Clinical features

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

• hyponatraemia is usually asymptomatic when plasma sodium is between 130-135mmol per litre (1).
• 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 (2).

Signs and symptoms of hyponatraemia are often initially non specific (3)

• the symptoms may range from lethargy and anorexia to agitation, disorientation, seizures and coma
• cerebral oedema may be seen in acute onset of hyponatraemia (3)

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 (3)
• coma, death

Patients with chronic hyponatraemia are unlikely to have any symptoms except during an acute exacerbation of the hyponatraemia (4).

Reference:

• (1) Reynolds RM, Padfield PL, Seckl JR. Disorders of sodium balance.BMJ.2006;332(7543):702-5
• (2) Smellie WS et al. Best practice in primary care pathology: review 8. J Clin Pathol. 2007;60(7):740-8
• (3) Guidelines and Audit Implementation Network (GAIN) 2010. Hyponatraemia in adults (on or after 16th birthday)
• (4) Reynolds RM, Seckl JR. Hyponatraemia for the clinical endocrinologist. Clin Endocrinol (Oxf). 2005;63(4):366-74.