Hyponatremia

Definitions & Key takeaways

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.

Chapters:

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Introduction0:00–0:21

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.

Physiology0:21–1:45

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.

Pathology1:45–2:07

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.

Hypervolemic hyponatremia2:07–3:11

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.
Even though there’s more water overall, there’s a decrease in the effective circulating volume, the amount of blood flowing in the body.
That, though, stimulates the release of antidiuretic hormone, which retains pure water, leading to an increase in water, as well as aldosterone which retains sodium...but remember since water follows sodium, the body retains even more water, so ultimately there’s a large increase in water, but a small increase in sodium, leading to hyponatremia.

Hypovolemic hyponatremia3:11–4:09

The second category is hypovolemic hyponatremia where there’s a small decrease in total body water with a large decrease in total body sodium.
This can occur in conditions like diarrhea or vomiting, where the cells lining the gastrointestinal tract actually pump sodium ions into the digestive juices, but then those ions don’t get reabsorbed because the undigested food and the digestive juices are tossed out of the body.
It can also develop in response to using certain medications like diuretics where sodium ions are pumped into the renal tubule and lost in the urine.
Another more nuanced condition is cerebral salt wasting which is when an intracranial injury like meningitis disrupts the normal sympathetic nervous system stimulation of the kidneys leading to disproportionate loss of sodium.

Euvolemic hyponatremia4:09–5:25

A third category is euvolemic hyponatremia, or normal volume hypovolemia, which is where there’s a normal body sodium with an increase in total body water, even though that’s contrary to the name.
The reason that it’s given that name is that you don’t have fluid pouring into the interstitial space, and therefore there’s no edema.
So no clinical signs of hypervolemia. Euvolemic hyponatremia can be split into cases with dilute urine and concentrated urine.
Cases that cause dilute urine include adrenal insufficiency, hypothyroidism, and drinking too much water called polydipsia or beer which is called potomania.
Basically the body has a lot of water and the kidneys are trying to get rid of it as best they can. The main condition that causes concentrated urine is syndrome of inappropriate antidiuretic hormone secretion, which can be shortened to SIADH.
This is where inappropriate presence of antidiuretic hormone causes water retention, which means the urine gets more concentrated.

Pseudohyponatremia5:25–6:08

There is one final type of hyponatremia and it’s sometimes called a false hyponatremia or pseudohyponatremia. This is where the body water and sodium levels are normal, but there’s an excessive amount of lipids, like in hypertriglyceridemia, or proteins, like in multiple myeloma.
High levels of lipids and proteins affects the laboratory instruments that measure the sodium concentration - making the instruments say the sodium concentration is too low, which is false.
Hyponatremia can cause symptoms like nausea, vomiting, muscle cramps. In severe hyponatremia, which is when the sodium concentration falls below 120 mEq/L, there can be cerebral edema which can cause confusion, coma and even death.

Symptoms6:08–6:55

The cerebral edema results from water shifting from the extracellular compartment to the intercellular compartment, causing the cells in the central nervous system to swell up and get damaged or die.
It can also cause increased intracranial pressure which can squash the blood vessels heading in and out of the brain causing ischemia, as well as possible brain herniation which can damage respiratory centers in the brain and cause respiratory failure.
Hyponatremia is usually diagnosed by looking at physical exam findings, as well as labs from the urine and blood. For example, a normal serum osmolality suggests true hyponatremia versus a pseudohyponatremia.

Diagnosis6:55–8:05

In addition, signs of edema suggest hypervolemic hyponatremia, whereas signs of dehydration suggest hypovolemic hyponatremia.
And urine osmolality can help differentiate between the two main types of euvolemic hyponatremia. If the urine osmolality is really concentrated, greater than 100 mOsm/kg, then the problem may be SIADH.
On the other hand, if the urine osmolality is dilute, less than 100 mOsm/kg, then the problem may be due to taking in too much fluid.
The urinary sodium concentration can also be helpful - with greater than 20-40 mEq/L suggesting SIADH and cerebral salt wasting, and less than 20 mEq/L suggesting hypovolemia.
Treatment of hyponatremia depends on the underlying cause. In patients with SIADH, fluid restriction is generally first-line therapy, whereas in patients that are hypovolemic, giving more fluid is helpful.

Treatment8:05–8:40

Generally speaking for severe hyponatremia, it’s important to give hypertonic saline, but it has to be done carefully to avoid complications like cerebral pontine myelinolysis - which is where there’s a loss of myelin in the pons, a structure in the brain, due to rapid shifts in sodium and water.
Alright, as a quick recap, hyponatremia is an electrolyte disorder that describes a low concentration of sodium in the blood, which can be divided into hypervolemic, hypovolemic and euvolemic causes.

Review8:40–9:07

Euvolemic causes can be split into those with concentrated urine, and those with dilute urine. Hyponatremia can cause water to shift into cells, and in severe cases it can lead to cerebral edema and death.
Increasing sodium levels can be done with hypertonic saline but needs to be done slowly to avoid cerebral pontine myelinolysis.