SIADH

Exam Cram

• SIADH: Syndrome of inappropriate anti-duiretic hormone, results in excessive anti-duiretic hormone causing excessive fluid retention- euvolaemia & hyponatraemia

• Presentation: symptomatic hyponatreaemia (confusion, headaches, drowsiness) with euvolaemic fluid status

• Diagnosis: low serum Na+ and osmolality, high urine sodium & osmolality

• Management: fluid restriction, address underlying cause

Outline

Background

• SIADH is caused by excessive ADH production, causing fluid retention and a dilutional hyponatraemia

• Clinically, it is hydrated individual who is unable to concentrate their urine, i.e. an inappropriately low urine osmolality in the context of low sodium

Assessment

History

• Asymptomatic

• Symptomatic hyponatraemia- confusion, seizures, coma

• Symptoms from cause- cough/ weight loss (lung ca), new drug introduction

Examination

• Euvolaemia

Investigations

• Hyponatraemia & low serum osmolality

• Urine: osmolality < serum osmolality

• Hyponatraemia work up- TFTs, consider cortisol, calcium, blood glucose

Diagnosis

Diagnostic criteria: (3/3)

• Hypotonic hyponatraemia with hypotonic urine

• Inappropirately raised urinary Na > 20

• Systolic BP > 120 (euvolaemia)

Classification (aetiology):

1. Malignancy:

   1. • Small cell lung cancer (most common)

      • Rarer: carcinoid, pancreatic, prostate, lymphoma, leukaemia

2. Drugs (in order of MCQ): 

   1. • SSRIs

      • Carbamazepine

      • Thiazide diuretics (particularly in elderly), 

      • Cyclophosphamide, 

      • Vincristine, 

3. Intracranial lesions & infections

Management

Acute (hyponatraemia)

1. Fluid restriction 1.5-1.2L if PO intake Na+ > 2g

2. Hypertonic saline If PO intake < 1L H20 or < 2g Na+, requires 

3. Fluid Balance: daily weights, monitor input/ output

4. Regular Na+ monitoring (6hrly -24hrly depending upon severity)

5. Investigate underlying cause: 

   1. Cancer: CXR, CT-T/ CTCAP, CT-H 

   2. Stop drug causes

6. Level of care

   1. Admission if Na < 126 or symptomatic

   2. HDU hypertonic saline

Inpatient reviews

• Monitor Na+, weight and urine/ serum osmolality

• Monitor symptoms- e.g. confusion

• FU underlying cause investigations

• Discharge once symptoms and Na+ stable & above 126

Chronic

• Fluid restriction as able 1.5-1.2L

• Monitor Na+ and weight

• Consider goals of care & escalation status

Med Reg Notes

Isotonic IVF in SIADH

Isotonic fluids in SIADH do not work and should be used with caution. This is because the kidneys will inappropriately retain the water and excrete the Na+, worsening the Na+. 

SIADH can commonly cause dehydration through vomiting and anorexia. Therefore, this makes the fluid restriction difficult and moves the patient towards a salt wasting diagnosis on most hyponatraemia algorithyms. That is why we have focused on the urinary Na+ first as it is objective. SIADH dehydration should be resolved with hypertonic saline because isotonic saline may worsen the dilutional affect of the Na+. It can be therefore reasonable and useful to trial isotonic IVF but if poor response, hypertonic fluids are required. 

Salt tablets can be used but there is little evidence. 

Resources and References

Written in 2024