SIADH Diagnosis and Management

Algorithm Steps

1. ▶

   Start

   Suspected SIADH

   Euvolemic hyponatremia with inappropriate urine concentration

   1. ●

      Action

      SIADH Diagnostic Criteria

      All criteria required for diagnosis

      • 1. Serum osmolality <275 mOsm/kg (hypotonic)
      • 2. Urine osmolality >100 mOsm/kg (inappropriately concentrated)
      • 3. Urine sodium >30 mmol/L (on normal salt intake)
      • 4. Clinical euvolemia (no edema, no dehydration)
      • 5. Normal thyroid and adrenal function
      • 6. No recent diuretic use
      1. ◆

         Decision

         Rule Out Other Causes

         SIADH is diagnosis of exclusion

         • Hypothyroidism: Check TSH
         • Adrenal insufficiency: Check cortisol/ACTH
         • Renal failure: Check creatinine
         • Psychogenic polydipsia: Urine osmolality <100
         • Diuretic use: Recent thiazide/loop diuretics
         1. ●

            Action

            Identify Underlying Cause

            Common SIADH etiologies

            • Malignancy: SCLC, head/neck, GI cancers
            • CNS: Stroke, SAH, meningitis, trauma, tumor
            • Pulmonary: Pneumonia, TB, COPD, positive pressure ventilation
            • Medications: SSRIs, carbamazepine, NSAIDs, cyclophosphamide
            • Post-surgical: Especially after pituitary surgery
            • Idiopathic (elderly)
            1. ◆

               Decision

               Assess Severity

               Guide treatment urgency

               • SEVERE symptoms: Seizures, coma, respiratory distress → Emergency treatment
               • MODERATE symptoms: Confusion, nausea, headache → Prompt treatment
               • MILD/Asymptomatic: Focus on underlying cause + gradual correction
               1. ●

                  Action

                  SEVERE Symptoms

                  Emergency treatment with 3% saline

                  • 3% Hypertonic saline 150 mL IV over 20 min
                  • Repeat up to 3 times in first hour if needed
                  • Target: 5 mmol/L rise in first 1-2 hours
                  • Max 10-12 mmol/L in 24h (avoid ODS)
                  • Transfer to ICU
                  1. ●

                     Action

                     1. Fluid Restriction

                     First-line for mild-moderate SIADH

                     • Restrict to 1000-1500 mL/day (or 500 mL below urine output)
                     • More restrictive if urine:serum osmolality ratio >1
                     • Include ALL fluids (oral + IV)
                     • Expected Na rise: 1-2 mmol/L/day
                     • Compliance is often challenging
                     1. ●

                        Action

                        3. Vasopressin Receptor Antagonists (Vaptans)

                        For refractory SIADH

                        • Tolvaptan: 15-60 mg PO daily
                        • Initiate in hospital (overcorrection risk)
                        • Do NOT use with hypertonic saline
                        • Monitor Na q6h for first 24-48h
                        • FDA: Avoid >30 days due to liver injury risk
                        • Conivaptan: 20 mg IV then 20-40 mg/day (short-term only)
                        1. ⚠

                           Warning

                           ⚠️ Vaptan Precautions

                           Important safety considerations

                           • Contraindicated in hypovolemic hyponatremia
                           • Risk of rapid overcorrection → ODS
                           • Hepatotoxicity with prolonged use (tolvaptan)
                           • Must initiate in monitored setting
                           • Do not combine with 3% saline
                        2. ●

                           Action

                           4. Treat Underlying Cause

                           Essential for long-term management

                           • Discontinue offending medications
                           • Treat infection (pneumonia, CNS infection)
                           • Cancer treatment if malignancy-related
                           • Manage CNS pathology
                           • SIADH often resolves when cause addressed
                           1. ●

                              Action

                              5. Monitoring

                              Ongoing assessment

                              • Serum Na: q4-6h during active treatment
                              • Urine osmolality and output
                              • Fluid intake (strict I/O)
                              • Neurological status
                              • Max correction: 10-12 mmol/L in 24h
                              1. ✓

                                 Outcome

                                 SIADH Resolved/Controlled

                                 Na normalized, underlying cause treated

                              2. ⚠

                                 Warning

                                 Chronic SIADH

                                 Long-term management needed

                                 • Continued fluid restriction
                                 • Salt tablets ± loop diuretic
                                 • Consider vaptans PRN
                                 • Regular Na monitoring
               2. ●

                  Action

                  2. Adjunct Therapies

                  If fluid restriction insufficient

                  • Salt tablets: 3-9 g NaCl/day (with loop diuretic)
                  • Urea: 15-60 g/day (osmotic diuresis)
                  • Loop diuretic + salt: Furosemide + NaCl tablets
                  • Demeclocycline: 300-600 mg BID (induces nephrogenic DI)