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)

• Hyponatraemia, 

• Euvolaemia, 

• Urine osmolality lower than serum

Classification (aetiology):

1. Malignancy:

   1. • Small cell lung cancer (most common)

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

2. Drugs (in order of MCQ): 

   1. • Carbamazepine

      • Thiazide diuretics (particularly in elderly), 

      • Cyclophosphamide, 

      • Vincristine, 

3. Intracranial lesions & infections

Management

Acute (hyponatraemia)

1. Fluid restriction 1.5-1.2L

2. Regular weights

3. Investigate underlying cause- (cancer): CXR, CT-T/ CTCAP, CT-H 

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

5. Hypertonic saline if seizures/ GCS < 8

6. Admission if Na < 126 or symptomatic

7. Stop drug causes

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

Resources and References

Written in 2024