In very warm weather there is a risk of water intoxication, particularly in dogs. This often occurs in dogs that play in and around water sources for a prolonged period such as ponds and paddling pools, become thirsty and drink an excessive amount of water. This results in dilutional hyponatraemia as a result of ingestion of water at a rate in excess of the maximal excretory capacity of the kidney, or by ingestion of lesser amounts of water with impaired water excretion by the kidney. Water intoxication results in a relative lack of electrolytes due to dilution rather than an absolute lack of electrolytes.
The onset of signs of water intoxication are usually rapid and occur within a few hours. Signs of hyponatraemia and water intoxication include vomiting, restlessness, muscle cramps and weakness. Seizures, coma, pulmonary and cerebral oedema occur in severe cases. Osmotic demyelination is a complication of hyponatraemia, more commonly seen after chronic rather than acute cases. Signs occur 3-4 days after the correction of hyponatremia and include lethargy, weakness, and ataxia that can progress to hypermetria and quadriparesis.
If water intoxication is suspected, monitor electrolytes, particularly sodium, serum osmolality, fluid status and urine output and osmolality. Gut decontamination is unlikely to be practical due to the rapid onset of signs.
In asymptomatic dogs with mild hyponatraemia, restriction of fluids is usually all that is required.
If the hyponatraemia is more significant or there are severe signs then the serum sodium should be increased slowly with a slow IV infusion of isotonic saline (0.9%). The aim is to raise the sodium no faster than 0.5 mmol/L/hour and by no more than 10-12 mmol/L/24 hours. Hypertonic saline (3%) should not be used as rapid correction of serum sodium can cause myelinolysis (demyelination of the brain). In severe cases administration of a loop diuretic such as furosemide may be combined with isotonic saline and potassium supplementation (to replace diuretic-induced potassium loss).