SIADH

Question 1

SIADH defined by

Answer 1

hyponatraemia (apparent hyponat bc of increase H2O in blood)
hypo-osmolality (decreased concentration of all bodily fluids)
due to crazy high ADH secretion

Question 2

causes

Answer 2

tumours
pneumonia
tb
meningitis
head injury
alcohol withdrawal
drugs (thiazide diuretics)
there are millions more

Question 3

osmolality = ?

Answer 3

measure of the osmoles of solute per kilogram of solvent i.e. the higher the number, the more salts there are dissolved in the plasma

Question 4

how does ADH usually work

Answer 4

1. ADH produced when there is a high serum osmolality. 2. Binds to distal convoluted tubule and allows water to be reabsorbed out of collecting ducts into blood stream
2. Results in decrease in volume and increase in osmolality of the urine excreted

Question 5

what happens to sodium in SIADH

Answer 5

low

Question 6

sx? worse if acute or chronic?

Answer 6

large range, depends on time taken for hyponatraemia to develop. i.e. if develops rapidly, worse symptoms

Mild – Nausea / Vomiting / Headache / Anorexia / Lethargy
Moderate – Muscle cramps / Weakness / Confusion / Ataxia
Severe – Drowsiness / Seizures / Coma

Question 7

fluid status in siadh

Answer 7

euvolaemic or hypervolaemic (if dehydrated, consider another cause)

Question 8

NB SIADH is one of the commoner causes of hyponatraemia

Answer 8

-

Question 9

plasma osmolality in siadh

Answer 9

reduced (due to the low sodium concentration)

Question 10

urine osmolality

Answer 10

relatively high (if serum osmolality is low, urine osmolality should also be low, because kidneys are trying to retain the solute. In SIADH the excess ADH causes water retention but NOT solute retention)

Question 11

is sodium lost in SIADH

Answer 11

no. it’s just that more water is retained leading to a lower concentration of sodium in the blood and therefor an apparently lower sodium if you analyse the amount in the blood. dilutional.
NB Movement of potassium from the intracellular space to the extracellular space prevents dilutional hypokalemia

Question 12

other ix other than U+E and plasma and urine osmolality

Answer 12

serum cortisol -> addison’s causes low Na. a low serum cortisol would suggest addison’s
tfts - hypothyroid can cause SIADH

Question 13

major diagnostic features (6)

Answer 13

Hyponatraemia
Low plasma osmolality
Inappropriately elevated urine osmolality (>plasma osmolality)
Urine [Na+] >40 mmol/L with normal salt intake
Euvolaemia
Normal thyroid and adrenal function

Question 14

rx

Answer 14

no step by step rx as so many causes. specialist endocrine input.
fluid restriction, sodium replacement

Question 15

really bad complication of sodium replacement

Answer 15

central pontine myelinolysis. permanent damage to myelin sheath in brainstem causing paralysis, dysphagia, dysarthria, diploplia, decreased conciousness

Question 16

diabetes insip vs SIADH
urine
ADH
Na
Hydration level
thirst

Answer 16

diabetes insip
++ urine, low ADH (or insensitive), apparent hypernat, dehydrated

SIADH
– urine, low ADH, apparent hyponat, over hydrated

BOTH ARE THIRSTY. diabetes insip as are dehydrated, SIADH as increased ADH makes you thirsty