Page 65 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Disorders of Sodium and Water: Hypernatremia and Hyponatremia 55

Increased serum sodium concentration
( >155 mEq/L in dogs; >162 mEq/L in cats)
Evaluation of volume status

Hypervolemia Normovolemia Hypovolemia

Impermeant Pure water
Hypotonic loss
solute gain deficit

Salt poisoning Primary hypodipsia Extrarenal Renal
Hyperaldosteronism Central diabetes insipidus (FC Na <1%) (FC Na >1%)
Hypertonic fluid Nephrogenic diabetes
administration insipidus Gastrointestinal Appropriate
High environmental • Vomiting • Osmotic diuresis
temperature • Diarrhea (mannitol, hyperglycemia)
Fever • Small intestinal • Chemical diuresis
Inadequate access to water obstruction (furosemide, ethacrynic acid)
Third space loss Inappropriate
• Pancreatitis • Chronic renal failure
• Peritonitis • Nonoliguric acute renal failure
• Post-obstructive diuresis
Cutaneous
• Burns
The following clinical information suggests hypovolemia: a history of hypotonic losses (e.g., vomiting, diarrhea), decreased skin
turgor, dry mucous membranes, delayed capillary refill time, tachycardia, flat jugular veins, systemic hypotension, low central
venous pressure. Hypervolemic patients may develop pulmonary edema, especially those with underlying cardiac disease.
Figure 3-7 Clinical approach to the patient with hypernatremia. FC Na , Fractional clearance of sodium.

Rarely, chronic hypernatremia may occur in fully con- adipsia, and diabetes insipidus have been observed in a
scious animals that have access to water. In these cases, young dalmatian dog with dysplasia of the rostral dien-
abnormal osmoregulation of ADH release caused by cephalon. 5,34 Hypernatremia also has been reported in a
underlying hypothalamic lesions results in hypodipsia. dog 63 and cat 115 with central nervous system lymphoma.
Animals that are unable to obtain water because central Hypodipsia, hypernatremia, and hypertonicity caused
nervous system disease has resulted in an altered senso- by an abnormal thirst mechanism have been reported in
rium may also be hypernatremic; but in these instances, young female miniature schnauzers and in a young Great
the hypernatremia is simply a result of water deprivation. Dane. 27,70,76,112,159 One miniature schnauzer with
Hypodipsic hypernatremia related to defective osmoreg- hypodipsic hypernatremia had severe behavioral
ulation of ADH has been reported in a dog with hydro- disturbances, and holoprosencephaly was found at nec-
cephalus and normal pituitary function. 31 In normal ropsy. 153 Another had dysgenesis of the corpus callosum
individuals, administration of hypertonic saline increases and other forebrain structures. 112 Grossly visible neuro-
plasma osmolality and simultaneously causes volume anatomic abnormalities were not identified in a previous
expansion. Osmoreceptors are stimulated by hyperos- report. 27 Whether a spectrum of neuroanatomic
molality but inhibited by volume expansion. Normally, abnormalities exists in these dogs (which appear to have
the response to hyperosmolality takes precedence, and a form of congenital adipsic hypernatremia) is not known.
ADH secretion increases, resulting in decreased urine vol- Infusion of hypertonic saline has been shown to lead to an
ume and increased urine osmolality. The affected dog increase in urine volume and a decrease in urine osmolal-
experienced increased urine volume and decreased urine ity compatible with defective osmoregulation of ADH. 27
osmolality in response to an infusion of hypertonic saline, Clinical signs in affected dogs are associated with hyper-
indicating defective osmoreceptor function as observed tonicity and include anorexia, lethargy, weakness,
in human patients with hypodipsic hypernatremia. Simi- disorientation, ataxia, and seizures. Affected dogs can
larly, destruction of osmoreceptors in the hypothalamus be managed clinically by addition of water to their food,
was thought to be responsible for adipsia and but hypernatremia and neurologic dysfunction recur
hypernatremia in a dog with focal granulomatous menin- whenever water supplementation is discontinued. In a
goencephalitis. 104 Weakness and polymyopathy have Norwegian elkhound with adipsic hypernatremia, the
been reported in a young cat with hypodipsia, adipsia resolved spontaneously at 2 years of age, 62 and
hypernatremia, and hypertonicity associated with hydro- an adipsic Labrador retriever with hypothyroidism
cephalus and hypopituitarism, and hypernatremia, responded to treatment with levothyroxine. 81