Hyponatraemia

🔍 When to Suspect

Non-specific symptoms like headache, nausea, vomiting, or muscle cramps; consider with new-onset confusion, drowsiness, seizures, or falls in older adults

From the full topic in The Ocean Library: Hyponatraemia

🧭 When to suspect

Hyponatraemia (serum sodium < 135 mmol/L) is the commonest electrolyte abnormality in clinical practice.

• It is usually a disorder of water excess rather than true sodium deficiency.

• Most cases are found incidentally on a routine U&E.

• Symptoms are often vague and easily attributed to age or comorbidity.

Suspect it in anyone with non-specific headache, nausea, vomiting, muscle cramps, lethargy or unsteadiness.

• Consider it actively in older adults with new confusion, drowsiness, gait instability or recurrent falls.

The two clinical priorities in primary care:

• Assess volume status (which directs the diagnosis).

• Review the drug chart, since medication is one of the most common and most reversible causes.

Biochemical severity	Serum sodium	Significance & action
Mild	130–135 mmol/L	• Often asymptomatic • Investigate the cause and recheck – usually managed in primary care
Moderate	125–129 mmol/L	• Investigate • Discuss with endocrinology if asymptomatic • Admit if symptomatic, acute or hypovolaemic
Severe (profound)	< 125 mmol/L	• High risk • Arrange urgent admission, particularly if acute or symptomatic

Classify each case three ways: by biochemical severity (above), by onset (acute < 48 hours versus chronic ≥ 48 hours or unknown), and by volume status (hypovolaemic, euvolaemic or hypervolaemic). Symptom severity drives urgency more than the absolute number.

⚠️ Common pitfall • Acting on the number before confirming it is real. • A low sodium may be spurious (a drip-arm sample), or reflect pseudohyponatraemia (very high lipids or protein, with a normal serum osmolality) or hyperglycaemia drawing water into the blood. • Exclude these first. • Remember that a profoundly low sodium can be asymptomatic, while a modest fall can cause significant symptoms if it developed quickly.

Source: Society for Endocrinology · European Hyponatraemia Guideline

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