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518 Hyponatremia

Hyponatremia Client Education
Sheet
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PHYSICAL EXAM FINDINGS
BASIC INFORMATION
natremia. Additional diagnostic efforts are
• Examination is typically unremarkable, or out pseudohyponatremia or spurious hypo-
Definition abnormalities are related to the disorder aimed at determining the underlying cause of
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Serum sodium (Na ) concentration below the responsible for hyponatremia. For example hyponatremia.
lower end of the reference range. Clinical signs ○ Hypoadrenocorticism (p. 512): dehydra-
are most common with rapid rate of decline in tion, bradycardia, weakness, prolonged Differential Diagnosis
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Na and are rarely seen until values fall below capillary refill time • Pseudohyponatremia due to hyperlipidemia
≈132 mEq/L, with the most dramatic signs ○ CHF (p. 408): murmur, tachycardia, or hyperparaproteinemia
apparent at concentrations < 120 mEq/L. tachypnea, audible pulmonary crackles • Spurious hyponatremia occurs with rapid-
(left-sided failure) or jugular pulses and onset hyperglycemia because of cellular water
Epidemiology diminished lung sounds (right-sided being dragged into the extracellular fluid
SPECIES, AGE, SEX failure) space. Serum sodium decreases an average
Hyponatremia can affect any animal. Young to ○ Hepatic cirrhosis (p. 174): icterus, of 1.7 mEq/L for every 100 mg/dL increase
middle-aged, female dogs may be predisposed abdominal effusion in blood glucose concentration.
to hypoadrenocorticism (p. 512), an important ○ Protein-losing nephropathy: peripheral
cause of hyponatremia. edema, ascites, weight loss Initial Database
• Gelatinous skin texture, transudative body • Serum chemistry profile
GENETICS, BREED PREDISPOSITION cavity abdominal effusions, crackles on ○ By definition, Na is below lower end of
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Dog breeds predisposed to hypoadrenocorti- thoracic auscultation, and jugular distention reference range.
cism: standard poodle, Portuguese water dog, can be signs of fluid retention. ○ Flame photometry or indirect poten-
Nova Scotia duck tolling retriever, bearded collie • Mental dullness can occur with severe hypo- tiometry measurements can result in
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natremia (especially if Na < 120 mEq/L). pseudohyponatremia if hyperproteinemia
RISK FACTORS or hyperlipidemia is present.
Parenteral administration of sodium-free fluids; Etiology and Pathophysiology ○ Ion-selective electrode measurements can
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disorders causing water gain or sodium loss • Na accounts for the vast majority of serum confirm true hyponatremia.
osmolality. Rapid onset of hyponatremia ○ Hypochloremia is common.
ASSOCIATED DISORDERS allows water to enter cells in the brain, ○ Other abnormalities reflect cause for
Hypoadrenocorticism (Addison’s disease); causing edema and neurologic signs or even hyponatremia (e.g., hyperkalemia in
renal tubular dysfunction; conditions leading death. If hyponatremia develops more slowly, hypoadrenocorticism, hypoalbuminemia
to appropriate or inappropriate secretion of brain cells can equilibrate (by loss of elec- due to protein-losing nephropathy, icterus
antidiuretic hormone (ADH) with resultant trolytes, amino acids, and other osmolytes) due to cirrhosis, hyperglycemia due to
water retention so that clinical signs are less likely. diabetes mellitus)
• Perhaps the most dangerous aspect of chronic • CBC: varies, depending on cause of
Clinical Presentation (>24 hours) hyponatremia is overly rapid hyponatremia
DISEASE FORMS/SUBTYPES correction (>0.5-1 mEq/L/h). The adap- ○ Normal eosinophil and lymphocyte numbers
• Hyponatremia may be acute (<24 hours) or tive changes that protect the brain against in a sick dog suggest hypoadrenocorticism.
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chronic and caused by water gain or Na loss excessive swelling also render it susceptible • Urinalysis and, ideally, urine osmolality:
• Patients with hyponatremia may be catego- to dehydration during the correction of variable
rized by osmolality and volume status hyponatremia. ○ If concentrated urine suggests fluid reten-
○ Normosmolar ○ Rapid correction can cause water to leave tion, calculation of urine sodium fractional
○ Hyperosmolar the brain cells, and brain shrinkage leads to excretion can help differentiate between
○ Hypoosmolar (most common): hypovo- vascular rupture and intracranial bleeding. appropriate and inappropriate ADH secre-
lemic, euvolemic, or hypervolemic ○ A delayed osmotic demyelination syn- tion. Urine Na > 25 mEq/L and increased
drome (i.e., central pontine myelinosis) urine osmolality suggests syndrome of
HISTORY, CHIEF COMPLAINT can cause irreversible neurologic damage, inappropriate ADH (SIADH).
• Most clinical signs are related directly to producing ataxia, paresis, coma, and even • Thoracic and abdominal imaging studies for
the underlying cause of hyponatremia (e.g., death. This may not be evident for up to underlying disease
gastrointestinal [GI] signs, cough and respira- 2 weeks after the episode of hyponatremia.
tory distress due to congestive heart failure • True hyponatremia results from sodium loss Advanced or Confirmatory Testing
[CHF], icterus due to hepatic cirrhosis). or water gain. As indicated by preliminary findings and
• Additional signs are related to chronicity and • Depending on the associated pathophysiol- necessary to confirm underlying disorder (e.g.,
severity of hyponatremia. ogy, it can be classified according to hydration ACTH stimulation test to confirm suspected
○ Acute hyponatremia can cause nausea, state, plasma tonicity (hypotonic, isotonic, hypoadrenocorticism; echocardiography to
vomiting, muscular weakness, mental hypertonic), and plasma volume status confirm cardiac disease; thyroid profile to
dullness, disorientation, and seizures. (hypervolemic, euvolemic, hypovolemic). confirm hypothyroidism)
○ Signs of chronic hyponatremia are often • See causes of hyponatremia elsewhere
subtle, but inappropriate medical manage- (p. 1241). TREATMENT
ment can cause devastating consequences.
Overly rapid correction can cause more DIAGNOSIS Treatment Overview
severe consequences than hyponatremia Goals of treatment are to correct hyponatremia
itself. Diagnostic Overview at a safe rate (0.5 mEq/L/h for hyponatremia
• Hyponatremia may not be suspected before Hyponatremia is confirmed by measure- of > 24 hours’ duration) while addressing the
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electrolytes are measured. ment of serum or plasma Na after ruling underlying disorder that originally caused
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