In SIADH the body releases antidiuretic hormone when it does not need to, so the kidneys keep too much water and blood sodium falls. Common causes are lung disease, brain disorders, small cell lung cancer, pain, nausea and drugs such as SSRIs and carbamazepine.
Key symptoms
- Often none if sodium falls slowly
- Nausea, headache and fatigue
- Confusion, unsteady walking and falls
- Muscle cramps
- Seizures or coma when sodium is very low
What you find on exam
- Normal fluid status: no edema and no dehydration
- Mental status and neuro checks
- Gait and balance
- Clues to the cause: lungs, brain, cancer, medication list
- Weight and fluid balance
Tests
- Sodium under 135 mmol/L with low serum osmolality
- Urine osmolality over 100 mOsm/kg and urine sodium over 30 mmol/L
- TSH and morning cortisol to exclude other causes
- Chest imaging and drug review to find the cause
Treatment basics
- Severe symptoms: 3 percent saline bolus with close sodium checks
- Raise sodium no more than 8 to 10 mmol/L in 24 hours
- Mild cases: fluid restriction to under 1 liter a day
- Oral urea or tolvaptan if restriction fails
- Stop the causing drug and treat the underlying disease
Red flags
- Seizure, vomiting or reduced consciousness
- Sodium under 120 mmol/L
- New weakness or trouble speaking days after a fast sodium rise (osmotic demyelination)
Practice it in Making Synapses
Making Synapses cases
Related guides
Sources: NCCPA PANCE Content Blueprint; NBOME COMLEX-USA clinical presentations; AAMC Core Entrustable Professional Activities (Core EPAs). Written by Making Synapses. Making Synapses is not affiliated with these organizations.