Sodium is a water problem, not a salt problem. The board pivot: how fast did it happen, and which way is water moving in the brain.
Board-Style Opener · Read Every Word
A 62-year-old woman is brought to the emergency department by her daughter because of confusion and repeated vomiting since this morning, followed by a generalized tonic-clonic seizure in triage. She was started on a thiazide diuretic five days ago for hypertension and has been drinking large amounts of water and iced tea since. On examination she is lethargic and disoriented, with moist mucous membranes, no focal deficit, and no peripheral edema. Temperature is 37.0 C, blood pressure 142/86 mm Hg, heart rate 95/min. Laboratory studies show serum sodium 115 mEq/L (135 to 145), serum osmolality 236 mOsm/kg (275 to 295), and urine osmolality 430 mOsm/kg. A noncontrast head CT shows effaced sulci without hemorrhage.
Which of the following best explains her seizure and neurologic findings?
AWater shifting into brain cells, producing cerebral edema
BOsmotic demyelination of the central pons
CHypertonic plasma drawing water out of neurons
DSpontaneous intracranial hemorrhage from hypertension
EHyperammonemia from hepatic encephalopathy
From the Attending
Two words sound alike and ruin students: edema and demyelination. They are opposite directions of water. Sodium dropped in five days. That is fast. Fast and low means plasma is dilute, so water rushes INTO neurons and the brain swells. Swelling seizes. Swelling herniates.
Demyelination is the mirror image: it comes from fixing a slow, chronic low too fast, pulling water back OUT. She has not been corrected yet. There is nothing to over-correct. The seizure is edema. Do not talk yourself out of the right answer.
The Mechanism
Water Moves Which Way
One neuron. Four moments in time. Tap each phase and watch the cell swell, adapt, then shrink. This is the entire acute-versus-chronic story in one picture.
From the Attending
The brain is not passive. Give it 48 hours and it pumps its own osmolytes out to stop the swelling. Now the cell is normal size but running low on solute. That is the trap waiting to spring.
Raise the sodium faster than the brain can pull those osmolytes back in, and water leaves the cell, the myelin tears, and you get a locked-in patient days later. Slow is safe. Roughly 6 to 8 a day. Every time.
The Timeline Fork
Acute vs Chronic vs Demyelination
Same low sodium number, three different emergencies. The only variable that matters is time. Tap each tab.
Acute
Chronic
Demyelination
Acute Hyponatremia · Water Floods In
Head CT · diffuse cerebral edema, effaced sulci · tap to expand
Timeframe
Under 48 hours. The brain has had no time to adapt.
Water move
Plasma is suddenly hypotonic, so water rushes into neurons. The cell swells.
Picture
Headache, nausea, vomiting, confusion, seizures, and risk of brainstem herniation.
Treat
3% hypertonic saline. A 4 to 6 mEq/L rise quickly reverses swelling and stops seizures.
Board trap
Do not slow-walk a seizing patient. Acute symptomatic low sodium is the one time you push sodium up fast.
Chronic Hyponatremia · The Brain Adapted
Timeframe
Over 48 hours, often weeks. SIADH, thiazides, heart failure, cirrhosis.
Water move
Neurons exported organic osmolytes, so cell volume is near normal. The brain looks calm.
Picture
Often mild: fatigue, gait unsteadiness, falls, poor concentration. The danger is now in the correction, not the number.
Treat
Fix the cause and raise sodium slowly: no more than about 6 to 8 mEq/L in 24 hours.
Board trap
A calm patient with sodium of 110 is chronic until proven otherwise. The osmolyte loss is exactly why fast correction is dangerous.
Osmotic Demyelination · Corrected Too Fast
Brain MRI · central pontine demyelination · tap to expand
Cause
Raising sodium faster than the brain can re-accumulate osmolytes in a chronic low. Water leaves the cell.
Water move
Plasma becomes relatively hypertonic, water exits neurons, and myelin in the central pons is stripped.
Picture
A classic biphasic course: the patient improves, then 2 to 6 days later develops dysarthria, dysphagia, spastic quadriparesis, and a locked-in state.
Highest risk
Alcohol use, malnutrition, liver disease, hypokalemia, and starting sodium very low.
Board trap
If correction overshoots, re-lower sodium with free water or desmopressin. Prevention beats rescue.
The Decision Tool
Find the Cause
Hyponatremia is not one disease. Walk the tree the way you would at the bedside: osmolality first, then volume status, then the urine.
Node 1 · Serum Osmolality
Start here. What is the measured serum osmolality?
Low, under 275 mOsm/kg
Normal or high, 275 or above
Node 2 · Volume Status
True hypotonic hyponatremia. Now examine the patient. What is the volume status?
Volume is down. The kidney either hoards sodium or leaks it. What is the urine sodium?
Urine sodium under 20: the kidney is holding on
Urine sodium over 20: the kidney is leaking
Node 4 · Euvolemic Urine Osmolality
Euvolemic. The urine tells you whether ADH is on. What is the urine osmolality?
Concentrated, over 100, with urine sodium over 30
Maximally dilute, under 100
Node 5 · Hypervolemic Urine Sodium
Edema and ascites. Effective circulating volume is low even though total body water is high. What is the urine sodium?
Urine sodium under 20: heart failure or cirrhosis
Urine sodium over 20: advanced kidney disease
Active Retrieval
Predict the Urine Studies
Cover the row in your head first. Say the volume status, the urine osmolality, and the urine sodium out loud. Then tap that one row to check yourself. One row at a time.
Tap a row to reveal it · predict before you tap
Cause
Volume
Urine osm
Discriminator
SIADH
Euvolemic
High, over 100
Urine Na over 30, LOW uric acid, normal thyroid and cortisol
Massive water intake, psychiatric history, beer potomania, tea and toast
Heart failure / cirrhosis
Hypervolemic (edema, ascites)
High
Urine Na UNDER 20 (avid retention), restrict sodium and water
GI volume loss
Hypovolemic
High
Vomiting or diarrhea, urine Na under 20, give isotonic saline
Hyperglycemia (pseudo)
Variable
Serum osm NORMAL or high
Glucose very high; add 1.6 to 2.4 Na per 100 glucose over 100; not truly hypotonic
From the Attending
Uric acid is the quiet tiebreaker nobody uses. SIADH expands volume just a little, so the kidney dumps urate. Low uric acid in a euvolemic patient with concentrated urine is SIADH until proven otherwise.
And before you ever call something SIADH, you must show me a normal cortisol and a normal TSH. Adrenal insufficiency and hypothyroidism wear the SIADH costume. Find the source.
Two Classic Traps
Cirrhosis and the Fake Lows
The hypervolemic patient is drowning in water but starving for effective volume. And not every low sodium is real.
Hypervolemic Hyponatremia: Cirrhosis and Heart Failure
In cirrhosis, splanchnic vasodilation drops the effective circulating volume even though total body water is high. The kidney senses underfill, so it activates renin, aldosterone, and ADH and retains sodium and water avidly. Urine sodium is low (under 20). Water is retained out of proportion to sodium, so serum sodium falls. The belly fills with ascites and the legs pit.
Tense ascites in cirrhosis · tap to expand
⚠The fix is sodium restriction, fluid restriction, and diuretics (spironolactone first in cirrhosis). Not protein restriction. Restricting protein worsens malnutrition and does nothing for the fluid.
The Bedside Sign: Pitting Edema
The same sodium and water retention shows up at the bedside as dependent pitting edema. With a urine sodium under 20, ascites and pitting edema together confirm the hypervolemic category and point to heart failure or cirrhosis.
Pitting edema · the hypervolemic bedside sign · tap to expand
From the Attending
A student tells me to restrict protein in the ascites patient. Stop. Where did the fluid come from. Sodium. Water follows sodium. So you restrict the sodium and the water, and you give spironolactone to block aldosterone.
Protein restriction is an old reflex from the wrong decade. These patients are already catabolic. Feed them protein, restrict their salt. That distinction drives everything.
Pseudohyponatremia and Hyperglycemia: The Fake Lows
Step one of every workup is serum osmolality precisely because some low sodiums are not real.
Hyperglycemia (translational): glucose is osmotically active and pulls water out of cells into plasma, diluting sodium. Serum osm is normal or high. Correct the reported sodium by adding about 1.6 to 2.4 mEq/L for every 100 mg/dL of glucose above 100. The true sodium is usually normal.
Pseudohyponatremia (lab artifact): severe hypertriglyceridemia or paraproteinemia (for example multiple myeloma) takes up plasma volume and fools older sodium assays. Measured osm is normal. The sodium concentration in plasma water is actually fine.
✅If serum osmolality is normal or high, it is not true hypotonic hyponatremia. Do not give hypertonic saline to a fake low.
Memory Hook
ACUTE swells, FAST strips. A sudden low SWells the brain (cerebral edema, seizures). Correcting a chronic low too FAst STrips myelin (osmotic demyelination). Two opposite directions of water.
Memory Hook
Eight is the gate. In a chronic low, do not raise sodium more than about 8 mEq/L in 24 hours. Slower in high-risk patients (alcohol, malnutrition, hypokalemia, very low starting sodium).
Board-Style Walkthrough
Test Yourself
Six original board-style vignettes. One at a time. Right-click or long-press a choice to cross it out, double-tap to highlight. Tap the chain after answering to walk the logic.
Medically reviewed by Fatima Ali, DO and Kaitlyn Cocuzzo, MD · Last reviewed June 2026
Public references: standard physiology, internal medicine, and nephrology texts, and Gray's Anatomy for structural grounding. This page is an independent educational resource and contains no real or recalled examination questions.
Bone Wizardry is an independent educational resource for visual learning in the medical sciences. It is not affiliated with, endorsed by, or sponsored by any licensing or examination board, contains no real or recalled examination questions, and does not guarantee any educational or examination outcome.
