Neurohypophysial disorders Flashcards
What does the posterior pituitary look like on an MRI?
It looks like a bright spot on the pituitary MRI
What are the two nuclei involved with the neurohypophysis? Including in the secretion of VP
Supraoptic nucleus
Paraventricular nucleus
What are the main two hormones involved in the neurohypophysis?
Vasopressin (ADH)
Oxytocin
What is the principle effect of vasopressin?
It acts as an antidiuretic increases water absorption from renal cortical and medullary collecting ducts via V2 receptors.
V1a - vasoconstrictor activity
V1b - corticotrophin (ACTH) release
V2 - also Factor VIII and von Willebrand factor secretion
What is function of oxytocin?
Constriction of myometrium at parturition
Milk ejection reflex
Central effects
Acts on oxytocin receptors
What measures the osmolality of the blood so that vasopressin can be stimulated to be released?
Osmoreceptors located in the organum vasculosum project to the PVN and SON
How do osmoreceptors cause the release of vasopressin?
When there is low water content in the blood there is a greater concentration of Na+ (extracellular Na+ is higher). As a result water flows out the cell by osmosis causing the osmoreceptor to shrink. This change in the shape causes increased osmoreceptor firing causing VP release from hypothalamic PVN and SON.
What is the main factor for water deprivation?
Serum osmolality
What are the types of diabetes insipidus?
Cranial/Central - Absence or lack of circulating vasopressin. Most common diabetes insipidus
Nephrogenic - End-organ (kidneys) resistance to vasopressin
What are the causes of cranial diabetes insipidus?
Damage to neurohypophysial system - Traumatic brain injury, pituitary surgery, pituitary tumours, craniopharyngioma, metastasis to the pituitary gland e.g Breast, granulomatous infiltration of median eminence e.g TB, sarcoidosis
Congenital is rare
What are the causes of nephrogenic diabetes insipidus?
Congential - rare (mutation in V2 receptor or in aquaporin type 2)
Acquired - Drugs (lithium - used to treat bipolar disorder)
Signs and symptoms of diabetes insipidus?
Polyuria, hypo-osmolar (dilute urine), polydipsia, nocturea, dehydration - can lead to death
Describe the mechanism of diabetes insipidus
See diagram on slides
What is psychogenic polydipsia?
Most frequently seen in psychiatric patients - Excess fluid intake and urine output but ability to secrete VP is preserved.
See notes
How to differentiate between diabetes insipidus and polydipsia? And cental vs nephrogenic
The normal range for plasma osmolality is 270 to 290 mOsm/kg H2O. Diabetes insipidus patients may have a higher plasma osmolality whereas psychogenic polydipia patient have a lower plasma osmolality
Injection of synthetic VP and see the changes.
Water deprivation test - see notes
What are the biochemical features of those with diabetes insipidus and psychogenic polydipsia?
Diabetes insipidus - [Hypernatraemia, raised urea, increased plasma osmolality] these features are all due to dehydration. Hypo-osmolar urine
Psychogenic polydipsia - Mild hyponatraemia - excess plasma osmolality, hypo-osmolar urine
What can be used to treat cranial diabetes insipidus?
Desmopressin - specific for V2 receptor in the kidneys (replaces VP). Doesn’t bind to V1 receptors which cause vasoconstriction. Terlipressin is used for GI bleeds because it causes vasocontriction.
How can you administer desmopressin?
Nasally (normal way), orally and as an injection.
Need to tell patients not to continue drinking large amounts of fluids, risks of hyponatraemia.
What can be used to treat nephrogenic diabetes insipidus?
Thiazide diruetics - not clear how it works.
bendroflumethiazide
Define SIADH
The plasma VP concentration is inappropriately high for the existing plasma osmolality
Describe the effects and mechanisms of increased VP
See diagram
euvolaemia - surprising. Stretch in heart due to increase water causing the release of an atrial natriuetic peptide (ANP) from the right atrium. This causes natriuresis in the urine.
Hyponatraemia - due to lots of water being retained
What are the clinical features of SIADH?
Raised urine osmolality, decreased urine volume (initially)
Hyponatreaemia
Opposite of diabetes insipidus
What are the symptoms of SIADH?
Can be symptomless
If p[Na+] < 120mM: generalised weakness, poor mental function, nausea = this just adds to the hyponatreamia because you losing sodium in the vomit
If p[Na+] <110mM: confusion, coma, death
What are the causes of SIADH?
CNS
Pulmonary disease
Malignancy
Drug related
See notes for specific causes
