T1 l10: Physiology of Thirst and Fluid Balance and its Disorders

Question 1

which ventricles are osmoreceptors a part of

Answer 1

The 3rd ventricle

Question 2

what receptors in the kidney does ADH activate

Answer 2

V2 receptors

Question 3

when there is low plasma osmolality AVP is

Answer 3

undetectable

Question 4

when there is high plasma osmolality AVP is

Answer 4

very high

Question 5

when there is low plasma osmolality urine is

Answer 5

Dilute

high output

Question 6

when there is high plasma osmolality urine is

Answer 6

Concentrated

low output

Question 7

when there is high plasma osmolality thirst is

Answer 7

Increased and drinking immediately suppresses AVP secretion

Question 8

what are DI related polydipsia

Answer 8

Cranial (central) diabetes insipidus (DI)
Lack of osmoregulated AVP secretion

Nephrogenic diabetes insipidus (DI)
Lack of response of the renal tubule to AVP

Primary polydipsia
Psychogenic polydipsia, social/cultural

Question 9

what occurs in cranial diabetes insipidious

Answer 9

Decreased osmoregulated AVP secretion

Excess solute-free renal water excretion
-polyuria

Provided thirst sensation remains intact and there is ready access to fluids, thirst is stimulated to maintain a stable, normal plasma osmolality
polydipsia

Question 10

what are the secondary effects to hypothalamic syndrome

Answer 10

Disordered thirst and DI

Disordered appetite (hyperphagia)

Disordered temperature regulation

Disordered sleep rhythm
Hypopituitarism

Question 11

what is nephrogenic diabetes insipidus

Answer 11

Renal tubules resistant to AVP
-Polyuria

Thirst stimulated
Polydipsia

Question 12

what is primary Polydipsia

Answer 12

Increased fluid intake
-polydipsia

Lower plasma osmolality

Suppressed AVP secretion

Low urine osmolality, high urine output
-Polyuria

Also lose renal interstitial solute, reducing renal concentrating ability

Question 13

what is a water deprivation test

Answer 13

Period of dehydration

Measure plasma and urine osmolalities & weight

Injection of synthetic vasopressin
-Desmopressin (DDAVP)

Measure plasma and urine osmolalities

Question 14

what are the different responses to the water deprivation test

Answer 14

Normal response to dehydration
-Normal plasma osmolality, high urine osmolality

Cranial diabetes insipidus

• Poor urine concentration after dehydration
• Rise in urine osmolality after desmopressin

Nephrogenic diabetes insipidus

• Poor urine concentration after dehydration
• No rise in urine osmolality after desmopressin

Question 15

what is the responses to the water test

Answer 15

Normal response to dehydration
-Normal plasma osmolality, high urine osmolality

Cranial diabetes insipidus

• Poor urine concentration after dehydration
• Rise in urine osmolality after desmopressin

Nephrogenic diabetes insipidus

• Poor urine concentration after dehydration
• No rise in urine osmolality after desmopressin

Question 16

what is the treatment for cranial diabetes insipidus

Answer 16

DDAVP (desmopressin)

Over-treatment can cause hyponatraemia

Question 17

what is the treatment for Nephrogenic diabetes insipidus

Answer 17

Correction of cause (metabolic / drug cause)

Thiazide diuretics / NSAIDs

Question 18

what is the treatment for Primary polydipsia

Answer 18

Explanation, persuasion

Psychological therapy

Question 19

define hyponatraemia

Answer 19

[Sodium] <135 mmol/L
Severe [Na] <125 mmol/L

Non-specific
Headache, nausea, mood change, cramps, lethargy
Severe / sudden
Confusion, drowsiness, seizures, coma

Question 20

how would you classify hyponatraemia

Answer 20

Exclude ‘drug’ causes 
-
Thiazide diuretics, others
Exclude high concentrations of 
-Glucose, plasma lipids or proteins

Classify by extracellular fluid volume status

• Hypovolaemia
• -Renal loss, non-renal loss (D&V, burns, sweating)
• Normovolaemia (euvolaemia)
• -Hypoadrenalism, hypothyroidism
• -Syndrome of inappropriate ADH secretion (SIADH)

-Hypervolaemia
Renal failure, cardiac failure, cirrhosis, excess IV dextrose

Question 21

what is Syndrome of inappropriate ADH secretion (SIADH)

Answer 21

Diagnosis

• Clinically euvolaemic patient
• Low plasma sodium and low plasma osmolality
• Inappropriately high urine sodium concentration and high urine osmolality

Assess renal, adrenal and thyroid function

Many causes
-Neoplasias, neurological disorders (CNS), lung disease, drugs, endocrine (hypothyroid/hypoadrenalism)

Question 22

what is SIADH treatment

Answer 22

Fluid restriction (<1000 ml daily)

• Induce negative fluid balance 500 ml
• Aim ‘low normal’ sodium

Demeclocycline
-Drug that induces mild nephrogenic DI

Vasopressin (V2 receptor) antagonists

• “Vaptans” – induce a water diuresis
• Expensive, variable responses, some attenuation
• Lack of clinically significant outcome data

Question 23

treatment for hyponatraemia

Answer 23

Correct severe hyponatraemia slowly
Rapid correction risks oligodendrocyte degeneration and CNS myelinolysis (osmotic demyelination)
Severe neurological sequelae, may be permanent
Alcoholics & malnourished particularly at risk

Intravenous fluids. IV sodium solution