T1 L10 Physiology of thirst, fluid balance and disorders

Question 1

What does regulation of water balance ensure?

Answer 1

Plasma osmolality and extracellular fluid osmolality remains stable

Question 2

What is the range of plasma osmolality?

Answer 2

285-295mosmol/kg

Question 3

What are the 3 key determinants of water homeostasis?

Answer 3

ADH
Kidney
Thirst

Question 4

What are osmoreceptors?

Answer 4

Groups of specialised cells which detect changes in plasma osmolality

Question 5

Where are osmoreceptors located?

Answer 5

In anterior wall of third ventricle

Question 6

How do osmoreceptors alter their volume?

Answer 6

By transmembrane flu of water in response to changes in plasma osmolality

Question 7

What is the effect of osmoreceptors altering their volume?

Answer 7

Initiates neuronal impulses that are transmitted to the hypothalamus to synthesise ADH and to the cerebral cortex to register thirst

Question 8

What is arginine vasopressin (AVP)?

Answer 8

Human form of ADH

Nonapeptide

Question 9

Where is AVP synthesised?

Answer 9

In neurons in supraoptic and paraventricular nuclei of hypothalamus

Question 10

What happens to the aquaporins when ADH is released?

Answer 10

Aquaporins are normally stored in cytoplasmic vesicle
Move and fuse with luminal membrane
Increases water permeability of renal collecting tubules to promote water reabsorption

Question 11

Describe low plasma osmolality

Answer 11

AVP is undetectable
Dilute urine
High urine output

Question 12

Describe high plasma osmolality

Answer 12

High AVP secretion
Concentrated urine
Low urine output

Question 13

What happens to thirst in high plasma osmolality?

Answer 13

Increased thirst sensation

Drinking immediately transiently suppresses AVP secretion and thirst to avoid overshoot

Question 14

What are the main causes of polyuria and polydipsia?

Answer 14

Diabetes mellitus
Cranial diabetes insipidus
Nephrogenic diabetes insipidus
Primary polydipsia

Question 15

What is cranial diabetes insidious?

Answer 15

Lack of osmoregulated AVP secretion

Question 16

What is nephrogenic diabetes insidious?

Answer 16

Lack of response of renal tubule to AVP

Thirst is still stimulated - polydipsia

Question 17

What are the genetic causes of cranial diabetes insipidus?

Answer 17

Familial AD mutation of AVP gene

DIDMOAD

Question 18

What are the secondary causes of cranial diabetes insipidus?

Answer 18

Post-surgical
Traumatic
Tumours
Histiocytosis
Sarcoidosis
Encephalitis
Meningitis
Vascular insults
Autoimmune

Question 19

Describe hypothalamic syndrome

Answer 19

Disordered thirst and diabetes insidious
Disordered appetite
Disordered temperature regulation 
Disordered sleep rhythm
Hypopituitarism

Question 20

What can cause nephrogenic diabetes insipidus?

Answer 20

Idiopathic
Genetic
Metabolic: high calcium concentration or low potassium concentration
Drugs: lithium
Chronic kidney disease

Question 21

What is primary polydipsia?

Answer 21

Psychogenic
Increased fluid intake
Lower plasma osmolality
Suppressed AVP secretion
Low urine osmolality, high urine output
Lose renal interstitial solute which reduces renal concentrating ability

Question 22

How is polyuria and polydipsia investigated?

Answer 22

Medical history
Exclude diabetes mellitus
24 hour fluid balance
Exclude hypercalcaemia / hypokalaemia
Water deprivation test

Question 23

What are the steps of the water deprivation test?

Answer 23

1) Period of dehydration
2) Measure plasma and urine osmolalities and weight
3) Injection of synthetic vasopressin
4) Measure plasma and urine osmolalities

Question 24

What is the normal response of the water deprivation test?

Answer 24

Normal plasma osmolality

High urine osmolality

Question 25

What is the response of the water deprivation test in cranial diabetes insipidus?

Answer 25

Poor urine concentration after dehydration

Rise in urine osmolality after desmopressin

Question 26

What is the response of the water deprivation test in nephrogenic diabetes insipidus?

Answer 26

Poor urine concentration after dehydration

No rise in urine osmolality after desmopressin

Question 27

What is the treatment for cranial diabetes insipidus?

Answer 27

DDAVP (desmopressin)

Over treatment can cause hyponatraemia

Question 28

What is the treatment for nephrogenic diabetes insipidus?

Answer 28

Correction of cause

Thiazide diuretics / NSAIDs

Question 29

What is the treatment for primary polydipsia?

Answer 29

Explanation, persuasion

Psychological therapy

Question 30

What is hyponatraemia?

Answer 30

Sodium concentration below 135mmol/L

<125mmol/L if severe

Question 31

What are the symptoms of hyponatraemia?

Answer 31

Asymptomatic
Non-specific: headache, nausea, mood change, cramps, lethargy
If severe / sudden: confusion, drowsiness, seizures, coma

Question 32

What drugs can cause hyponatraemia?

Answer 32

Thiazide diuretics

Question 33

How can hyponatraemia be classified?

Answer 33

Hypovolaemia
Normovolaemia
Hypervolaemia

Question 34

What are the causes of hypovolaemic hyponatraemia?

Answer 34

Renal loss

Non-renal loss: D&V, burns, sweating

Question 35

What are the causes of normovolaemic hyponatraemia?

Answer 35

Hypoadrenalism
Hypothyroidism
SIADH

Question 36

What are the causes of hypervolaemic hyponatraemia?

Answer 36

Renal failure
Cardiac failure
Cirrhosis
Excess IV dextrose

Question 37

What can happen if hyponatraemia is corrected too quickly?

Answer 37

Risk of oligodendrocyte degeneration and CBS myelinolysis

Question 38

What is the diagnosis of SIADH?

Answer 38

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

Question 39

What are the causes of SIADH?

Answer 39

Neoplasias
Neurological disorders
Lung disease
Drugs
Endocrine

Question 40

What is the treatment of SIADH?

Answer 40

Identify and treat underlying cause
Fluid restriction
Demeclocycline
Vasopressin antagonists: vaptans

Question 41

What does demeclocycline do?

Answer 41

Induces mild nephrogenic diabetes insipidus