Differentials
Pseudohyponatraemia
SIGNS / SYMPTOMS
Many patients are asymptomatic. Symptoms, if present, depend on the nature and severity of the underlying cause.
Possible symptoms or signs of poorly controlled diabetes mellitus (polyuria, polydipsia) may be present if hyperglycaemia is the cause.
Eruptive xanthoma may be seen in patients with severe hyperlipidaemia.
Patients with hyperproteinaemia may show signs of multiple myeloma or other rarer causes.
INVESTIGATIONS
Suspicion is raised if measurement of serum glucose, lipids, and protein reveals one of these to be elevated.
Elevated serum glucose: diagnosis is confirmed if calculation of the corrected serum sodium value reveals a normal sodium level. The equation for SI units is: corrected sodium (mmol/L) = measured sodium (mmol/L) + 0.016{(glucose [mmol/L] x 18)-100}. The equation for conventional units is: corrected sodium (mEq/L) = measured sodium (mEq/L) + 0.016 [glucose (mg/dL) - 100].
Elevated serum lipid level, particularly triglyceride: normal serum osmolarity confirms diagnosis.
Elevated serum protein, particularly, globulin (multiple myeloma): normal serum osmolarity confirms diagnosis.
Hypovolaemia
SIGNS / SYMPTOMS
History of poor oral intake, vomiting, diarrhoea, or diuretic use help to distinguish hypovolaemia from SIADH.
Evidence of dry mucous membranes, skin tenting, and flat neck veins can accompany hypovolaemia, although clinical assessment is poor at predicting volume status.[17]
INVESTIGATIONS
Central venous pressure (CVP) <8.
Urine sodium <20 mmol/L (<20 mEq/L).
Elevation in serum sodium with a diagnostic trial of 1 to 2 L of normal saline infusion.
Cerebral salt-wasting
Hypervolaemia (e.g., CHF, cirrhosis, pregnancy)
SIGNS / SYMPTOMS
Dyspnoea, pulmonary oedema, elevated jugular venous pressure, ascites, lower extremity oedema.
INVESTIGATIONS
CVP elevated.
Urine sodium <40 mmol/L (<40 mEq/L).
Psychogenic polydipsia
SIGNS / SYMPTOMS
There may be a psychiatric history or excess fluid intake elicited during history.
INVESTIGATIONS
Urine osmolality <100 mmol/kg H₂O (<100 mOsm/kg H₂O).
24-hour urine osmoles >600 mmol (>600 mOsm).
Poor solute intake (e.g., beer potomania, low-protein diet)
SIGNS / SYMPTOMS
Beer drinking or malnourished patients with poor dietary solute intake and high water intake. Low solute excretion limits water excretion, causing water retention.
INVESTIGATIONS
Urine osmolality <100 mmol/kg H₂O (<100 mOsm/kg H₂O).
24-hour urine osmoles <300 mmol (<300 mOsm).
Renal failure
SIGNS / SYMPTOMS
Hypertension; oedema may be present.
INVESTIGATIONS
Elevated creatinine.
Addison's disease
SIGNS / SYMPTOMS
Pigmentation of skin and mucosa, weight loss, hypotension.
INVESTIGATIONS
Low cortisol levels.
Responds to steroid administration.
Hypothyroidism
SIGNS / SYMPTOMS
Dry coarse skin, myxoedema, hair loss, weight gain.
INVESTIGATIONS
High TSH and low serum thyroid hormone levels.
Responds to treatment with thyroid hormone.
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