A clear distinction between patients with hypothalamic diabetes insipidus, patients with nephrogenic diabetes insipidus or dipsogenic diabetes insipidus and normal persons could be performed by two different dynamic function tests: the water deprivation test or the infusion of hypertonic saline (Hickey-Hare test) [[1]]. We agree with the statement of Dr. Ritter that the saline infusion test (with 5% NaCl) is the gold standard to differentiate polydipsia/polyuria from diabetes insipidus centralis. In our opinion the saline infusion test should not be done in patient with a cerebral lymphoma who has an increased risk of hypertonic encephalopathy by this test. In our opinion this test should only be used when the water deprivation test can not clearly discriminate between partial diabetes insipidus and primary polydipsia. In our patient the serum sodium was at the beginning of the test 145 mmol/l and at the end after 10 h 152 mmol/l. The patient lost in this period a weight of 4 kg (ca. 6% of the body weight of 90 kg). We are sorry that both facts were not mentioned in our case report. The water deprivation test was stopped after 10 h because Uosm (urine osmolality) did not change more than 30 mOsm/h. Patients with a central diabetes insipidus concentrate their urine when given an injection of arginine vasopressine. This fact has also be seen in our patient.

Moreover, it is important to mention that urine osmolality of patients with primary polydipsia and partial diabetes insipidus centralis are similar but only patients with a complete diabetes insipidus centralis have an increase of Uosm after exogenous ADH administration [[2]]. In dipsogenic diabetes insipidus prolonged fluid ingestion results in a chronic suppression of vasopressin secretion.

We do not agree with the statement of Dr. Ritter that the saline infusion test was necesssary in this case. The combination of the morphological correlate and the pathological dynamic function test (water deprivation) revealed diabetes insipidus centralis.

We would like to thank Dr. Ritter for his constructive critique and we hope that he will be satisfied with our reply.

References

• 2 Baylis P H, Thompson C L. Diabetes insipidus and hyperosmolar syndromes. In: Principles and Practice of Endocrinology and Metabolism. Ed: Becker KL Philadelphia: J B Lipincott Company 257-265 1995
• 3 Zerbe R L, Robertson G L. A comparison of plasma vasopressin measurement with a standard indirect test in the differential diagnosis of polyuria.  N Engl J Med 305. 305 1539 1981;