Use of T1-weighted MR Imaging to Differentiate between Primary Polydipsia and Central Diabetes Insipidus

Title: Use of T1-weighted MR Imaging to Differentiate between Primary Polydipsia and Central Diabetes Insipidus
Authors: Moses, Arnold M.; Clayton, Barbara; Hochhauser, Leo
Publisher: AJNR. American Journal of Neuroradiology
Date Published: September 01, 1992
Reference Number: 301
PURPOSE: To investigate the value of MR in differentiating patients with primary polydipsia, who have an intact neurohypophyseal system, from those with central diabetes insipidus, who have impaired synthesis and/or release of vasopressin.
METHODS: Eighteen patients with clinically significant hypotonic polyuria were diagnosed endocrinologically as having primary polydipsia or diabetes insipidus (central or nephrogenic). These patients, and 92 patients without sellar disease, were then imaged with 1.5-T, T1-weighted, thin sagittal sections without gadolinium contrast.
RESULTS: Normal hyperintense signal of the neurohypophysis was present in 90 of 92 patients without sellar disease. The signal was also present in all six patients with primary polydipsia. In contrast, the hyperintense signal was absent in all eight patients with central diabetes insipidus. Three of the four patients with nephrogenic diabetes insipidus also had an absent hyperintense signal.
CONCLUSION: T1-weighted MR may prove important in differentiating patients with central diabetes insipidus from those with primary polydipsia.

This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)

One function of the neurohypophyseal system (which is the back lobe of the pituitary gland) is to produce and secrete the antidiuretic hormone, arginine vasopressin (AVP). One of the functions of AVP is to signal the kidney to reabsorb water and concentrate urine. People with central diabetes insipidus (CDI) have impaired synthesis and/or release of AVP. People with congenital nephrogenic diabetes insipidus (NDI) have adequate levels of AVP that are appropriately released, but their kidneys cannot respond to it. In both cases, the affected person exhibits polyuria (chronic passage of large volumes of urine) and polydipsia (chronic, excessive thirst). People with primary polydipsia (PP) consume excessive amounts of water, often for unknown reasons. This chronic water intake inhibits the release of AVP; one result of this is polyuria. Distinguishing CDI from PP can be difficult.

Magnetic resonance (MR) is a diagnostic procedure analogous to X-rays used to obtain internal images of targeted areas of the body. MR will pick up a hyperintense signal (HIS) (sometimes referred to as a "bright spot") in a person with a normally functioning posterior pituitary (i.e. the neurohypophysis). The cause of the HIS is disputed but is probably due to some constituent of the neurosecretory vesicles within the normal neurohypophsis.

Moses, et al., investigated the value of MR in differentiating patients with PP from those with CDI. They performed MR on the neurohypophyseal system of 18 patients with significant polyuria and dilute urine who were diagnosed by other means as either having PP, NDI or CDI. They also performed MR on 92 patients with normally functioning neurohypophysis.

Of the 92 control subjects, 90 had normal HIS emanating from the neurohypophysis. The same was found in all six patients with PP. However, none of the eight CDI patients produced the HIS, and neither did three out of the four NDI patients. These observations strongly support the probability that the lack of the HIS in CDI patients reflects the fact that CDI patients have impaired abilities to synthesize, store and release AVP. There is no information on pituitary AVP content in NDI patients, but the authors' MR images suggest they are unable to maintain normal amounts of AVP in their neurohypophysis, perhaps due to their chronic mild dehydration which depletes pituitary AVP. The fact that all six PP patients produced HIS reflects the fact that they have normal amounts of AVP in their neurohypophysis.

The results of the authors' work suggests that MR can be useful in distinguishing between CDI and PP, though they mention that there may be rare cases where a CDI patient could produce HIS because he or she might be able to produce, but not release, AVP. Also, newborns with congenital CDI may have an HIS for several months after birth.