Hyponatraemia: Causes, Symptoms, Diagnosis, and Treatment

Hyponatraemia is a very common clinical problem that is defined as the low levels of sodium in the blood. Sodium is an essential electrolyte which is used to maintain water balance, nerve activity and muscle activity in the body. In hyponatraemia, the value of serum sodium is less than 135 mEq/L. The normal range of sodium in blood is 135-145 mEq/L.

Where patients are under diuretics, the definitive diagnosis is often not possible and it is essential that the diuretic be terminated. In case this step is not feasible, then hyponatraemia is likely to be caused by an underlying cardiac, renal or hepatic disease. Early exclusion of pseudo- or dilutional hyponatraemia is important. Clinical evaluation should be concerned with volume status, i.e. an assessment of the extent of oedema, the presence of fluid loss (e.g. diarrhoea, fistula loss), and the presence of signs of dehydration (e.g. postural fall in blood pressure). A urine Na⁺ level and a thyroid-stimulating hormone (TSH) are especially useful at this point. Hypoadrenalism is the diagnosis that should not be overlooked since the untreated cases may be life-threatening. There is therefore a justification of a low threshold to use a Short Synacthen Test. Hypoadrenalism due to pituitary failure does not necessarily include hyperkalaemia, hypotension or hyperpigmentation and can be easily ignored. Cerebral salt wasting usually follows in days after brain injury (e.g. subarachnoid haemorrhage, neurosurgery, or stroke) and is believed to be mediated by brain natriuretic peptides.

Causes of pseudohyponatraemia

Causes of pseudohyponatraemia with normal serum osmolality:

1. Hyperproteinaemia (e.g. myeloma)
2. Hyperlipidaemia (hypertriglyceridaemia)
3. Glycine or sorbitol (from bladder irrigant)

Causes of pseudohyponatraemia with raised serum osmolality:

• Hyperglycaemia
• Glycerol
• Mannitol

The syndrome of inappropriate antidiuretic hormone (SIADH) is an exclusionary diagnosis.

Diagnostic criteria of SIADH

• Hyponatraemia
• Absence of diuretics
• Absence of oedemaNormal renal function
• Normal adrenal function
• Normal thyroid function
• Urine Na⁺ >20 mmol/L
• Euvolaemic

Each requirement in above table has to be met. There is usually no particular cause and multiple precipitating factors can simultaneously occur. Chronic SIADH is relatively prevalent in older people and could explain hyponatraemia that remains in many years without apparent aetiology. Such patients are to be recommended to take less than a litre of fluid in a day and only when thirsty.

The characteristics of SIADH/hyponatraemia that are commonly underrated

1. Unless there is high clinical suspicion, no radiological investigation to establish underlying malignancy is required. When malignancy is present, it is typically massive, visible and untreatable (e.g. diffuse small-cell lung cancer).
2. The urine osmolality does not have to be significantly high. Urine osmolality of less than 250 mOsmol/L (i.e. less than plasma) in people consuming large amounts of fluid may represent true SIADH.
3. Cases that were once referred to as the sick-cell syndrome are now believed to be an indication of SIADH in severely ill patients.
4. Water intoxication is typically a mixture of SIADH and overhydration. Healthy people do not consume enough fluid to surpass the renal water excretion capacity (712 L per day), and this is why isolated over-drinking does not often result in hyponatraemia. Potomaniacs have to be hyponatraemic, and this is possible only in the presence of SIADH.
5. The post-operative environment is especially prone to SIADH triggers-nausea, pain, opiates, post-operative pneumonia and hypovolaemia caused by blood loss, and administration of large volumes of intravenous fluids may predispose to hyponatraemic events.
6. Drowsiness, coma or seizures related to hyponatraemia are dependent on the rate of fall in serum Na⁺, not on the absolute concentration. Those who are alert with serum Na⁺ <125 mmol/L have probably been chronically hyponatraemic and only need to be corrected gradually but a sudden drop to serum Na⁺ <130 mmol/L, usually caused by a huge infusion of hypotonic fluid into the bladder, can trigger severe neurological deterioration and needs emergency resuscitation with hypertonic saline.

Symptoms of Hyponatraemia

Symptoms depend on how quickly and how severely sodium levels drop:

Mild to moderate symptoms:

• Nausea
• Headache
• Fatigue or weakness
• Confusion or forgetfulness

Severe symptoms:

• Seizures
• Coma
• Respiratory arrest
• Death (in extreme, untreated cases)

Treatment

Depending on the cause and severity, treatment is done:

• Mild cases: fluid restriction, underlying causes treatment
• Moderate to severe:
  • IV (e.g., hypertonic saline) saline solutions
  • Drugs such as vasopressin receptor antagonists (vaptans)
  • Management of associated conditions (heart failure, SIADH, and so on)

Quick normalization of sodium is dangerous and may cause osmotic demyelination syndrome (ODS) which is severe neurological disorder.

Takeaways

• Hyponatraemia = low sodium in the blood (<135 mEq/L)
• When severe or fast developing, it can be life-threatening. It is necessary to diagnose it in time and treat it properly to prevent severe complications.