Elpidoforos S. Soteriades, M.D., Jane C. Evans, D.Sc., Martin G. Larson, Sc.D., Ming Hui Chen, M.D., Leway Chen, M.D., Emelia J. Benjamin, M.D., and Daniel Levy, M.D.
Background Little is known about the epidemiology and prognosisof syncope in the general population.
Methods We evaluated the incidence, specific causes, and prognosisof syncope among women and men participating in the FraminghamHeart Study from 1971 to 1998.
Results Of 7814 study participants followed for an average of17 years, 822 reported syncope. The incidence of a first reportof syncope was 6.2 per 1000 person-years. The most frequentlyidentified causes were vasovagal (21.2 percent), cardiac (9.5percent), and orthostatic (9.4 percent); for 36.6 percent thecause was unknown. The multivariable-adjusted hazard ratiosamong participants with syncope from any cause, as comparedwith those who did not have syncope, were 1.31 (95 percent confidenceinterval, 1.14 to 1.51) for death from any cause, 1.27 (95 percentconfidence interval, 0.99 to 1.64) for myocardial infarctionor death from coronary heart disease, and 1.06 (95 percent confidenceinterval, 0.77 to 1.45) for fatal or nonfatal stroke. The correspondinghazard ratios among participants with cardiac syncope were 2.01(95 percent confidence interval, 1.48 to 2.73), 2.66 (95 percentconfidence interval, 1.69 to 4.19), and 2.01 (95 percent confidenceinterval, 1.06 to 3.80). Participants with syncope of unknowncause and those with neurologic syncope had increased risksof death from any cause, with multivariable-adjusted hazardratios of 1.32 (95 percent confidence interval, 1.09 to 1.60)and 1.54 (95 percent confidence interval, 1.12 to 2.12), respectively.There was no increased risk of cardiovascular morbidity or mortalityassociated with vasovagal (including orthostatic and medication-related)syncope.
Conclusions Persons with cardiac syncope are at increased riskfor death from any cause and cardiovascular events, and personswith syncope of unknown cause are at increased risk for deathfrom any cause. Vasovagal syncope appears to have a benign prognosis.
Syncope, defined as a sudden loss of consciousness associatedwith the inability to maintain postural tone, followed by spontaneousrecovery, is relatively common. However, the epidemiology andprognosis of syncope in the community have not been well described.Data are limited on the cumulative incidence1,2 and lifetimeprevalence3,4,5,6,7 of syncope.
Although syncope has many possible causes, several studies haveused three categories of cause — cardiac, noncardiac,and unknown — to examine the prognosis of syncope prospectively.8Previous studies of syncope were conducted in emergency departments,9,10,11,12in general hospitals,13,14,15,16,17 and among highly selectedsubgroups of patients with syncope of unknown cause.18,19,20,21,22,23,24,25,26,27Little is known about the prognosis of syncope due to specificcauses in the general population. We conducted a study to evaluatethe incidence and prognosis of syncope due to specific causesamong participants in the Framingham Heart Study.
Methods
Study Sample
The study sample consisted of participants in the original FraminghamHeart Study and in the Framingham Offspring Study who underwentroutine clinical examinations between 1971 and 1998. The detailsof the study design and selection criteria have been describedelsewhere.28,29 We examined all reports of syncope in studyparticipants undergoing a qualifying clinical examination. Syncopereports were coded as "yes," "no," or "maybe." A committee ofphysicians reviewed all episodes coded as "maybe." Possibleepisodes of syncope were coded as definite only if the participant'schart contained additional medical information confirming asyncopal event that had not been available at the time of theinitial clinical evaluation. We excluded 120 equivocal reportsof syncope, 101 reports of syncope from participants who hadnot had an examination within four years before the index report(to minimize possible recall bias), 47 reports of syncope dueto head trauma, and 7 reports of syncope with incomplete records.All participants undergoing examination in the Framingham HeartStudy provided written informed consent; the content and proceduresof the examinations were reviewed and approved by the institutionalreview board of Boston Medical Center.
Incidence
The overall incidence rate of syncope and the incidence ratesfor syncope with specific causes were calculated by dividingthe number of participants with syncope by the total numberof person-years of follow-up in each subgroup. For the calculationof incidence rates in different age groups, we categorized participantsaccording to their age halfway between the time of the clinicalexamination preceding the report of syncope and the time ofthe index examination for syncope. We calculated the cumulativeincidence by assuming a constant incidence over time, usingthe following formula:
cumulative incidence = 1 – e–It,
where e is the natural logarithm constant (2.718), I denotesthe incidence rate, and t denotes the duration of follow-up.
Causes of Syncope
The cause of syncope was assigned according to a clinical diagnosismade by two physicians (an internist and a cardiologist) onthe basis of documentation provided by the examining FraminghamHeart Study physician and information obtained during routineclinic visits from the medical history, physical examination,and electrocardiographic examination. In addition, the physiciansreviewed all available hospital and outpatient medical recordspertaining to each instance of syncope. Generally, the dischargediagnosis was used for participants evaluated at an outsidehospital, as long as this diagnosis was supported by associatedrecords. For other participants, the diagnosis of the examiningFramingham Heart Study physician was used. If a cardiovascularcause was suspected, a second evaluation was performed by anotherFramingham Heart Study physician. If these evaluations did notagree, the physician reviewers evaluated available data to assigna cause.
Reports of syncope were classified into eight categories accordingto cause: cardiac cause (including ischemia and arrhythmias),unknown cause, stroke or transient ischemic attack, seizuredisorder, vasovagal cause, orthostatic cause, medication, andother, infrequent causes (including cough syncope, micturitionsyncope, and situational syncope). These eight categories werereduced to four etiologic categories for the outcome analysis:cardiac cause, unknown cause, neurologic cause (including syncopedue to stroke, transient ischemic attack, and seizure), andvasovagal or other cause (including vasovagal syncope, orthostaticsyncope, medication-induced syncope, and syncope due to otheridentified causes). Secondary analyses were also performed afterwe excluded syncope due to seizure from the category of neurologicsyncope. When a participant did not seek medical attention forsyncope and the history, physical examination, and electrocardiographicfindings were not indicative of any of the specific causes,the cause was classified as unknown. The physicians who confirmedthe diagnoses of syncope and assigned the causes were unawareof subsequent outcomes.
Recurrence of Syncope
The first episode of syncope reported by a participant duringa clinical examination was treated as the first syncopal event.If more than one syncopal event was reported between clinicvisits, we counted only one episode of syncope. Episodes reportedat subsequent clinical examinations were considered recurrencesof syncope.
End Points
The participants in the follow-up sample were followed for upto 25 years. A prospective cohort design was used to study threeoutcomes in relation to syncope: death from any cause, myocardialinfarction or death from coronary heart disease (which includedboth sudden and nonsudden death from coronary heart disease),and fatal or nonfatal stroke. All outcome events were reviewedwith the use of all pertinent available medical records andadjudicated on the basis of previously published criteria.30Data from 95 participants who reported syncope during theirlast index examination were excluded from the outcome analysis,because no follow-up data were available for them. For eachparticipant with syncope, we randomly selected two participantswithout syncope who underwent clinical examination at the FraminghamHeart Study clinic during the same examination cycle, matchedaccording to age and sex.
Statistical Analysis
All statistical analyses were performed with SAS software (version6.12).31 The age-, sex-, and category-specific incidence ratesof syncope were computed. The survival rates among participantswith syncope due to specific causes were examined with Kaplan–Meiersurvival curves.32 Age- and multivariable-adjusted Cox proportional-hazardregression models33 were used to analyze the risk of each outcomeof interest among participants with cause-specific syncope (fouretiologic categories) as compared with participants withoutsyncope. We evaluated the outcomes of interest by using dummyvariables for each of the four etiologic categories. Multivariablemodels were adjusted for age, sex, presence or absence of ahistory of cardiovascular disease (myocardial infarction, coronaryheart disease, stroke or transient ischemic attack, congestiveheart failure, atrial fibrillation, and intermittent claudication),smoking status, presence or absence of hypertension, systolicblood pressure, presence or absence of diabetes, total cholesterollevel, heart rate, and use or nonuse of cardiac medication,including antihypertensive medications. Hypertension was a dichotomousvariable defined as a systolic blood pressure of at least 140mm Hg, a diastolic blood pressure of at least 90 mm Hg, or currentuse of antihypertensive medication. The presence or absenceof a history of cardiovascular disease, smoking status, presenceor absence of diabetes, use or nonuse of cardiac medication,and normal or elevated total cholesterol level (with an elevatedlevel defined as at least 200 mg per deciliter [5.2 mmol perliter]) were also included in the model as dichotomous variables.Age, systolic blood pressure, and heart rate were included ascontinuous variables.
Results
We followed 7814 participants (3563 men and 4251 women) foran average of 17.0 years (median, 19.1; interquartile range,10.6 to 23.6) during 133,164 person-years of follow-up. Theparticipants had a mean (±SD) age of 51.1±14.4years (range, 20 to 96). A total of 822 participants (348 menand 474 women; mean age, 65.8 years) reported syncope. Afterexcluding participants without follow-up (those whose firstepisode of syncope was reported at the last index examination),we included 727 participants in the outcome analysis.
Incidence
The overall incidence rate of a first report of syncope was6.2 per 1000 person-years. The incidence rates increased withage, with a sharp rise at 70 years (Figure 1). The age-adjustedincidence was 7.2 per 1000 person-years among both men and women.The age-adjusted incidence rate among participants with cardiovasculardisease was nearly twice that among participants without cardiovasculardisease (10.6 vs. 6.4 per 1000 person-years). The incidencerates of first reports of cardiac syncope, syncope of unknowncause, syncope due to stroke or transient ischemic attack, syncopedue to seizure, vasovagal syncope, orthostatic syncope, medication-inducedsyncope, and syncope of other causes were 0.59, 2.26, 0.26,0.30, 1.31, 0.58, 0.42, and 0.47 per 1000 person-years, respectively.Assuming a constant incidence rate over time, we calculateda 10-year cumulative incidence of syncope of 6 percent.
Figure 1. Incidence Rates of Syncope According to Age and Sex.
The incidence rates of syncope per 1000 person-years of follow-up increased with age among both men and women. The increase in the incidence rate was steeper starting at the age of 70 years. Syncope rates were similar among men and women.
Causes of Syncope
The distributions of causes in men and women, respectively,were as follows: cardiac causes (13.2 percent and 6.7 percent),unknown cause (31.0 percent and 40.7 percent), stroke or transientischemic attack (4.3 percent and 4.0 percent), seizure disorder(7.2 percent and 3.2 percent), vasovagal cause (19.8 percentand 22.2 percent), orthostatic cause (8.6 percent and 9.9 percent),medication (6.3 percent and 7.2 percent), and other causes (9.5percent and 6.1 percent). The causes of syncope according tocardiovascular disease status at base line are shown in Table 1.Overall, 56 percent of participants with a syncopal episodereported seeing a doctor or visiting a hospital for evaluation.
Table 1. Causes of Syncope According to Sex and the Presence or Absence of Cardiovascular Disease at Base Line.
Recurrence of Syncope
Five hundred seventy participants (78.4 percent) reported onlyone episode of syncope. One hundred fifty-seven participants(21.6 percent) reported one or more recurrences of syncope (17.6percent reported one recurrence, 3.3 percent two recurrences,and 0.7 percent three or more recurrences). The rate of recurrenceamong participants with a history of syncope was higher thanthe incidence of a first episode of syncope among those withoutsuch a history (multivariable-adjusted hazard ratio, 23.2; 95percent confidence interval, 14.2 to 37.9). The risk of recurrencewas especially high among participants with cardiac syncope(multivariable-adjusted hazard ratio, 30.0; 95 percent confidenceinterval, 14.9 to 60.3).
Follow-up
The characteristics of the 2181 study participants who wereeligible for follow-up are shown in Table 2. As compared withthose without syncope, participants with syncope of any cause,and in particular those with cardiac syncope, were more likelyto have a history of coronary artery disease or cerebrovasculardisease and to take cardiac or antihypertensive medications.During a mean follow-up of 8.6 years (median, 7.7), there were847 deaths from all causes, 263 myocardial infarctions or deathsfrom coronary heart disease, and 178 fatal or nonfatal strokes.
Figure 2. Overall Survival of Participants with Syncope, According to Cause, and Participants without Syncope.
P<0.001 for the comparison between participants with and those without syncope. The category "Vasovagal and other causes" includes vasovagal, orthostatic, medication-induced, and other, infrequent causes of syncope.
Table 3. Hazard Ratios for the Outcomes of Interest in Participants with Syncope as Compared with Participants without Syncope.
Secondary analyses were also performed to examine the relationsof cause-specific syncope to the outcomes of interest afterexclusion of data from 40 participants who had syncope due toseizures. The findings were similar to the results shown inTable 3, except that the association between neurologic syncopeand death from any cause was no longer statistically significant(hazard ratio, 1.24; 95 percent confidence interval, 0.81 to1.89).
Because cardiac syncope might have been preferentially diagnosedin participants with syncope who were already known to havecardiovascular disease, we also performed secondary analysesstratified according to the presence or absence of known cardiovasculardisease. The results were similar in the two groups (data notshown). In addition, to address the possibility that syncopeof unknown cause might have been associated with poor outcomesbecause persons with this diagnosis received a different levelof medical care, we performed analyses stratified accordingto whether the participant sought or did not seek medical carefor the syncopal event (130 and 131 participants, respectively).The data from 11 participants were excluded from this analysisbecause information on whether they sought medical care wasnot available. As compared with participants without syncope,participants with this diagnosis who sought medical care hada higher risk of death from any cause (multivariable-adjustedhazard ratio, 1.47; 95 percent confidence interval, 1.14 to1.90), although their risk of myocardial infarction or deathfrom coronary heart disease was not significantly elevated (adjustedhazard ratio, 1.07; 95 percent confidence interval, 0.63 to1.83). Participants with syncope of unknown cause who did notseek medical care had a higher risk of myocardial infarctionor death from coronary heart disease (adjusted hazard ratio,1.60; 95 percent confidence interval, 1.04 to 2.48), but theirrisk of death from any cause was not significantly elevated(adjusted hazard ratio, 1.23; 95 percent confidence interval,0.94 to 1.60).
Discussion
In this population-based study, we found a similar incidence of syncope in men and women.The incidence was almost doubledin participants with a history of cardiovascular disease. Mortalitywas about 30 percent higher among all participants with syncopeand among those with syncope of unknown cause than among thosewithout syncope; this increase was due largely to an increasedrisk of death from myocardial infarction or coronary heart disease.Cardiac syncope doubled the risk of death from any cause andincreased the risk of fatal and nonfatal cardiovascular events.Neurologic syncope was associated with an increased risk ofdeath from any cause and an increased risk of stroke.
Although the incidence of syncope rose sharply with advancingage, the age-specific annual incidence of syncope for thoseabove 80 years of age in our study (about 2 percent) was considerablylower than that reported by Lipsitz et al. (6 percent) in anelderly institutionalized population (mean age, 87 years).1Overall, 37 percent of syncopal events in our sample were due to unknown causes.The distribution of causes of syncope thatwe observed is similar to that reported elsewhere.34,35 Forexample, Linzer et al. reported that, on average, 34 percentof syncopal episodes could not be assigned to an etiologic categoryafter a standard evaluation with a detailed history taking,physical examination, and electrocardiography.36
Our findings are consistent with those of Kapoor15 in suggestingthat cardiac syncope is associated with increased risks of prematuredeath and cardiovascular events. Whether or not there was ahistory of cardiovascular disease, we found that participantswith a diagnosis of cardiac syncope were more likely to havea poor outcome than those without syncope.
Several other studies have evaluated the outcome of syncopeof unknown cause.18,19,20,21,22,23,24,25,26,27 The results suggestedthat among subjects with syncope that was initially classifiedas being of unknown cause and that was later found to have acardiac cause, the risk of death was higher than in the generalpopulation. Electrophysiological studies and electrocardiographicmonitoring in highly selected subgroups of patients37,38,39,40,41have suggested that 45 to 80 percent of cases of syncope classifiedas being of unknown cause could be assigned a cardiac cause.It is likely that many participants in our study with syncopeof unknown cause had unrecognized cardiac syncope, with itsattendant risk of an adverse outcome.
Participants with syncope of neurologic cause had more thantwice as high a risk of fatal or nonfatal stroke as subjectswithout syncope. The increased risk is probably attributableto a high risk of stroke, often recurrent, in participants withneurologic syncope who had preexisting cerebrovascular disease.
Among participants with syncope of identified cause, by farthe largest subgroup consisted of those with syncope of vasovagalor other causes, including orthostatic and medication-inducedsyncope. Among these participants we found no increase in therisk of death from any cause or in the risk of myocardial infarctionor death from coronary heart disease, in comparison to thosewithout syncope, even though the statistical power to detectsuch increases was high (96 percent power to detect a 50 percentincrease in the risk of death from any cause; 84 percent powerto detect a 75 percent increase in the risk of myocardial infarctionor death from coronary heart disease).
This study involved a population-based sample free of selectionbias and an extensive period of follow-up. However, some limitationsshould be noted. We attempted to minimize misclassificationof participants according to whether or not they had previouslyhad an episode of syncope by excluding those who had not undergonea clinical examination within four years before an examinationat which they reported syncope. However, a four-year intervalmay constitute a long recall period, especially for older patients.Differential recall bias is unlikely to have affected our findings,because our prospective cohort design ensured that exposureswere ascertained before the outcomes. Problems with recall,however, may have contributed to underestimation of the incidenceof syncope and hazard ratios for adverse outcomes associatedwith this diagnosis.42
Another concern is possible misclassification of participantsaccording to etiologic categories of syncope.43 Although wedid not develop explicit criteria for confirming a given causeof syncope, we sought to minimize misclassification by havingdiagnoses assigned by the consensus of two physician reviewers,unaware of the study outcomes, who reviewed all related outsiderecords and Framingham Heart Study examination records. Thepresence of other risk factors for cardiovascular disease orthe presence of cardiovascular disease may have contributedto cardiovascular risk among participants with syncope, butour findings persisted after we adjusted for these factors inmultivariable models and in secondary analyses stratified accordingto the presence or absence of known cardiovascular disease.Finally, our study sample consisted almost entirely of middle-agedand older white men and women, and thus the results may notbe generalizable to other populations.
Our findings suggest that persons with cardiac syncope constitutea high-risk group predisposed to morbidity and premature mortalityfrom cardiovascular disease who should be monitored closely.Persons with syncope of unknown cause appear to constitute amixed group at increased risk for death; this result suggeststhat further diagnostic testing may be warranted in these patients.The increased risk of stroke in participants with neurologicsyncope may be attributable to preexisting cerebrovascular disease.Our results also provide reassurance that vasovagal syncope has a benign prognosis.
Supported by grants (N01-HC-38038 and NS-17950) from the NationalHeart, Lung, and Blood Institute.
Source Information
From the National Heart, Lung, and Blood Institute Framingham Heart Study, Framingham, Mass. (E.S.S., J.C.E., M.G.L., M.H.C., L.C., E.J.B., D.L.); the Departments of Cardiology and Preventive Medicine, Boston Medical Center, Boston (E.J.B.); the National Heart, Lung, and Blood Institute, Bethesda, Md. (D.L.); and the Division of Cardiology, Beth Israel Deaconess Medical Center, Boston (D.L.).
Address reprint requests to Dr. Levy at the Framingham Heart Study, 73 Mt. Wayte Ave., Suite 2, Framingham, MA 01702-5827.
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