Syncope

Definition

Syncope is a symptom defined as an abrupt, transient, complete loss of consciousness associated with inability to maintain postural tone, with rapid and spontaneous recovery. The presumed mechanism is cerebral hypoperfusion. There should be no clinical features of other nonsyncope causes of loss of consciousness (seizure, antecedent head trauma, pseudosyncope). sources/syncope-aha-acc-hrs-2017 (rating: very high)

Transient loss of consciousness (TLOC) is the broader category: it encompasses syncope (due to cerebral hypoperfusion) and nonsyncope conditions (seizures, hypoglycemia, metabolic, drug/alcohol intoxication, concussion).

Presyncope: symptoms before syncope (extreme lightheadedness, tunnel vision/graying out, altered consciousness without complete LOC) — may abort or progress to full syncope.

Key Concepts

Classification of Syncope

• Reflex (neurally mediated) syncope: Reflex causing vasodilation, bradycardia, or both
  • Vasovagal syncope (VVS) — most common overall; see concepts/Vasovagal-Syncope
  • Carotid sinus syndrome — see concepts/Carotid-Sinus-Syndrome
  • Situational syncope — micturition, defecation, cough, laugh, swallow
• Cardiac (cardiovascular) syncope: Due to bradycardia, tachycardia, or hypotension from low cardiac output, obstruction, vasodilatation, or acute vascular dissection
• Orthostatic hypotension syncope: Neurogenic OH, dehydration, drug-induced; see concepts/Orthostatic-Hypotension
• Unexplained syncope: No cause determined after thorough initial evaluation; ~37% of ED presentations

Epidemiology

• Prevalence up to 41%; recurrent syncope in 13.5% sources/syncope-aha-acc-hrs-2017 (rating: very high)
• Female predominance in prevalence (22% vs 15%; p<0.001)
• Trimodal age distribution in both sexes: ~20yr, ~60yr, ~80yr; third peak 5–7yr earlier in males
• Etiology in the community: reflex syncope 21% > cardiac 9% = OH 9% > unexplained 37%
• ED burden: 0.77% of all ED patients; 58% of patients >80yr admitted to hospital
• NYHA Class III–IV HF: syncope present in 12–14% of patients

Initial Evaluation

History and Physical Examination (Class I, B-NR) sources/syncope-aha-acc-hrs-2017 (rating: very high)

• Aim: identify prognosis, diagnosis, reversible factors, comorbidities, medications
• Features favoring cardiac syncope: age >60yr, male sex, known cardiac disease, brief/no prodrome, exertional syncope, syncope supine, abnormal cardiac exam, family history of premature SCD, congenital heart disease
• Features favoring noncardiac (reflex/OH) syncope: younger age, no cardiac disease, syncope only standing, positional change trigger, nausea/vomiting/warmth prodrome, specific environmental triggers, situational triggers, frequent recurrence
• Physical examination: orthostatic BP/HR in lying, sitting, immediate standing, and after 3 minutes; cardiac auscultation; basic neurological exam

12-Lead ECG (Class I, B-NR) sources/syncope-aha-acc-hrs-2017 (rating: very high)

• Mandatory in all patients with syncope
• May reveal: arrhythmogenic substrate (WPW, BrS, LQTS, HCM pattern, ARVC), conduction disease (heart block, bifascicular block), active ischemia
• Abnormal ECG (AF, IVCD, LVH, ventricular pacing) = increased 1-year all-cause mortality

Risk Stratification

Short-term risk factors (≤30 days) sources/syncope-aha-acc-hrs-2017 (rating: very high)

• Male sex, age >60yr, no prodrome, palpitations preceding LOC, exertional syncope
• Structural heart disease, HF, cerebrovascular disease, family history of SCD, trauma
• Evidence of bleeding, persistent abnormal vital signs, abnormal ECG, positive troponin

Long-term risk factors (>30 days) sources/syncope-aha-acc-hrs-2017 (rating: very high)

• Male sex, older age, structural heart disease, HF, cerebrovascular disease, VA, cancer, diabetes, high CHADS-2, abnormal ECG, lower GFR

Risk scores (OESIL, SFSR, STePS, ROSE, Boston Syncope Rule): validated but have NOT outperformed unstructured clinical judgment (Class IIb, B-NR) sources/syncope-aha-acc-hrs-2017 (rating: very high)

Disposition

• Hospital required (Class I, B-NR): presence of a serious medical condition — arrhythmic, structural cardiac, vascular, or major noncardiac comorbidity
• Outpatient acceptable (Class IIa, C-LD): presumptive reflex-mediated syncope without serious conditions; VVS mortality = same as matched controls
• Structured ED observation protocol (Class IIa, B-R): intermediate-risk patients; reduces hospitalization without harm vs unstructured admission
• Outpatient for suspected cardiac syncope (Class IIb, C-LD): in absence of serious medical conditions; expedited specialist referral as alternative

Additional Diagnostic Testing

Implantable cardiac monitor superiority sources/syncope-aha-acc-hrs-2017 (rating: very high):

• Diagnosis established in 55% ICM vs 19% conventional testing (external loop recorder → tilt table → EPS); p=0.0014
• Cost per diagnosis is lower with ICM despite higher device cost
• Battery life 2–3 years; remote monitoring and automatic arrhythmia detection

Cause-Specific Management: Cardiac Conditions

• Bradycardia (Class I, C-EO): GDMT per device guidelines; permanent pacemaker reasonable for unexplained syncope + chronic bifascicular block when other causes excluded
• SVT/AF (Class I, C-EO): GDMT per SVT/AF guidelines; syncope from rapid ventricular response uncommon; WPW syncope requires history + possible EPS
• Ventricular arrhythmia (Class I, C-EO): GDMT; ICD for lethal VA; VA ≥200 bpm → 65% syncope rate
• Ischemic/nonischemic cardiomyopathy (Class I, C-EO): GDMT; ICD reasonable for unexplained syncope + significant VA on EPS; ICD reasonable for unexplained syncope + nonischemic dilated CMP with significant LV dysfunction
• Valvular heart disease (Class I, C-EO): exertional AS syncope = hemodynamic mechanism; AVR after excluding other causes
• HCM (Class I, C-EO): ICD reasonable for ≥1 recent arrhythmic syncope episode; unexplained syncope = independent SCD predictor
• ARVC: ICD for documented sustained VA (Class I, B-NR); ICD reasonable for suspected arrhythmic syncope (Class IIa, B-NR)
• Cardiac sarcoidosis: ICD for sustained VA (Class I, B-NR); pacing for conduction abnormalities (Class I, C-EO); ICD reasonable for suspected arrhythmic syncope + LV dysfunction (Class IIa, B-NR)

Cause-Specific Management: Channelopathies

• Brugada: ICD reasonable for suspected arrhythmic syncope (IIa); NOT for reflex-mediated syncope (III); cardiac event rate 1.9%/yr in syncope, 0.5% asymptomatic
• Short-QT (QTc ≤340 ms): ICD may be considered for suspected arrhythmic syncope (IIb)
• LQTS: Beta-blockers = Class I; syncope → 6–12× risk of fatal/near-fatal events; ICD on failing/intolerant beta-blocker (IIa); LCSD for recurrent refractory (IIa)
• CPVT: Exercise restriction (Class I); beta-blockers (Class I); flecainide for breakthrough (IIa); ICD for refractory (IIa); LCSD for refractory (IIb)
• Early repolarization: ICD may be considered with FHx cardiac arrest (IIb); EPS = Class III Harm

Psychogenic Pseudosyncope

• Apparent but not true LOC; suspect in frequent (even daily) events mimicking VVS
• Tilt-table testing: apparent unconsciousness with normal hemodynamics + normal EEG = pseudosyncope (Class IIa, B-NR)
• Psychiatric evaluation and treatment indicated

Uncommon Conditions Associated with Syncope

• Subclavian steal syndrome, pulmonary hypertension, pacemaker syndrome, cardiac tamponade, aortic dissection — uncommon but important structural causes requiring targeted investigation

Contradictions / Open Questions

• Risk scores from multiple studies fail to outperform clinical judgment — reason unclear; possibly reflects the heterogeneity of syncope populations and composite endpoint issues sources/syncope-aha-acc-hrs-2017 (rating: very high)
• Optimal threshold for ICM implantation vs continued external monitoring is not defined; cost varies substantially by setting
• Role of structured syncope management units in North American healthcare systems: promising diagnostic yield reduction in European data but not studied in NA healthcare context
• ICD vs observation for unexplained syncope in non-ischemic CMP is debated (tension with DANISH trial showing no all-cause mortality benefit for primary prevention in NICM) — see concepts/ICD
• Cardiac syncope in Brugada with reflex features: distinguishing arrhythmic vs vasovagal mechanism can be challenging; overlap not fully characterized

Connections

• Related to concepts/Vasovagal-Syncope — most common form of syncope
• Related to concepts/Orthostatic-Hypotension — second major noncardiac cause
• Related to concepts/Carotid-Sinus-Syndrome — less common reflex cause
• Related to concepts/POTS — related autonomic syndrome; not true syncope per se
• Related to entities/ICD — key therapy for cardiac syncope in structural/inheritable disease
• Related to concepts/Permanent-Pacing-Indications — bradycardic and reflex-mediated syncope
• Related to concepts/Sudden-Cardiac-Death — cardiac syncope risk stratification overlap
• Related to concepts/HCM-Risk-SCD — syncope as SCD predictor in HCM
• Related to concepts/Arrhythmogenic-Cardiomyopathy — syncope triggers ICD evaluation
• Related to concepts/Sinus-Node-Dysfunction — bradycardic syncope
• Related to concepts/Atrioventricular-Block — bradycardic syncope

Sources

• sources/syncope-aha-acc-hrs-2017