ENDOCRINE: Diabetes insipidus

Question 1

What is osmolality?

Answer 1

How salty the blood is, low osmolality means less salt

Question 2

What is diabetes insipidus caused by?

Answer 2

Vasopressin deficiency - cranial DI,

or vasopressin resistance - nephrogenic DI.

Question 3

What is seen in biochemistry of DI?

Answer 3

High serum osmolality, low urine osmolality and high urine volume

Question 4

What is cranial DI caused by?

Answer 4

Pituitary disease 
Brain tumours 
Head injury 
Brain malformation 
Brain infections 
Genetic causes - strong FHx

Question 5

What is nephrogenic DI caused by?

Answer 5

Metabolic and electrolyte disturbance
Renal disease
Drugs affecting the kidney e.g. lithium

Question 6

What values of urine volume confirm DI?

Answer 6

> 3L in 24hrs in the presence of high serum osmolality and low urine osmolality

Question 7

What values of serum osmolality and urine osmolality confirm DI?

Answer 7

serum osmolality >295mosmol/kg and urine osmolality <300mosmol/kg

Question 8

What value of urine osmolality or serum osmolality can exclude diagnosis of DI?

Answer 8

Urine osmolality >600mosmol/kg

Double serum osmolality

Question 9

What investigation might you do for suspected DI?

Answer 9

Water deprivation test

Question 10

How do you conduct the water deprivation test?

Answer 10

Patient fluid deprived for 8 hours
Then urine osmolality is measured
Synthetic ADH is administered
8 Hours later urine osmolality is measure again

Question 11

What results would you expect on the water depravation test for cranial DI?

Answer 11

Initially low urine osmolality, then post-ADH administration high urine osmolality as cells of collecting duct are responding to the synthetic ADH

Question 12

What results would you expect on the water depravation test for nephrogneic DI

Answer 12

Low urine osmolality both pre and post ADH, due to the cells of the collecting duct not being able to respond to ADH

Question 13

How is cranial DI managed?

Answer 13

Investigate for pituitary disease

Give synthetic vasopressin - desmopressin

Question 14

How does overtreatment of DI with desmopressin present?

Answer 14

Get dilutional hyponatrameia - so get headaches, reduced cognitive ability, seizures

N.B.desmopressin= synthetic ADH, overtreatment leads to increased reabsorption of water therefore leading to the dilutional hyponatraemia

Question 15

How does undertreatment of DI with desmopressin present?

Answer 15

Excessive thirst, polyuria

Question 16

How is nephrogenic DI managed?

Answer 16

Find cause and reverse if possible.
Use of low salt, low protein diet, diuretics, NSAIDs
Keep on top of the thirst and replace water lost

Question 17

What are the differentials of Diabetes Insipidus clinical presentation?

Answer 17

Psychogenic polydipsia, DM, Diuretic use, Hypercalcaemia ( as serum calcium is elevated)