Diabetes Insipidus and SIADH

Question 1

Diabetes insipidus
-pathophysiology and types

Answer 1

90% acquired, 10% inherited

1. lack production of ADH
   -idiopathic
   -head trauma, surgery
   -haemochromatosis
2. Kidney fails to respond
   -genetic
   -high Ca, low K
   -lithium

Question 2

Diabetes insipidus
-presentation

Answer 2

Polyuria, polydipsia, nocturia, bed wetting in children
-quantify fluid intake
-palpable distended bladder

Dehydration - fatigue, dizzy, weak
-dry mucous membrane, poor skin turgor
-long CRT, high HR, OH

Space occupying lesion - headache, visual changes, seizures
-test visual fields

Recent head trauma/surgery
Lithium?
FHx

Question 3

Diabetes insipidus
-investigations to rule in and out differentials

Answer 3

24hr urine collection - 3L+
Urine serum osmolality - U2:P1 dilute urine
BM - rule out DM
Ca - rule out high Ca
U&E - High Na if inadequate water consumed

Question 4

Diabetes insipidus
- how to do a water deprivation test

Answer 4

Water deprivation test - avoid if hypovolemic/high Na

Water deprivation
-empty bladder, only eat dry foods
-weight hourly
-2hourly urine osmolality and volume
-4hourly serum osmolality

If urine still dilute (U600mOsmol/kg) => desmopressin
-can drink
-hourly urine osmolality for 4 hours

Question 5

Diabetes insipidus
-interpretation of results from water deprivation test

Answer 5

Normal
Serum - 285-295
Urine - 600+

Psychogenic DI - impaired thirst mechanism => excess drinking
Serum - U300
Urine - 400-600
No change with desmopressin

Cranial DI
Serum - 300+
Urine - U300
Desmopressin concentrates urine

Nephrogenic DI
Serum - 300+
Urine - U300
No change with desmopressin

Question 6

Diabetes insipidus mimics

Answer 6

DM - high BM and HbA1c

Psychogenic polydipsia - more likely to experience low Na

Diuretic overuse - review dose

UTI - dysuria, fever

Hypercalcemia - constipation, abdo pain, muscle weakness, delirium, psychosis

Question 7

Diabetes insipidus
-management

Answer 7

Treat reversible causes
-Lithium toxicity
-hypercalcemia

Conservative
-drink water, low solute diet to avoid hypernatremia

Medical
Central - desmopressin
Nephrogenic - treat underlying cause, but supported by thiazides
-lowers serum Na => lowers osmolality

Question 8

SIADH
-what is it
-causes
-presentation

Answer 8

Excess release of ADH => volume expansion, water retention, dilutional low Na

SCLC
Stroke, SAH, SDH, meningitis, encephalitis, abscess
TB, pneumonia
SSRIs, carbamazepine

V high urine osmolality
V high urine Na - from action of ADH on renal tubules

Correct slowly to prevent CPM
Fluid restriction
Demeclocycline