DR Medhat MRCP

Investigations :
1) High plasma osmolality , low urine osmolality
2) Water deprivation test (to R/O psychogenic polydipsia i.e compulsive water
drinking) .
3) Pitressin (DDAVP) test (to differentiate between cranial & nephrogenic DI).
4) Hypertonic saline (3%) test (to diagnose hypothalamic cause).
5) CT,MRI/brain , visual acuity & field.
Water deprivation test
o Method :
‏.(تصوم المريض 21 ساعة - Prevent patient drinking water (i.e deprivation
- Ask patient to empty bladder.
- Hourly urine and plasma osmolality.
DDAVP test
- Injection of desamno-D- arginin vasopressin [ADH analogue].
- If the patient improved ( U.O.P, urine osmolality) cranial DI.
- If no improvement (still polyuric,low urine osmolality) nephrogenic DI.
: Interpretation -
سؤال مهم Starting plasma
osm.
Final urine
osm.
Urine osm. post-
DDAVP
Normal Normal (275-299) > 600 > 600
Psychogenic
Low > 400 > 400
polydipsia
Cranial DI High < 300 > 600
Nephrogenic DI High < 300 < 300
SIADH Low High ----
Hypertonic saline test
- Normally ,Injection of hypertonic saline (3%) plasma osmolality ++
hypothalamic osmoreceptors ++ posterior pituitary to ADH UOP.
- If no urine output indicates hypothalamic lesion.