Page 66 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

56 ELECTROLYTE DISORDERS

ECF ICF treated group, suggesting that the dogs with untreated
CDI drank sufficient water to maintain relatively normal
plasma osmolality. The transient nature of CDI after
hypophysectomy may result from the fact that some of
600 1200 the vasopressin-producing neurons from the hypothala-
2 L 4 L
mOsm mOsm mus terminate in the median eminence.
Animals with CDI have severe polydipsia and polyuria.
Their urine typically is hyposthenuric (urine osmolality,
60 to 200 mOsm/kg), but urine osmolality may
300 300 approach 400 to 500 mOsm/kg in the presence of dehy-
mOsm/Kg mOsm/Kg
dration. Variability in USG and urine osmolality values at
the time of presentation in dogs and cats with diabetes
ECF ICF
insipidus presumably is related to hydration status and
severity of vasopressin deficiency. In one study, dogs were
1 L H O classified as having complete or partial CDI based on the
2
magnitude of increase in their USG and urine osmolality
1200 4 L
mOsm after induction of 5% dehydration. 65 Dogs with complete
600 CDI had USG values of 1.001 to 1.007 that did not
1 L
mOsm change substantially after induction of 5% dehydration,
whereas dogs with partial CDI had USG values of
1.002 to 1.016 that increased to 1.010 to 1.018 after
ECF ICF induction of 5% dehydration. In both groups, there was
a substantial (>50%) increase in USG 2 hours after
administration of 1 to 5 U of aqueous arginine vasopres-
sin. Affected dogs responded well to administration of
600 1200
1.67 L 3.33 L DDAVP acetate (1 to 2 drops in both eyes every 12 to
mOsm mOsm
24 hours), but the prognosis was dependent on the
underlying cause of CDI. Many older dogs with CDI
had tumors in the region of the pituitary gland and devel-
oped neurologic signs.
360 360
mOsm/Kg mOsm/Kg Increased plasma osmolality and hypernatremia may
Figure 3-8 Effect of loss of 1 L of water on volume and tonicity of occur in dogs and cats with CDI. These results suggest
extracellular fluid (ECF) and intracellular fluid (ICF). (Drawing by that some affected dogs and cats do not obtain enough
Tim Vojt.) water to maintain water balance and are presented in a
hypertonic state. Severe hypernatremia and neurologic
Central or pituitary diabetes insipidus (CDI) is caused dysfunction may occur if the animal cannot maintain ade-
by a partial or complete lack of vasopressin production quate water intake. 36,133 In contrast, with psychogenic
and release from the neurohypophysis. 65 It may result polydipsia, plasma osmolality and serum sodium concen-
from trauma or neoplasia or may be idiopathic in dogs tration may be lower than normal at presentation. 91
and cats.* Visceral larva migrans also has been reported Administration of vasopressin leads to an increase in urine
to cause CDI in a dog. 99 In one dog with hypernatremia, osmolality or specific gravity in dogs and cats with CDI,
hypertonicity, and gastric dilation-volvulus, CDI was but the initial response may be less than expected because
present and caused by neurohypophyseal atrophy second- of renal medullary washout of solute. In one study, USG
ary to a cystic craniopharyngeal duct. 36 Congenital CDI values increased to 1.018 to 1.022 after vasopressin
is rare 58,86,163 but has been reported in two sibling administration in dogs with complete CDI and to
Afghan pups. 131 Traumatic CDI may be transient in 1.018 to 1.036 in dogs with partial CDI. 65
nature. Hypophysectomy for treatment of hyperadreno- Treatment with vasopressin restores medullary hyper-
corticism results in transient CDI that may take several tonicity and normal urinary concentrating ability. Histor-
weeks to resolve. 102 Marked hypernatremia occurs in ically, vasopressin tannate in oil (pitressin tannate) has
dogs in the first 24 hours after hypophysectomy and been used to treat CDI in small animal practices.
can be prevented by prophylactic treatment with The dosage is 3 to 5 U for dogs or 1 to 2 U for cats given
desmopressin (DDAVP). 64 In the month after surgery, intramuscularly or subcutaneously every 24 to 72 hours as
serum sodium concentrations in control dogs were not needed to control polyuria and polydipsia. To avoid the
markedly different from those observed in the DDAVP- possibility of water intoxication, it is recommended that
the treatment interval be determined by recurrence of
*References 4, 18, 29, 45, 57, 58, 72, 106, 119, 120, 130, 133, 140. polyuria.Thisproductisnolongercommercially available.