Database searching, supplemented by manual screening of reference lists and by recent publications recommended during peer review, identified the literature included in this synthesis, comprising original cohort studies and clinical trials, systematic reviews and meta-analyses, clinical practice guidelines and consensus documents, and narrative reviews.

The prevalence of syncope in DCM depends on the study population and heart failure (HF) severity. In a retrospective analysis of 897 patients with DCM, syncope occurred in 30 (3.3%). (6) Among patients with advanced HF (New York Heart Association [NYHA] class III–IV), prevalence rises to 12–14%; (5, 17) Middlekauff et al reported syncope in 12% of 491 patients referred for transplant evaluation. (5) Syncope recurs in approximately 35% of patients after an initial episode, and 29% sustain physical injury. (3, 4) In the general population, the incidence of syncope increases sharply after the age of 70 years, reaching 19.5 per 1,000 person-years after 80 years, (4) a pattern likely amplified in DCM by concurrent comorbidities.

Cardiac mechanisms account for 50–60% of syncopal episodes in DCM. (6, 8)

Ventricular arrhythmias are the most common identified cause, accounting for approximately 33% of cases. (6, 18) Ventricular tachycardia (VT) and ventricular fibrillation (VF) cause abrupt reductions in cardiac output, and the DCM substrate—fibrosis, chamber dilation, and abnormal conduction—predisposes to sustained ventricular arrhythmias. (19, 20) Among 14 patients with nonischemic DCM and unexplained syncope who received an implantable cardioverter-defibrillator (ICD), Knight et al observed appropriate ICD therapy in 57% during a mean follow-up of 25 months. (7) Similarly, among 103 patients with nonischemic advanced HF and syncope, ICD therapy improved survival compared with historical medically treated controls. (8)

Conduction disorders account for approximately 23% of cardiac syncope in DCM (6, 21) and include complete or high-grade atrioventricular (AV) block, sinus node dysfunction with prolonged pauses, and bundle branch block with intermittent progression to complete heart block. The fibrotic process frequently involves the conduction system, explaining the high incidence of conduction abnormalities in this population. (22)

Low cardiac output may precipitate syncope during exertion or increased demand, when the failing ventricle cannot augment output; in advanced HF, syncope may occur even in the absence of documented arrhythmia. (5)

Other cardiac causes include atrial fibrillation with rapid ventricular response or long pauses, pulmonary embolism, and, rarely, dynamic outflow tract obstruction in specific DCM subtypes. (1, 2)

Non-cardiac mechanisms are identified in a substantial minority of patients with DCM and syncope. (6) Neurally mediated (reflex) syncope, including vasovagal syncope, situational syncope, and carotid sinus hypersensitivity, accounts for approximately 7% of cases. (1, 2, 6, 23) Orthostatic hypotension accounts for another 7% (6, 24) and results from HF-related autonomic dysfunction, (25) diuretic-induced volume depletion, vasodilator therapy (angiotensin-converting enzyme [ACE] inhibitors, angiotensin receptor blockers [ARBs], and nitrates), a blunted heart rate response under beta-blockade, and age-related impairment of baroreceptor function. (24)

Environmental temperature extremes. Ambient temperature extremes may precipitate or exacerbate syncope and related symptoms in DCM. Heat exposure induces cutaneous vasodilation, sweat-related volume loss, and a fall in central venous pressure and stroke volume, (26) which a failing ventricle with limited preload reserve compensates poorly; these effects are amplified by diuretics, renin–angiotensin system inhibitors, angiotensin receptor–neprilysin inhibitors, sodium–glucose cotransporter 2 inhibitors, and beta-blockers, which limit volume or heart-rate compensation and predispose to orthostatic hypotension. (24, 27) In older adults with chronic conditions, heatwaves are associated with increased heat-related hospitalizations, with the risk modified by commonly used medications. (27) Cold exposure, conversely, increases sympathetic tone, afterload, and myocardial oxygen demand and may promote ventricular arrhythmias and HF decompensation. (28) In a multinational analysis of 27 countries, both extreme heat and extreme cold were associated with increased cardiovascular mortality, with the greatest relative excess for heart failure. (29) Direct evidence linking temperature extremes to syncope specifically in patients with DCM, however, remains limited, and these associations are largely extrapolated from broader heart failure and older-adult populations. Patients with DCM should therefore receive individualized advice on heat avoidance and fluid intake, and diuretic and vasodilator doses should be reviewed during heatwaves, taking into account congestion status and renal function.

Several expected predictors, including LVEF, (6, 38) NYHA class, (6) ischemic versus non-ischemic etiology, (38) atrial fibrillation, and dyslipidemia, (6) have not shown consistent associations with syncope; Abdel-Hadi et al found no significant difference in LVEF between patients with and without syncope. (6) This likely reflects the multifactorial nature of syncope, in which arrhythmogenic substrate, autonomic tone, medication effects, and comorbidities play roles beyond pump function. Myocardial fibrosis detected by late gadolinium enhancement (LGE) on CMR has emerged as a major predictor of ventricular arrhythmias and SCD, with incremental prognostic value beyond LVEF, (39, 40) and LMNA, FLNC, DSP, and PLN variants mark heightened arrhythmic risk, particularly when associated with conduction disease, nonsustained VT (NSVT), or unexplained syncope. (22)

Arterial hypertension (61.5% [8/13] vs 23.5% [4/17], P = .08) and diabetes mellitus (46.2% [6/13] vs 5.9% [1/17], P = .03) were more frequent in patients with non-cardiac than with cardiac syncope, (6) suggesting that autonomic neuropathy and orthostatic mechanisms may be particularly important in diabetic patients. (6, 25) General predictors of recurrent syncope include structural heart disease, (1, 2) impaired renal function, (41) AV block or left bundle branch block, (1) male sex, (4) HF, (1, 5) age >70 years, (3, 4) and medications predisposing to orthostatic hypotension. (1, 24)

Initial evaluation includes detailed history taking, physical examination with orthostatic blood pressure measurement, 12-lead electrocardiogram (ECG), echocardiography, and targeted laboratory testing; (1, 2) history alone establishes the diagnosis in up to 50% of cases. (46) High-risk features warranting hospitalization include syncope during exertion or while supine, associated chest pain or dyspnea, preceding palpitations, family history of sudden death before 50 years of age, LVEF <35%, new or significant ECG abnormalities (bifascicular block, corrected QT interval [QTc] >500 ms), significant injury, and known structural heart disease, including DCM. (1, 2) The San Francisco Syncope Rule, the Osservatorio Epidemiologico sulla Sincope nel Lazio (OESIL) score, and the Evaluation of Guidelines in Syncope Study (EGSYS) score have been validated for risk stratification, although their performance in DCM requires further study. (1, 47)

Rhythm monitoring. In-hospital telemetry is indicated for high-risk patients. (1, 2) After discharge, options include Holter monitoring (24–72 hours) when syncope is frequent, (2) external event recorders (2–4 weeks) for less frequent symptoms, and implantable loop recorders for infrequent, unexplained, recurrent syncope, with a diagnostic yield of 35–43% over 1–2 years. (9, 10) Last-generation insertable cardiac monitors offer a comparable diagnostic yield (42%) with a lower clinical workload, (11) and in cardiomyopathy patients at borderline arrhythmic risk at least one ILR-guided diagnosis was made in 44% during a mean follow-up of 19 months, most frequently de novo atrial fibrillation. (12) In DCM, an ILR is particularly useful when LVEF is only mildly or moderately reduced and the arrhythmic origin of syncope remains uncertain. (12, 13)

Electrophysiological study. Programmed ventricular stimulation can identify inducible VT, (30, 48, 49) but its predictive value differs by etiology: sensitivity for subsequent arrhythmic events is approximately 50–80% in ischemic cardiomyopathy (48) but only 20–30% in non-ischemic DCM, so a negative study does not reliably exclude arrhythmic risk. (30, 49) Indications for EPS in patients with syncope and structural heart disease, including DCM, have been summarized in dedicated recommendations. (50)

Cardiac magnetic resonance. Beyond assessment of ventricular size and function, CMR enables tissue characterization through LGE, which reflects myocardial fibrosis and arrhythmogenic substrate. (39) LGE independently predicts ventricular arrhythmias, SCD, appropriate ICD therapies, and all-cause mortality in DCM, (40) and its prognostic value extends beyond severely reduced LVEF. (51, 52)

Tilt-table testing and carotid sinus massage. Tilt testing is indicated when reflex syncope is suspected and has approximately 90% specificity but variable sensitivity (30–70%), depending on the protocol. (1, 2, 53) Carotid sinus massage is recommended in patients >40 years of age with unexplained syncope; carotid sinus hypersensitivity is diagnosed when massage produces asystole >3 seconds or a systolic blood pressure decrease >50 mmHg. (1, 2)

Device therapy. ICD implantation is indicated in patients with DCM and syncope with LVEF ≤35% after at least 3 months of guideline-directed medical therapy (GDMT) (class I, primary prevention), documented or inducible sustained VT/VF (class I, secondary prevention), or unexplained syncope with high suspicion of arrhythmic etiology. (44, 45, 54) In the Defibrillators in Non-Ischemic Cardiomyopathy Treatment Evaluation (DEFINITE) trial, ICD therapy showed a trend toward reduced mortality (hazard ratio 0.65, P = .08) and significantly reduced sudden death. (55) Pacemaker implantation is indicated for documented bradyarrhythmias or conduction system disease, (21, 56) and cardiac resynchronization therapy with a defibrillator (CRT-D) is preferred when indications for both CRT and an ICD exist. (14, 54) CRT is recommended in symptomatic patients with LVEF ≤35% and left bundle branch block with QRS duration ≥150 ms despite optimal medical therapy, and should be considered at QRS durations of 130–149 ms; (15) the choice between CRT-D and CRT with a pacemaker (CRT-P) may be refined by CMR tissue characterization, genotype, and electrophysiological testing. (16)

Medical therapy. GDMT for HF, including ACE inhibitors, ARBs, or angiotensin receptor–neprilysin inhibitors (ARNIs), beta-blockers, mineralocorticoid receptor antagonists, and sodium–glucose cotransporter 2 (SGLT2) inhibitors, should be optimized. (54, 57) Amiodarone or sotalol may be used for VT suppression in selected cases, typically as an adjunct to ICD therapy. (44, 58) Medications should be adjusted when orthostatic hypotension is identified, and volume should be repleted when hypovolemia contributes. (24)

Catheter ablation. Ablation is generally considered for recurrent monomorphic VT despite ICD therapy. (44, 59, 60) Success rates are lower in non-ischemic DCM (approximately 50–70%) than in ischemic cardiomyopathy (approximately 70–80%) because of diffuse, often epicardial or mid-myocardial substrate, and epicardial access is frequently required. (59, 60)

Treatment of non-cardiac causes. Vasovagal syncope is managed with education, trigger avoidance, counter-pressure maneuvers, and adequate hydration; (1, 23) orthostatic hypotension with medication adjustment, compression stockings, and increased salt and fluid intake. (24) Pacemaker implantation is recommended for recurrent cardioinhibitory carotid sinus hypersensitivity or vasovagal syncope with documented asystole. (1, 56)

The main characteristics of syncope in DCM are summarized in Table 1, key practical considerations for its evaluation in Table 2, and a diagnostic and management algorithm is presented in Figure 1.