SIADH

Overview

No definite cause can be found in many cases, typically around half or more of all patients with SIADH (Shapiro et al, 2010).

SIADH is the commonest cause of hyponatraemia (Clayton et al, 2006; Shapiro et al, 2010). Potential underlying causes include malignancy (eg bronchogenic carcinoma, lymphoma, sarcoma), respiratory disease (eg pneumonia, pneumothorax), neurological disease (eg stroke, subarachnoid haemorrhage, meningitis) and drugs (including many chemotherapy agents and psychiatric drugs). The most commonly implicated drugs are antidepressants, antipsychotics, anticonvulsants and chemotherapy agents. However, many other commonly used drugs can occasionally cause SIADH, including antidiabetic drugs, proton pump inhibitors, opiates and non-steroidal anti-inflammatory drugs (NSAIDs).

SIADH can also be precipitated by pain and nausea, and can be seen in psychosis even in patients yet to start antipsychotic drugs (Soiza et al, 2008).

The water retention and sodium loss both cause hyponatremia, which is a key feature in SIADH.

Hyponatremia and concentrated urine (Uosm> 300 mOsm) are seen, as well as no signs of edema or dehydration (euvolemic).

When hyponatremia is severe (sodium <120 mOsm), or acute in onset, symptoms of cerebral edema become prominent (irritability, confusion, seizures, and coma).

Stepwise assessment of SIADH

1. Confirm hyponatraemia
   • Serum Na+ is less than 135 mmol/L.
2. Assess serum osmolality
   • Determine serum osmolality to confirm it is low (<275 mOsm/kg), indicating hypotonic hyponatremia.
3. Evaluate volume status
   • Conduct a clinical assessment to determine if the patient is hypovolaemic, euvolaemic or hypervolaemic.
4. Measure urine osmolality
   • If urine osmolality is >100 mOsm/kg, it suggests inappropriate concentration of urine.
5. Measure urine sodium concentration
   • Check urine sodium concentration. A level >30 mmol/L supports the diagnosis of SIADH.
6. Rule out other causes
   • Exclude other potential causes of euvolaemic hyponatraemia such as hypothyroidism, adrenal insufficiency and renal failure through appropriate tests (thyroid function tests, cortisol levels and assessment of renal function).

Diagnostic criteria for SIADH:

Confirm the following criteria:

• hypotonic hyponatraemia