Clinical features

Symptoms depend on the severity of hyponatraemia and the rate of decrease in the plasma sodium concentration (1,2).

• hyponatraemia is usually asymptomatic when plasma sodium is between 130-135mmol per litre.
• features of severe hyponatraemia may occur when the plasma sodium falls rapidly towards 125 mmol/l and symptoms usually appear when the sodium level is below 115 mmol/l.

Signs and symptoms of hyponatraemia are often initially non specific

• the symptoms may range from lethargy and anorexia to agitation, disorientation, seizures and coma
• cerebral oedema may be seen in acute onset of hyponatraemia

Features of moderately severe hyponatraemia i.e. plasma sodium less than 120 mmol/l):

• headache, nausea, vomiting
• malaise, irritability, depression, personality change
• cramps, muscle weakness

Features of severe hyponatraemia i.e. plasma sodium less than 110 mmol/l):

• confusion, drowsiness, convulsions
• diminished reflexes, extensor plantar response
• Cheyne-Stokes respiration
• coma, death

Patients with chronic hyponatraemia are unlikely to have any symptoms except during an acute exacerbation of the hyponatraemia.

Reference:

1. Adrogué HJ, Tucker BM, Madias NE. Diagnosis and management of hyponatremia: a review. JAMA. 2022 Jul 19;328(3):280-91.
2. Spasovski G, Vanholder R, Allolio B, et al; Hyponatraemia Guideline Development Group. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014 Feb 25;170(3):G1-47.