SIADH: symptoms, treatment, diagnosis and causes of low sodium

NICE CKS says most people with hyponatraemia are asymptomatic, particularly if it is mild and of gradual onset. Symptoms, if present, are often non-specific and related to the severity of the low sodium, how fast it fell, its duration and other conditions. The severity of symptoms may not correlate with the degree of hyponatraemia.

Rapid changes or severe hyponatraemia can cause vomiting, headache, drowsiness, seizures, coma and cardio-respiratory arrest. Chronic hyponatraemia can lead to gait disturbance, a risk of falls, reduced concentration and other cognitive deficits.

Because symptoms are subtle, low sodium is frequently found on a blood test arranged for another reason. NICE CKS calls hyponatraemia the most common electrolyte disorder in clinical practice and says it is usually an incidental finding on routine blood tests.

• Often none, if mild and gradual

• Headache, nausea and vomiting

• Tiredness, drowsiness and poor concentration

• Unsteadiness and falls, especially in older people

• Confusion, seizures or coma if severe or fast

Treatment depends on severity, speed of onset, symptoms and the cause. NICE CKS advises arranging emergency hospital admission if there is acute-onset, severe or symptomatic hyponatraemia, discussing with an endocrinology specialist if it is asymptomatic but moderately severe, and stopping or reducing the dose of medicines that may cause or contribute, depending on clinical judgement. Sodium is monitored, and any acute illness is managed.

Patient (professional reference) says that in SIADH with normal fluid volume the first step is fluid restriction of 500 to 1,000 ml a day for adults. If that is inadequate, demeclocycline, which blocks ADH, may be considered; it is licensed for SIADH secondary to malignant disease where fluid restriction is ineffective. Vaptans such as tolvaptan may be useful but can induce thirst, are expensive and have limited availability, and may raise sodium too rapidly.

Correction needs care. NICE CKS lists osmotic demyelination syndrome, caused by over-rapid correction of chronic hyponatraemia, among the complications, which is why sodium is raised under controlled conditions in hospital rather than by home remedies. Never stop a prescribed medicine without advice.

NICE CKS says assessment includes asking about recent illness or surgery, fluid intake, thirst, urine output, medicines, other conditions, exercise, diet and alcohol, and any previous episodes. The doctor examines for signs of fluid status and arranges a repeat serum sodium, urea and electrolytes, serum osmolality, and urine osmolality and sodium.

Patient (professional reference) lists the major diagnostic features of SIADH: hyponatraemia, low plasma osmolality in proportion to the sodium, urine osmolality inappropriately high (above 100 mOsmol/kg), persistent urine sodium above 30 mmol/L with normal salt intake, normal fluid volume, and normal thyroid and adrenal function. That is why thyroid and adrenal hormones are checked and medicines reviewed before the diagnosis is made.

NICE CKS classifies hyponatraemia as mild (130 to 135 mmol/L), moderate (125 to 129) or severe (below 125), and as acute (less than 48 hours) or chronic (48 hours or more).

NICE CKS explains that SIAD can be caused by drugs that increase the production or potentiate the action of ADH, by malignancy, by central nervous system disorders and by lung disease, and sometimes no cause is found. The cause of low sodium is often multifactorial.

Antidiuretic hormone (ADH, also called vasopressin) is made in the brain and released by the pituitary gland. It tells the kidneys to hold on to water. Normally ADH is turned down when the body has enough water.

In SIAD, NICE CKS says ADH is released in an excessive, unsuppressible way from the posterior pituitary or from an abnormal non-pituitary source, so the kidneys retain water that the body does not need. This does not mean that the body has gained sodium; the sodium in the blood is diluted. Hyponatraemia is a water balance disorder.

The term SIADH (syndrome of inappropriate antidiuretic hormone secretion) is used widely. NICE CKS uses syndrome of inappropriate antidiuresis (SIAD) because ADH levels are not always raised.

NICE CKS lists possible complications of hyponatraemia: cerebral oedema (a potentially life-threatening complication of severe and/or acute-onset hyponatraemia), osmotic demyelination syndrome after over-rapid correction, a greater risk of fragility fractures and osteoporosis, and higher mortality and length of stay in hospitalised patients.

Patient (professional reference) says that mild hyponatraemia is associated with gait disturbance in the elderly and with falls, and can reduce bone mass, increasing the incidence of fractures. Outlook depends mainly on the cause: when SIADH is triggered by a medicine or a short illness it often settles once that is dealt with, whereas longer-term causes may need ongoing monitoring.

Call 999 or go to A&E for new confusion, a seizure, severe headache with repeated vomiting, or marked drowsiness, particularly if you have been told your sodium is low or you take medicines that can lower it. NICE CKS says acute-onset, severe or symptomatic hyponatraemia needs emergency hospital admission.

See your GP promptly if you have a low sodium result and symptoms such as new unsteadiness, falls or poor concentration, or if you are told to restrict fluids and cannot manage. Do not drink large volumes of water to try to flush it out; in SIADH that makes sodium lower.

Low sodium is usually picked up on NHS blood tests in primary care or hospital. NICE CKS says a GP may refer to endocrinology or another specialist depending on the likely cause. Acute or severe cases go through the NHS emergency pathway, with endocrinology, renal or oncology teams involved depending on the cause.

Private endocrinology reviews are an option for non-urgent assessment, for example persistent mildly low sodium. Because treatment decisions depend on a full record of blood results and medicines, ask for results to be shared with your GP.