16. Salt and water disorders Flashcards
if plasma osmolality increases (usually due to increased sodium levels) or body water decreases what happens
receptors in the hypothalamus. cause ‘thirst’ to increase water uptake
also cause posterior pituitary to release vasopressin aka ADH (antidiuretic hormone)
how does ADH (anti-diuretic work)
acts on the collecting ducts of the kidney to increase the number of aquaporin channels allowing water to be absorbed in the blood
What is diabetes insipidus and what can it be classified into
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.
what is primary polydipsia
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
What is nephrogenic diabetes insipidus
this is when the collecting ducts of the kidney do not respond to ADH
what can cause nephrogenic diabetes insipidus
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)
what is cranial diabetes insipidus
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
what can cause cranial diabetes insipidus
- Brain tumours
- Head injury
- Brain malformations
- Brain infections (meningitis, encephalitis and tuberculosis)
- Brain surgery or radiotherapy
what is the presentation symptoms for diabetes insipidus
- Polyuria (excessive urine production)
- Polydipsia (excessive thirst)
- Dehydration
- Postural hypotension
- Hypernatraemia
what investigations would you carry out in someone with suspected diabetes insipidus
- Low urine osmolality
- High serum osmolality
- Water deprivation test
What is the water deprivation test aka desmopressin stimulation test.
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
what is the urine osmolality in patients after deprivation, and after ADH if they have cranial diabetes insipidus and why.
After deprivation - low
after ADH- high
what is the urine osmolality in patients after deprivation, and after ADH if they have nephrogenic diabetes insipidus and why.
after deprivation.- low
after ADH- low
the patient is unable to respond to ADH
what is the urine osmolality in patients after deprivation, and after ADH if they have primary polydypsia and why
after deprivation- high
after ADH- high
they are drinking so much water that urine
what is the treatment for diabetes insipidus
desmopressin (synthetic ADH)
What is SIADH
syndrome of inappropriate ADH release
what is the issue in SIADH
posterior pituitary secreting too much ADH or ADH may be coming from elsewhere such as small cell lung cancer
what does excessive ADH cause
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
in excessive ADH release what happens to the osmolality of the urine
high urine osmolality
what are the symptoms of SIADH
non specific signs • Headache • Fatigue • Muscle aches and cramps • Confusion • Severe hyponatraemia can cause seizures and reduced consciousness
name some causes of SIADH
- 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
in SIADH what will the following results show.
U+Es
urine sodium and osmolality
U&E will show hyponatramia
Urine sodium and osmolality will be high
what are some other causes of hyponatramia that need to be excluded
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
What do you have to suspect in someone with persistent hyponatriemia with no clear cause
malignancy
It is essential to correct sodium slowly to prevent which disease
central pontine myelinnolysis
aim for a change in sodium of less than 10 mmol/l per. 24 hours
Name some common management options to treatment hyponatraemia
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
what is central pontine myelinolysis (CPM)
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
Name some signs of dehydration/hypovolaemia
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
name some signs of fluid overload/hypervolaemia
o Tachycardia / bounding pulse o Raised JVP o Pulmonary oedema or pleural effusions o Ascites o Peripheral oedema
hyponatraemia
If the patient is dehydrated and has high sodium in the urine what are some causes of renal Na+ loss
Addisons
renal failure
diuretic excessive
osmoloar diuresis
hyponatraemia
if the patient is dehydrated and has low urinary sodium then what are some of the reasons for loss elsewhere
Diarrhoea vomiting fistulae burns small bowel obstruction trauma CF heat exposure
hyponatraemia
if the patient is not dehydrated and is oedematous what conditions could they have
nephrotic syndorme
cardiac failure
cirrhosis
renal failure
hyponatraemia
if the patient is not dehydrated, not oedematous but has high urine osmolality what condition do they have
inappropriate ADH
hyponatraemia
if the patient is not dehydrated, not oedematous and doesn’t have high urine osmolality what could they have
water overload
severe hypothyroidism
glucocorticoid insufficiency
what could be the cause of hypernatraemia
only really associated with dehydration where the person has been unable to access water
what are some causes of hypernatraemia and euvolaemia
inadequate water intake
fever/burns
diabetes insipidus.
what are some causes of hypernatraemia and hypovolaemia
very rare !!!
iatrogenic- IV sodium bicarbonate infusion
salt ingestion- accidental or sea water drowning
Name some causes of hypernatraemia and hypovolaemia
vomiting, diarrhoea, fistulae
excessive sweating
hyperosmolar diabetic coma
renal failure
