⚠️ Must-Not-Miss Diagnoses
Severe symptomatic hyponatremia (seizures, coma) requiring hypertonic saline, osmotic demyelination syndrome from overcorrection, Addisonian crisis masquerading as hyponatremia, SIADH from occult malignancy.
Differential Diagnosis by Key Features
| Clinical Feature | Think Of | Priority |
|---|---|---|
| Seizures, coma, severe neurological symptoms | Severe symptomatic hyponatremia — emergency | HIGH |
| Hypovolemic: vomiting, diarrhea, diuretics, Addison's | Hypovolemic hyponatremia | Moderate |
| Euvolemic: recent surgery, medications (SSRIs, diuretics), pulmonary/CNS disease | SIADH | Moderate |
| Hypervolemic: edema, heart failure, cirrhosis, nephrotic syndrome | Hypervolemic hyponatremia | Moderate |
| Hyperglycemia, mannitol, contrast | Pseudohyponatremia / redistributive | Low — correct glucose first |
Systematic Approach
Symptoms
Severity determines urgency
Mild (>125): often asymptomatic. Moderate (120-125): nausea, malaise, headache. Severe (<120 or rapid): seizures, coma, herniation. Symptoms drive treatment urgency more than the absolute value.
Volume Status
The critical first branch point
Assess volume status clinically: skin turgor, mucous membranes, JVD, edema, orthostatic vitals. This determines the dominant mechanism: hypovolemic (renal or extrarenal loss), euvolemic (SIADH), or hypervolemic (heart failure, cirrhosis, nephrotic).
Urine Studies
Urine sodium and osmolality
Urine Na <20: kidneys conserving sodium → extrarenal loss or low effective circulating volume. Urine Na >40: kidneys wasting sodium → SIADH, cerebral salt wasting, diuretics, Addison's. Urine Osm >100: appropriate ADH effect or SIADH.
Correction Rate
Never correct faster than 8-10 mEq/L per 24 hours
Overcorrection causes osmotic demyelination syndrome (central pontine myelinolysis) — irreversible neurological damage. In chronic hyponatremia (<48h not established), limit correction to 8 mEq/L/day. In acute symptomatic, 1-2 mEq/L/hour until symptoms resolve.
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