Diabetes Insipidus

Symptoms

• Polyuria, polydipsia
• Dehydration
• Hypernatraemia

Causes of cranial DM;

• Congenital – defects in the ADH gene
• Tumour – craniopharyngioma, metastases, pituitary tumour
• Trauma – hypophysectomy, head injury
• Infiltration – histiocytosis, sarcoidosis
• Vascular – Sheehan’s syndrome, haemorrhage
• Infection – meningoencephalitis

Causes of nephrogenic DI;

• Inherited
• Metabolic – low K+, high Ca2+
• Drugs – lithium
• Chronic renal disease
• Post-obtructive uropathy

Tests

• U&Es, Ca2+, glucose (exclude DM)
• Serum and urine osmolalities (serum osmolality is 2 x (Na + K) + glucose + urea)

Diagnosis

• The water deprivation test aims to test the ability of the kidneys to concentrate urine for the diagnosis of DI and then localise the cause
• Don’t do this test until you have established that the urine output is >3L/day
• Stage 1 of test – fluid deprivation for 8hr for diagnosis of DI
  • Empty bladder and the no drinks and only dry food
  • Weight hourly – if >3% weight loss during test order urgent serum osmolality, if greater then 300mOsmol/kg proceed to Stage 2, if <300 continue test
  • Collect urine every 2 hour, measure its volume and osmolality
  • Venous sample for osmolarity every 4hr
  • Stop test after 8hr is urine osmolality is >600mOsmol/kg (i.e. normal)
• Stage 2 – differentiate between cranial and nephrogenic DI
  • Proceed if urine still dilute i.e. urine osmolality <600mOsmol/kg
  • Give intranasal desmopressin. Water can be drunk at this stage
  • Measure urine osmolality hourly for the next 4hr

Interpretation

• Normal – urine osmolality >600mOsmol/kg in Stage I i.e. normal concentrating ability
• Primary polydipsia – urine concentrates but less than normal e.g. >400-600 mOsmol/kg
• Cranial DI – urine osmolality increases to >600 mOsmol/kg after desmopressin is given
• Nephrogenic DI – no increase in urine osmolality after desmopressin

Treatment

• Cranial DI
  • Find the cause e.g. MRI head, do pituitary function
  • Give desmopressin, a synthetic analogue of ADH
• Nephrogenic DI
  • Treat cause
  • If it persists try bendroflumethiazide
  • NSAIDs lower urine volume and plasma Na by inhibiting prostaglandin synthase. Prostaglandins locally inhibit the action of ADH