Endocrinology - Diabetes insipidus Flashcards

1
Q

What is diabetes insipidus?

A

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

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2
Q

Classes of diabetes insipidus

A

Nephrogenic

Central

Primary polydipsia - normal ADH system but patient is drinking excessive quantities of water leading to polyuria

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3
Q

What is nephrogenic DI?

A

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)

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4
Q

What is central DI?

A

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
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5
Q

Presentation of diabetes insipidus

A
Polyuria (excessive urine production)
Polydipsia (excessive thirst)
Dehydration
Postural hypotension
Hypernatraemia
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6
Q

Investigations to do in suspected DI

A

Urine osmolality - low
Serum osmolality - high
Water deprivation test

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7
Q

What is the water deprivation test?

A

Diagnostic test for DI

  1. Avoid fluids for 8h
  2. Then, urine osmolality is measured and synthetic ADH (desmopressin) is administered.
  3. 8 hours later urine osmolality is measured again.
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8
Q

What are the meaning of the results of water deprivation test?

A

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

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9
Q

Management of DI

A

Treat the underlying cause

Desmopressin can be used in cranial DI and in nephrogenic DI (in higher doses under close monitoring)

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