Polyuria - Teaching by Dr Cox

Question 1

How can you classify polyuria (specify)?

Answer 1

Volume (over 3.5L per day)
Frequency - day or night

Question 2

Causes of polyuria as increased frequency.

Answer 2

Obstructions/prostate hyperplasia
Detrusor insufficiency
Medications - diuretics etc
Infections / UTI

Noctural polyuria:
Due to third space mobilisation from a posturally dependent place, esp in older people

Question 3

Causes of polyuria as increased volume.

Answer 3

Drinking more:

Psychogenic polydipsia (increased thirst)
Increased thirst due to medications - TCAs,

Retaining less:

Diabetes insipidus (cranial or nephrogenic)

Producing more:

Diabetes
Diabetic drugs - SGLT2 inhibitors
Solute diaresis (e.g. to get rid of excess salt)

Question 4

List some medications which can increase thirst

Answer 4

Anticholinergics e.g. donepezil
TCAs

Question 5

Causes of cranial diabetes insipidus / ADH deficiency

Answer 5

V - vascular - Sheehan’s
I - infection - meningitis, TB
T - trauma - pituitary stalk shearing, often secondary to second Transphenoidal surgeries
A - autoimmune - TB, sarcoidosis, IGG4 diesase, Langerhans histiocytosis (due to chemo/post pregancy)
M - metabolic
I - iatrogenic
N - neoplasm - craniopharyngiomas
C - congenital

Question 6

Causes of nephrogenic diabetes insipidus / ADH deficiency

Answer 6

Kidneys aren’t responding to anti-diuretic hormone

Post-AKI (resolves in 2-3 days)
Congenital mutations
Drugs - Lithium
Primary hyperparathyroidism (hypercalcemia)
Tubular interstitial disease

Question 7

Why doesn’t kidney damage cause nephrogenic DI?

Answer 7

Because kidney damage normally causes your kidneys to just shut down completely rather than produce urine.

Question 8

Investigations for polyuria

Answer 8

Urine dip - for glucose and infection
Bladder diary
Bloods - check sodium levels to see if dehydrated and hypernatremic (>145) due to loss of water

Question 9

What is the normal plasma osmolality

Answer 9

285 - 295

Question 10

What is the minimum and maximum concentration of dilution of urine

Answer 10

100-1000

Question 11

What further tests would you do after the GP tests

Answer 11

Measure early morning urine and plasma osmolality
Water deprivation test
Desmopressin/DDAVP - to distinguish cranial vs nephrogenic

Question 12

What results would you see for normal and abnormal early morning urine and plasma osmolality

Answer 12

Normally:

Plasma - normal
Urine - concentrated >800 as you’ve retained water through the night

If abnormal:
Urine - <600 = dilute (lost water due to ADH not working properly)
Plasma - >295 = concentrated; sodium very high

Question 13

Water deprivation test is how useful?

Answer 13

Not that useful as patients often compensate for the lack of drinking throughout the day by drinking huge volumes of water before the test, so you can’t accurately see if they’re losing water/retaining water.

Question 14

What can you use instead / new tests upcoming?

Answer 14

Arginine - co-peptin (arginine stimulates ADH production as well as GH)
Co-peptin is something linked to ADH that can be measured, since ADH can’t be measured.

Question 15

Management of cranial DI

Answer 15

Desmopressin - nocturnal nasal spray to last throughout the night, and then a tablet for the morning

Question 16

Management of nephrogenic DI

Answer 16

Harder to manage

LOW SODIUM DIET - Advise to reduce salt intake as too much salt would mean your body would need to hold onto water to dilute the sodium, so this unmeetable need is eradicated.

Thiazide diuretics (counter intuitive but knocking out the Na/Cl channels on the distal tubule means that the kidneys respond by reabsorbing sodium via the ascending limb, which therefore reabsorbs water by osmosis.

You can try high dose desmopressin spray.

Question 17

What other hormones are necessary to pee?

Answer 17

Thyroxine and cortisol!