Page 946 - Small Animal Internal Medicine, 6th Edition

918 PART VII Metabolic and Electrolyte Disorders

balance (principally a defect in renal water excretion) rather plasma water remains the same. Methods that measure the
than from abnormalities in sodium balance. Causes of hypo- amount of sodium in a specific volume of plasma (e.g., flame
VetBooks.ir natremia in dogs and cats are listed in Box 53.2. photometry) result in falsely low sodium values, whereas
methods that determine the sodium concentration in the
Hyponatremia must be differentiated from pseudohypo-
natremia, which is a decrease in the serum sodium concen-
ion-specific electrodes) yield an accurate sodium value.
tration that results from laboratory methods used in the aqueous phase of plasma (e.g., direct potentiometry using
presence of normal plasma osmolality. Pseudohyponatremia Pseudohyponatremia can usually be identified if the method
occurs in the presence of hyperlipidemia or severe hyperpro- used to measure the sodium concentration is known, a blood
teinemia. An increase in the concentration of triglycerides sample is examined for the presence of gross lipemia, and a
or proteins in plasma reduces the sodium concentration in CBC and a serum biochemistry panel are performed.
the total plasma volume, but the sodium concentration in Hyponatremia may also occur after the concentration of
osmotically active solutes (e.g., glucose, mannitol) in the
ECF is increased. An increase in the concentration of osmot-
BOX 53.2 ically active solutes in the ECF causes a fluid shift from the
intracellular to the extracellular compartment and a corre-
Causes of Hyponatremia in Dogs and Cats sponding decrease in the serum sodium concentration. For
example, the serum sodium concentration decreases by
With Normal Plasma Osmolality approximately 1.6 mEq/L for every 100-mg/dL increase in
Hyperlipidemia serum glucose concentration, and this decrease may become
Hyperproteinemia more severe when the blood glucose concentration exceeds
With High Plasma Osmolality 500 mg/dL. Estimation of the plasma osmolality is helpful in
Hyperglycemia* differentiating the cause of hyponatremia. Hyponatremia is
Mannitol infusion usually associated with hypo-osmolality (<290 mOsm/kg),
Severe azotemia whereas pseudohyponatremia is associated with normal
plasma osmolality, and hyponatremia caused by an increase
With Low Plasma Osmolality in osmotically active solutes in the ECF is associated with
And hypervolemia hyperosmolality. Plasma osmolality can be estimated using
Advanced liver failure* the following formula:
Advanced chronic kidney disease*
=
(
[
2
Nephrotic syndrome* Plasma osmolality mOsmkg) ( × NamEq L])
Congestive heart failure GlucosemgdL) Ureaanitrogen mg dL)
(
(
Accidental water injection or ingestion (water + +
intoxication) 18 . 28
And normovolemia
Primary (psychogenic) polydipsia Normal plasma osmolality in dogs and cats is approxi-
Inappropriate antidiuretic hormone (ADH) secretion mately 280 to 310 mOsm/kg.
(SIADH) Clinical Features
Myxedema coma of hypothyroidism
Iatrogenic causes Clinical signs of hyponatremia include lethargy, anorexia,
Hypotonic fluid administration vomiting, weakness, muscle fasciculations, obtundation, dis-
Antidiuretic drugs (e.g., barbiturates, β-adrenergics) orientation, seizures, and coma. CNS signs are the most con-
And hypovolemia cerning, occur when hyponatremia is severe (<120 mEq/L),
Hypoadrenocorticism* and develop as changes in plasma osmolality cause fluid to
Gastrointestinal fluid loss* shift from the extracellular to the intracellular space, result-
Third-space loss ing in neuronal swelling and lysis. The onset and severity of
Pleural effusions (e.g., chylothorax)
Peritoneal effusions clinical signs depend on the rapidity with which hyponatre-
Pancreatitis mia develops as well as on the degree of hyponatremia. The
Cutaneous burns more chronic the hyponatremia and the more slowly it
Diuretic administration develops, the more capable the brain is of compensating for
Salt losing nephropathy changes in osmolality through the loss of potassium and
Cerebral salt wasting syndrome organic osmolytes from cells. Clinical signs develop when
Shock the decrease in plasma osmolality occurs faster than the
Renal insufficiency brain’s defense mechanisms can counter the influx of water
into the neurons.
*Common causes.
Modified from DiBartola SP: Disorders of sodium and water: Diagnosis
hypernatremia and hyponatremia. In DiBartola SP, editor: Fluid,
electrolyte, and acid-base disorders in small animal practice, ed 4, Hyponatremia is readily evident from measurement of
St Louis, 2012, Saunders/Elsevier. serum electrolyte concentrations. However, hyponatremia