The symptoms of chronic hyponatremia may take a long time to develop, not becoming apparent until the blood level of sodium has fallen significantly.

In cases of dilutional hyponatremia or water intoxication, there is an increase in the volume of blood fluid which leads to a relative reduction in the concentration of sodium. This can lead to symptoms such as headache, nausea, vomiting, disorientation and frequent urination.

Some of the symptoms of hyponatremia include:

• Mild to moderate hyponatremia may be asymptomatic, although there may be signs of another underlying condition that has led to the hyponatremia such as recurrent vomiting or diarrhea.
• Acute hyponatremia leads to osmotic shifting of the water from the blood or plasma to the brain cells which can lead to serious complications including cerebral edema and herniation of the brain. Symptoms of accelerating cerebral edema include nausea, vomiting, headache, fatigue and listlessness. Acute hyponatremia can also cause cardiopulmonary arrest, seizure, coma and even death if not appropriately treated.
• The symptoms of hyponatremia worsen, the longer the condition goes untreated. Symptoms may be mild at first, starting with confusion or the need to urinate more frequently, but eventually there is a risk of delirium, seizure, stupor, coma or even death. Furthermore, nausea is can stimulate the release of antidiuretic hormone (ADH), which may worsen the condition by promoting the retention of water and further diluting the sodium level.

The patient may also be thirsty and try to consume large quantities of water when their sodium levels have not yet been adequately restored.

• Some of the neurological symptoms of hyponatremia include:
  • Headache
  • Nausea and vomiting
  • Confusion
  • Lethargy, malaise and fatigue
  • Irritability and mood swings
  • Seizures
  • Loss of consciousness
  • Coma

In many people, symptoms may be absent even when the blood sodium level has fallen to below 115 mmol/l. These patients are at a particularly high risk because failure to detect and treat the hyponatremia can lead to life-threatening consequences.