Start with the nephron, then the osmoregulatory axis, then the brain. Every therapy maps to one of these three compartments.
Every sodium disorder localizes to a specific tubular segment. Hover or click a segment in the diagram to see its transport machinery, drug targets, and how it contributes to free-water balance.
Free-water handling is split between three jobs: generating the medullary gradient (TAL), diluting tubular fluid (TAL + DCT), and finally recovering or excreting water at the collecting duct (AQP2 ± ADH). Hyponatremia emerges when either diluting fails or ADH holds water inappropriately.
Plasma osmolality is kept within ≈1% of set-point (≈285 mOsm/kg) by a tight feedback loop centered on hypothalamic osmoreceptors, ADH release from the posterior pituitary, thirst, and renal water handling.
| Plasma osm | ADH | Behavior |
|---|---|---|
| < 280 | Suppressed (<0.5 pg/mL) | Maximal aquaresis, U osm <100 |
| 280–285 | Threshold for release | — |
| 285–290 | Linear ↑ with osm | Urine concentrates |
| ≈ 295 | Thirst threshold | Drink to restore |
| > 295 | Max ADH (≈5–10 pg/mL) | U osm up to 1200 |
Baroreceptor-driven ADH beats osmoreceptor-driven ADH — the body prioritizes volume over osmolality. A hypovolemic patient will happily dilute to Na 115 to defend circulation.
Measured plasma osmolality includes all osmoles — Na, glucose, urea, mannitol, ethanol, etc. But only effective osmoles (those that don't cross cell membranes freely) actually pull water across. Tonicity = effective osmolality.
| Measured Na | Plasma osm | Effective tonicity | Scenario | Cell behavior |
|---|---|---|---|---|
| Low | Low (<275) | Low | True hypotonic hyponatremia — SIADH, HF, hypovolemia, etc. | Cells swell |
| Low | Normal (280–295) | Normal | Pseudohyponatremia — severe hyperlipidemia, hyperproteinemia (multiple myeloma, IVIG) | No change |
| Low | High (>295) | High | Translocational — hyperglycemia (DKA, HHS), mannitol, glycine (TURP), sorbitol | Cells shrink (water drawn out by effective osmole) |
Urea crosses membranes freely (via UT-A transporters). It raises measured osmolality but equilibrates across the cell membrane within minutes — no water movement, no tonicity gradient. A uremic patient with BUN 140 and Na 130 has a measured osm of ≈300 but effective osm of 270 (true hypotonic). Effective osm ≈ 2·[Na] + glucose/18 (not + BUN/2.8).
When measured osm exceeds calculated osm by >10, there is an unmeasured osmole present (methanol, ethylene glycol, isopropanol, mannitol, or severe ketoacidosis). In hyponatremia work-up, a gap of >20 with a high calculated osmolality points to a translocational (not dilutional) process.
The single most useful number for managing ongoing hyponatremia therapy. Tells you, right now, whether the patient is net-excreting or net-retaining free water — and predicts whether fluid restriction will work.
| EFWC | What it means | Action |
|---|---|---|
| Positive (>0) | Kidney excreting net free water → Na will rise on restriction | Fluid restrict, expect Na to improve |
| Slightly negative (−500 to 0) | Slight water retention — restriction alone marginal | Restrict <800 mL + salt tabs or urea |
| Strongly negative (<−500) | Drinking water lowers Na further ("water flushing") | Add urea, vaptan, or solute load; restriction alone will fail |
If (UNa + UK) / PNa > 1.0, urine is more concentrated in cations than plasma — every mL of urine actually removes Na/K faster than water, leaving serum more dilute. Restriction alone will not fix this patient.
The kidney's maximum free-water excretion is solute-limited. Minimum achievable urine osm ≈ 50 mOsm/kg. So:
| Daily solute intake | Max urine output | Threshold for hyponatremia |
|---|---|---|
| Normal diet (≈800 mOsm) | 16 L/day | Polydipsia >15 L |
| Tea & toast (≈250 mOsm) | 5 L/day | Normal water intake tips into hyponatremia |
| Beer-only (≈100 mOsm) | 2 L/day | Even modest intake overwhelms capacity |
Understanding the brain's two-phase adaptation is what makes the "acute vs. chronic" distinction life-or-death. Acute hyponatremia kills via edema. Chronic hyponatremia is safer — but corrects dangerously.
Oligodendrocytes maintain one of the highest metabolic rates in the CNS. Their myelin membranes demand constant ATP. They also have fewer aquaporin-4 channels than astrocytes, so they can't efflux water as quickly. When osmotic stress hits, they can't shed water fast enough to survive.
The pons is disproportionately affected because its oligodendrocytes outnumber astrocytes by a huge margin, and the pontine microvasculature doesn't offer the same reserve buffering the cortex has. Hence "central pontine myelinolysis" — now preferred as "osmotic demyelination syndrome" because up to 50% of cases have extrapontine involvement.
| Context | Dominant threat | Rule |
|---|---|---|
| Acute (<48 h) | Cerebral edema | Correct fast — reverse symptoms |
| Chronic (>48 h) | ODS from correction | Correct <8–10/24 h; <18/48 h |
| Unknown | Assume chronic | Cap correction; re-lower if exceeded |
| Severe acute symptoms + chronic | Both | 3% NaCl bolus to reverse symptom; then revert to chronic caps |
ODS isn't random cell death. It's a predictable cascade: rapid osmotic stress → oligodendrocyte water efflux → apoptosis → myelin breakdown → inflammation → axonal injury. Understanding the molecular sequence explains why DDAVP rescue works and why speed matters.
| Site | Frequency | Clinical correlate |
|---|---|---|
| Central pons (CPM) | ~50% | Dysarthria, dysphagia, quadriparesis, "locked-in" |
| Basal ganglia / thalamus (EPM) | ~30% | Parkinsonism, dystonia, movement disorders |
| Cerebellum / lateral geniculate (EPM) | ~15% | Ataxia |
| Combined CPM + EPM | ~20% | Variable, typically severe |
MRI T2/FLAIR hyperintensity lags clinical onset by 1–2 weeks. A negative MRI in the first few days does not rule out ODS. Diffusion-weighted imaging may catch it earlier but sensitivity remains limited. The "trident" sign (sparing of corticospinal tracts producing a three-pronged bright pontine signal) is characteristic but late.
If clinical picture is consistent after over-correction, treat presumptively. Don't wait for imaging.
3% NaCl (513 mEq Na/L) is one of the most potent interventions available, and one of the most dangerous if mis-dosed. The teaching "100 mL bolus raises Na by ≈2" is derived directly from Adrogué–Madias.
Example: 70-kg man, TBW = 42 L, serum Na 120, 3% NaCl (Na 513):
In a smaller person or at lower Na, the yield per bolus is proportionally higher — which is why fixed-volume boluses work across patient sizes.
Before 2013, 3% was typically given as a continuous 0.5–2 mL/kg/h infusion. Garrahy, Sterns, and others documented repeated over-correction because the infusion was titrated too slowly against rising Na and the underlying cause (e.g., hypovolemia) was quietly improving.
The modern 100 mL bolus × up to 3 approach — endorsed by the 2013 Expert Panel and 2014 European Consensus — achieves the same Na rise in minutes, is easier to titrate against clinical response, and reduces the cumulative volume administered.
100 mL 3% IV over 10 min → recheck in 20 min. If symptoms persist, repeat ×2 (total 300 mL, Na rise ≈ 5–6 mEq/L). Switch to 0.9% NS or oral when symptoms resolve.
ADH binds V2 receptors on the basolateral membrane of collecting duct principal cells, triggers a cAMP cascade, and drives AQP2 vesicles to fuse with the apical membrane. Vaptans block V2, prevent AQP2 insertion, and produce aquaresis — pure water loss without sodium loss.
| Drug | Route | Selectivity | Use |
|---|---|---|---|
| Tolvaptan | PO 15–60 mg | V2 selective | Euvol/hyperol hyponatremia; ADPKD (separate indication) |
| Conivaptan | IV 20–40 mg | V1a + V2 | Short-term hospital only (≤4 d) |
| Mozavaptan | PO | V2 | SIADH (Japan only) |
Tolvaptan is limited to 30 days maximum for hyponatremia; contraindicated in liver disease after the TEMPO/trial hepatotoxicity signal. Do not combine with fluid restriction in the first 24 h — risk of over-correction.
Oral urea 15–30 g/day induces an osmotic diuresis via obligate solute load. Non-inferior to tolvaptan at 7 days in Garrahy 2020. Cheap, no hepatotoxicity, no 30-day ceiling, no liver restriction. Dysgeusia is the practical downside — now reduced by flavored formulations.