodium concentration, Na < 135 mEq/L) based on tonicity and volume status:

Isotonic means what

Indicates that the serum osmolarity is within the normal range, meaning the concentration of solutes in the blood is balanced.

Hypertonic means what

Indicates a higher concentration of sodium or solutes in the blood compared to normal.

Hypotonic means what

Refers to a lower concentration of sodium in the blood compared to normal.

Euvolemic/isovolemic means what

normal blood volume

Hypervolemic means what

Refers to an increased total body fluid volume

Hypovolemic means what

decreases in total body fluid

What is isotonic hyponatremia?

measured sodium concentration is low, but the true sodium concentration in the body is not actually reduced

Think of it like this: sodium levels in the blood appear low, but the overall concentration of particles in the blood is normal

• Isotonic: Indicates that the serum osmolarity is within the normal range, meaning the concentration of solutes in the blood is balanced.

• Hyponatremia: Refers to low sodium concentration in the blood.

What is isotonic hyponatremia caused by?

Hyperlipidemia / Hyperproteinemia

Note: These conditions can interfere with lab measurements, leading to a "pseudohyponatremia," where sodium appears low due to lab errors rather than actual low sodium in the blood.

How to treat isotonic hyponatremia?

Repeat lab test

What is hypertonic hyponatremia?

low sodium levels in the blood due to increased concentration of solutes (like glucose or mannitol) in the extracellular fluid, which draws water into the extracellular space and dilutes the sodium concentration

the sodium appears low due to dilution, but the actual total body sodium may not be significantly reduced.

What is hypertonic hyponatremia caused by?

Hyperglycemia

How to treat hypertonic hyponatremia?

reduce serum glucose and re-check sodium

How to find the corrected Sodium if someone has hypertonic hyponatremia?

Corrected Sodium = Measured Sodium + 1.6 x (Glucose - 100) / 100

Example of finding corrected sodium: Blood glucose 925mg/dL, serum sodium 124mEq/L

Corrected Sodium = 124 + 1.6 x (925 - 100) / 100 = 124 + 13.2 = 137.2 mEq/L

• This means that after glucose is reduced to normal (around 100 mg/dL), the sodium will likely be around 137 mEq/L, which is normal.

What are the subcategories of hypotonic hyponatremia

Euvolemic→ increase in total body fluid but no change in sodium levels

Hypovolemic→ increase sodium levels(↑) but large increase in total body water(↑↑)

Hypervolemic→ large decrease in sodium levels(↓↓) and some decrease in total body water(↓)

Note: this is based on volume status

What is Hypotonic Euvolemic Hyponatremia?

This occurs when there is normal body water content but an abnormal retention of water, which dilutes sodium.

↑TBW relative to no change in Na

the body has too much water, but sodium levels stay the same, so the sodium appears low due to dilution.

• Hypotonic: Refers to a lower concentration of sodium in the blood compared to normal.

• Euvolemic: Indicates a normal total body fluid volume, meaning there is no significant fluid overload or deficit.

• Hyponatremia: Low sodium concentration in the blood.

Causes of Hypotonic Euvolemic Hyponatremia

SIADH-Syndrome of Inappropriate AntiDiuretic Hormone

• its the overactivity of ADH which results in concentrated urine + excess water in ECF)

Drug-induced SIADH

Psychogenic polydipsia – intake >20L free water per day

Treatment for Hypotonic Euvolemic Hyponatremia steps

1. Underlying disorder

2. Water restriction 1000-1200mL/day

3. use Vasopressin receptor antagonists:

   1. Tolvaptan, conivaptan

   2. Demeclocycline

4. Sodium chloride + loop diuretic

What drugs induce Hyponatremia?

• SSRIs

• Carbamazepine

• Opiates

Hypotonic Euvolemic Hyponatremia impact on compartments in the body

Expansion of ECF is minimal – not clinically evident

Demeclocycline induces what

nephrogenic diabetes insipidus (blocks AVP at collecting duct)

Demeclocycline dose

300mg PO BID-QID

Demeclocycline carries the risk of what?

Hepatoxicity

Demeclocycline should be avoided

in children <8 years and pregnancy d/t changes in tooth development

What receptors does Conivaptan work on?

V1 & V2 receptor antagonist; IV only

Tolvapaptan brand name

Samsca

Tolvapaptan works on what targets?

V2 receptor antagonist; PO

Indication for tolvaptan

Hyper and euvolemia hyponatreamia due to HF, cirrhosis, or SIADH; autosomal dominant polycystic kidney disease

Interactions with Tolvaptan

an substrate of CYP3A4 and P-glycoprotein

Dose of tolvaptan

15-60mg PO once daily

ADRs of tolvaptan

thirst, polyuria, constipation

BBW for tolvaptan

Hepatotoxicity

What is Hypotonic Hypervolemic Hyponatremia?

This occurs when there is excess fluid retention, often associated with conditions where the kidneys are unable to excrete enough water(↑↑ TBW relative to ↑ Na)

• Hypotonic: Indicates a lower concentration of sodium in the blood compared to normal.

• Hypervolemic: Refers to an increased total body fluid volume.

• Hyponatremia: Low sodium concentration in the blood.

Causes of hypertonic hypovolemia hyponatremia

Cirrhosis, Heart Failure (HF), Nephrotic syndrome

Treatment steps for hypertonic hypovolemia hyponatremia?

1. Treat the Underlying disorder

2. Water restriction 1000-1200mL/day + Salt restriction 2000mg/day

3. Loop diuretics

4. use V2 receptor antagonist(tolvaptan, conivaptan,

What is Hypotonic Hypovolemic Hyponatremia?

This occurs when there is a loss of both sodium and water, but the loss of sodium is greater than the loss of water, leading to a dilution of sodium.

↓TBW relative to ↓ ↓ Na

• Hypotonic: Refers to a lower concentration of sodium in the blood compared to normal.

• Hypovolemic: Indicates a reduced blood volume or fluid deficit.

• Hyponatremia: Low sodium concentration in the blood.

Causes of hypotonic hypovolemic hyponatremia

Non-Renal Causes:

• GI losses: Conditions like diarrhea or vomiting lead to isotonic or hypotonic fluid loss.

  • This fluid loss stimulates thirst and the release of arginine vasopressin (AVP), causing the administration of hypotonic fluids while continuing to lose fluids.

  • Urinary Sodium: < 20 mEq/L, indicating low sodium excretion due to volume contraction.

Renal Causes:

• Thiazide diuretics: These medications can cause the kidneys to excrete more sodium.

  • Urinary Sodium: > 20 mEq/L, indicating that the kidneys are actively losing sodium.

Rapid correction of hyponatremia can lead to

OSMOTIC DEMYELINATION SYNDROME

• AKA: central pontine myelinolysis

Steps to correct hyponatremia:

What is Acute, Severe, Hypotonic Hypovolemic Hyponatremia?

rapid decline in serum [Na] <110mEq/L

• Severe symptoms: seizures

How to treat Acute, Severe, Hypotonic Hypovolemic Hyponatremia

Hypertonic saline (3% or 23.4% sodium chloride) can be used initially to manage this hyponatremia

• calculate sodium deficit

• May combine with loop diuretic to enhance free water clearance

• Stop once serum [Na]≥120mEq/L or severe symptoms resolve

How to calculate sodium deficit

([Na desired]-[Na measured]) x Total Body Water

• [Nadesired] = 125-130mEq/L to avoid rapid over correction