Effect of Arginine on the Hypothalamic-Pituitary-Adrenal Axis in Individuals With and Without Vasopressin Deficiency.

CONTEXT: Arginine stimulates pituitary hormones, like growth hormone and vasopressin, but its effect on the hypothalamic-pituitary-adrenal (HPA) axis is unknown. Arginine may also stimulate the HPA axis, possibly through a mechanism involving vasopressin. OBJECTIVE: To investigate the effect of arginine on adrenocorticotropic hormone (ACTH) and cortisol in subjects with and without vasopressin deficiency. DESIGN: Prospective study, University Hospital Basel. PARTICIPANTS: 38 patients with central diabetes insipidus, 58 patients with primary polydipsia, and 50 healthy controls. INTERVENTION: Arginine infusion with measurement of ACTH, cortisol and copeptin at baseline and 30, 45, 60, 90, and 120 minutes. RESULTS: We found different response patterns to arginine: in patients with diabetes insipidus (and low stimulated copeptin levels) median (interquartile range [IQR]) ACTH and cortisol increased from 22.9 (16.8, 38.7) to 36.6 (26.2, 52.1) ng/L and from 385 (266, 463) to 467 (349, 533) nmol/L, respectively. In contrast, median (IQR) ACTH and cortisol levels decreased in patients with primary polydipsia (despite high stimulated copeptin levels): ACTH from 17.3 (12.3, 23) to 14.8 (10.9, 19.8) ng/L and cortisol from 343 (262, 429) to 272 (220.8, 360.3) nmol/L; likewise, in healthy controls: ACTH from 26.5 (17.6, 35.7) to 14.8 (12.1, 22.7) ng/L and cortisol from 471 (393.3, 581.8) to 301.5 (206.5, 377.8) nmol/L. CONCLUSION: Diabetes insipidus is associated with increased responsiveness of ACTH/cortisol to arginine. In contrast, arginine does not stimulate the HPA axis in healthy controls or in primary polydipsia.

Copeptin and its role in the diagnosis of diabetes insipidus and the syndrome of inappropriate antidiuresis.

Copeptin is secreted in an equimolar amount to arginine vasopressin (AVP) but can easily be measured in plasma or serum with a sandwich immunoassay. The main stimuli for copeptin are similar to AVP, that is an increase in osmolality and a decrease in arterial blood volume and pressure. A high correlation between copeptin and AVP has been shown. Accordingly, copeptin mirrors the amount of AVP in the circulation. Copeptin has, therefore, been evaluated as diagnostic biomarker in vasopressin-dependent disorders of body fluid homeostasis. Disorders of body fluid homeostasis are common and can be divided into hyper- and hypoosmolar circumstances: the classical hyperosmolar disorder is diabetes insipidus, while the most common hypoosmolar disorder is the syndrome of inappropriate antidiuresis (SIAD). Copeptin measurement has led to a "revival" of the direct test in the differential diagnosis of diabetes insipidus. Baseline copeptin levels, without prior thirsting, unequivocally identify patients with nephrogenic diabetes insipidus. In contrast, for the difficult differentiation between central diabetes insipidus and primary polydipsia, a stimulated copeptin level of 4.9 pmol/L upon hypertonic saline infusion differentiates these two entities with a high diagnostic accuracy and is clearly superior to the classical water deprivation test. On the contrary, in the SIAD, copeptin measurement is of only little diagnostic value. Copeptin levels widely overlap in patients with hyponatraemia and emphasize the heterogeneity of the disease. Additionally, a variety of factors lead to unspecific copeptin elevations in the acute setting further complicating its interpretation. The broad use of copeptin as diagnostic marker in hyponatraemia and specifically to detect cancer-related disease in SIADH patients can, therefore, not be recommended.

A COMBINED OUTPATIENT AND INPATIENT OVERNIGHT WATER DEPRIVATION TEST IS EFFECTIVE AND SAFE IN DIAGNOSING PATIENTS WITH POLYURIA-POLYDIPSIA SYNDROME.

OBJECTIVE: The water deprivation test (WDT) is widely used for the differential diagnosis of the polyuria-polydipsia syndrome (PPS). However, it is inconvenient and may not always be precise in differentiating partial forms of diabetes insipidus (DI) from primary polydipsia (PP). The aim of this study was to evaluate the results of a combined outpatient and inpatient overnight WDT protocol that included an overnight unsupervised period concerning its feasibility and safety. METHODS: We performed a retrospective analysis of clinical data and laboratory results of 52 patients with PPS undergoing WDT at a single center. RESULTS: PP was the most frequent diagnosis, followed by complete central DI (cCDI), partial central DI (pCDI), and nephrogenic DI (NDI). Over 90% of the patients showed an expected increase in serum osmolality at the end of the dehydration period. There were no reports of complications during the overnight deprivation period. Post-dehydration urine osmolality and urine-to-serum osmolality ratio significantly differentiated all the groups ( P<.05), except for cCDI and NDI, which could be differentiated by basal and post-dehydration vasopressin (AVP) levels ( P<.05 for both). Although these measurements were useful for differentiating patients according to their allocation groups, results from WDT and direct AVP levels may often require a comprehensive diagnostic approach, particularly in the challenging groups of PP and pCDI. CONCLUSION: A combined outpatient and inpatient overnight WDT protocol is safe and feasible when the test is performed with special care at experienced centers. Newer diagnostic tools are expected to improve the accuracy of PPS diagnosis. ABBREVIATIONS: AQP2 = aquaporin-2; AVP = vasopressin; CDI = central diabetes insipidus; cCDI = complete central diabetes insipidus; DDAVP = desmopressin; DI = diabetes insipidus; IQR = interquartile range; MRI = magnetic resonance imaging; Na+ = sodium; NDI = nephrogenic diabetes insipidus; pCDI = partial central diabetes insipidus; PP = primary polydipsia; PPS = polyuria-polydipsia syndrome; S_osm = serum osmolality; U_osm = urine osmolality; WDT = water deprivation test.

Diagnostic value of the water deprivation test in the polyuria-polydipsia syndrome.

OBJECTIVE: Diabetes insipidus (DI) and primary polydipsia (PP) are characterised by polyuria and polydipsia. It is crucial to differentiate between these two disorders since the treatment is different. The aim of this study was to evaluate the diagnostic value of the short and an extended variant of the water deprivation test (WDT) and of measuring urinary vasopressin (AVP) in patients with polyuria and polydipsia. DESIGN: A retrospective, single-centre study based on WDTs performed between 2004 and 2014 including 104 consecutive patients with the polyuria-polydipsia syndrome. During a strict water deprivation, weight, urinary osmolality, urinary vasopressin and specific gravity were collected until one of the following was reached: i) >3% weight reduction, ii) Urinary specific gravity >1.020 or, urinary osmolality >800 mOsm/L, iii) Intolerable adverse symptoms such as excessive thirst. RESULTS: Out of 104 patients (67 women, 37 men), 21 (20%) were diagnosed with DI and 83 (80%) with PP. The median (interquartile range; range) test duration was 14 hours (10-16; 3-36) in patients with DI and 18 hours (14-24; 7-48) in patients with PP (P=0.011). Of those diagnosed with PP, 22 (26%) did not reach urinary specific gravity >1.020 nor urine osmolality >800 mOsm/L. Urine AVP did not overlap between patients with PP and patients with central DI. CONCLUSIONS: The short WDT is of limited value in the diagnostic work-up of polydipsia and polyuria and a partial DI may have been missed in every fourth patient diagnosed with PP. Urinary AVP has excellent potential in discriminating PP from central DI.