7.3 DI, SIADH & Cerebral Salt Wasting

Why these three disorders dominate the pediatric neuro ICU

After traumatic brain injury, intracranial infection, hypoxic-ischemic injury, or neurosurgery—especially craniopharyngioma, pituitary, or hypothalamic procedures—children frequently develop disorders of antidiuretic hormone (ADH, vasopressin) or renal salt handling. The CCRN Pediatric exam stresses that serum sodium alone cannot distinguish the cause. You must integrate volume status, urine output, serum osmolality, urine osmolality, and urine sodium before choosing fluids, restriction, or desmopressin.

Antidiuretic hormone from the posterior pituitary promotes free-water reabsorption in the collecting duct. Too little effective ADH → water loss (diabetes insipidus). Too much ADH → water retention (SIADH). A separate entity, cerebral salt wasting, is a brain-mediated natriuresis with extracellular volume depletion—clinically easy to confuse with SIADH because both lower serum sodium.

Diabetes insipidus (DI)

Central DI is the usual PICU form: inadequate ADH release after pituitary/hypothalamic injury or surgery. Nephrogenic DI (ADH resistance) is less common but can follow lithium exposure, severe hypokalemia, or hypercalcemia.

Hallmarks of DI:

• Polyuria — often >4 mL/kg/hr (can be massive, e.g., hundreds of mL/hr in larger children)
• Dilute urine — urine specific gravity often <1.005, urine osmolality typically <200–300 mOsm/kg and lower than serum
• Rising serum sodium and high serum osmolality as free water is lost
• Thirst in awake children; in intubated/sedated patients the only clues may be escalating urine output and climbing Na+

Nursing priorities: hourly urine output, matching replacement of free-water losses (often with enteral water or IV dextrose-containing hypotonic fluid per neurosurgery protocol), and desmopressin (DDAVP) for central DI. After DDAVP, watch closely for overshoot into hyponatremia. Strict intake-and-output and q4–6h (or more frequent) sodium checks are standard after pituitary surgery. Untreated DI rapidly produces hypovolemic hypernatremic encephalopathy and hemodynamic instability.

SIADH

Syndrome of inappropriate ADH secretion produces water retention, dilutional hyponatremia, and concentrated urine despite low plasma osmolality. Pediatric triggers include meningitis, encephalitis, subarachnoid hemorrhage, TBI, pneumonia, positive-pressure ventilation, pain/nausea, and medications (e.g., some anticonvulsants, vincristine).

Hallmarks of SIADH:

• Euvolemia or mild fluid excess (not frankly edematous like nephrotic syndrome, but not volume depleted)
• Low serum Na+ and low serum osmolality
• Inappropriately high urine osmolality (often >100–300 mOsm/kg when it should be maximally dilute)
• Urine sodium usually elevated (commonly >40 mEq/L) while the child is euvolemic
• Low urine output relative to DI

Treatment: fluid restriction is first-line for mild-moderate asymptomatic SIADH. For seizures, deep coma, or severely symptomatic hyponatremia, give 3% hypertonic saline in controlled doses and correct sodium gradually (commonly avoid rises >8–10 mEq/L in 24 hours) to reduce osmotic demyelination risk. Loop diuretics are sometimes added when volume expansion is evident. Demanding a single “correct” sodium number without neuro status context is a common exam trap—treat the brain first.

Cerebral salt wasting (CSW)

Cerebral salt wasting occurs with intracranial disease when renal sodium wasting causes true volume depletion and hyponatremia. Proposed mechanisms include excessive natriuretic peptides and neural influences on the kidney. CSW is particularly discussed in children with TBI, brain tumors, and neurosurgical patients.

Hallmarks of CSW:

• Hypovolemia — tachycardia, dry mucous membranes, poor perfusion, elevated hematocrit/BUN suggesting hemoconcentration, negative fluid balance
• Hyponatremia with high urine sodium (often strikingly elevated)
• High urine output that is salty, not the massive dilute water loss of DI
• Low or normal serum osmolality reflecting hyponatremia, but the key discriminator versus SIADH is volume depletion

Treatment is the opposite of SIADH: restore intravascular volume with isotonic (sometimes hypertonic) saline, replace ongoing urinary sodium losses, and consider fludrocortisone in refractory cases under specialist guidance. Fluid restriction is harmful in CSW because it worsens hypovolemia and cerebral perfusion.

Side-by-side comparison for rapid recognition

Neuro ICU nursing scenarios

Scenario A — night of craniopharyngioma resection: urine output jumps from 1 mL/kg/hr to 8 mL/kg/hr, urine SG 1.002, serum Na+ rises from 140 to 151 mEq/L. This is classic central DI. Notify the team, replace losses, and prepare DDAVP while protecting against rapid overcorrection.

Scenario B — meningitis day 3: serum Na+ 122 mEq/L, child appears euvolemic, urine osmolality high, urine output modest. Think SIADH. Restrict free water; if seizing, give hypertonic saline per protocol.

Scenario C — severe TBI: serum Na+ 125 mEq/L, tachycardia, sunken eyes, net negative fluid balance, urine Na+ 120 mEq/L. This pattern favors CSW. Give isotonic/volume expansion and sodium replacement—do not fluid-restrict as if it were SIADH.

Practical nursing bundle for all three: labeled bags and burettes for precise pediatric volumes, q1h intake/output during instability, paired serum/urine studies when sodium shifts, neurologic exams timed with sodium changes, and clear handoff of whether the current plan is restriction, free-water replacement, or salt/volume replacement. Misclassification between SIADH and CSW is one of the highest-yield cognitive traps on the exam and at the bedside.