In a child presenting with polyuria and headache, what is the clinical significance and which investigations are indicated?

Polyuria with Headache in Children: Clinical Significance and Investigation

Clinical Significance

The combination of polyuria and headache in a child is a red flag that demands urgent evaluation for central diabetes insipidus (DI) secondary to intracranial pathology, particularly craniopituitary lesions, until proven otherwise. 1, 2, 3

This presentation is particularly concerning because:

• Craniopharyngiomas, germinomas, and other sellar/suprasellar masses frequently present with both symptoms—polyuria from posterior pituitary dysfunction and headache from mass effect or increased intracranial pressure 1, 2
• Post-traumatic or post-surgical DI following head trauma or pituitary surgery manifests with polyuria and may have concurrent headache from the underlying injury 2
• The temporal relationship matters: headache preceding polyuria suggests an expanding mass lesion, while simultaneous onset may indicate acute trauma or hemorrhage 1, 3

Immediate Diagnostic Approach

Step 1: Confirm True Polyuria (Do Not Rely on History Alone)

• Obtain a 24-hour urine collection or accurate fluid intake/output diary showing >2 L/m²/day (or >3 L/day in adolescents) to confirm polyuria versus pollakiuria 3, 4
• Measure spot urine osmolality: <300 mOsm/kg indicates water diuresis (DI or primary polydipsia), while >300 mOsm/kg suggests osmotic diuresis (diabetes mellitus, renal disease) 3, 5, 6
• Check serum osmolality and sodium: elevated levels (>295 mOsm/kg or Na >145 mEq/L) with dilute urine confirm inadequate ADH effect 3, 5, 7

Step 2: Urgent Neuroimaging (Do Not Delay)

Brain MRI with and without contrast, including dedicated pituitary views, is mandatory in any child with confirmed polyuria and headache. 1, 2

• MRI is superior to CT for visualizing the pituitary stalk, posterior pituitary bright spot (absent in central DI), and small sellar/suprasellar masses 1
• Do not perform a water deprivation test before imaging if headache is present—this delays diagnosis of potentially life-threatening lesions 1, 2, 3
• CT without contrast may be appropriate only in the acute setting if thunderclap headache suggests subarachnoid hemorrhage, but MRI should follow 1

Step 3: Simultaneous Laboratory Evaluation

While arranging imaging, obtain:

• Serum glucose to exclude diabetes mellitus (osmotic diuresis) 3, 4
• Serum calcium and potassium to exclude hypercalcemia or hypokalemia causing nephrogenic DI 3, 4
• Blood urea nitrogen and creatinine to assess renal function 3, 4
• Paired serum and urine osmolality during polyuria: serum osm >295 mOsm/kg with urine osm <300 mOsm/kg confirms DI 3, 5, 6
• Copeptin or plasma vasopressin (if available) can differentiate central from nephrogenic DI without water deprivation 5

Step 4: Ophthalmologic Examination

• Fundoscopic examination to assess for papilledema (increased intracranial pressure) or optic atrophy (chronic compression from craniopharyngioma) 1
• Visual field testing if cooperative, as bitemporal hemianopsia indicates chiasmal compression 1

Diagnostic Algorithm After Initial Workup

If MRI Shows Sellar/Suprasellar Mass:

• Urgent neurosurgical and endocrinology consultation 1, 2
• Complete pituitary hormone panel (TSH, free T4, cortisol, LH, FSH, IGF-1, prolactin) to assess anterior pituitary function 2
• Desmopressin trial (5-10 mcg intranasal or 0.1-0.2 mg oral) to confirm central DI: urine osmolality should increase >50% and urine output decrease 2, 3, 5

If MRI is Normal but Polyuria Confirmed:

• Proceed with supervised water deprivation test (8-12 hours) with hourly urine osmolality and body weight monitoring 3, 5, 7
• Stop test if weight loss >5%, serum Na >145 mEq/L, or serum osm >295 mOsm/kg 5, 7
• Desmopressin challenge after deprivation: urine osm increase >50% confirms central DI; <10% increase confirms nephrogenic DI; intermediate response suggests partial defects or primary polydipsia 3, 5, 7

If Headache Pattern Suggests Primary Headache Disorder:

• Do not assume benign etiology without imaging when polyuria is present 1
• Migraine and tension-type headaches are diagnoses of exclusion in this context 1
• Even if headache characteristics seem typical for migraine, the presence of polyuria mandates structural imaging 1

Critical Pitfalls to Avoid

• Never attribute polyuria to "behavioral" causes (excessive water drinking) without excluding central DI from intracranial pathology—primary polydipsia is a diagnosis of exclusion 3, 5, 7
• Do not perform water deprivation testing before neuroimaging in children with headache—this delays diagnosis of treatable tumors 1, 2, 3
• Do not rely on CT alone for pituitary evaluation; MRI with contrast is essential for small adenomas, craniopharyngiomas, and pituitary stalk lesions 1
• Avoid assuming diabetes mellitus without checking glucose—central DI can present with similar polyuria/polydipsia but requires entirely different management 3, 4
• Do not miss nephrogenic causes: hypercalcemia, hypokalemia, and certain medications (lithium, amphotericin) can cause polyuria but would not explain headache unless coincidental 3, 4

When to Refer Urgently

• Any child with polyuria + headache requires same-day evaluation and imaging coordination 1, 2
• Immediate neurosurgical referral if MRI shows mass lesion, hydrocephalus, or hemorrhage 1
• Endocrinology referral for all confirmed cases of DI to manage desmopressin therapy and monitor for evolving pituitary dysfunction 2
• Ophthalmology referral if visual field defects or papilledema detected 1