SIADH

Question 1

What is SIADH?

Answer 1

SIADH is caused by over secretion of ADH
ADH is released by the eposteriro pituitary gland in response to normally increases water reabsortpion in the kidney (aquaporij 2 channels move onto the apical membrane of the tubules). In SIADH there can be e topic ADH release or the this negative feedback mechanism is lost and ADH is inappropriately released, this causes hyponatraemia.

In SIADH the patient remains euvolaemic – ADH causes fluid retention, thus increases fluid levels which is detected by the heart muscle and case natriurtietc peptide release (thus reduces fluid volume back to normal levels through increased gfr and Na excretion). This causes euvolaemic with hyponatramia, a highurine osmolarity and a low serum osmolarity

Question 2

What are the causes of SIADH

Answer 2

Neurological – Meningitis, Haemorrhage. Abscess, stroke
Hypothyroidism
Respiratory (Tb, atypical pneumonia, lung abscess)
Malignancy (Small cell lung cancer, Pancreatic, Prostate, Lymphoma)
Drugs (Opiates, Amitriptyline, SSRI’s, Carbimazole)
Other – Trauma/Large surgery

Question 3

What will you find in a history taking of SIADH?

Answer 3

Symptoms:
Mild – Nausea, Vomiting, headache, anorexia, lethargy
Moderate – Cramps, Weakness, Confusion, Ataxia
Severe – drowsiness, seizure , coma

Risk factors:

Specific questions to ask:
Is there any oedema? – if there is oedema then it is not SIADH (while there is increased fluid levels in SIADH, there is no change in the osmolarity of fluid compartments and thus it is evenly spread, with most going into intravascular spaces like most fluid, this means that very little fluid moves into the extracellular compartment and so. O signs of fluid overload are seen.

Differentials:
Diuretic
Renal failure
Addison’s – Hyponatraemia + Hyperkalaemia

Question 4

What will you find on examination of SIADH?

Answer 4

End of the bed:
Reduced consciousness
Seizures
Assess fluid status – If SIADH patient should be euvolaemic and thus have no signs of fluid overload or dehydration
Chest:
Respiratory arrest – hyponatraemia causes raised ICP and brainstem herniation
Look for an underlying cause – fever, respiratory signs, weight loss, neurological signs

Question 5

What investigations will you order in SIADH?

Answer 5

Bedside:
Urine osmolarity – raised as water is being reabsorbed
Urinary sodium – raised

Bloods:
U&E – hyponatraemia, rule out renal failure as a cause of fluid overload
Plasma osmolarity – reduced as fluid levels dilute
TFT – hypothyroidism is a cause
Serum cortisol/Short Synachten test – ruling out Addison’s
FBC - Looking for infection
Serum Ca2+ - Looking for cancer

Imaging:
CXR – if lung disease indicated
Head CT - if neurological cause indicated

SIADH is diagnosed based off of clinical findings of hyponatraeima, concentrated urine and low plasma osmolality

Question 6

What is the treatment of SIADH?

Answer 6

A-E approach
Get IV Access/Give O2 to maintain sats of 94+
Assessment with AMPLE history and brief examination
Get help - Consider ITU referral
Frequent Observations - Constant or 15 minutely

Medical:
Symptomatic - Refer to ITU for hypertonic (3%) saline infusion, patient may require vaptans (vasopressin receptor antagonists that cause loss of fluid with no change to electrolyte levels) or loop diuretics
Asymptomatic - Fluid Restriction of 1L/day for first day, then 500 mL less than daily urine volume from then on.