Endocrinology - Diabetes insipidus Flashcards
What is diabetes insipidus?
Lack of ADH or lack of response to ADH
This prevents the kidneys from being able to concentrate the urine leading to polyuria and polydipsia
Classes of diabetes insipidus
Nephrogenic
Central
Primary polydipsia - normal ADH system but patient is drinking excessive quantities of water leading to polyuria
What is nephrogenic DI?
When the collecting ducts of the kidneys don’t respond to ADH
Can also be caused by:
Drugs, particularly lithium used in bipolar affective disorder
Mutations in the AVPR2 gene on the X chromosome that codes for the ADH receptor
Intrinsic kidney disease
Electrolyte disturbance (hypokalaemia and hypercalcaemia)
What is central DI?
When the hypothalamus doesn’t produce enough ADH for the pituitary to secrete
Can be idiopathic or caused by:
- Brain tumours
- Head injury
- Brain malformations
- Brain infections
- Brain surgery/radiotherapy
Presentation of diabetes insipidus
Polyuria (excessive urine production) Polydipsia (excessive thirst) Dehydration Postural hypotension Hypernatraemia
Investigations to do in suspected DI
Urine osmolality - low
Serum osmolality - high
Water deprivation test
What is the water deprivation test?
Diagnostic test for DI
- Avoid fluids for 8h
- Then, urine osmolality is measured and synthetic ADH (desmopressin) is administered.
- 8 hours later urine osmolality is measured again.
What are the meaning of the results of water deprivation test?
Cranial DI -
- Urine osmolality is low after fluid deprivation
- Urine osmolality is high after the desmopressin
- As the kidneys still respond to ADH and reabsorb water, concentrating the urine
Nephrogenic DI:
- Urine osmolality is low after fluid deprivation
- Urine osmolality is still low after the desmopressin
- Kidneys don’t respond to ADH and don’t reabsorb water, so urine remains dilute
In primary polydipsia the urine osmolality will be high both after fluid deprivation and after desmopressin
Management of DI
Treat the underlying cause
Desmopressin can be used in cranial DI and in nephrogenic DI (in higher doses under close monitoring)