Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 17 Fluid, Electrolyte, and Acid-Base Balance
FIGURE 17-6 The urinary sodium concentration as an aid in the differentiation of possible causes of
hyponatremia. Urinary electrolyte concentration is also influenced by dietary intake.
A form of moderate to pronounced hyponatremia associated
with elevated levels of ADH, which develops in
the absence of an appropriate osmotic or volume stimuli,
has been well recognized in human subjects. This syndrome
has been associated with several systemic disorders
including malignant neoplasms that produce an ADH-like
material, with a variety of brain diseases that apparently
stimulate synthesis and release of ADH, or in association
with certain pulmonary diseases that may result in abnormal
neural inputs from the lung that trigger the inappropriate
ADH release from the pituitary.
The diagnosis of SAIDH in the absence of readily
available procedures for ADH determination depends on
ruling out a number of other potential causes for persistent
hyponatremia. The diagnostic criteria for SIADH for
human subjects include the following ( McKeown, 1986 ):
1. Demonstration of a hypotonic hyponatremia without
hypovolemia or edema
2. Normal renal, adrenal, and thyroid function
3. Inappropriately elevated urine osmolality relative to
plasma osmolality
4. Relatively high urine sodium concentration
5. Correction of the hyponatremia by strict fluid
restriction
The SIADH has been reported in the dog ( Breitschwerdt
and Root, 1979 ; Brofman et al ., 2003 ; Crow and Stockham,
1985 ; Fleeman et al ., 2000 ; Giger and Gorman, 1984 ;
Houston et al ., 1989 ), but the incidence and importance of
this problem in other animal species are uncertain. A variant
of SIADH has been described in certain chronically ill and
malnourished human subjects that maintain a mild but persistent
hyponatremia. In these individuals, ADH secretion
remains under osmotic control, but it would appear that
the osmoreceptor threshold functions at a lower value than
normal ( McKeown, 1986 ). Thus, it is the resetting of the
osmostat that results in persistent chronic hyponatremia.
Resolution of the underlying medical problem usually
results in correction of this variant form of SIADH.
2 . Hypernatremia
Hypernatremia almost always is associated with elevation
of serum osmolality. Hypernatremia occurs in dehydrated
subjects when water losses exceed losses of sodium and
potassium and should be considered as an indication of a
relative water deficit ( Scribner, 1969 ). This situation can
occur in the initial stages of diarrhea, vomiting, or renal
disease if losses of water exceed the electrolytes lost (see
Table 17-6 ) ( Saxton and Seldin, 1986 ). As water losses
are replaced by increased water consumption or enhanced
renal water retention, serum sodium concentration tends
to decline into or below the normal range. Hypernatremia
also develops as the result of an essentially pure water
loss, such as the evaporative respiratory water loss in panting
animals ( Tasker, 1980 ). Hypernatremia is associated
with excessive renal free water loss with either central or
nephrogenic diabetes insipidus if water intake is restricted
( Breukink et al ., 1983 ). Food and water deprivation in normal
individuals is associated with substantial reduction of