Autonomics and Orthostatic Hypotension
Gary Salomon
Background
- Orthostatic hypotension (OH): sustained ↓SBP ≥20 mmHg (or ≥30 mmHg in patients with supine hypertension) or↓ DBP ≥10 mmHg after at least 2 minutes of standing or head-up tilt to 60 degree.
- Pathophysiology: Standing causes 500-1000 mL venous pooling; normally compensated by baroreceptor-triggered sympathetic vasoconstriction and increased heart rate, but this fails in OH due to autonomic dysfunction (neurogenic OH) or inadequate intravascular volume (non- neurogenic OH).
| Classification | Mechanism | Key Features | Causes |
|---|---|---|---|
| Neurogenic OH (nOH) | Autonomic dysfunction → impaired baroreflex-vasoconstriction | Blunted heart rate response (ΔHR/ΔSBP 0.5 beats/min per mmHg has 91% sensitivity and 88% specificity) | Primary causes: Parkinson, MSA, LBD, Pure autonomic failure Secondary causes:Diabetic neuropathy, Amyloidosis, Spinal cord disorders, Autoimmune neuropathy |
| Non-neurogenic OH | Reversible factors impairing hemodynamic compensation | Preserved/exaggerated heart rate response (ΔHR/ΔSBP >0.5) Note: ΔHR/ΔSBP ratio is invalid in patients with atrial fibrillation or on beta-blockers | Hypovolemia (dehydration, blood loss), medications (diuretics, vasodilators, alpha-blockers), cardiac disease, anemia |
Evaluation
- Evaluate for orthostatic symptoms (dizziness, lightheadedness, vision changes, syncope, falls), timing (worse in morning, after meals, in heat), medication review.
- Obtain orthostatic Vital Signs - Active Stand Test (AST).
- Measure BP/HR after 5 minutes supine.
- Measure BP/HR immediately upon standing, then at 1, 2, and 3 minutes.
- Assess for symptoms during testing.
- Optimal timing: Morning testing (when symptoms most prominent) - Alternative: Sit-to-stand test (lower sensitivity but useful when supine position not feasible).
Labs
- CBC, CMP, EKG, TSH, B12, LFTs.
- Consider SPEP/UPEP, paraneoplastic panel, autonomic function testing depending on clinical context.
Special Considerations
- Delayed OH: BP drop occurring >3 minutes after standing; associated with progression to autonomic failure.
- Postprandial hypotension: Screen in all nOH patients; common comorbidity.
- Supine hypertension: Present in ~50% of nOH patients; critical to identify as it affects treatment selection.
Management
- Medication Review: Discontinue or reduce causative medications when possible.
- Avoid alpha-blockers, diuretics, and long-acting vasodilators in nOH.
- Hydration and Salt: Fluid intake: 2-2.5 L/day. Salt intake: increase to 2.3-4.6g/day.
- Acute intervention: Drink 500 mL (16 oz) water 15-30 minutes before standing (can raise SBP by ~30 mmHg).
- Compression Garments: Waist-high compression stockings (30-40 mmHg) or abdominal binder (most effective).
- HOB 30-45 degrees always for supine HTN.
- Small, frequent meals to minimize postprandial hypotension.
Pharmacologic therapies ideally should be used only after conservative measures fail and patient has undergone formal autonomic testing:
| Drug | Dose | Mech | Side effects |
|---|---|---|---|
| Fludrocortisone (Florinef) | 0.1mg QD ↑ by 0.1 mg Max: 0.3 mg QD | Mineralocorticoid → increase blood volume. Enhances sensitivity to catecholamines | Edema, HTN, ↓K Contraindications: - CHF |
| Midodrine (genera) | 2.5mg TID ↑ by 2.5mg Up to 10mg TID | Peripheral-selective α1 agonist → constricts both aa & vv | Supine HTN, Pilomotor reactions, Pruritus, GI sx Contraindications: - uncontrolled HTN - urinary retention - Heart disease |
| Droxidopa | 100mg ↑ by 100mg Max: 600mg TID | NE precursor that crosses BBB | Supine HTN less than midodrine |
| Atomoxetine | 10mg or 18mg | SNRI | Contraindications: - glaucoma - MAOI |