Endocrine · Year 2 · from Endocrine

Case 1: Central Diabetes Insipidus

Patient Demographics

  • Age: 32 years
  • Sex: Male
  • Occupation: Construction worker

Chief Complaint

"I'm urinating constantly and can't stop drinking water - I'm up every hour at night."

History of Present Illness

A 32-year-old man presents with a 3-week history of excessive thirst (polydipsia) and frequent urination (polyuria). He reports drinking approximately 8-10 liters of fluid daily and urinating every 30-60 minutes, including multiple times at night (nocturia). He prefers ice-cold water. He denies dysuria, hematuria, or recent medication changes. He reports fatigue and difficulty concentrating at work due to sleep disruption. Of note, he was involved in a motor vehicle accident 6 weeks ago with head trauma requiring hospitalization.

Physical Examination

  • Vital Signs: BP 118/76 mmHg, HR 88 bpm, Temp 37.0°C
  • General: Alert, mildly fatigued, carrying large water bottle
  • HEENT: Mucous membranes moist (due to continued drinking)
  • Cardiovascular: Normal
  • Neurologic: Alert and oriented, cranial nerves intact
  • Skin: Normal turgor

Workup

  • Laboratory Studies:
  • Serum sodium: 147 mEq/L (high-normal/elevated)
  • Serum osmolality: 298 mOsm/kg (elevated, normal 275-295)
  • Urine osmolality: 85 mOsm/kg (inappropriately dilute, should be >600 if dehydrated)
  • Urine specific gravity: 1.002
  • Glucose: 92 mg/dL (rules out diabetes mellitus)
  • Calcium: 9.2 mg/dL (rules out hypercalcemia)
  • Potassium: 3.8 mEq/L
  • 24-hour urine volume: 9.5 liters
  • Water Deprivation Test:
  • After water restriction: Urine osmolality remains <300 mOsm/kg
  • After desmopressin administration: Urine osmolality increases to 650 mOsm/kg (>50% increase)
  • MRI Pituitary/Hypothalamus: Loss of posterior pituitary bright spot on T1-weighted imaging, thickened pituitary stalk

Diagnosis

Central diabetes insipidus (post-traumatic, secondary to head injury)

Treatment

  1. Desmopressin (DDAVP):
  • Intranasal: 10-20 mcg once or twice daily
  • Oral: 0.1-0.4 mg two to three times daily
  • Subcutaneous: For acute situations
  1. Titrate dose to control polyuria while avoiding hyponatremia
  2. Free access to water - patients must be able to drink to thirst
  3. Monitor: Serum sodium regularly, especially when starting therapy
  4. Medical alert identification

Clinical Pearl

Central diabetes insipidus results from deficiency of ADH (vasopressin) secretion from the posterior pituitary. The posterior pituitary "bright spot" on T1-weighted MRI represents stored vasopressin in neurosecretory granules; its absence suggests central DI. The water deprivation test differentiates central DI from nephrogenic DI: in central DI, the kidneys can concentrate urine when given exogenous desmopressin, while in nephrogenic DI, the kidneys are resistant to ADH and do not respond. Post-traumatic central DI may be transient (recovering in weeks to months) or permanent, depending on the extent of hypothalamic/pituitary stalk injury.

Clinical Image

Sagittal T1-weighted MRI comparing normal posterior pituitary bright spot (left) with absence of bright spot in central diabetes insipidus (right).

Image Source: Radiopaedia - "Posterior pituitary bright spot" License: CC BY-NC-SA 3.0 URL: https://radiopaedia.org/cases/posterior-pituitary-bright-spot

All cases for this lecture as Markdown