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A 47-year-old male presents to the emergency department with altered mental status. According to the paramedics, the patient was found obtunded in an alley with empty beer cans at his side. While nursing staff attempt to establish an IV, the patient begins seizing. Temperature is 37.0°C (98.6°F), pulse is 101/min, respirations are 11/min, blood pressure is 152/74 mmHg, and O2 saturation is 95% on room air. On physical exam, the patient appears disheveled with tetany in the upper and lower extremities bilaterally. The abdomen is soft and nondistended. The patient’s laboratory findings are demonstrated below:  
 Laboratory Value, Serum  Result 
 Sodium   105 mEq/L 
 Potassium   4.2 mEq/L 
 Chloride   92 mEq/L 
 HCO3-  20 mEq/L 
 BUN  12 mg/dL 
 Creatinine   1.1 mg/dL 

Which of the following clinical findings is most likely to be present?   

External References

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hyponatremia p. 615

Hyponatremia p. 615

cirrhosis and p. 398

MDMA as cause p. 595

as paraneoplastic syndrome p. 221

thiazides p. 633

SIADH (hyponatremia)

paraneoplastic syndrome p. 221


With hyponatremia, hypo- means under or low, and -natrium is latin for sodium, often written as Na plus, and -emia refers to the blood, so hyponatremia means a lower than normal concentration of sodium in the blood, generally below 135 mEq/L.

The concentration of sodium depends on both sodium and water levels in the body.

About 60% of our body weight comes from just water, and it basically sits in two places or fluid compartments—one third of it is in the extracellular fluid, meaning outside the cells, and two thirds of it is in the intracellular fluid, or inside cells.

The extracellular fluid includes the fluid in blood vessels, lymphatic vessels, and the interstitial space, which is the space between cells that is filled with proteins and carbohydrates.

Normally, the two compartments have the same osmolarity -- total solute concentration -- and that allows water to move freely between the two spaces.

But the exact composition of solutes differs quite a bit.

The most common cation in the extracellular compartment is sodium, whereas in the intracellular compartment it’s potassium and magnesium.

The most common anion in the extracellular compartment is chloride, whereas in the intracellular compartment it’s phosphate and negatively charged proteins.

Of all of these, sodium is the ion the moves back and forth across cell membranes, and subtle changes in sodium concentration tilts the osmolarity balance in one direction or another and that moves water.

This is why we say “wherever salt goes, water flows”.

That being said, hyponatremia, or low concentration of sodium in the extracellular fluid and therefore the blood, can be caused by either losing more sodium than water, or gaining more water than sodium.

Broadly speaking, hyponatremia can be divided into three categories based on water volume status.

The first is hypervolemic hyponatremia where there’s an enormous increase in total body water with a less significant increase in total body sodium.

Typically this is seen in conditions like congestive heart failure, cirrhosis, or nephrotic syndrome where a lot of fluid leaks out of the blood vessels and into the interstitial space, causing edema especially in the ankles.


Hyponatremia refers to a blood sodium level that is below the normal range, specifically below 135 mEq/L. This can cause several symptoms, including nausea, vomiting, headache, confusion, and fatigue. In severe cases, hyponatremia can lead to hyporeflexia, seizures, coma, and even death.

Common causes of hyponatremia include drinking too much water or other fluids without enough salt, use of certain diuretics, renal failure, syndrome of inappropriate antidiuretic hormone secretion (SIADH), etc.


  1. "Robbins Basic Pathology" Elsevier (2017)
  2. "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
  3. "Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)
  4. "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
  5. "Clinical practice guideline on diagnosis and treatment of hyponatraemia" European Journal of Endocrinology (2014)
  6. "Management of Hyponatremia in the ICU" Chest (2013)

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