Focused screening for occult carotid artery disease: patients with known heart disease are at high risk☆

Article Outline

• Abstract
• Patients and methods
  • Screening protocol
  • Statistical analysis
• Results
  • Patient demographic data
  • Overall screening results
  • CAS screening results
  • Relationship between type of heart disease and CAS among screened patients
  • Relationship between risk factors and presence of bilateral carotid artery disease
  • Multivariate analysis
  • Screening costs
• Discussion
• Conclusions
• References
• Copyright

Abstract 

Objectives

Stroke puts a major financial burden on our healthcare system. However, carotid duplex scanning performed as a screening test for occult carotid artery stenosis (CAS) currently is not reimbursed by Medicare. The goals of this study were to develop a cost-effective stroke screening program, to determine the prevalence of potential causes of stroke in this population, and to define a population at high risk in which screening would be most effective.

Methods

In a community-based stroke screening program, patients were eligible if they were older than 60 years and had a history of either hypertension, heart disease, or cigarette smoking, or a family history of stroke. Screening included blood pressure determination, an electrocardiographic rhythm strip, and a previously validated modified carotid duplex ultrasound examination to detect CAS 50% or greater. The relationships between standard demographic risk factors and screening outcomes were analyzed.

Results

Screening was performed in 610 patients. Unilateral or bilateral CAS was detected in 66 patients (10.8%). The finding of occult CAS was more prevalent than that of new hypertension (2.6%) or new atrial fibrillation (0.5%). Patients with known hypertension were significantly more likely to have CAS than were those without hypertension (12.7% vs 7.8%; P = .05). Patients with heart disease were significantly more likely to have CAS than were those without heart disease (18.2% vs 8%; P < .0001). Patients with both risk factors were significantly more likely to have occult carotid artery disease than were patients without either risk factor (22.1% vs 8.5%; P < .0001). Multivariate analysis with logistic regression revealed a history of heart disease as an independent predictor of occult carotid artery disease (odds ratio 95% confidence interval, 1.4-4.4). Type of heart disease was not a significant factor in predicting occult CAS. Direct cost of the screening, including community outreach, nurses, technicians, support staff, and miscellaneous expenses, was less than $75 per patient.

Conclusions

In a screening program for treatable causes of potential stroke, CAS was the most commonly diagnosed disease. More than one of every five patients with known hypertension and heart disease had occult CAS. Known heart disease of any type was a significant independent predictor of occult CAS. Screening for treatable causes of potential stroke can be cost-effective. This information could help to further target populations to screen for occult CAS and to justify reimbursement for screening carotid duplex scanning examinations.

Stroke is the third leading cause of death in the United States, second only to heart disease and cancer.1 Approximately 83% of all strokes are ischemic, and 7.6% of these ischemic strokes result in death within 30 days.1 Perhaps even more important, stroke is a leading cause of serious, long-term disability. The estimated direct and indirect costs of caring for patients after stroke in the United States in 2002 were $49.4 billion.1 Certainly, no one would deny that stroke is a devastating illness, both from the patient's perspective and a national health care cost perspective.

Treatment of stroke, once it has occurred, is generally unsuccessful.1 Approximately 30% of patients who have a stroke will require long-term institutionalization, and an additional 30% will require significant assistance with activities of daily living. Although early thrombolytic treatment of ischemic stroke has been reported,2 this intervention is practically limited to the relatively small number of patients who can be treated at a major medical center within several hours of their symptoms. It is clear that the optimal treatment for stroke is primary prevention.

Despite these staggering statistics and extensive research in this area, there is no consensus on how to best prevent ischemic stroke. Weinberger3 estimates that the incidence of stroke could possibly be reduced by as much as 50% with improved attention to risk factor modification and identification of direct predisposing conditions such as atrial fibrillation and carotid artery stenosis (CAS). However, Hart et al,4 in a systematic review of 22 published guidelines for prevention of ischemic stroke, found that current stroke prevention guidelines do not provide adequate methodologic information to permit assessment of their quality and clinical applicability. Management recommendations in these reports varies widely, particularly with regard to the issue of carotid endarterectomy (CEA) to treat asymptomatic stenosis.4

Two additional important factors that enter this equation are cost-effectiveness and reimbursement. Widespread screening of neurologically asymptomatic patients for occult carotid artery disease has not been performed, generally because of the cost. Carotid duplex ultrasound scanning is time-intensive and requires use of a trained vascular technologist and expensive equipment. Cost analyses have varied extensively in their assumptions, assessments, and conclusions regarding screening for and treating asymptomatic carotid artery disease in a variety of patient populations.5, 6, 7, 8, 9, 10, 11, 12 Because widespread screening in the general population is generally thought to be of low value and not cost-effective, the cost of examinations performed to screen for occult CAS currently is not reimbursed by Medicare. However, as Zwolak13 has stated, “The ultrasound studies performed to evaluate vascular disease today meet all expectations for safety and accuracy, and cost thousands of dollars less than their contrast counterparts, yet few sectors in medicine have been challenged so regularly by the Health Care Financing Administration.” Several authors have attempted to develop more rapid and cost-effective protocols for carotid duplex ultrasound scanning to address this problem.14, 15

With these issues in mind, the objectives of this study were to develop a cost-effective stroke screening program, to determine the prevalence of potential causes of stroke in this population, and to define a population at high risk in which screening would be most effective.

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Patients and methods 

Screening protocol 

A community-based stroke prevention screening protocol was developed at New York University Medical Center. The screening was advertised in flyers and on billboards in the immediate area of our hospital, and was additionally advertised in several local community newspapers. Screening was free to subjects. Subjects were eligible for screening if they were at least 60 years old and had at least one of the following risk factors: history of cardiac disease, diagnosed hypertension, history of smoking, and family history of stroke. Subjects could have no history of transient ischemic attack, stroke, or carotid artery disease. Medical history and medications were prospectively recorded. Patients were considered to have hypercholesterolemia if they had a history of elevated cholesterol concentrations or were taking a lipid-lowering medication. Patients were considered to have coronary artery disease if they had a history of coronary artery disease, angina, or myocardial infarction or if they had undergone coronary artery bypass grafting or coronary artery angioplasty. Patients were considered to have other cardiac disease if they had a history of or had undergone an intervention for congestive heart failure, valvular heart disease, or cardiac arrhythmia or had a pacemaker or defibrillator implanted.

Once screened, each subject underwent modified duplex scanning by a registered vascular technician, electrocardiography, and blood pressure measurement. The protocol for our modified duplex scanning and its validation study have been published.16 Stenosis of 50% or greater was defined by peak systolic velocity greater than 125 cm/s. The finding of carotid artery occlusion was considered to be a positive result as far as screening was concerned. Experienced registered vascular technicians in a laboratory accredited by the Intersocietal Commission for the Accreditation of Vascular Laboratories performed the modified carotid duplex scanning.16

Screening tests were free to patients, and were available on several days between June 2001 and September 2002. The New York University School of Medicine Institutional Review Board approved the study, and all patients signed informed consent forms.

Statistical analysis 

Data were maintained in a computerized database, and were analyzed with SPSS statistical software (SPSS Inc, Chicago, Ill). Statistical comparisons were performed with the two-tailed Student t test for comparison of continuous variables, and either χ² analysis or the Fisher exact test for comparison of discrete variables. Multivariate analysis was performed with binary logistic regression. P ≤ .05 was considered statistically significant. Variables that reached significance of 0.2 or less at univariate analysis were entered into the multivariate model.

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Results 

Screening was performed in 610 patients. Unilateral or bilateral CAS was detected in 66 patients (10.8%). The finding of occult CAS was more prevalent than that of new hypertension (2.6%) or new atrial fibrillation (0.5%).

Patient demographic data 

Demographic data for the 610 patients who underwent screening are presented in Table I.

Table I. Demographic data for patient population

Overall screening results 

Of 610 patients who underwent screening, results were positive in 82 patients (13.4%), indicating newly diagnosed atrial fibrillation, hypertension, or CAS. The relationships between various demographic factors and positive screening test results are given in Table II. Patients with heart disease were significantly more likely to have positive screening results than were patients without heart disease (19.4% vs 11.1%; P = .007).

Table II. Relationship between patient variables and positive screening test results

CAS screening results 

Of 610 patients who underwent screening, 66 patients (10.8%) had newly diagnosed unilateral or bilateral CAS. The relationship between various demographic factors and the finding of occult CAS are shwon in Table III. Patients with hypertension were significantly more likely to have positive carotid screening results than were those without hypertension (12.7% vs 7.8%; P = .05). Patients with heart disease were significantly more likely to have occult CAS than were those without heart disease (18.2% vs 8.0%; P < .001). Patients with both hypertension and heart disease (n = 104) were significantly more likely to have occult CAS than were those without either of these risk factors (22.1% vs 8.5%; P < .001). There was no relationship between systolic or diastolic blood pressure measured at the screening and the finding of occult CAS (Table III).

Table III. Relationship between patient variables and occult carotid artery stenosis

CAS, Carotid artery stenosis.

Of the 66 patients with newly diagnosed unilateral or bilateral CAS, 57 patients (86.4%) underwent follow-up formal carotid duplex scanning. One carotid occlusion was identified; severity of the remainder of the lesions was in the range of 50% to 79% with University of Washington criteria.

Relationship between type of heart disease and CAS among screened patients 

Of 170 patients with known heart disease, coronary artery disease was present in 105 patients (61.8%), atrial fibrillation in 30 patients (17.6%), other arrhythmias or pacemaker insertion in 26 patients (15.3%), and congestive heart failure or valvular heart disease in 9 patients (5.3%). Although a history of coronary artery disease in and of itself was predictive of CAS at univariate analysis (17.9% vs 9.3%; P = .01), there was no particular relationship overall between type of heart disease and presence of CAS (Fig 1). At multivariate analysis, coronary artery disease was not a significant independent predictor of CAS, whereas heart disease was.

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• Fig 1. 

  Relationship between types of heart disease and occult CAS among screened patients (n = 170). Gray bars, Number of patients; black diamonds, percent of patients with CAS. CAD, Coronary artery disease; CAS, carotid artery stenosis.

Relationship between risk factors and presence of bilateral carotid artery disease 

Of the 610 patients screened, 17 patients (2.8%) had bilateral CAS. An analysis was performed to determine the relationship of various demographic risk factors and presence of bilateral carotid artery disease (Fig 2). Patients with occult bilateral CAS had an increased incidence of atherosclerotic risk factors compared with patients with unilateral occult CAS and patients without CAS. For heart disease, 3 × 2 contingency table analysis was statistically significant, at P = .001.

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• Fig 2. 

  Comparison of prevalence of demographic risk factors in patients without CAS (black bars), with unilateral CAS (light gray bars), and with bilateral CAS (dark gray bars). CAS, Carotid artery stenosis; Hi chol, hypercholesterolemia; Htn, hypertension.

Multivariate analysis 

To determine the significant independent predictors of occult CAS, multivariate analysis with a binary logistic regression model was performed. All variables that approached statistical significance at univariate analysis with a P value ≤ 0.2 were included in the multivariate analysis. Significant independent predictors of CAS included heart disease, current cigarette smoking, and hypertension (Table IV). Although a family history of stroke appeared to have a negative association with occult CAS at univariate analysis, this factor was not significant at multivariate analysis.

Table IV. Significant independent predictors of carotid artery stenosis at multivariate analysis

Screening costs 

Direct costs of the screening included personnel (nurses, vascular technologists, electrocardiographic technicians), printing (pamphlets, posters, stationary), advertising, postage, and supplies. Equipment and space costs were not included. The number of patients screened and the direct costs of screening per patient for three representative screening sessions is shown in Fig 3. As the program developed, efficiency improved such that more patients could be screened in the same amount of time. This and other factors resulted in a reduced direct cost of screening per patient as the program progressed. Mean direct screening cost per patient over these three representative sessions was $75.

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• Fig 3. 

  Direct costs per patient (black squares) per screening session. Gray bars indicate number of patients screened.

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Discussion 

To prevent stroke, some type of screening or assessment is required to identify patients who may be at risk for stroke. It is estimated that 20% to 75% of strokes in the United States are caused by cervical carotid artery disease, with the remainder caused by atrial fibrillation and hypertension.14, 32 These three immediate causes of stroke often do not produce initial premonitory symptoms; in addition, appropriate early management of these conditions can prevent future strokes.14 However, there remains considerable controversy as to which patients are appropriate for screening and how the evaluations should be performed. The major unresolved issues include defining an appropriate population for screening and achieving cost-effective screening. It is clear that if a population with an increased prevalence of these diseases could be defined, and the costs of screening could be reduced, ultimately cost-effectiveness would improve.

The epidemiology of carotid atherosclerosis has been extensively studied. In a subset of the Framingham Study17 involving 1116 patients, multivariate logistic regression revealed that age, cigarette smoking, systolic blood pressure, and cholesterol levels were independently related to carotid atherosclerosis. However, the prevalence of significant CAS in the general population was low, approximately 8%. In the Tromso Study18 an analysis of 6727 patients who were screened for CAS reached similar conclusions as in the Framingham Study, but additionally found that male gender was an independent predictor. In a study of risk factors associated with internal carotid artery occlusion, Bogousslavsky et al19 found that smoking, family history of stroke, and obesity were more prevalent than in age-matched and sex-matched patients without cerebrovascular disease. Lernfelt et al20 found that bilateral carotid atherosclerosis was correlated with systolic blood pressure and ischemic heart disease, and predicted the risk for stroke and mortality in men. Finally, the SMART study21 found that the prevalence of CAS increased as much as 50% in patients with peripheral arterial disease. It is apparent from these studies that traditional atherosclerotic risk factors are clearly associated with development of carotid atherosclerotic disease.

Screening for carotid artery disease has been examined widely as well. A variety of populations have been selected for screening purposes. Cheng et al22 found that routine carotid screening is worthwhile in elderly men with peripheral vascular disease. A number of other reports have defined additional appropriate screening populations, including all patients older than 60 years undergoing open heart surgery,23, 24 all patients older than 65 years referred to a vascular surgeon,25 all male patients with symptomatic lower extremity atherosclerosis,26 elderly patients with peripheral vascular disease with atherosclerotic risk factors,27 and patients with ankle-brachial index less than 0.7.28 However, recent extensive data from the SMART Study29 suggest that, although carotid screening is worthwhile in patients with known peripheral arterial disease or abdominal aortic aneurysm, it cannot be endorsed for patients who merely have risk factors for atherosclerosis alone. Finally, one study30 suggested that investigating for CAS in subjects without symptoms may actually be harmful; these authors believe the risks of angiography and subsequent CEA essentially outweigh the benefits of diagnosing asymptomatic CAS.

The cost-effectiveness of screening remains an unavoidable issue. Holloway et al,9 in a systematic review of cost-effectiveness research of stroke evaluation and treatment, found that the reports that addressed the cost-effectiveness of screening for asymptomatic CAS reached strikingly divergent conclusions, from extremely cost-effective to exceedingly detrimental. Yin and Carpenter12 found that screening for asymptomatic CAS was cost-effective when the disease prevalence was 4.5% or more, specificity of the test was 91% or more, stroke rate in medically treated patients was 3.3% or more, relative risk reduction of surgery was 37% or more, stroke rate associated with surgery was 160% or less than that of the North American Symptomatic Carotid Endarterterectomy Trial31 or the Asymptomatic Carotid Atherosclerosis Study,32 and cost of ultrasound screening was $300 or less. Derdeyn et al8 also found that a one-time screening program in an asymptomatic population with a high prevalence of CAS may be cost-effective. The cost-effectiveness of CEA itself has been examined, and found favorable in most circumstances.7, 10

Carotid duplex ultrasound scanning is arguably time-intensive, and requires considerable technical expertise and specialized equipment. When these examinations are performed as a screening test for occult CAS, they are currently not reimbursed by Medicare. Authors examining this issue have reported that the Health Care Financing Administration has substantially undervalued the costs of performing duplex scanning33 and that Medicare denial of noninvasive vascular diagnostic tests can lead to uncompensated physician and hospital technical fees.34 Passman et al34 also found that during one year in a hospital-based vascular laboratory 408 carotid duplex scanning examinations were performed. Of these 408 tests, 19.6% were denied by Medicare; however, 72.5% of these same scans demonstrated abnormal results.34 Clearly some sort of reevaluation or reform is required. The performance of abbreviated duplex scanning as a screening test, with more formal scanning reserved for patients with 50% or greater stenosis, may be a more cost-effective strategy.14, 15, 16

The current report differs from many of the cited reports on carotid screening in several ways. Most of the other “screening”-type studies reported in the literature were performed in patients with known peripheral vascular disease or who were already referred to a vascular surgeon. The goal of this study was primarily to prospectively identify an unselected patient population that would benefit from stroke screening. To make screening both efficient and cost-effective, a population with a high prevalence of disease must be defined. We deliberately did not choose patients referred to our practices, because we believed this would bias patients toward a higher prevalence of carotid artery disease. The patients came from the community, and were chosen on the basis of risk factors alone. We believe this information is crucial to eventually justify Medicare reimbursement of such screening. Certainly a prevalence of 20% in subsets of our screening population is a higher prevalence of disease than would be expected with screening mammography or Papanicolaou testing. Finally, our strategy was intended only as a stroke prevention screening program, whereas the recent national screening program of the American Vascular Association screened for all types of peripheral arterial occlusive disease, including CAS, abdominal aortic aneurysm, and lower extremity arterial occlusive disease.

The most common treatable cause of stroke discovered in this selected screening population was occult CAS. This, then, is presumed to be the most crucial area in which there is potential for aggressive intervention to prevent future stroke. Hypertension and atrial fibrillation are more likely to be noted in a routine visit to a primary care practitioner, whereas CAS cannot be definitively diagnosed at physical examination alone. Our abbreviated duplex scanning enabled diagnosis of stenosis of 50% or greater, and certainly most of these patients are not immediate candidates for carotid artery intervention. However, natural history studies have shown that in many patients with a moderate degree of CAS the disease will progress to severe stenosis or become symptomatic within a relatively short time.35, 36 Certainly the diagnosis of occult CAS, even if moderate, should prompt more vigorous risk factor modification and perhaps use of anti-platelet medications.

Second, a high-risk population has been clearly defined. Patients with any history of medications or interventions for cardiac disease were significantly more likely to have both a positive screening test overall, as well as to have occult CAS. Patients with both cardiac disease and hypertension represent an extremely high-risk cohort of subjects with regard to occult CAS. In a typical cardiology practice, one can imagine that at least 75% of patients would fit this category. Additional independent risk factors for occult CAS at multivariate analysis included current cigarette smoking and hypertension, factors that have been noted in previous comparable epidemiologic studies. Similar to the results in the current report, the Western New York Stroke Screening Program37 found that significant variables associated with CAS include age greater than 65 years, current smoking, coronary artery disease, and hypercholesterolemia. Although family history of stroke appeared to be negatively associated with CAS, we believe this may be an inaccurate variable; family history of stroke was not a significant independent predictor at multivariate analysis.

Screening a population with a high prevalence of disease will lead to both more effective and more efficient screening processes. Finally, this screening program has successfully decreased the time required to test for the three most common causes of stroke; therefore it can be presumed that the cost should be decreased as well. Increased efficiency on the part of our staff, nurses, and vascular technologists with abbreviated scanning has further reduced the time and expenditure required in the most recent screening sessions.

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Conclusions 

In a screening program for treatable causes of potential stroke, CAS was the most commonly diagnosed entity. More than one of every five patients within known hypertension and heart disease had occult CAS. Known heart disease of any type was a significant independent predictor of occult CAS, as were current cigarette smoking and hypertension. This information could help to further target populations to screen for occult CAS and to justify reimbursement for screening carotid duplex scanning examinations. If it is assumed that CEA and other stroke prevention measures are ultimately both beneficial and cost-effective, screening for treatable causes of potential stroke can also be cost-effective.

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