#vasovagal-syncope #autonomic-dysfunction #orthostatic-intolerance #syncope

Vasovagal Syncope (VVS)

Definition

Syncope: Transient loss of consciousness with inability to maintain postural tone, rapid and spontaneous recovery, and absence of clinical features specific to another cause (e.g., epileptic seizure).

VVS: Syncope syndrome that usually (1) occurs with upright posture held >30 seconds OR exposure to emotional stress, pain, or medical settings; (2) features diaphoresis, warmth, nausea, and pallor; (3) is associated with hypotension and relative bradycardia; (4) is followed by fatigue.

Key Concepts

Epidemiology

Cumulative incidence by age 60: 42% women, 32% men (actuarial methodology) sources/POTS-IST-VVS-HRS-2015 (rating: high)
Incidence increases markedly around age 11; median first syncope age ~14 years; most first episodes before age 40
Manifested as pallid syncope in ~1–3% of toddlers
~30–50% of syncope in emergency department settings is autonomically mediated (most vasovagal); ~35% of ED syncope patients remain undiagnosed
Outcome generally benign — no increased mortality; but high recurrence rate (~25–35% at 1 year) sources/POTS-IST-VVS-HRS-2015 (rating: high)
1-year recurrence strongly predicted by prior-year syncope burden: 7% if no syncope in past year vs 40% if ≥1 episode (POST study) sources/POTS-IST-VVS-HRS-2015 (rating: high)

Pathophysiology

Upright position → gravitational pooling of 500–800 mL blood (venous, pelvic/splanchnic, lower limbs) → reduced venous return → decreased cardiac output and BP → baroreceptor-triggered sympathetic noradrenergic vasoconstriction + heart rate increase
During VVS episode: ineffective reflex response → venous pooling → paradoxical vasodilation → hypotension + vagal cardioinhibition (relative or absolute bradycardia; sometimes prolonged asystole in sinus and AV nodes) sources/POTS-IST-VVS-HRS-2015 (rating: high)
Initial BP reduction during VVS is primarily driven by a ~50% decline in cardiac output; peripheral vasodilatation occurs in only a subset of patients
MSNA persistence during VVS recently reported by 2 independent groups — challenges the classical hypothesis that abrupt sympathetic withdrawal is the final precipitating event sources/POTS-IST-VVS-HRS-2015 (rating: high)
Two physiologic phenotypes (classified by supine systolic BP):
- Low-pressure phenotype (SBP <100 mmHg): low tyrosine hydroxylase → reduced NE synthesis → reduced NE availability
- Normal-pressure phenotype (SBP >100 mmHg): increased norepinephrine transporter → augmented NE reuptake → reduced NE availability
- Both phenotypes have reduced NE availability, ultimately impairing the neurocirculatory response to orthostatic stress sources/POTS-IST-VVS-HRS-2015 (rating: high)

Diagnosis

Based on clinical history: predisposing situation (prolonged standing ≥2–3 minutes; emotional/pain/medical context), prodromal features (diaphoresis, warmth, flushing, nausea, abdominal discomfort, visual blurring), postictal features (fatigue for minutes to hours), unconsciousness usually <1–2 minutes
Fine or coarse myoclonic movements in ~10% of episodes — can mimic epilepsy; videometric analysis and home videos helpful
Diagnostic scores available (high accuracy); useful clinical reminders but require further validation
Tilt-table testing:
- Sensitivity 78–92% in high-pretest probability populations; specificity ~90% vs asymptomatic controls
- Positive response: clinically reminiscent presyncope/syncope with hypotension ± bradycardia
- Does NOT establish causation — indicates predisposition/substrate for VVS
- Most useful specific indications: differentiating convulsive syncope from true seizure; unclear diagnosis after careful history; establishing pseudosyncope diagnosis
- ISSUE-3 subanalysis: positive tilt test result predicts patients who will NOT benefit from cardiac pacing sources/POTS-IST-VVS-HRS-2015 (rating: high)
Implantable loop recorder (ILR):
- Gold standard for documenting cardiac rhythm during spontaneous syncope
- Diagnostic yield ~35% over device lifetime (up to 3 years)
- RCTs consistently show earlier ILR use improves diagnosis and care in older patients with unexplained syncope (predominantly age 70s–80s in trials)
- ILR benefit for directing pacing therapy best established in older patients with documented asystole + negative tilt test sources/POTS-IST-VVS-HRS-2015 (rating: high)

Conservative and Medical Treatment

Patients with occasional syncope: Reassure; promote salt/fluid intake; teach physical counterpressure maneuvers; do NOT initiate pharmacotherapy if no syncope in the past year sources/POTS-IST-VVS-HRS-2015 (rating: high)
Patients with recurrent syncope: Begin conservatively as above; review and reduce hypotension-causing medications if possible
Physical counterpressure maneuvers: Isometric exercise of large muscles (leg crossing, hand-grip, arm-tensing) during impending syncope prodrome; 39% relative risk reduction vs controls in prospective parallel RCT; risk-free; recommended as core management for all severity levels sources/POTS-IST-VVS-HRS-2015 (rating: high)
Tilt training: Home standing protocols NOT beneficial; clinic-based monitored protocols possibly beneficial but poor long-term compliance and unproven biologic mechanism; no formal recommendation possible sources/POTS-IST-VVS-HRS-2015 (rating: high)
Beta-blockers: Adequately powered double-blind RCTs (POST I — metoprolol; atenolol trial) show NOT effective overall; signal of benefit in patients >40 years from POST meta-analysis (prespecified prestratified substudy); POST 5 (prospective multicenter RCT in older patients) was ongoing; reasonable to use metoprolol in older patients, avoid in younger patients sources/POTS-IST-VVS-HRS-2015 (rating: high)
Fludrocortisone: POST 2 (unpublished at time of guideline) showed strong trend to benefit only; pediatric trial showed placebo superior to fludrocortisone; reasonable to try in patients with sufficient symptom severity sources/POTS-IST-VVS-HRS-2015 (rating: high)
Midodrine: ~70% risk reduction across 4 RCTs; all with significant design limitations (pediatric populations, tilt test primary outcomes, open-label design, or extraordinarily symptomatic patients); no conventional placebo-controlled RCT in moderate-to-severe adult VVS (POST 4 was ongoing); caution in older men (urinary retention) and women of childbearing age (teratogenic effects unknown) sources/POTS-IST-VVS-HRS-2015 (rating: high)
Serotonin transporter inhibitors: Biologically plausible (serotonin regulates midbrain HR/BP); 3 small RCTs; considerable uncertainty about efficacy sources/POTS-IST-VVS-HRS-2015 (rating: high)

ACC/AHA/HRS 2017 Guideline Treatment Recommendations (Class/LOE)

sources/syncope-aha-acc-hrs-2017 (rating: very high)

Treatment	COR	LOE	Notes
Patient education (diagnosis, triggers, prognosis)	I	C-EO	Explain benign nature; identify/avoid triggers; core therapy for all patients
Physical counter-pressure maneuvers	IIa	B-R	First-line for any patient with a prodrome; leg crossing, limb contraction, squatting
Midodrine (recurrent VVS, no HTN/HF/urinary retention)	IIa	B-R	43% reduction in syncope recurrence across 5 RCTs; peripheral alpha-agonist
Orthostatic training	IIb	B-R	Uncertain benefit; RCTs do not show sustained reduction in syncope recurrence
Fludrocortisone (inadequate response to salt/fluid)	IIb	B-R	POST II: 31% risk reduction (marginally insignificant); pediatric trial favored placebo
Beta-blockers in patients ≥42 years	IIb	B-NR	Age-dependent benefit confirmed only ≥42yr; avoid in younger patients
Salt and fluid intake (2–3 L/day + 6–9 g salt/day)	IIb	C-LD	In absence of HTN/renal disease/HF
Reduce or withdraw hypotensive medications	IIb	C-LD	Review with prescribing provider
Selective serotonin reuptake inhibitors (recurrent VVS)	IIb	C-LD	Contradictory evidence from 3 small RCTs

ACC/AHA/HRS 2017 Pacemaker Recommendation for VVS (Class IIb, B-R [SR])