T1 - L10. Thirst Flashcards
Where are osmoreceptors located?
anterior wall of 3rd ventricle
Where is vasopressin made?
in neurons in supraoptic and paraventricular nuclei of the hypothalamus
ADH action at the kidney
mediated via V2 receptors
aquaporins normally stored in cytoplasmic vesicles, they move to and fuse with the luminal membrane
increases water permeablity of renal collecting tubules to promote water reabsorption
when ADH is cleared, water channels are removed from the surface by endocytosis and returned to the cytoplasm
Causes of polyuria/polydipsia
Cranial diabetes insipidus - lack of osmoregulated AVP secretion
diabetes mellitus
nephrogenic diabetes insipidus - lack of response of renal tubule to AVP
cranial diabetes insipidus causes
idiopathic genetic - familial mutation of AVP gene Usually secondary causes - commonest post surgical traumatic - head injury rarer causes - tumours, meningitis
Hypothalamic syndrome
Disordered thirst and DI hyperphagia - disordered appetite disordered temperature regulation disordered sleep rhythm hypopituitarism
Nephrogenic diabetes insipidus
renal tubules resistant to AVP causing polyuria
thirst stimulated - polydipsia
causes of nephrogenic DI
idiopathic
rare - genetics - mutations with v2 receptor gene or aquaporin gene
metabolic - high calcium or low potassium
drugs - lithium
CKD
What is primary polydipsia?
psychogenic increased fluid intake low plasma osmolality supressed AVP secretion low urine osmolality high urine output reduced renal concentrating ability as loss of renal interstitial solute
How would you investigate polyuria and polydipsia
history exclude DM document 24 hour fluid balance - urine output and fluid intake day and night exclude hypercalcemia and hypokalemia water deprivation test
water deprivation test
periods of dehydration
measure plasma and urine osmolalities and weight
injection of synthetic vasopressin - desmopressin DDAVP
measure plasma and urine osmolalities
Results of water deprivation test
NORMAL response - normal plasma osmolality and high urine osmolalilty
CRANIAL DI - poor urine conc after dehydration and rise in urine osmo after desmopressin
NEPHROGENIC DI - poor urine conc after dehydration and no rise in urine osmo after desmopressin
Treatment
CRANIAL DI - DDAVP
over treatment can cause hyponatremia
NEPHROGENIC DI - correct the cause - metabolic/drug
thiazide like diuretic / NSAIDs
PRIMARY POLYDIPSIA
explanation, persuasion, psychological therapy
How do you classify hyponatraemia?
classify by extracellular fluid volume status
- hypovolaemia - renal loss, non renal loss, burns, sweating
Normovolaemia - hypoadrenalism, hypothyroidism, SIADH
Hypervolaemia - renal failure, cardiac failure, cirrhosis
SIADH diagnosis
euvolaemic
low plasma sodium and low plasma osmolality
inappropriately high urine sodium concentration and high urine osmolality
Assess renal, adrenal and thyroid function
