SIADH

Question 1

What does SIADH stand for?

Answer 1

Syndrome of Inappropriate ADH

Question 2

SIADH refers to the decreased/increased release of ADH from the anterior/posterior pituitary?

Answer 2

increased release
posterior pituitary

Question 3

SIADH leads to increased water reabsorption from the urine, diluting the blood and leading to h_____

Answer 3

hyponatraemia

Question 4

Where is ADH made?

Answer 4

Hypothalamus

Question 5

Where is ADH stored and secreted from?

Answer 5

Posterior pituitary gland

Question 6

ADH stimulate water reabsorption from where in the kidneys?

Answer 6

Collecting ducts
Inserts more aquaporin II channels to reabsorb water

Question 7

What causes SIADH?

Answer 7

Think SIADH

Small cell lung cancer

Infection (eg. TB, pneumonia, meningitis, HIV)

Abscesses (other tumours)

Drugs (SSRIs - selective serotonin reuptake inhibitors, antidepressant)

Head trauma / Post-operative (stress response increase ADH release)

Question 8

What are 2 potential sources of excessive ADH?

Answer 8

Increased secretion by posterior pituitary

Ectopic ADH

Question 9

What is the most common cause of ectopic ADH?

Answer 9

small cell lung cancer

Question 10

True of false: SIADH causes so much extra water that there is fluid overload?

Answer 10

False
Enough to reduce sodium concentration (hyponatraemia), not much to change volume of blood by much.

Question 11

SIADH results in e______ hyponatraemia

Answer 11

euvolemic
(normal volume of blood)

Question 12

What happens to the urine in SIADH?

Answer 12

It becomes more concentrated
High urine osmolality and high urine sodium

Question 13

How may a patient with SIADH present?

Answer 13

May be asymptomatic

Vomiting
Headache
Muscle cramps
Fatigue
Confusion / decreased GCS (Glasgow coma scale)
Brain stem herniation

Question 14

What can severe hyponatraemia cause?

Answer 14

Seizures and reduced consciousness

Question 15

Why can brainstem herniation be caused by SIADH?

Answer 15

Low sodium means increased water enters skull to compensate. This increases the intra-cranial pressure and can cause hyponatraemic encephalopathy with risk of the brainstem herniating through foramen magnum.

Question 16

How do you diagnose SIADH?

Answer 16

Euvolaemia
Hyponatraemia
Low serum osmolality
High urine sodium
High urine osmolality

Question 17

What other causes of hyponatraemia need to be ruled out?

Answer 17

adrenal insufficiency (less aldosterone)- short synacthen test

Diuretic meds

D&V, burns, fistula, excessive sweating

No excessive water intake

No CKD or AKI (more water to sodium)

No heart failure (fluid retention) or liver disease

Na+ depletion - give 0.9% saline and should normalise, won’t in SIADH.

Question 18

What other condition also causes euvolaemic hyponatraemia?

Answer 18

Primary polydipsia

Excessive water consumption with no cause diluting the blood and urine.

Question 19

How can you differentiate primary polydipsia and SIADH?

Answer 19

primary polydipsia has low urine sodium levels and urine osmolality

SIAD has high urine sodium and urine osmolality

Question 20

How do you manage SIADH?

Answer 20

Treat the underlying cause (eg stop causative SSRI or treat infection)

Fluid restriction

Hypertonic saline (to concentrate the blood)

For chronic cases, use a vasopressin receptor antagonist eg tolvaptan

Question 21

Why is it important to correct the sodium slowly?

Answer 21

To prevent osmotic demyelination (don’t really need to know yet)

Na+ concentration shouldn’t change more than 10mmol/l in 24 hours