Neurohypophysial disorders Flashcards
how can the posterior pituitary be identified on the MRI?
‘bright spot’ on pituitary MRI
what are the two nucleus associated with the hypothalamo-neurohypophysial system
paraventricular and supraoptic nucleus
what hormones does the neurohypophysis secrete?
oxytocin and vasopressin
what is the principal effect of vasopressin?
= ANTI-DIURETIC ie increases water reabsorption from renal cortical and medullary collecting ducts via V2 receptors
what is vasopressin also known as?
Vasopressin also known as ADH – Anti Diuretic Hormone
Diuresis = increase in urine production
describe the actions of vasopressin?
- acts on the renal cortical and medullary collecting ducts (SPECIFICALLY: DCT + CT)
- acts on V2 receptors
- increases cAMP
- activated PKA
- stimulates synthesis and assembly of aquaporin 2
- increased water transport
- this has an antidiuretic effect
what does oxytocin do?
contraction of myometrium at parturition
milk ejection reflex
central effects
acts on oxytocin receptors
where are the osmoreceptors located?
organum vasculosum (devoid of the BBB) osmoreceptors then project to the hypothalamic PVN and SON
what are osmoreceptors very sensitive to?
changes in extracellular osmolality
what happens to the osmoreceptors when there is an increase in extracellular sodium?
Water moves out of the osmoreceptors when the extracellular sodium conc increases and the osmoreceptors shrink
this leads to increased osmoreceptor firing
VP is then released from hypothalamic PVN and SON neurones
what is the normal response to water deprivation?
lack of water leads to increased serum osmolality
this stimulates the osmoreceptors (leading to thirst)
this increases VP release
this increases water re-absorption from renal collecting ducts
this leads to a reduction in serum conc as well sa reduced urine volume and an increase in urine osmolality
what is diabetes insipidus?
Absence or lack of circulating vasopressin
what are the two types of diabetes insipidus and what is the difference between the two?
cranial diabetes insipidus: lack of VP being produced from the brain
nephrogeic diabetes insipidus: this is when the kidneys are resistant to the vasopressin, there is still vasopressin being produced but it cannot carry out its function
what are the acquired and congenital causes of cranial diabetes insipidus?
Acquired (more common)
Damage to Neurohypophysial system
Traumatic brain injury
Pituitary surgery
Pituitary tumours, craniopharyngioma
Metastasis to the pituitary gland eg breast
Granulomatous infiltration of median eminence eg TB, sarcoidosis
Congenital - rare
what are the acquired and congenital causes of nephrogenic diabetes insipidus?
Congenital - rare (e.g. mutation in gene encoding V2 receptor, aquaporin 2 type water channel)
Acquired - Drugs (e.g. lithium)
what are the signs and symptoms of diabetes insipidus?
Large volumes of urine (polyuria)
Urine very dilute (hypo-osmolar)
Thirst and increased drinking (polydipsia)
Dehydration (and consequences) if fluid intake not maintained - can lead to DEATH
Possible disruption to sleep with associated problems
what is the process of diabetes insipidus?
inadequate production/response to VP
large volumes of dilute (hypotonic urine)
increase in plasma osmolality and sodium
reduction in EC fluid volume
thirst-polydipsia
EC fluid volume expansion
-HOWEVER-
if there is no access to water then it can lead to dehydration and death
what is psychogenic polydipsia?
Excess fluid intake (polydipsia) and excess urine output (polyuria) – BUT unlike DI, ability to secrete vasopressin in response to osmotic stimuli is preserved
what are the causes of psychogenic polydipsia?
Most frequently seen in psychiatric patients – aetiology unclear, may reflect anti-cholinergic effects of medication – ‘dry mouth’
Can be in patients told to ‘drink plenty’ by healthcare professionals
what happens in psychogenic polydipsia?
increases drinking expansion of EC fluid volume, reduction in plasma osmolality less VP secreted by posterior pituitary large volumes of dilute urine EC volume return to normal
what is the difference in plasma osmolality in diabetes insipidus and psychogenic polydipsia?
diabetes insipidus= plasma osmolality too high
psychogenic polydipsia= plasma osmolality too low
how would you test for diabetes insipidus?
-water deprivation test
no water allowed and the urine osmolality is measured
then administer DDAVP
in a water deprivation test what happens in normal, p polydipsia, central DI and nephrogenic DI?
- urine osmolality
normal: increases (as more water is retained)- urine is more concentrated
p polydipsia: increases
central DI: v little change
nephrogenic DI: v little change
in a water deprivation test what happens when you add DDAVP in normal, p polydipsia, central DI and nephrogenic DI?
- urine osmolality
normal: drops slightly
p polydipsia: drops slightly
central DI: urine osmolality increases
nephrogenic DI: stays the same
