Neurohypophysial Disorders Flashcards
Name the two main nuclei within which neurones of the neurohypophysis have their cell bodies.
Paraventricular Nucleus
Supraoptic Nucleus
What two hormones are produced by the neurohypophysis?
Vasopressin
Oxytocin
What is the principal action of vasopressin and how does it carry out this action?
Vasopressin’s main action is on the V2 receptors in the renal cortical and medullary collecting ducts
It stimulates the synthesis and assembly of aquaporin 2, which then increases water reabsorption and has an antidiuretic effect
State some other actions of vasopressin.
Vasoconstriction
Corticotrophin release
Factor VIII and von Willebrand factor
Central effects
What are the main actions of oxytocin?
It is a contractile molecule that binds to oxytocin receptors
It causes contraction of the myometrium during parturition and is involved in milk ejection
It also has central effects
What are the consequences of a lack of the neurohypophysial hormones?
Lack of Oxytocin – not clinically significant
Lack of Vasopressin – Diabetes Insipidus
What are the two forms of diabetes insipidus?
Central (cranial) and Nephrogenic Diabetes Insipidus
What can cause central diabetes insipidus?
Damage to neurohypophysial system (injury, surgery, cerebral thrombosis, tumours, granulomatous infiltration)
Idiopathic
Familial (rare)
What can cause nephrogenic diabetes insipidus?
Familial (rare)
Drugs e.g. lithium, dimethyl chlortetracycline (DMCT)
State some signs and symptoms of diabetes insipidus.
Polyuria Polydipsia Hypo-osmolar urine Dehydration Possible disruption of sleep Possible electrolyte imbalance
State another cause of polydipsia that isn’t diabetes.
Psychogenic polydipsia
This is a central disturbance that increases the drive to drink
What test can be used to distinguish between normal, psychogenic polydipsia, central DI and nephrogenic DI? Describe the results you would expect.
Fluid deprivation test
Normals and psychogenic polydipsia will show a rise in urine osmolality
Central and nephrogenic diabetes insipidus will show little or no change in urine osmolality
Fluid deprivation with administration of DDAVP (Desmopressin)
Central diabetes insipidus will show a rise in urine osmolality
Nephrogenic DI will still have a low urine osmolality (because of end-organ resistance)
Why is the urine osmolality of someone with psychogenic polydipsia lower (in the fluid deprivation test) than a normal subject?
Over time, the constant passage of large volumes of water through the kidneys will wash out the osmotic gradient that is necessary for AVP to exert its diuretic effect
Describe the normal change in urine osmolality as plasma osmolality increases.
Normally, urine osmolality will increase as plasma osmolality increases (in a graph of urine osmolality against plasma osmolality it will show a sigmoid shape)
In DI, there is little change in urine osmolality as plasma osmolality increases
Describe changes in plasma vasopressin following administration of hypertonic saline in a normal subject, psychogenic polydipsia, central DI and nephrogenic DI.
Hypertonic saline will increase the plasma osmolality and hence will increase the vasopressin secretion in patients that have the capacity to produce vasopressin (normal, psychogenic polydipsia and nephrogenic DI)
Patients with central DI can’t produce vasopressin at all so the hypertonic saline will show no change in plasma vasopressin