Endo - Disorders of vasopressin

Question 1

Where does vasopressin come from?

Answer 1

The posterior pituitary

Question 2

What stimulates AVP release?

Answer 2

Osmotic factors:

• Rise in plasma osmolality
• Sensed by osmoreceptors

Non-osmotic:

• Decrease in atrial pressure
• Sensed by atrial stretch receptors

Question 3

What are the names of the osmoreceptors that detect the osmolality of blood plasma and what is special about their blood supply?

Answer 3

Organum vasculosum & subfornical organ

Their blood supply has no blood-brain barrier, can detect changes in systemic blood

Question 4

How do the osmoreceptors work?

Answer 4

If there is lots of salt in the blood, water will leave the receptor and cause shrinkage which will lead to increased signal firing

Question 5

What are the symptoms of diabetes insipidus?

Answer 5

• Polyuria
• Nocturia
• Thirst - Often extreme
• Polydipsia
  Happens in diabetes mellitus also - much more common

Question 6

What is cranial diabetes insipidus?

Answer 6

Where there is a problem with the hypothalamus &/or the posterior pituitary which leads to the inability to produce AVP

Question 7

What is nephrogenic diabetes insipidus?

Answer 7

The body is still able to make AVP but the kidney (collecting duct) is unable to respond to the AVP

Question 8

What are the causes of cranial diabetes insipidus?

Answer 8

Acquired:

• Traumatic brain injury
• Pituitary surgery
• Pituitary tumours
• Metastasis to the pituitary gland
• Granulomatous infiltration of pituitary stalk
• Autoimmune

Congenital causes are rare

Question 9

What are the causes of nephrogenic diabetes insipidus?

Answer 9

Congenital causes are rare

Acquired:
- Drugs (e.g Lithium)

Question 10

What are the effects of diabetes insipidus on urine and plasma?

Answer 10

Urine:
- Very dilute (hypo-osmolar)
- Large volumes
Plasma:
- Increased concentration (hyper-osmolar) as patient becomes dehydrated
- Increased sodium (hypernatraemia)
- Glucose levels are normal (ALWAYS CHECKED WITH THESE SYMPTOMS)

Question 11

What is psychogenic polydipsia?

Answer 11

• Has similar symptoms as diabetes insipidus
• No problem with AVP
• Patient drinks all the time, so passes large volumes of dilute urine

Question 12

How do you distinguish between diabetes insipidus and psychogenic polydipsia?

Answer 12

Water deprivation test:

• Place them somewhere with no access to any water
• Overtime measure:
  - Urine volumes
  - Urine concentration (osmolality)
  - Plasma concentration (osmolality)
• A patient with psychogenic polydipsia will increase their urine osmolality with time, but not as high as a healthy patient
• A patient with diabetes insipidus will never concentrate their urine
• Weight is measured as if weight loss >3% then the test is stopped

Question 13

How do you distinguish between cranial and nephrogenic diabetes insipidus?

Answer 13

Give the patient ddAVP (synthetic AVP) and if the patients urine osmolality increases over time, it is cranial as this shows they can respond to AVP, whereas if they had nephrogenic DI then urine osmolality would not as they cannot respond to AVP or the ddAVP

Question 14

What is the treatment for cranial diabetes insipidus?

Answer 14

Give desmopressin:

• only acts on V2 receptors and not V1 receptors
• can be given as a tablet or nasal spray

Question 15

What is the treatment for nephrogenic diabetes insipidus?

Answer 15

Difficult to treat

Give thiazide diuretics

Question 16

What is syndrome of inappropriate ADH?

Answer 16

Too much AVP:

• reduced urine output
• water retention
• High urine osmolality
• Low plasma osmolality
• Dilutional hyponatraemia

Question 17

What is the management of SIADH?

Answer 17

Fluid restriction or vasopressin antagonist - binds to V2 receptors in the kidney