Therapeutic Trial of ddAVP
Used when partial response to water deprivation test to differentially diagnose Primary Polydipsia (PP) and partial Cranial Diabetes Insipidus (CDI) or Nephrogenic Diabetes Insipidus (NDI).
Water intoxication in PP
- Admit to hospital
- Monitor daily: fluid input and output, body weight, U+Es and urine osmolality.
- Patient observed for 2 days and then 10 mcg DDAVP given intranasally od for at least 2-3 days.
- Partial CDI: prompt improvement in thirst and polyuria
- NDI: no effect; can be treated for further 2-3 days with a 10 fold increased dose to see if defect partial or complete
- PP: decreased polyuria with no change in polydipsia. Causes weight gain, increased urine osmolality and progressive dilutional hyponatraemia, which may develop rapidly and severely (hence need for hospitalisation)
SENSITIVITY AND SPECIFICITY
Small possibility of false diagnosis of PP as hyponatraemia may occur in 5% of CDI who continue to drink excessively on DDAVP because of associated abnormal thirst or prolonged habit
Vokes et al., Endo. Metab. Clin. N. Amer. 17(2), 281 (1988).