Hyponatremia

Key takeaways

Hyponatremia belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Hyponatremia to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Hyponatremia from memory before moving on to harder problems.

Reference excerpt

Hyponatremia or hyponatraemia is a low concentration of sodium in the blood. It is generally defined as a sodium concentration of less than 135 mmol/L (135 mEq/L), with severe hyponatremia being below 120 mEq/L. Symptoms can be absent, mild or severe. Mild symptoms include a decreased ability to think, headaches, nausea, and poor balance. Severe symptoms include confusion, seizures, and coma; death can ensue. The causes of hyponatremia are typically classified by a person's body fluid status into low volume, normal volume, or high volume. Low volume hyponatremia can occur from diarrhea, vomiting, diuretics, and sweating. Normal volume hyponatremia is divided into cases with dilute urine and concentrated urine. Cases in which the urine is dilute include adrenal insufficiency, hypothyroidism, and drinking too much water or too much beer. Cases in which the urine is concentrated include syndrome of inappropriate antidiuretic hormone secretion (SIADH). High volume hyponatremia can occur from heart failure, liver failure, and kidney failure. Conditions that can lead to falsely low sodium measurements include high blood protein levels such as in multiple myeloma, high blood fat levels, and high blood sugar. Treatment is based on the underlying cause. Correcting hyponatremia too quickly can lead to complications. Rapid partial correction with 3% normal saline is only recommended in those with significant symptoms and occasionally those in whom the condition was of rapid onset. Low volume hyponatremia is typically treated with intravenous normal saline. SIADH is typically treated by correcting the underlying cause and with fluid restriction while high volume hyponatremia is typically treated with both fluid restriction and a diet low in salt. Correction should generally be gradual in those in whom the low levels have been present for more than two days. Hyponatremia is the most common type of electrolyte imbalance, and is often found in older adults. It occurs in about 20% of those admitted to hospital and 10% of people during or after an endurance sporting event. Among those in hospital, hyponatremia is associated with an increased risk of death. The economic costs of hyponatremia are estimated at $2.6 billion per annum in the United States.

Signs and symptoms Signs and symptoms of hyponatremia include nausea and vomiting, headache, short-term memory loss, confusion, lethargy, fatigue, loss of appetite, irritability, muscle weakness, spasms or cramps, seizures, and decreased consciousness or coma. Lower levels of plasma sodium are associated with more severe symptoms. However, mild hyponatremia (plasma sodium levels at 131–135 mmol/L) may be associated with complications and subtle symptoms (for example, increased falls, altered posture and gait, reduced attention, impaired cognition, and possibly higher rates of death). Neurological symptoms typically occur with very low levels of plasma sodium (usually <115 mmol/L). When sodium levels in the blood become very low, water enters the brain cells and causes them to swell (cerebral edema). This results in increased pressure in the skull and causes hyponatremic encephalopathy. As pressure increases in the skull, herniation of the brain can occur. This can lead to headache, nausea, vomiting, confusion, seizures, brain stem compression and respiratory arrest, and non-cardiogenic accumulation of fluid in the lungs. This is usually fatal if not immediately treated. Symptom severity depends on how fast and how severe the drop in blood sodium level is. A gradual drop, even to very low levels, may be tolerated well if it occurs over several days or weeks, because of neuronal adaptation. The presence of underlying neurological disease such as a seizure disorder or non-neurological metabolic abnormalities, also affects the severity of neurologic symptoms. Hyponatremia, by interfering with bone metabolism, has been linked with a doubled risk of osteoporosis and an increased risk of bone fracture.

Causes The specific causes of hyponatremia are generally divided into those with low tonicity (lower than normal concentration of solutes), without low tonicity, and falsely low sodiums. Those with low tonicity are then grouped by whether the person has high fluid volume, normal fluid volume, or low fluid volume. Hyponatremia can also be a possible side effect of Pfizer COVID-19 vaccination.

High volume Both sodium and water content increase: Increase in sodium content leads to hypervolemia and water content to hyponatremia.

Cirrhosis of the liver Congestive heart failure Nephrotic syndrome in the kidneys Excessive water consumption (Water intoxication)

Normal volume There is volume expansion in the body, no edema, but hyponatremia occurs

SIADH (and its many causes) Hypothyroidism Not enough ACTH Beer potomania Normal physiologic change of pregnancy Reset osmostat

Low volume Hypovolemia (extracellular volume loss) is due to total body sodium loss. Hyponatremia is caused by a relatively smaller loss in total body water.

Any cause of hypovolemia such as prolonged vomiting, decreased oral intake, severe diarrhea Diuretic use (due to the diuretic causing a volume depleted state and thence ADH release, and not a direct result of diuretic-induced urine sodium loss) Addison's disease and congenital adrenal hyperplasia in which the adrenal glands do not produce enough steroid hormones (combined glucocorticoid and mineralocorticoid deficiency) Isolated hyperchlorhidrosis (Carbonic anhydrase XII deficiency), a rare genetic disorder which results in a lifelong tendency to lose excessive amounts of sodium by sweating. Pancreatitis Prolonged exercise and sweating, combined with drinking water without electrolytes is the cause of exercise-associated hyponatremia (EAH). It is common in marathon runners and participants of other endurance events. The use of MDMA (ecstasy) can result in hyponatremia.

Medication Antipsychotics have been reported to cause hyponatremia. Available evidence suggests that all classes of psychotropics, i.e., antidepressants, antipsychotics, mood stabilizers, and sedative/hypnotics can lead to hyponatremia. Age is a significant factor for drug induced hyponatremia.

Other causes Miscellaneous causes that are not included under the above classification scheme include the following:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Hyponatremia

Start with the simplest possible case. Write down what Hyponatremia claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Hyponatremia before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Hyponatremia ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Hyponatremia

In research: Hyponatremia appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Hyponatremia in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Hyponatremia is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electrolyte disturbances, Mineral deficiencies, Sodium, so understanding it makes those chapters shorter.
In everyday life: Look for Hyponatremia outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Hyponatremia in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Hyponatremia means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Hyponatremia out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Hyponatremia in simple terms?

Hyponatremia or hyponatraemia is a low concentration of sodium in the blood. It is generally defined as a sodium concentration of less than 135 mmol/L (135 mEq/L), with severe hyponatremia being below 120 mEq/L.

Why does Hyponatremia matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Hyponatremia?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Hyponatremia.