3) Neurohypophysial disorders Flashcards
What is the neurohypophysis + what type of tissue is it made up of?
PPG
Neural tissue
What are the 2 PPG hormones?
Vasopressin
Oxytocin
How does the PPG appear on an MRI?
Bright spot
What does Vasopressin cause?
Water reabsorption
What does an anti-diuretic do?
Reduces urine production by increasing water reabsorption
How does vasopressin actually increase water reabsorption?
1) Vasopressin binds to V2 receptors on collecting duct
2) intracellular signalling cascade
3) aquaporins inserted into apical membrane
How is vasopressin release regulated?
Osmoreceptors in hypothalamus detect osmorality
Define osmolarity:
How many solutes there are (concentration)
How do osmoreceptors respond to increased plasma osmolality?
1) water leaves osmoreceptors
2) osmoreceptors shrink –> increases their firing rate
3) Vasopressin release increases
What is the normal response to a water deprivation test?
Increased plasma osmolality
Detected by osmoreceptors-increased VP secretion + thirst
Increased water reabsorption + reduced urination=plasma osmorality decreases
What 2 things can osmoreceptors cause?
Increased vasopressin secretion
Thirst
2 types of Diabetes insipidus:
Cranial-no vasopressin released
Neprogenic-vasopressin resistance
2 causes of diabetes insipidus:
Acquired-tumour/trauma/lithium
Congenital-mutation in V2R
5 symptoms of diabetes insipidus:
Polyuria Polydipsia Hypo-osmolar Dehydration Sleep disturbance
So what do patients with diabetes insipidus do when plasma osmorality increases?
DRINK
What is psychogenic polydipsia?
Polyuria + polydipsia but no issue with the vasopressin system
What is normal, diabetes insipidus and psychogenic polydipsia plasma osmolaltiy?
Diabetes insipidus=above normal (290)
Normal=280
Psychogenic polydipsia=below normal (270)-as you’re diluting yourself
Compare the primary issue in diabetes insipidus and psychogenic polydipsia:
DI: urination causes thirst
PP: thirst causes urination
Compare biochemical features of diabetes insipidus and psychogenic polydipsia:
DI:
Increased plasma osmolality-increased urea + hypernatremia
Hypo-osmolar urine
PP:
Decreased plasma osmolality-hyponatremia
Hypo-osmolar urine
What 2 things are measured during a water deprivation test?
Urine volume
Urine + blood osmolality
State the results of a water deprivation test for a normal person vs psychogenic polydipsia vs diabetes insipidus:
Normal=little concentrated urine produced
PP: normal result as issue is drinking which has now stopped
DI: no VP so can’t reabsorb water so still urinating
How can you use a water deprivation test to identify whether the diabetes insipidus is cranial or nephrogenic?
Give DDAVPA (vasopressin analogue) Cranial-normal response Nephrogenic-kidneys can't respond to VP so response doesn't change
What must you remember to do during a water deprivation test?
Measure body weight every hour-if more than 3% lost=clinical dehydration
Summarise the issue in SIADH vs DI vs PP:
SIADH=excess vasopressin
DI=lack of/resistance to vasopressin
PP=drinking alot
Define SIADH (syndrome of inappropriate ADH):
Excess vasopressin
Vasopressin level is inappropriate for plasma osmolality
5 causes of SIADH:
Tumour-PPG or ectopic
CNS/pulmonary disease/drugs/idiopathic
Biochemical features of SIADH:
Hyponatremia
Symptoms of SIADH:
Symptomless
When Na+ levels get v low=CNS affected
Treatment of SIADH:
IMMEDIATELY REDUCE FLUID INTAKE
Surgery-remove tumour
V2 receptor antagonist-VAPTAINS
How is diabetes insipidus treated?
Cranial-desmopressin (V2 R agonist)
- nasally administered
- must tell patients to decrease fluid intake
Nephrogenic=thiazide diuretics
