Polyuria and polydipsia

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Polyuria and/or polydipsia (PU/PD) can be a frustrating presentation for clients trying to cope with increased volumes of urine from their pet. A logical approach is required. This chapter looks at the underlying mechanisms of thirst, urine production and urine concentration, before considering the pathophysiology and diagnostic approach of PU/PD and associated conditions. Polydipsia and muscle weakness in a Domestic Shorthaired Cat; Polydipsia and polyuria in a Labrador Retriever with a cough.

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2.1 Arginine vasopressin production and action in the kidney. 1 = Antidiuretic hormone (ADH), otherwise known as arginine vasopressin (AVP), is synthesized in the hypothalamus as a preprohormone and stored in vesicles within the posterior pituitary. Secretion of AVP from the posterior pituitary is stimulated by increasing plasma osmolality sensed by osmoreceptors in the hypothalamus or decreased total circulating plasma volume sensed by change in pressure within the atria, veins and the carotid sinus. 2 = Circulating AVP has its primary action in the kidney, more specifically in the distal tubule and the collecting ducts. Here AVP interacts with its receptor (V2) and via a cascade of events facilitates the transient insertion of water channels (aquaporin-2) which increases the permeability of these epithelial cells to water. By an independent pathway, AVP also regulates urea transport within the inner medullary collecting duct. AVP results in an increase in transepithelial transport of urea, important for maintenance of the urea gradient. 3 = The fluid in the distal tubule is dilute, allowing passive movement of water from the tubule to the hypertonic medullary interstitium along an osmotic gradient. 4 = Maintenance of the osmotic gradient is dependent on the counter-current multiplier system between the loop of Henle and the vasa recta blood supply, which concentrates solutes (urea and sodium) within the medullary interstitium. cAMP = cyclic adenosine monophosphate. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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2.7 Flow diagram for the diagnostic investigation of a patient with polyuria and polydipsia. ACTH = adrenocorticotrophic hormone; GFR = glomerular filtration rate; PD = polydipsia; PU = polyuria; SDMA = symmetric dimethylarginine.
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2.13 Modified water deprivation test (WDT). USG = urine specific gravity.
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2.15 Schedule for performing a desmopressin trial. CDI = central diabetes insipidus; NDI = nephrogenic diabetes insipidus.
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