16. Salt and water disorders

Question 1

if plasma osmolality increases (usually due to increased sodium levels) or body water decreases what happens

Answer 1

receptors in the hypothalamus. cause ‘thirst’ to increase water uptake

also cause posterior pituitary to release vasopressin aka ADH (antidiuretic hormone)

Question 2

how does ADH (anti-diuretic work)

Answer 2

acts on the collecting ducts of the kidney to increase the number of aquaporin channels allowing water to be absorbed in the blood

Question 3

What is diabetes insipidus and what can it be classified into

Answer 3

lack of antidiuretic hormone (ADH) or a lack of response to ADH. This prevents the kidneys from being able to concentrate the urine leading to polyuria (excessive amounts of urine) and polydipsia (excessive thirst). It can be classified as nephrogenic or cranial.

Question 4

what is primary polydipsia

Answer 4

when a patient has a normally functioning ADH system. but they are drinking excessive quantities of. water leading to excessive urine production

they don’t have diabetes insipidus

Question 5

What is nephrogenic diabetes insipidus

Answer 5

this is when the collecting ducts of the kidney do not respond to ADH

Question 6

what can cause nephrogenic diabetes insipidus

Answer 6

Drugs, particularly lithium used in bipolar affective disorder
Mutations in the AVPR2 gene on the X chromosome that codes for the ADH receptor
Intrinsic kidney disease
Electrolyte disturbance (hypokalaemia and hypercalcaemia)

Question 7

what is cranial diabetes insipidus

Answer 7

when the hypothalamus does not produce ADH for the poituairy gland to secrete

it can be idiopathic, without a clear cause or a known cause

Question 8

what can cause cranial diabetes insipidus

Answer 8

• Brain tumours
• Head injury
• Brain malformations
• Brain infections (meningitis, encephalitis and tuberculosis)
• Brain surgery or radiotherapy

Question 9

what is the presentation symptoms for diabetes insipidus

Answer 9

• Polyuria (excessive urine production)
• Polydipsia (excessive thirst)
• Dehydration
• Postural hypotension
• Hypernatraemia

Question 10

what investigations would you carry out in someone with suspected diabetes insipidus

Answer 10

• Low urine osmolality
• High serum osmolality
• Water deprivation test

Question 11

What is the water deprivation test aka desmopressin stimulation test.

Answer 11

patient. doesn’t have any fluids for 8 hours and the urine osmollity is then measured
patient then given synthetic ADH (desomopressin) and 8 hours later their urine osmolality measured again

Question 12

what is the urine osmolality in patients after deprivation, and after ADH if they have cranial diabetes insipidus and why.

Answer 12

After deprivation - low

after ADH- high

Question 13

what is the urine osmolality in patients after deprivation, and after ADH if they have nephrogenic diabetes insipidus and why.

Answer 13

after deprivation.- low
after ADH- low

the patient is unable to respond to ADH

Question 14

what is the urine osmolality in patients after deprivation, and after ADH if they have primary polydypsia and why

Answer 14

after deprivation- high
after ADH- high

they are drinking so much water that urine

Question 15

what is the treatment for diabetes insipidus

Answer 15

desmopressin (synthetic ADH)

Question 16

What is SIADH

Answer 16

syndrome of inappropriate ADH release

Question 17

what is the issue in SIADH

Answer 17

posterior pituitary secreting too much ADH or ADH may be coming from elsewhere such as small cell lung cancer

Question 18

what does excessive ADH cause

Answer 18

results in excessive water reabsorption in the collecting ducts.
this can cause hyponatriemia
excessive reabsorption is often not enough to cause fluid overload therefore you end up with a euvolaemic hyponatramia
urine becomes more concentrated as less. water is excreted by the kidneys- therefore high urine osmolality and high urine sodium

Question 19

in excessive ADH release what happens to the osmolality of the urine

Answer 19

high urine osmolality

Question 20

what are the symptoms of SIADH

Answer 20

non specific signs 
•	Headache
•	Fatigue
•	Muscle aches and cramps
•	Confusion
•	Severe hyponatraemia can cause seizures and reduced consciousness

Question 21

name some causes of SIADH

Answer 21

• Post-operative from major surgery
• Infection, particularly atypical pneumonia and lung abscesses
• Head injury
• Medications (thiazide diuretics, carbamazepine, vincristine, cyclophosphamide, antipsychotics, SSRIs, NSAIDSs,)
• Malignancy, particularly small cell lung cancer
• Meningitis

Question 22

in SIADH what will the following results show.
U+Es
urine sodium and osmolality

Answer 22

U&E will show hyponatramia

Urine sodium and osmolality will be high

Question 23

what are some other causes of hyponatramia that need to be excluded

Answer 23

Negative short synacthen test to exclude adrenal insufficiency
No history of diuretic use
No diarrhoea, vomiting, burns, fistula or excessive sweating
No excessive water intake
No chronic kidney disease or acute kidney injury

Question 24

What do you have to suspect in someone with persistent hyponatriemia with no clear cause

Answer 24

malignancy

Question 25

It is essential to correct sodium slowly to prevent which disease

Answer 25

central pontine myelinnolysis

aim for a change in sodium of less than 10 mmol/l per. 24 hours

Question 26

Name some common management options to treatment hyponatraemia

Answer 26

Fluid restriction to 500-1L may be enough to not need medications
Tolvaptan is a ADH receptor blocker that causes. a rapid increase in sodium- this is dangerous and should only be initiated by a specialist endocrinologist

Question 27

what is central pontine myelinolysis (CPM)

Answer 27

aka osmotic demyelination syndrome and is usually a complication of long term severe hyponatraemia (less than 120 mmol/L) being treated too quickly (greater than a 10 mmol/L increase over 24 hours

Question 28

Name some signs of dehydration/hypovolaemia

Answer 28

o Cool peripheries / prolonged capillary refill
o Tachycardia / can be weak, thread pulse
o Postural hypotension
o Confusion
o Dry mucous membranes
o Reduced skin turgor

Question 29

name some signs of fluid overload/hypervolaemia

Answer 29

o	Tachycardia / bounding pulse
o	Raised JVP
o	Pulmonary oedema or pleural effusions
o	Ascites
o	Peripheral oedema

Question 30

hyponatraemia

If the patient is dehydrated and has high sodium in the urine what are some causes of renal Na+ loss

Answer 30

Addisons
renal failure
diuretic excessive
osmoloar diuresis

Question 31

hyponatraemia

if the patient is dehydrated and has low urinary sodium then what are some of the reasons for loss elsewhere

Answer 31

Diarrhoea 
vomiting 
fistulae 
burns 
small bowel obstruction 
trauma 
CF 
heat exposure

Question 32

hyponatraemia

if the patient is not dehydrated and is oedematous what conditions could they have

Answer 32

nephrotic syndorme
cardiac failure
cirrhosis
renal failure

Question 33

hyponatraemia

if the patient is not dehydrated, not oedematous but has high urine osmolality what condition do they have

Answer 33

inappropriate ADH

Question 34

hyponatraemia

if the patient is not dehydrated, not oedematous and doesn’t have high urine osmolality what could they have

Answer 34

water overload
severe hypothyroidism
glucocorticoid insufficiency

Question 35

what could be the cause of hypernatraemia

Answer 35

only really associated with dehydration where the person has been unable to access water

Question 36

what are some causes of hypernatraemia and euvolaemia

Answer 36

inadequate water intake
fever/burns
diabetes insipidus.

Question 37

what are some causes of hypernatraemia and hypovolaemia

Answer 37

very rare !!!
iatrogenic- IV sodium bicarbonate infusion
salt ingestion- accidental or sea water drowning

Question 38

Name some causes of hypernatraemia and hypovolaemia

Answer 38

vomiting, diarrhoea, fistulae
excessive sweating
hyperosmolar diabetic coma
renal failure