11.2 SIADH and Cerebral Salt Wasting (01.R.5–6)

Hyponatremia is the most common electrolyte disorder in the neuro ICU. After SAH, TBI, meningitis, and pituitary surgery it is also a management trap, because two named syndromes share a high urine sodium and a concentrated urine and then demand opposite water orders. This section covers the syndrome of inappropriate antidiuretic hormone secretion (SIADH) and cerebral salt wasting (CSW). Independent OpenExamPrep material here follows topics listed under 01.R.5–6 in the ABPN Content Specifications. It is not an ABIM or ABPN product.

Why the distinction is an examination item

A day-7 aneurysmal SAH patient with sodium 128 mEq/L, urine sodium 90 mEq/L, and urine osmolality 520 mOsm/kg can be SIADH or CSW on those two numbers alone. If you restrict free water in a volume-depleted patient, you thin the circulating volume at the same moment the brain is most vulnerable to delayed cerebral ischemia (DCI). If you pour isotonic saline into true SIADH, the salt may be excreted and the water retained, and sodium may not rise. The examination is asking whether you will look at volume and whether you know that SAH patients are not medical-ward SIADH.

Some endocrine series (for example Hannon and colleagues, JCEM 2014, in mild-to-moderate SAH) found glucocorticoid deficiency and SIADH far more often than classic CSW. The existence and frequency of CSW remain debated. That debate does not license automatic restriction. Operationally you treat hypovolemic hyponatremia with volume and sodium, and you do not restrict a SAH patient who looks dry.

Start with tonicity, then volume

Confirm hypotonic hyponatremia (low plasma osmolality). Hyperglycemia, mannitol, and recent contrast can give a low measured sodium that is not a water-excess problem. Then ask whether the patient is hypovolemic, euvolemic, or hypervolemic.

Hypovolemia at the bedside: orthostatic drop, dry mucosa, tachycardia, low CVP or collapsing IVC, rising hematocrit and albumin, falling weight, net negative fluid balance, low urine output that then becomes a high-sodium diuresis. Euvolemia in SIADH is the absence of those findings and the absence of edema. Hypervolemia (heart failure, cirrhosis, nephrosis) is a different ADH-physiology story and is not CSW.

Secondary adrenal insufficiency belongs on the same list as SIADH. Cortisol deficiency impairs free-water excretion and looks like SIADH until hydrocortisone is given. After pituitary surgery or apoplexy, replace cortisol before you argue about vaptans.

SIADH

SIADH is ADH activity that is inappropriate for a low plasma osmolality. Water is retained, the urine is concentrated, and a modest natriuresis follows volume expansion of the intravascular compartment — so urine sodium is high even though the patient is not "salt wasting" in the CSW sense.

Usual laboratory pattern:

• Low plasma osmolality.
• Urine osmolality >100 mOsm/kg, and typically higher than plasma osmolality (often >300–500).
• Urine sodium typically >40 mEq/L on a normal salt intake (lower if the patient is salt-restricted or volume-depleted from another cause).
• Low BUN and low uric acid.
• Clinically euvolemic: no edema, no frank dehydration.

Neuro ICU precipitants: SAH, TBI, stroke, meningitis and encephalitis, and drugs (carbamazepine, oxcarbazepine, SSRIs, ecstasy, some antipsychotics, cyclophosphamide). Small-cell lung cancer remains the classic medical SIADH, and a heavy-smoking ICU patient with hyponatremia still deserves that thought once the acute brain injury is stable.

Treatment when the patient is truly euvolemic, mildly symptomatic, and not in the DCI window of SAH:

• Fluid restriction, often 800–1000 mL/day, with the salt in the diet continued.
• Oral urea (typical adult ranges 15–30 g/day, titrated) as an osmotic way to drive water off.
• Loop diuretic plus salt tablets in selected euvolemic patients.
• Vaptans (tolvaptan, conivaptan) block V2 receptors. They are tools for euvolemic or hypervolemic SIADH, not for hypovolemia, not for a seizing patient who needs a controlled 3% bolus, and not as casual first-line therapy after SAH. Tolvaptan carries a liver-injury warning and needs a setting where sodium can be checked often so you do not overcorrect.
• Hypertonic saline when symptoms are severe, regardless of the label SIADH versus CSW.

Isotonic 0.9% saline is a weak SIADH treatment. If urine osmolality is higher than the infusate, the kidney can dump the sodium and keep the water. That is why a "just give normal saline" reflex fails in SIADH and why 3% saline or a solute load is the honest sodium-raising plan when you need the number to move.

Cerebral salt wasting

Cerebral salt wasting is renal sodium loss driven by a brain injury, producing volume depletion and hyponatremia. Proposed mechanisms include natriuretic peptides (BNP/ANP) and reduced sympathetic tone to the kidney, so proximal sodium reabsorption falls. The urine looks like SIADH: high sodium, high osmolality. The patient does not look like SIADH: they are losing weight, running a negative sodium balance, and looking intravascularly dry.

CSW is described after SAH, TBI, neurosurgery, and CNS infection. Polyuria in CSW is a salt diuresis, so urine specific gravity is not the <1.005 of DI. Plasma sodium falls or stays low while urine output is high — the opposite of untreated DI.

Treatment is the reverse of restriction:

• Restore intravascular volume with isotonic crystalloid first if the patient is frankly hypovolemic.
• Replace sodium with salt tablets, hypertonic saline infusions, or a sodium-acetate/chloride mix in the maintenance fluid, targeting euvolemia and a controlled sodium rise.
• Fludrocortisone (often 0.1–0.2 mg daily, sometimes up to about 0.4 mg) can reduce natriuresis after SAH when salt wasting is ongoing. Watch hypokalemia and volume overload.
• Do not start a vaptan in a hypovolemic patient. Blocking ADH when the kidney is already wasting sodium and the patient is dry is the wrong direction.

Comparison table (memorize this one)

Urine sodium and urine osmolality do not separate the two. Volume status does. If you cannot tell, look at yesterday's weights, the I/O sheet, the hematocrit trend, and the CVP or bedside echo — not at a single urine sodium printed in isolation.

Hyponatremia after SAH: do not default to restriction

Aneurysmal SAH care is built on euvolemia, early aneurysm securement, and nimodipine. Historical "triple-H" (hypertension, hypervolemia, hemodilution) as a prophylactic package is retired; induced hypertension is used for symptomatic DCI, and volume is kept at euvolemia rather than flooding. Hyponatremia in this population peaks in the same epoch as vasospasm and DCI (often days 4–14).

Fluid restriction in that window, if the patient actually has CSW or even if they are merely tending dry from natriuresis and cerebral salt losses, reduces cerebral perfusion. DCI is a preventable secondary injury. Independent OpenExamPrep teaching for this examination is: do not treat post-SAH hyponatremia as ward SIADH. Maintain volume, replace sodium (often with 1.5–3% saline titrated to sodium and volume), consider fludrocortisone for ongoing natriuresis, and use hypertonic saline for a dangerous sodium or a worsening exam. Urea is used in some SAH protocols as a way to raise sodium without drying the patient out; it is still not a license to clamp intake at 800 mL.

If the patient is clearly euvolemic, the aneurysm is secured, DCI risk is low, and the picture is textbook SIADH, modest restriction can be reasonable. That is a later, quieter patient — not the restless day-7 MCA-territory-at-risk patient with a flat IVC.

Severe symptoms: hypertonic saline first

Seizure, coma, vomiting with a rapidly falling sodium, or signs of herniation are sodium emergencies. The treatment is hypertonic (3%) saline, not a consult and a fluid-restriction sign on the door.

Two common bolus recipes, both acceptable to know:

• European hyponatremia guidance: 150 mL of 3% saline over 20 minutes, repeat while checking sodium, aiming for a small early rise (about 5 mmol/L) and symptom improvement.
• Expert US practice (Verbalis/Sterns-style): 100 mL of 3% saline over 10 minutes, repeat up to two more times as needed.

Either bolus is meant to increase sodium enough to reduce cerebral edema, not to normalize the laboratory overnight. After the emergency rise of about 4–6 mmol/L, you stop chasing and you respect the 24-hour cap.

A 3% saline infusion (for example 20–30 mL/h, adjusted) is used when the sodium needs a controlled climb and the patient is not seizing. Central access is preferred for prolonged 3% infusions; a short emergency bolus can go through a large peripheral line while you watch the site.

Osmotic demyelination: the correction-rate problem

Osmotic demyelination syndrome (ODS), including central pontine myelinolysis and extrapontine myelinolysis, is the delayed (often 2–6 days) complication of overcorrecting chronic hyponatremia. Oligodendrocytes in the pons and elsewhere cannot keep up with a sudden rise in extracellular tonicity. The clinical picture is dysarthria, dysphagia, spastic quadriparesis, locked-in syndrome, movement disorders, and a patient who looked better when the sodium was 118 and looks devastated at 135.

Risk is highest when hyponatremia has been present >48 hours and the patient has alcohol use, malnutrition, or liver disease. Acute hyponatremia (true <24–48 h, as in postoperative hypotonic fluids or MDMA) is more a herniation risk than an ODS risk; you may need a faster early rise to protect the brain.

Practical caps used on this examination:

• European guidance: do not exceed 10 mmol/L in the first 24 hours, then 8 mmol/L per 24 hours thereafter, until the sodium is near 130 mEq/L.
• Many US experts aim 4–8 mmol/L/day, and 4–6 mmol/L/day (not more than about 8) in high-risk patients.

If the sodium overshoots — a DDAVP-withdrawn postoperative patient who suddenly makes dilute urine, or a vaptan that worked too well — stop the saline, give electrolyte-free water, and consider desmopressin to clamp further water loss. Relowering an overcorrected sodium is how you prevent ODS after the fact.

Check sodium frequently (every 2–4 hours during active 3% therapy, then at 6–12 hours) until the trajectory is boring.

Worked bedside scenarios

A 56-year-old on day 6 after coiled ACom aneurysm has Na 127 mEq/L, urine Na 88 mEq/L, urine osm 480, CVP 3 mm Hg, and a 1.4 kg weight loss. This is treated as salt wasting / hypovolemic hyponatremia: volume and sodium, not 800 mL restriction.

A 70-year-old on oxcarbazepine, euvolemic, Na 128, urine osm 620, no SAH, normal exam: SIADH physiology. Restriction, hold the drug if you can, urea if needed. No vaptan as a first reflex if the number is mild.

A 44-year-old seizes with Na 112 mEq/L two days after SAH: 100–150 mL of 3% saline now. Argue SIADH versus CSW after the seizure stops.

Exam traps

Using urine sodium alone to name SIADH. Fluid-restricting a dry SAH patient. Giving a vaptan in hypovolemia. Infusing 0.45% saline into SIADH. Correcting chronic Na 108 to 132 in an afternoon. Forgetting cortisol deficiency as an SIADH mimic after pituitary surgery. Treating DI (rising Na, SG <1.005) as CSW because "the urine output is high."