(Renal & Urology)

Overview

• Condition in which kidneys are unable to concentrate urine due to inadequate secretion or sensitivity to ADH, resulting in the production of large quantities of dilute urine
  • Central DI (More common) → insufficient levels of ADH from posterior pituitary. Causes = pituitary tumour/surgery, traumatic brain injury, infection (meningitis), sarcoidosis, TB, SAH, hereditary haemochromatosis
  • Nephrogenic DI → defective ADH receptors in the distal tubules and collecting ducts. Causes = lithium therapy, electrolyte imbalances (hypercalcaemia or hypokalaemia), idiopathic, inherited (AVP2 gene)

<aside> 💡 ADH → increases water reabsorption into blood in collecting duct via V2 receptors

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Making Diagnosis

Clinical Features:

• Polyuria → with very dilute urine
• Nocturia → restless sleep, daytime sleepiness
• Polydipsia → excessive thirst
• Dehydration → tachycardia, reduced tissue turgor, dry mucous membranes

(No symptoms suggestive of DM)

Investigations:

• Low urine osmolality + High serum osmolality
• Water Deprivation Test → confirmatory test. If osmolality corrects itself then may be psychogenic polydipsia. If not, after 8 hours administer desmopressin to differentiate between CDI and NDI. Urine osmolality will rise in CDI but remain low in NDI.
  • Water Deprivation Test Results
• MRI → look for brain/pituitary tumour