Page 967 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

942 CHAPTER 7

VetBooks.ir DIABETES INSIPIDUS testing should be performed as described earlier and
should never be performed in a dehydrated or azo-
Definition/overview
Diabetes insipidus is an uncommon cause of PU/PD taemic horse. Horses should be closely monitored
during the water-deprivation test to avoid severe
in horses. hypertonic dehydration. Horses with central or
nephrogenic diabetes insipidus cannot concentrate
Aetiology/pathophysiology urine during water deprivation. Psychogenic poly-
Antidiuretic hormone (ADH) (vasopressin) is a pow- dipsia with medullary interstitial osmotic gradient
erful effector of the feedback system for regulating cannot be ruled out initially in horses not responding
plasma osmolarity and sodium concentration. It oper- to water deprivation. In such horses, partial depriva-
ates by altering renal excretion of water independently tion of water intake at 40 ml/kg/day should be per-
of the rate of solute excretion. Diabetes insipidus occurs formed prior to repetition of the water-deprivation
when inadequate ADH is produced (neurogenic or test. If urine concentration still does not occur, then
central diabetes insipidus) or when the distal tubules, a diagnosis of diabetes insipidus can be made.
collecting tubules and collecting ducts are unable to Alternative diagnostic methods include infusion of
respond to ADH (nephrogenic diabetes insipidus). hypertonic saline (0.25 ml/min/kg i/v for 45–60 min-
Central diabetes insipidus can develop secondary utes), which in normal horses should stimulate urine
to head trauma, encephalomyelitis or PPID (equine concentration across renal tubules, and the ADH
Cushing disease). Nephrogenic diabetes insipidus (vasopressin) challenge (2.5 mU/kg via constant rate
can develop secondary to many types of renal dis- infusion over 60 minutes or 0.5 U/kg i/m), which
ease, especially those that damage the renal medulla. should stimulate urine concentration and is used to dif-
A hereditary basis to the disease is possible. ferentiate nephrogenic from central diabetes insipidus.

Clinical presentation Management
PU/PD should be the sole presenting complaint, Secondary diabetes insipidus should be managed via
unless water intake has been restricted and dehydra- treatment of the primary disease. Successful treat-
tion has developed. ment of primary or idiopathic diabetes insipidus has
not been reported.
Differential diagnosis
Differential diagnoses include CKD, psychogenic Prognosis
polydipsia, diabetes mellitus, Cushing’s disease. The prognosis for diabetes insipidus not associated
with underlying renal or neurological disease is fair
Diagnosis if access to water is available at all times. Affected
Physical examination is unremarkable. Urinalysis horses are unable to concentrate urine and are prone
should be normal apart from a lack of concentration to dehydration if water is restricted. The progno-
of urine. Blood urea and creatinine levels are normal sis for secondary diabetes insipidus depends on the
unless dehydration is present. Water-deprivation prognosis for the primary disease.

DISEASES OF THE URETERS

ECTOPIC URETERS Endoscopic examination of the vagina and distal uri-
nary tract may reveal the orifice of the ectopic ureter
Ectopic ureter is rare deformation in horses and is but visualisation is often difficult. Speculum exami-
most often reported in fillies. It is usually noted in nation of the vagina can also be diagnostic. An excre-
foals with a complaint of persistent urine dribbling tory urogram may aid in diagnosis and is most useful
and perineal dermatitis (urine scalding). There should in young animals where direct visualisation is more
be no other clinical or haematological abnormalities. difficult due to the size of the animal. Depending