Hyponatremia
Na⁺ < 135 mEq/L -the most common electrolyte abnormality in hospitalized patients. The approach is systematic: (1) serum osmolality, (2) volume status, (3) urine studies. Correct slowly -osmotic demyelination syndrome is iatrogenic and devastating.
🔍 Systematic Approach
Step 1 -Serum Osmolality
| Serum Osm | Category | Causes |
|---|---|---|
| < 275 (low) | Hypotonic (true hyponatremia) | Most cases. Proceed to Step 2. Includes post-TURP (transurethral resection of the prostate) absorption of glycine or sorbitol irrigant, which is genuinely hypo-osmolar and therefore swells the brain, unlike the mannitol row below. |
| 275–295 (normal) | Isotonic (pseudohyponatremia) | Hyperlipidemia, hyperproteinemia (multiple myeloma). Lab artifact -true Na is normal. Check lipid panel + protein. |
| > 295 (high) | Hypertonic (translocational) | Hyperglycemia (#1 -correct Na for glucose: add 1.6 mEq/L Na for every 100 mg/dL glucose above 100), mannitol, IV contrast. The water is pulled out of cells, so the patient is hypertonic despite the low sodium: do NOT give hypertonic saline. |
Iatrogenic Hyponatremia: Mannitol and Post-TURP Irrigant Absorption
Why these two sit together: both are hyponatremias you cause, and in both the tonicity is not what the sodium number implies. Mannitol produces a hypertonic hyponatremia, so reaching for hypertonic saline pushes in the wrong direction. Glycine absorption during a TURP produces a genuinely hypotonic hyponatremia that can seize and kill within hours. Identical low sodium on the chart, opposite physiology, opposite treatment. Measure the osmolality before you treat any hyponatremia you may have created.
Mannitol: Hypertonic (Translocational) Hyponatremia
- Where you will actually meet it: mannitol given for raised intracranial pressure or cerebral edema (severe TBI, large hemispheric stroke, tumor with mass effect), for acute angle-closure glaucoma to drop the intraocular pressure, and mannitol used as the irrigant during monopolar TURP or hysteroscopy. The neuro-ICU patient on repeated dosing is the classic setup, because the risk is cumulative.
- Mechanism: mannitol is an effective osmole, meaning it stays outside cells. It raises plasma osmolality, which pulls intracellular water out into the extracellular space, and that water dilutes the serum sodium. The sodium falls while the patient is hypertonic, which is the opposite of ordinary hyponatremia.
- How to recognize it: the measured osmolality exceeds the calculated osmolality (2 × Na + glucose/18 + BUN/2.8), producing an osmolal gap, because mannitol is a solute the calculation does not include. A low sodium plus a high measured osmolality plus an osmolal gap in someone who received mannitol is the entire diagnosis.
- Do not give hypertonic saline: the brain cells are already shrunken by the hypertonicity, so adding more tonicity worsens the problem you are trying to treat. The treatment is to stop the mannitol and let the kidney clear it, and dialysis if it has accumulated and the patient is symptomatic.
- Accumulation is the setup, and renal impairment is the risk factor: mannitol is freely filtered and not reabsorbed, so clearance depends entirely on GFR. In AKI or CKD it builds up in the circulation and keeps pulling water out of cells. Follow the osmolal gap rather than the serum osmolality alone before each repeat dose, since a rising gap means drug is accumulating even when the calculated numbers look acceptable.
- The second phase flips to hypernatremia: once mannitol reaches the tubule it drives an osmotic diuresis that loses proportionally more water than sodium. The same drug therefore causes hyponatremia early and hypernatremia later, so a patient on repeated dosing needs the sodium followed in both directions rather than checked once.
- Watch the potassium too: water leaving cells carries potassium with it, so the identical shift that lowers the sodium can raise the potassium, and it does so fastest in the patients with the worst renal clearance.
Post-TURP (Transurethral Resection of the Prostate) Irrigant Absorption Syndrome
- Why the irrigant is electrolyte-free in the first place: monopolar transurethral resection of the prostate (TURP) returns current through the patient's body to a grounding pad, so a conductive fluid such as saline would disperse the current and stop the loop cutting. That forces an electrolyte-free irrigant: glycine 1.5%, sorbitol, or mannitol. The hyponatremia is a direct consequence of the electrosurgical design, not of the surgery itself.
- How it gets in: resection opens prostatic venous sinuses, and irrigation pressure exceeding venous pressure drives fluid straight into the circulation at roughly 20 mL/min, which is several liters across a long resection. About 1 L absorbed in an hour drops the sodium 5 to 8 mEq/L.
- The triad is hyponatremia, hypo-osmolality, and hypervolemia. Glycine 1.5% runs about 200 mOsm/L and contains no sodium. The hypo-osmolality is what swells the brain (headache, confusion, seizures, and in severe cases herniation), while the hypervolemia is what floods the lungs.
- The vital signs run backwards from what you expect: intraoperative trouble usually means tachycardia and hypotension, but irrigant absorption gives hypertension with bradycardia because the problem is volume overload. In an awake patient under spinal anesthesia, restlessness, nausea, confusion, and visual disturbance arrive first and give you a warning. Under general anesthesia that warning is erased, and the presentation may be an unexplained blood pressure rise, ECG changes, or a seizure. That early-warning value is a large part of why spinal anesthesia is favored for TURP.
- Glycine carries two toxicities beyond the sodium: it is metabolized to ammonia, so hyperammonemic encephalopathy can persist after the sodium is fixed and explains the patient who stays confused despite a corrected number; and it is an inhibitory neurotransmitter in the retina, so it causes transient blurred vision or frank blindness. Transient blindness after a TURP is the glycine, not a stroke, and it resolves as the glycine clears, which saves the patient an emergent stroke workup.
- Management: tell the surgeon to stop and terminate the resection, secure the airway, and send an immediate sodium and osmolality. Give 3% hypertonic saline for seizures or severe neurologic symptoms, and add a loop diuretic when there is pulmonary edema, since this patient is volume overloaded rather than dry. Use dialysis or CRRT for profound hyponatremia with volume overload, where hypertonic saline alone would worsen the fluid load.
- Anticipate spontaneous overcorrection: as glycine is metabolized and the absorbed water is excreted, the sodium climbs on its own. Layer hypertonic saline on top of that and you can overshoot into osmotic demyelination. Recheck the sodium every 1 to 2 h and stop the hypertonic saline the moment symptoms resolve. This hyponatremia developed over minutes to hours, so the brain has not adapted and the demyelination risk is lower than in chronic hyponatremia, but respect the 24-hour ceiling anyway because you can rarely be certain the baseline was normal.
- Prevention, and why this is now uncommon: bipolar resection keeps the current flowing between two electrodes millimeters apart on the resectoscope tip, so it never crosses the patient and normal saline can be used as the irrigant, which has made classic TURP syndrome rare. Also limit resection time, keep the irrigation bag height low to reduce driving pressure, and check a sodium during long resections rather than waiting for symptoms.
The Irrigant Determines the Physiology
| Irrigant | Resulting picture | Extra toxicity beyond the sodium |
|---|---|---|
| Glycine 1.5% (~200 mOsm/L, most common in monopolar TURP) | Hypotonic hyponatremia with true hypo-osmolality, so cerebral edema is the danger | Metabolized to ammonia (encephalopathy outlasting the sodium correction) and transient visual disturbance or blindness from retinal inhibition |
| Sorbitol 3% | Hypotonic hyponatremia, same cerebral edema risk | Metabolized to fructose and glucose, so hyperglycemia and lactic acidosis, which matters most in diabetics |
| Mannitol 5% (~275 mOsm/L, near iso-osmolar) | Dilutional hyponatremia without hypo-osmolality, so volume overload dominates rather than cerebral edema | Intravascular expansion and pulmonary edema, then an osmotic diuresis as it is excreted |
| Normal saline (possible only with bipolar resection) | No hyponatremia, which is the entire reason bipolar equipment displaced monopolar | Large absorbed volumes can still cause hyperchloremic metabolic acidosis and fluid overload |
Step 2 -Volume Status (for Hypotonic)
| Volume Status | Urine Na | Causes | Treatment |
|---|---|---|---|
| Hypovolemic (dry mucous membranes, orthostasis, tachycardia, skin tenting) | < 20: extrarenal losses (GI: vomiting, diarrhea; 3rd spacing: burns, pancreatitis) > 20: renal losses (diuretics, adrenal insufficiency, cerebral salt wasting) | GI losses, diuretics, adrenal crisis, burns | Volume resuscitation with NS. Na will correct as volume is restored. Watch for overcorrection -once ADH stimulus (hypovolemia) is removed, kidneys dump free water rapidly. |
| Euvolemic (no edema, no orthostasis -hardest to assess) | > 40 (inappropriately concentrated urine) | SIADH (#1), hypothyroidism, adrenal insufficiency, psychogenic polydipsia (Uosm < 100), beer potomania, tea-and-toast | Fluid restriction (SIADH). Treat underlying cause. See SIADH topic for details. |
| Hypervolemic (edema, JVD, ascites, anasarca) | < 20: CHF, cirrhosis, nephrotic syndrome (effective hypovolemia → ADH release → water retention) > 20: CKD/ESKD (kidneys can't excrete water) | HF, cirrhosis, nephrotic, CKD | Fluid restriction + treat underlying disease. Diuretics for HF/cirrhosis. Dialysis if ESKD. |
| Hypervolemic, acute and iatrogenic post-TURP or operative hysteroscopy | > 20 (the sudden volume expansion drives a natriuresis, which lowers the sodium further) | Absorbed hypotonic irrigant: glycine 1.5% or sorbitol during monopolar resection. Suspect it on hypertension with bradycardia, the reverse of the usual intraoperative crash, plus confusion or visual disturbance in an awake patient | Stop the resection first. This is the one hypervolemic hyponatremia where you still give 3% hypertonic saline for seizures or severe symptoms, adding a loop diuretic for pulmonary edema. Full section |
Step 3 -Key Labs to Order
- Serum osmolality (first -classifies the hyponatremia)
- Urine osmolality (Uosm < 100 = appropriate dilution → water overload/polydipsia. Uosm > 100 = kidneys inappropriately concentrating → ADH-mediated)
- Urine sodium (renal vs extrarenal losses in hypovolemic; confirms SIADH if > 40 in euvolemic)
- Serum glucose (correct for hyperglycemia: add 1.6 per 100 above 100)
- TSH, AM cortisol (rule out hypothyroidism and adrenal insufficiency before diagnosing SIADH)
- Osmolal gap (measured minus calculated osmolality) whenever the patient received mannitol or absorbed irrigant -an unmeasured osmole means you are dealing with a translocational or iatrogenic hyponatremia, not a volume problem, and the treatment changes completely
🚨 Management
Correction Rates
The danger of hyponatremia treatment is NOT the low sodium -it's correcting too fast. Osmotic demyelination syndrome (ODS) is iatrogenic, irreversible, and devastating. ODS Prevention Study, 2010
| Scenario | Max Correction Rate | Treatment |
|---|---|---|
| Chronic (> 48h or unknown duration) | ≤ 8 mEq/L in 24h (some guidelines use ≤ 10). High-risk for ODS: ≤ 6 mEq/L in 24h. | Fluid restriction (SIADH), NS (hypovolemic), treat underlying cause. Check Na q4–6h. |
| Acute (< 48h, known onset) | Can correct faster -brain hasn't adapted. Still aim for ≤ 10–12 mEq/L in 24h. | More aggressive treatment acceptable. Still monitor closely. |
| Symptomatic (seizures, coma, severe AMS) | Immediate goal: raise Na by 4–6 mEq/L in first 6h to stop symptoms. Then ≤ 8 total in 24h. | 3% hypertonic saline 100–150 mL IV bolus over 10–20 min. SALSA, 2021 May repeat × 2. ICU. Check Na q2h. |
Osmotic Demyelination Syndrome (ODS)
- Occurs 2–6 days after overcorrection -not immediately
- Central pontine myelinolysis → "locked-in syndrome" (quadriplegia, inability to speak/swallow, preserved consciousness)
- Risk factors for ODS: chronic hyponatremia (> 48h), alcoholism, malnutrition, hypokalemia (correct K⁺ simultaneously -K⁺ correction counts toward Na correction!), liver disease, Na < 105
- If overcorrecting (Na rising too fast): (1) D5W infusion (free water to re-lower Na), (2) DDAVP 2 mcg IV q8h (clamp urine output → stop Na from rising further), (3) target: bring Na back within safe correction range
🔄 Updated Practice: Old teaching: correct sodium no more than 10-12 mEq/L in 24 hours. Updated: the recommended limit is now ≤8 mEq/L in any 24-hour period for patients at high risk for osmotic demyelination syndrome (ODS) -chronic hyponatremia (>48h), alcoholism, malnutrition, hypokalemia, liver disease. For truly acute hyponatremia (<48h, e.g., exercise-associated, postoperative), faster correction is safe. If overcorrected: give back free water (D5W) and consider desmopressin (DDAVP) 2 mcg IV to re-lower sodium.
Danger point: hypovolemic hyponatremia corrected with NS. Once volume is restored, the ADH stimulus disappears → kidneys suddenly excrete massive free water → Na shoots up rapidly → osmotic demyelination syndrome (ODS) risk. Prevention: anticipate it, cap correction ≤ 8 mEq/L in any 24h period in high-risk patients (chronic hyponatremia, alcoholic, malnourished, liver disease, hypokalemia). If overshooting: D5W infusion ± DDAVP 2 mcg IV to re-lower Na into the safe range.
Swipe for more examples
📋 Case 1, SIADH from SCLC
Patient: 68F with SCLC, found lethargic, Na⁺ 118 mEq/L.
Step 1 -Is this real? Serum osm: 248 (< 280 = true hypoosmolar hyponatremia). Not pseudohyponatremia.
Step 2 -Assess volume status:
- Exam: Mucous membranes moist, no edema, no JVD, skin turgor normal → euvolemic
- Urine Na: 45 mEq/L (> 40 = kidneys wasting Na → SIADH)
- Urine osm: 520 (> 100 = kidneys concentrating inappropriately → SIADH)
- TSH: normal. AM cortisol: normal (rules out hypothyroid and adrenal insufficiency)
Diagnosis: SIADH from SCLC (paraneoplastic ADH secretion)
Step 3 -Treatment:
- Symptomatic (lethargic) → 3% hypertonic saline 100 mL IV bolus over 10 min. Recheck Na in 2h.
- Na went 118 → 121 (+3 in 2h) -good. Symptoms improving.
- Target: correct ≤ 8 mEq/L in first 24h (risk of osmotic demyelination if faster)
- Fluid restriction 1L/day for maintenance
- If refractory SIADH: tolvaptan (Samsca) 15mg PO daily (check Na q6h -potent)
📋 Case 2, Beer Potomania
Patient: 48M heavy beer drinker (12+ beers/day), found confused. Na⁺ 108 mEq/L. No edema. Uosm 58 (maximally dilute).
Diagnosis: Beer potomania, massive free water intake with minimal solute intake → kidneys cannot excrete the water load despite maximally dilute urine.
Treatment:
- STOP free water intake. Normal saline is usually sufficient (not hypertonic) because once solute intake resumes, kidneys correct rapidly.
- Monitor Na q2h closely.
- If correcting > 8 mEq/day → give DDAVP 2 mcg IV q8h + D5W to slow correction.
⚠️ DANGER: These patients auto-correct too fast once admitted and beer is stopped. The kidneys suddenly have enough solute to excrete free water → Na shoots up.
Key lesson: Beer potomania corrects itself dangerously fast when you admit the patient and stop beer. DDAVP rescue may be needed to SLOW correction.
📋 Case 3, SIADH from SSRI
Patient: 72F started on sertraline 3 weeks ago, presents with nausea and confusion. Na⁺ 122 mEq/L. Euvolemic.
Labs: Uosm 580 (inappropriately concentrated). Urine Na 65. TSH and cortisol normal.
Diagnosis: Classic SIADH from SSRI.
Treatment:
- Fluid restriction 1–1.2 L/day (first-line for SIADH).
- If not improving → salt tabs 1–2 g TID + loop diuretic (furosemide 20 mg, forces dilute urine).
- Consider stopping sertraline or switching to a less SIADH-prone antidepressant.
- Avoid hypertonic saline unless symptomatic (seizures, severe AMS).
Key lesson: Always check the med list for SIADH causes. SSRIs are the #1 medication cause. Fluid restriction is first-line, not hypertonic saline.
Related Topics
Acid-Base DisordersCalciphylaxis (Calcific Uremic Arteriolopathy)Chronic Kidney Disease (CKD)Contrast-Induced NephropathyDialysis IndicationsHypernatremia
🧪 Workup
Diagnostic Workup -Hyponatremia
Systematic approach: (1) Serum osmolality → (2) Volume status → (3) Urine studies. Do NOT diagnose SIADH without first ruling out hypothyroidism and adrenal insufficiency.
| Test | Rationale | Key Values / Interpretation |
|---|---|---|
| Serum osmolality | First step -classifies the hyponatremia. | < 275 = hypotonic (true hyponatremia -proceed to step 2). 275-295 = isotonic (pseudohyponatremia -check lipids, protein). > 295 = hypertonic (hyperglycemia -correct Na: add 1.6 mEq/L per 100 mg/dL glucose above 100). |
| Urine osmolality | Distinguishes ADH-mediated from water overload. | < 100 mOsm/kg = kidneys appropriately diluting (polydipsia, beer potomania, tea-and-toast). > 100 = ADH-mediated (inappropriate concentration → SIADH, hypovolemia, hypervolemia). |
| Urine sodium | Differentiates renal from extrarenal sodium losses in hypovolemic states; confirms SIADH in euvolemic. | UNa < 20 = extrarenal losses (GI, third-spacing) or effective hypovolemia (CHF, cirrhosis). UNa > 40 in euvolemic state = SIADH. UNa > 20 in hypovolemic = renal losses (diuretics, adrenal insufficiency). |
| Volume status (clinical) | Essential physical exam -guides entire differential and treatment. | Hypovolemic (orthostasis, dry mucous membranes, tachycardia) → NS. Euvolemic → SIADH workup. Hypervolemic (edema, JVD, ascites) → CHF/cirrhosis/nephrotic. |
| TSH | Hypothyroidism is a reversible cause -must exclude before diagnosing SIADH. | Severe hypothyroidism → decreased free water clearance → hyponatremia. Treat thyroid disease first. |
| AM cortisol | Adrenal insufficiency mimics SIADH (euvolemic, high UNa). Must exclude before SIADH diagnosis. | AM cortisol < 3 = adrenal insufficiency likely. 3-15 = indeterminate → ACTH stimulation test. > 15 = AI unlikely. |
| Serum glucose | Correct Na for hyperglycemia (translocational hyponatremia). | Corrected Na = measured Na + 1.6 × [(glucose - 100) / 100]. If corrected Na is normal → not true hyponatremia. |
💊 Medications
Drug Therapy -Hyponatremia
| Drug | Dose | Route | Indication | Key Notes |
|---|---|---|---|---|
| 3% Hypertonic saline | 100-150 mL IV bolus over 10-20 min. May repeat × 2 (max 3 boluses). | IV | Severe symptomatic hyponatremia (seizures, coma, severe AMS) | ICU setting. Goal: raise Na by 4-6 mEq/L in first 6h to stop symptoms. Check Na q2h. Rapid intermittent bolus is as effective and safer than continuous infusion SALSA, 2021. |
| Fluid restriction | 1-1.5 L/day (all PO and IV fluids combined) | - | SIADH (first-line), hypervolemic hyponatremia (HF, cirrhosis) | Effective if urine osmolality is not extremely high. Poor compliance limits effectiveness. Calculate free water clearance to predict response. |
| NaCl tablets (salt tabs) | 1-3 g PO TID | PO | Chronic SIADH (with or without loop diuretic) | Often combined with furosemide 20 mg daily -the diuretic promotes free water excretion while salt tabs replenish sodium. Effective outpatient strategy. |
| Furosemide | 20-40 mg PO daily | PO | Combined with salt tabs for chronic SIADH | Impairs urinary concentration → promotes electrolyte-free water excretion. Only effective when combined with adequate sodium intake (salt tabs). |
| Tolvaptan (Samsca) | 15 mg PO daily (may increase to 30-60 mg) | PO | Refractory SIADH or hypervolemic hyponatremia not responding to fluid restriction | V2 receptor antagonist ("vaptan") -blocks ADH at collecting duct → aquaresis (free water loss). Must initiate inpatient. Check Na q6h for first 24h. Hepatotoxicity risk -do not use > 30 days. Do NOT use in hypovolemic hyponatremia. |
| Demeclocycline | 300-600 mg PO BID | PO | Alternative for chronic SIADH (if tolvaptan not available/tolerated) | Tetracycline that induces nephrogenic DI. Slow onset (3-5 days). Nephrotoxic -avoid in liver disease. Largely replaced by tolvaptan. |
| DDAVP (desmopressin) | 2 mcg IV q8h | IV | ODS rescue -given when Na is overcorrecting too rapidly | Clamps urine output → stops further Na correction. Combine with D5W (3-6 mL/kg/hr) to actively re-lower Na. Target: bring correction rate back to ≤ 8 mEq/L in 24h. |
| D5W (5% dextrose) | 3-6 mL/kg/hr | IV | ODS rescue -free water to re-lower sodium if overcorrecting | Used with DDAVP. The dextrose is metabolized, leaving free water. Start immediately if Na rising > 8-10 mEq/L in 24h. |
📋 On Rounds
Why does correcting hypokalemia matter in hyponatremia?
Potassium is osmotically active and contributes to the Na correction rate. When you give IV KCl, K⁺ enters cells and Na⁺ comes out (to maintain electrochemical balance via the Na⁺/K⁺-ATPase). This means every mEq of K⁺ you give raises the serum Na by approximately the same amount as giving a mEq of Na. If you're correcting hypokalemia aggressively alongside hyponatremia, the Na may rise faster than you expect.
A urine osmolality of 50 mOsm/kg in a hyponatremic patient tells you what?
The kidneys are doing their job -maximally diluting urine. This means ADH is appropriately suppressed and the kidneys are trying to excrete free water. The hyponatremia is from water intake exceeding the kidney's diluting capacity. Differential: psychogenic polydipsia (> 15–20 L/day overwhelms even normal kidneys), beer potomania (low solute intake → low obligate urine output → can't excrete enough water), tea-and-toast diet (same mechanism).
You are correcting hyponatremia and realize you've already gone up 10 mEq in 18 hours. What do you do?
ODS rescue protocol -act immediately: (1) Stop all hypertonic saline and salt tabs, (2) Give D5W at 3-6 mL/kg/hr (free water to re-lower sodium), (3) Give DDAVP 2 mcg IV q8h (makes kidneys retain free water, prevents further correction). (4) Recheck Na q2h until it falls back to the safe correction rate (≤ 8 mEq/24h, some experts say ≤ 6 in high-risk patients).
What are the most dangerous medications that cause SIADH?
SSRIs are the #1 drug cause of SIADH (sertraline, fluoxetine, paroxetine -onset typically 1-4 weeks). Other common offenders: carbamazepine/oxcarbazepine (direct ADH-like effect on collecting duct), cyclophosphamide (especially with high-volume hydration), vincristine (neuropathic ADH release), opioids, PPIs, NSAIDs, ecstasy/MDMA (causes acute hyponatremia from both ADH release and excessive water intake → can be fatal).
Clinical Examples
📋 Case 1, SIADH from SSRI with Overcorrection Risk
Patient: 72 y/o F on sertraline x3 weeks, presents with confusion. Na⁺ 112 (was 128 two weeks ago).
Key findings: Serum osm 238, urine osm 480, UNa 52, euvolemic. TSH and cortisol normal. SIADH from sertraline.
Management:
- Stop sertraline, removing the SIADH stimulus may cause rapid free water excretion
- Fluid restriction < 1L/day + salt tabs 1g TID
- Na rose 10 mEq in 18h (overcorrection), start ODS rescue: D5W + DDAVP 2 mcg IV q8h
- Re-lower Na back to safe correction rate (≤ 8 mEq/24h); recheck Na q2h
Teaching point: When the SIADH stimulus is removed, ADH drops and free water is excreted rapidly, causing overcorrection. ODS is devastating and irreversible. High-risk patients need proactive DDAVP clamping.
📋 Case 2, Severe Symptomatic Hyponatremia with Seizures
Patient: 55 y/o M with SCLC, found seizing at home. Na⁺ 104, obtunded.
Key findings: Serum osm 218, urine osm 640, UNa 68, GCS 8. Euvolemic. Ectopic ADH from SCLC.
Management:
- 3% NaCl 100-150 mL bolus over 10 min, repeat x1 if still seizing
- Target 4-6 mEq/L acute rise to stop seizures, then ≤ 8 mEq/24h total
- Check Na q2h during correction; DDAVP 2 mcg IV q8h proactively to prevent overcorrection
- ICU admission, oncology for underlying SCLC treatment
Teaching point: Seizures from hyponatremia require emergent 3% NaCl, the one scenario where rapid correction is indicated. Target only enough rise to stop symptoms, then strictly limit total correction.
📋 Case 3, Beer Potomania (Low Solute Hyponatremia)
Patient: 48 y/o M, chronic alcohol use, Na⁺ 118. Drinks 12+ beers daily, minimal food intake.
Key findings: Serum osm 248, urine osm 52 (maximally dilute, ADH suppressed), UNa 8. Malnourished.
Management:
- Urine osm 52 = ADH appropriately suppressed, this is NOT SIADH
- Low solute intake → low obligate urine output → cannot excrete enough free water
- Treatment: normal diet (increase solute) + fluid restriction
- HIGH overcorrection risk when diet resumes, use prophylactic DDAVP
Teaching point: Urine osm < 100 rules out SIADH. Beer potomania corrects rapidly when normal diet resumes, proactive DDAVP is essential to prevent ODS. SALT-1/2, 2006 showed tolvaptan is effective for SIADH but is NOT appropriate for low-solute states.
📣 Sample Presentation
One-Liner
"Mrs. Lee is a 68-year-old with SCLC presenting with Na⁺ 118, serum osm 252, urine osm 520, urine Na 45, euvolemic on exam. Consistent with SIADH from malignancy."
Key Points to Cover on Rounds
Euvolemic hypoosmolar hyponatremia. Urine osm 520 (inappropriately concentrated), UNa 45 (>40). TSH and cortisol normal. Etiology: SIADH from SCLC. Asymptomatic (no seizure, no AMS). Treatment: fluid restriction <1L/day. Na trending 118→120→122 over 24h (rate 4 mEq/24h -safe). Salt tabs 1g TID added. If refractory → tolvaptan 15 mg daily SALT-1/2, 2006 (monitor in-hospital, check Na q6h). Goal: ≤8 mEq/L correction per 24h to avoid ODS. Plan: treat underlying malignancy, continue fluid restriction.
Monitoring -Hyponatremia Correction
| Parameter | Frequency | Target / Action |
|---|---|---|
| Serum sodium | q2h if on hypertonic saline; q4-6h during active correction; q6-8h once stable | Max correction ≤ 8 mEq/L in any 24h period (≤ 6 in high-risk: alcoholism, malnutrition, hypokalemia, liver disease, Na < 105). If overcorrecting → DDAVP + D5W rescue immediately. |
| Serum osmolality | At baseline, then q12-24h during correction | Should rise proportionally with Na. Guides assessment of correction adequacy. |
| Urine output | q1-2h during active treatment | Sudden brisk water diuresis (> 200 mL/hr of dilute urine) = danger sign for overcorrection. This happens when ADH stimulus is removed (e.g., volume resuscitation in hypovolemic hyponatremia). Start DDAVP preemptively if UOP surges. |
| Neurological status | q2-4h during active correction | Improving: resolution of confusion, seizures, lethargy = adequate correction. New dysarthria, dysphagia, quadriparesis 2-6 days after correction = ODS -MRI brain, neurology consult. |
| Potassium | With each Na check | K⁺ correction counts toward Na correction (K⁺ enters cells, Na⁺ comes out). If repleting K⁺ aggressively, account for this in your correction rate calculation. |
| I&Os | Strict q1h during active treatment | Track all free water intake (IV and PO). Ensure fluid restriction is enforced if indicated. Document urine osmolality if available (Uosm < 200 on tolvaptan = expected). |
Overcorrection rescue protocol: If Na rises > 8-10 mEq/L in 24h → (1) Stop all hypertonic saline and salt tabs, (2) D5W at 3-6 mL/kg/hr, (3) DDAVP 2 mcg IV q8h, (4) Recheck Na q2h until back within safe range. Act immediately -ODS is irreversible.
⚡ Summary
Summary
Symptoms Outrank Etiology
Seizures or severe obtundation mean hypertonic saline now: 3% NaCl 100 to 150 mL over 10 to 20 min, repeatable twice. Target an acute rise of only 4 to 6 mEq/L, because that is enough to shrink the brain and stop the seizing, and any more starts spending the 24-hour correction budget. The workup runs after the brain is safe, not before.
First Lab Question: Is It Even Hypotonic?
Check serum osmolality before anything else. Below 275 is true hypotonic hyponatremia. Normal osmolality is pseudohyponatremia, a lab artifact from paraproteins, severe lipemia or IVIG: the sodium is fine, so treating it harms. High osmolality is translocational, glucose or mannitol pulling water out of cells; correct the sodium up 2.4 mEq/L per 100 mg/dL of glucose above 100, because fixing the glucose fixes the sodium.
Urine Osmolality Asks: Is ADH On?
Urine osm below 100 means ADH is appropriately off and the kidney is dumping water as fast as it can: the problem is intake outrunning it (primary polydipsia, beer potomania, tea-and-toast low solute), fixed by restricting water and feeding solute. Urine osm above 100 means ADH is acting, and the question becomes whether that ADH release is appropriate (hypovolemia) or not (SIADH).
Urine Sodium Splits the ADH-On Group
Urine Na below 20 to 30 means the kidney is clinging to sodium: true hypovolemia, or the low effective arterial volume of CHF, cirrhosis and nephrotic syndrome, where total body water is up but the kidney perceives emptiness. Urine Na above 30 to 40 in a euvolemic patient points to SIADH, but only after excluding hypothyroidism and adrenal insufficiency (check TSH and cortisol), which mimic it and have their own fixes. Diuretic use muddies the urine Na, so interpret it off diuretics when possible.
Treatment Follows the Category
Hypovolemic: isotonic fluid, and expect the sodium to jump once volume is restored and ADH switches off abruptly, so recheck early. Hypervolemic: fluid and sodium restriction plus loop diuretics, treating the failing organ. SIADH: fluid restriction under 1 L/day first (water is the poison, so remove the water), then salt tabs with a loop diuretic, then tolvaptan, inpatient only with sodium checks every 6 h, because vaptans can overcorrect rapidly. Always hunt the SIADH cause: drugs, small cell lung cancer, CNS and pulmonary disease.
The Ceiling: 8 mEq/L per 24 h
Never correct more than 8 mEq/L in 24 h, and hold to 4 to 6 in high-risk patients: sodium 105 or below, alcohol use disorder, malnutrition, hypokalemia, advanced liver disease. The brain has already adapted by shedding osmolytes, so a fast rise dehydrates oligodendrocytes and causes osmotic demyelination: quadriparesis, pseudobulbar palsy, locked-in syndrome, appearing 2 to 6 days later and largely irreversible. Chronic hyponatremia is the dangerous one to correct; the adaptation is complete.
Overcorrection Rescue
If the sodium is rising past the limit, actively re-lower it: D5W plus DDAVP 2 mcg IV q8h. This is not optional damage control; re-lowering within the same day prevents demyelination. The classic setting is the hypovolemic patient whose ADH shut off after fluids, producing a brisk free-water diuresis: a rising urine output with dilute urine is the early warning to act.
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Nephrology · One Pager
Hyponatremia
Check serum osm → volume status → urine studies. Correct ≤ 8 mEq/24h. Overcorrection → ODS (devastating). Severe symptoms → 3% NaCl 100-150 mL bolus.
Approach
Step 1: Serum osm (< 275 = true). Step 2: Volume status (hypo/eu/hyper). Step 3: UNa + urine osm. Euvolemic + UNa > 40 + concentrated urine = SIADH.
Acute Severe Symptoms
Seizures or AMS → 3% NaCl 100-150 mL bolus over 10 min (can repeat ×1). Target 4-6 mEq rise to stop seizures. Then slow correction to ≤ 8/24h total.
ODS Prevention
Correct ≤ 8 mEq/24h (≤ 6 if high-risk: Na < 105, chronic, alcoholism, malnutrition, liver disease). If overcorrecting: D5W + DDAVP 2 mcg IV q8h immediately.
Key Drugs
3% NaCl100-150 mL bolus (severe)
DDAVP2 mcg IV q8h (ODS rescue)
Tolvaptan15 mg daily (SIADH, inpatient only)
Salt tabs1g TID (chronic SIADH)
Pitfalls
- Overcorrection → ODS (irreversible pontine demyelination)
- Not checking TSH and cortisol (must exclude before diagnosing SIADH)
- Free water restriction while on tolvaptan (dangerous combo)
- IV NS for hypovolemic hyponatremia in SIADH (worsens it)