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BRS Phase 1b > Disorders of Vasopressin > Flashcards

Disorders of Vasopressin Flashcards

1
Q

What type of tissue is the posterior pituitary?

A

Neural tissue. anatomically continuous with hypothalamus.

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2
Q

What is the posterior pituitary made up of?

A

Hypothalamic magnocellular neurons, that are long and originate in the supraoptic and paraventricular hypothalamic nuclei.

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3
Q

What are the two main hormones in the posterior pituitary?

A

Arginine Vasopressin (AVP), Oxytocin.

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4
Q

What receptor does AVP act through in the kidney?

A

V2 receptor - stimulation of water reabsorption.

V1 receptor - vasoconstriction, ACTH release.

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5
Q

What is the main stimulus for the release of ACTH from the posterior pituitary?

A

CRH

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6
Q

Is it normal not to see the posterior pituitary (bright spot) in an MRI?

A

Yes, not visible in all healthy individuals.

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7
Q

What are the stimuli for Vasopressin release?

A

Osmotic - Rise in plasma osmolarity sensed by osmoreceptors.

Non-osmotic - Decrease in atrial pressure sensed by atrial stretch receptors.

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8
Q

What groups of cells sense plasma osmolarity?

A

Organum vasculosum

Subfornical organ

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9
Q

What is special about the Organum vasculosum & Subfornical organ?

A
  • both nuclei which sit around the 3rd ventricle
    (‘circumventricular’)
  • no blood brain barrier – so neurons can respond to
    changes in the systemic circulation
  • highly vascularised
  • neurons project to the supraoptic nucleus - site of
    vasopressinergic neurons
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10
Q

How do Osmoreceptor’s regulate vasopressin?

A 
Increase in extracellular Na+
Water flows out of Osmoreceptor
Osmoreceptor shrinks
Change in shape triggers osmoreceptor firing
AVP release from hypothalamic neurons
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11
Q

Describe Non-Osmotic stimulation of vasopressin release?

A

Atrial stretch receptors detect pressure in the right atrium,
Inhibit vasopressin release via vagal afferents to hypothalamus,
Reduction in circulating volume eg haemorrhage means less stretch of these atrial receptors, so less inhibition of vasopressin.

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12
Q

What is the difference between diabetes mellitus and diabetes insipidus?

A

Symptoms that typically present due to diabetes mellitus, do so due to osmotic diuresis whereas in diabetes insipidus it is due to problems with vasopressin.

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13
Q

What are the two types of diabetes insipidus?

A

Cranial

Nephrogenic

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14
Q

What happens in Cranial diabetes insipidus?

A

Problem with hypothalamus &/or posterior pituitary
Unable to make arginine vasopressin
‘VASOPRESSIN INSUFFICIENCY’

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15
Q

What happens in Nephrogenic diabetes insipidus?

A

Can make arginine vasopressin (normal hypothalamus & posterior pituitary)
Kidney (collecting duct) unable to respond to it
‘VASOPRESSIN RESISTANCE’

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16
Q

What are the causes of Cranial diabetes insipidus?

A

Acquired (more common)
Traumatic brain injury
Pituitary surgery
Pituitary tumours
Metastasis to the pituitary gland eg breast
Granulomatous infiltration of pituitary stalk eg TB, sarcoidosis
Autoimmune

Congenital rare

17
Q

What are the causes of Nephrogenic diabetes insipidus?

A

Much less common than cranial diabetes insipidus
Congenital
rare (e.g. mutation in gene encoding V2 receptor, aquaporin 2 type water channel)
Acquired
Drugs (e.g. lithium)

18
Q

How does someone with diabetes insipidus present?

A

Polyuria
Polydipsia
Nocturia
Thirst - often extreme

19
Q

What are the physiological presentations of diabetes insipidus?

A 
Urine
-Very dilute (hypo - osmolar)
-Large volumes
Plasma
-Increased concentration (hyper-osmolar) as patient 
 becomes dehydrated
-Increased sodium (hypernatraemia) 
-Glucose normal (make sure you ALWAYS check this in a 
 patient with these symptoms)
20
Q

What is Psychogenic Polydipsia?

A

Problem that the patient drinks all the time, so passes large volumes of dilute urine. Presentation is very similar to diabetes insipidus, but there is no problem with vasopressin.

21
Q

How do we distinguish between diabetes insipidus & psychogenic polydipsia?

A 
Water deprivation test 
No access to anything to drink
Over time, measure:
- Urine volumes  
- Urine concentration (osmolality)
- Plasma concentration (osmolality).
22
Q

What would the water deprivation test look like for someone with diabetes insipidus?

A

urine osmolarity would stay the same and would not return to normal after water deprivation. Increase in plasma osmolality.

23
Q

Why do you need to weigh someone regularly during the Water deprivation test?

A

Stop the test if they lose >3% of body weight, which would be a marker of significant dehydration.

24
Q

How do you distinguish between Cranial and Nephrogenic diabetes insipidus?

A

Give ddAVP injection (V2 receptor agonist)
This will work ‘like’ vasopressin
Cranial diabetes insipidus – response to ddAVP – urine concentrates
Nephrogenic diabetes insipidus – no increase in urine osmolality with ddAVP, as kidneys can’t respond

25
Q

Would Plasma osmolality be greater in diabetes insipidus of psychogenic polydipsia?

A

Diabetes Insipidus

26
Q

What is the treatment for Cranial Diabetes Insipidus?

A

Want to replace vasopressin
Desmopressin
Selective for V2 receptor (V1 receptor activation would be unhelpful)
Different preparations
- Tablets
- Intranasal (this can be thought of not important in hospital admissions, but it is critical to know it is).

27
Q

What is the treatment for Nephrogenic Diabetes Insipidus?

A

Luckily this is very rare – difficult to treat successfully
Thiazide diuretics eg bendofluazide
Paradoxical! Mechanism unclear

28
Q

What is Syndrome of Inappropriate Anti-Diuretic Hormone SIADH?

A

Too much arginine vasopressin so you have;

  • Reduced urine output
  • water retention
  • High urine osmolality
  • low plasma osmolality
  • Dilutional hyponatraemia
29
Q

What are the causes of SIADH?

A 
CNS
- Head injury, stroke, tumour, 
Pulmonary disease
- Pneumonia, bronchiectasis
Malignancy
- Lung cancer (small cell)
Drug-related
- Carbamazepine, Serotonin Reuptake Inhibitors (SSSRIs)
Idiopathic
- most common
30
Q

What is the management of SIADH?

A

Fluid restrict

You can use a vasopressin antagonist (vaptan) - binds to the V2 Receptors in the kidney ($$$$$)

BRS Phase 1b (80 decks)

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