This post presents diagnostic and therapeutic approaches to canine and feline patients with hyponatremia. As we all remember, the plasma sodium concentration represents a ratio of plasma sodium content to the water content. As a result, the plasma sodium concentration depends heavily on water balance. Therefore, hyponatremia may develop secondary to either excess of free water in the body or lack of sodium ions relatively to the free water. Also, it is helpful to remember that plasma sodium concentration is the key determinant of plasma osmolality, and the majority of hyponatremic patients will have low plasma osmolality. However, this is not always the case. For example, patients with hyperglycemia or those who received mannitol may develop hyponatremia in conjunction with high plasma osmolality caused by excessive glucose concentration or the presence of other osmotically active substances (e.g. mannitol). Hyponatremic patients with normal plasma osmolality typically have spurious hyponatremia (aka pseudohyponatremia).
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