Diabetes Insipidus Flashcards
Types of DI.
Vasopressin def. aka cranial DI
Vasopressin resistance aka nephrogenic DI
Features of DI.
Passing of large volumes of dilute urine
Profound unquenchable thirst
Biochemical hallmarks of DI.
High serum osmolality (due to polyuria)
Low urine osmolality (due to polyuria)
High urine volume (due to polyuria)
What can happen in severe cases of DI?
Hypernatraemia
Dehydration
Death
What usually causes cranial DI?
Pituitary disease
If there is a strong family history genetic causes should be considered.
What usually cause nephrogenic DI?
Metabolic or electrolyte disturbances
Renal disease
Drugs affecting the kidney such as lithium.
What is primary polydipsia?
A behavioural condition leading to polydipsia which in its turn drives polyuria.
How can you differentiate between primary polydipsia and DI?
Primary polydipsia is not associated with hypernatraemia and may infact cause dilutional hyponatraemia.
Why might it be hard to distinguish between primary polydipsia and DI?
Due to similar symptoms.
and
Some patients with PPD have an impaired ability to concentrate their urine. This is due to a down-regulation of vasopressin release.
Investigations of DI.
Urine volume
Serum osmolality
Urine osmolality
U&Es
Glucose to exclude DM
Ca2+
When is DI confirmed due to investigations?
Urine volume > 3L per 24 hours with high serum osmolality and low urine osmolality.
or
Serum osmolality >295 mosmol/kg and urine osmolality < 300 mosmol/kg
What urine osmolalities or serum osmolalities will exclude DI?
If urine osmolality is >600 mosmol/kg
If the serum osmolality is doubled.
If the urine to plasma osmolality ratio (U:P) is more than 2:1 provided that plasma osmolality is no greater than 295mosmol/kg significant DI can be excluded.
In DI despite raised plasma osmolality, uine is dilute with a U:P ratio <2
In partial DI it might be hard to confirm a diagnosis.
What can be done to confirm it?
Water Deprivation Test (WDT)
Explain WDT
Patients with a frank DI will have an unacceptable thirst and lose significant weight due to the water loss.
When should WDT be stopped?
If excessive weight loss occurs or symptoms are too severe.
When is DI excluded in WDT?
If patients concentrate urine osmolality > 600 mosmol/kg
and
serum osmolality remains < 300 mosmol/kg.
Once symptoms have been established in WDT and DI can be confirmed…
What is the next step?
Synthetic vasopressin (DDAVP) administration.
Why is DDAVP administered?
To distinguish between cranial DI and nephrogenic DI
What happens in cranial DI when DDAVP is given?
Leads to reduced urine volume and increased urine osmolality.
Symptoms start to fade.
What happens in nephrogenic DI when DDAVP is given?
No response.
When is it not needed to do a WDT?
If DI is clinically obvious
What is new way of diagnosing DI without WDT?
To use co-peptin which is an AVP precursor.
Management of cranial DI.
Investigation of pituitary disease and treat it accordingly.
Can also give DDAVP such as desmopressin to relieve symptoms.
How can desmopressin be administered?
Intra-nasally
Orally
Sublingually
Parenterally
Complications of over-treatment with DDAVP.
Dilutational hyponatraemia
Clinical features of over-treatment with DDAVP.
Headache
Reduced cognitive ability
Seizures if there is sudden drop in sodium.
Signs of under-treatment of DDAVP.
Excessive thirst and polyuria.
Sometimes but very rarely patients with DI may have an impaired thirst mechanism, meaning they won’t have the commonly associated polydipsia.
What is it called and what causes it?
Hypodipsic DI
Can be seen in hypothalamic infiltrative disorders.
This requires specialist care.
Risks of hypodipsic DI.
Severe hypernatraemia
Dehydration
Management of nephrogenic DI.
Underlying cause should be considered and dealt with if possible.
If symptoms persist after primary fault has been dealt with, how can nephrogenic DI be treated?
Low salt diet
Low protein diet
Diuretics such as thiazides
NSAIDS
(Possibly DDAVP)
Preparations and precautions of the 8h water deprivation test.
Do not do the test before establishing that the urine volume is >3L/d
Stop the test if the urine osmolality >600 mOsmol/kg in Stage 1 and stop the test if >3% body weight is lost during the test.
Before initiating free fluids are okay until 07.30. Light breakfast at 06.30, no tea, no coffee and no smoking.
Explain how to perform the 8 hour water deprivation test.
Stage 1:
Fluid deprivation for 8 hours and start it at 08:00.
Empty bladder, then no drinks and only dry food.
Weigh hourly
Collect urine every 2h, measure volume and osmolality.
Venous sample for osmolality every 4h
Stop test after 8h if urine osmolality >600 mosmol/kg (ie normal)
Stage 2:
Differentiate between cranial and nephrogenic
Proceed if urine still dilute (I.e. urine < 600mosmol/kg)
Give desmopressin 2mcg IM. Water can be drunk now.
Measure urine osmolality hourly for the next 4h.
