DISORDERS OF VASOPRESSIN

Question 1

What are the 3 effects of AVP and their respective receptors?

Answer 1

Water reabsorption in renal collecting duct by V2 receptor
Vasoconstrictor via V1 receptor
Stimulates ACTH release from anterior pituitary

Question 2

What are the 2 stimuli for AVP release?

Answer 2

• Rise in plasma osmolality sensed by osmoreceptors

- Decrease in atrial pressure sensed by atrial stretch receptors

Question 3

What are the names of the two nuclei for the osmotic stimulation of vasopressin release?

Answer 3

Organum vasculosum

Subfornical organ

Question 4

Describe how the organum vasculosum and subfornical organ carry out their function.

Answer 4

They sit around the 3rd ventricle where there is no blood barrier so they can respond to systemic changes. Neurones project down to the supraoptic nucleus.

Question 5

How do osmoreceptors work?

Answer 5

When plama osmolality increases, water diffuses out of osmoreceptor cell and it shrinks causing increase osmoreceptor firing

Question 6

Where do atrial stretch receptors detect pressure?

Answer 6

Right atrium

Question 7

How do atrial stretch receptors inhibit AVP release?

Answer 7

Inhibit AVP release via vagal afferents to hypothalamus

e.g. haemorrhage = less circulating volume = less inhibition of AVP

Question 8

What are the 2 effects of osmoreceptor stimulation?

Answer 8

Thirst and AVP release

Question 9

What is the difference and similarity between diabetes insipidus and mellitus?

Answer 9

similarities: polyuria, nocturia, thirst, polydispia
differences: in mellitus symptoms due to hyperglycaemia and osmotic diuresis, in insipidus symptoms due to problem with AVP

Question 10

What are the 2 types of diabetes insipidus?

Answer 10

Cranial and Nephrogenic

Question 11

What is the difference between CDI and NDI?

Answer 11

CDI - problem with hypothalamus/pituitary causing inability to make AVP

NDI - problem with kidney (collecting duct) where its unable to respond to AVP

Question 12

List causes of CDI

Answer 12

Brain trauma, pituitary surgery, adenoma or metastasis from another site, granulomatous infiltration of pituitary stalk (TB, sarcoidosis…), autoimmune, congenital

Question 13

List causes of nephrogenic diabetes insipidus

Answer 13

Drugs e.g. lithium (used in mental health)

Congenital e.g. mutation in gene encoding V2 receptor/aquaporin 2

Question 14

Clinical presentations of DI?

Answer 14

Dilute, large volume of urine
Increased plasma conc.
Hypernatraemia (increased sodium conc in plasma)
Normal blood glucose level

Question 15

What condition often mimics diabetes insipidus?

Answer 15

Psychogenic polydipsia

Question 16

What is psychogenic polydipsia?

Answer 16

Patient has a psychological problem involving the consumption of vast amounts of water which can present the same symptoms as DI

Question 17

Name the test used to distinguish between DI and PP and explain it

Answer 17

Water deprivation test - Don’t let patient drink and fluid and overtime measure urine volume and concentration of urine and plasma. Weigh the patient regularly and if > 3% loss of weight stop test (significant dehydration which occurs in DI)

Question 18

How do we distinguish between CDI and NDI?

Answer 18

Give ddAVP (synthetic AVP - V2 receptor agonist) during water deprivation test.
If CDI urine osmolality should increase and volume decrease
If NDI nothing will happen

Question 19

What is the difference in plasma osmolality between DI and psychogenic polydipsia?

Answer 19

DI - high osmolality (290)

PP - low osmolality (270)

Question 20

What is the treatment for cranial DI?

Answer 20

Replace AVP with desmopressin (oral/intranasal)

Question 21

What is the treatment for nephrogenic DI?

Answer 21

Give thiazide diuretics e.g. benzofluazide (mechanism is unclear)

Very difficult to treat successfully

Question 22

What is SIADH?

Answer 22

Syndrome of Inappropriate Anti-Diuretic Hormone

- too much AVP causing low, conc. urine and water retention

Question 23

What are some causes of SIADH?

Answer 23

Head injury, stroke, tumour
Pulmonary disease (pneumonia, bronchiectasis)
Lung cancer
Drug related e.g. carbamazepine, serotonin reuptake inhibitors (SSSRIs)
Idiopathic

Question 24

Describe the management of SIADH

Answer 24

Prolonged hospital stay until sodium conc. in plasma increases
Fluid restrict
Vasopressin antagonist (vaptan) - binds to V2 receptor however extremely expensive