Diabetes Insipidus

Question 1

Types of DI.

Answer 1

Vasopressin def. aka cranial DI

Vasopressin resistance aka nephrogenic DI

Question 2

Features of DI.

Answer 2

Passing of large volumes of dilute urine

Profound unquenchable thirst

Question 3

Biochemical hallmarks of DI.

Answer 3

High serum osmolality (due to polyuria)

Low urine osmolality (due to polyuria)

High urine volume (due to polyuria)

Question 4

What can happen in severe cases of DI?

Answer 4

Hypernatraemia

Dehydration

Death

Question 5

What usually causes cranial DI?

Answer 5

Pituitary disease

If there is a strong family history genetic causes should be considered.

Question 6

What usually cause nephrogenic DI?

Answer 6

Metabolic or electrolyte disturbances

Renal disease

Drugs affecting the kidney such as lithium.

Question 7

What is primary polydipsia?

Answer 7

A behavioural condition leading to polydipsia which in its turn drives polyuria.

Question 8

How can you differentiate between primary polydipsia and DI?

Answer 8

Primary polydipsia is not associated with hypernatraemia and may infact cause dilutional hyponatraemia.

Question 9

Why might it be hard to distinguish between primary polydipsia and DI?

Answer 9

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.

Question 10

Investigations of DI.

Answer 10

Urine volume

Serum osmolality

Urine osmolality

U&Es

Glucose to exclude DM

Ca2+

Question 11

When is DI confirmed due to investigations?

Answer 11

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

Question 12

What urine osmolalities or serum osmolalities will exclude DI?

Answer 12

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

Question 13

In partial DI it might be hard to confirm a diagnosis.

What can be done to confirm it?

Answer 13

Water Deprivation Test (WDT)

Question 14

Explain WDT

Answer 14

Patients with a frank DI will have an unacceptable thirst and lose significant weight due to the water loss.

Question 15

When should WDT be stopped?

Answer 15

If excessive weight loss occurs or symptoms are too severe.

Question 16

When is DI excluded in WDT?

Answer 16

If patients concentrate urine osmolality > 600 mosmol/kg

and

serum osmolality remains < 300 mosmol/kg.

Question 17

Once symptoms have been established in WDT and DI can be confirmed…

What is the next step?

Answer 17

Synthetic vasopressin (DDAVP) administration.

Question 18

Why is DDAVP administered?

Answer 18

To distinguish between cranial DI and nephrogenic DI

Question 19

What happens in cranial DI when DDAVP is given?

Answer 19

Leads to reduced urine volume and increased urine osmolality.

Symptoms start to fade.

Question 20

What happens in nephrogenic DI when DDAVP is given?

Answer 20

No response.

Question 21

When is it not needed to do a WDT?

Answer 21

If DI is clinically obvious

Question 22

What is new way of diagnosing DI without WDT?

Answer 22

To use co-peptin which is an AVP precursor.

Question 23

Management of cranial DI.

Answer 23

Investigation of pituitary disease and treat it accordingly.

Can also give DDAVP such as desmopressin to relieve symptoms.

Question 24

How can desmopressin be administered?

Answer 24

Intra-nasally

Orally

Sublingually

Parenterally

Question 25

Complications of over-treatment with DDAVP.

Answer 25

Dilutational hyponatraemia

Question 26

Clinical features of over-treatment with DDAVP.

Answer 26

Headache

Reduced cognitive ability

Seizures if there is sudden drop in sodium.

Question 27

Signs of under-treatment of DDAVP.

Answer 27

Excessive thirst and polyuria.

Question 28

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?

Answer 28

Hypodipsic DI

Can be seen in hypothalamic infiltrative disorders.

This requires specialist care.

Question 29

Risks of hypodipsic DI.

Answer 29

Severe hypernatraemia

Dehydration

Question 30

Management of nephrogenic DI.

Answer 30

Underlying cause should be considered and dealt with if possible.

Question 31

If symptoms persist after primary fault has been dealt with, how can nephrogenic DI be treated?

Answer 31

Low salt diet

Low protein diet

Diuretics such as thiazides

NSAIDS

(Possibly DDAVP)

Question 32

Preparations and precautions of the 8h water deprivation test.

Answer 32

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.

Question 33

Explain how to perform the 8 hour water deprivation test.

Answer 33

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.