Standards in noninvasive cerebrovascular testing

Transcription

1 Standards in noninvasive cerebrovascular testing Report from the Committee on Standards for Noninvasive Vascular Testing of the Joint Council of the Society for Vascular Surgery and the North American Chapter of the International Society for Cardiovascular Surgery. Brian L. Thiele, MB, BS, FRACS, FACS, Anne M. Jones, RN, BSN, RVT, Robert W. Hobson, MD, Dennis F. Bandyk, MD, William H. Baker, MD, David S. Sumner, MD, and Robert B. Rutherford, MD The Joint Council of the Society for Vascular Surgery and the International Society for Cardiovascular Surgery in response to a perceived need for standardization in the field of noninvasive vascular testing, established the Com~ttee on Standards to formulate guidelines in this field. As a result of the deliberations of this committee, it was decided that issues referable to standards to be addressed should include the following: the nomenclature to be used for description of test methods, the principles of test methodologies, the diagnostic criteria to be used, the advantages and disadvantages of each test method, and also the issue of appropriate use of the various test methods. In addition to providing specific information about these issues, it was also thought that several indirect benefits would occur. In light of the fact that training in vascular technology can now be obtained formally under the guidance of essentials developed by the Joint Review Committee on Cardiovascular Technology (JRC- CVT) within the framework of the American Council of Graduate Medical Education (ACGME), the publication of standards could serve as an important reference for this training. It was also envisaged that standards as developed by professional organizations would provide a model for establishing some uniformity in both state and federal reimbursement pro- From the Committee on Standards for Noninvasive Vascular Testing of the Joint Council of the Society for Vascular Surgery and the North American Chapter of the International Society for Cardiovascular Surgery. Reprint requests: Brian L. Thiele, MD, Department of Surgery, The Milton S. Hershey Medical Center, P.O. Box 850, Hershey, PA /1/31718 posals. A major problem encountered with the current lack of standardization is the difficulty of comparing results obtained from one institution to those obtained from another. The adoption of uniform reporting standards would do much to obviate this difficulty. With the adoption of standards in this field, the organization ofmulticenter studies to investigate specific clinical problems would be much easier, and finally, the issue of laboratory accreditation could be facilitated by a development of this type. The general principles involved in the formulation of standards were that only those tests that are widely used would be considered, and that an attempt would be made to provide major and minor diagnostic criteria. The initial and major area of testing to be discussed was that of cerebrovascular noninvasive testing, and it is this that serves as the topic for consideration in this report. GOALS FOR DIAGNOSIS The primary requirement of noninvasive tests developed for the diagnosis of disease capable of producing cerebral ischemia is that they should be able to detect all forms of pathologic change that have been implicated in the causation of cerebral ischemia. The primary focus, therefore, should be intramural disease at the carotid bifurcation, 1 particularly of the internal carotid artery but including the whole of the cervical carotid system. Although it is less important, the capability should also exist of evaluating the subclavian vertebral vessels. 2 In considering the pathologic features of disease of the internal carotid artery, those of major impor- 495

2 496 Thiele et al. lottrnal of VASCULAR SURGERY Table I. Disease classification Hemodynamic classification I = 0%-20% II = 20%-60% III= 60%-80% IV = 80%-99% V = Occluded Plaque characteristics P1 = Homogeneous Pe = Heterogeneous (Diameter Reduction) Normal to mild (Diameter Reduction) Moderate (Diameter Reduction) Severe (Diameter Reduction) Critical S 1 = Smooth S 2 = Defect < 2 mm depth S s = Defect > 2 mm depth tance include the severity of stenosis, 3 the surface characteristics of the lesion, 4 and the morphologic components of the plaque itself, s'6 In the other extracranial vessels, the severity of stenotic lesions appears to be the primary feature to be identified. 3 In addition to the actual pathology of the disease itself, it is important to consider the clinical background of the patient. For example, in asymptomatic patients, several studies implicate primarily the degree of stenosis, 7's with the issues of surface characteristics and plaque morphology being much more controversial. 9,1 In the symptomatic patient, the degree of stenosis and the surface characteristics of the lesion appear to be of equal importanc@ 1 whereas the presence of hemorrhage in particular appears to be a precursor of ulceration.12 With regard to the degree of stenosis, it is recognized that lesions from 20% diameter reduction up to occlusion are capable of producing symptoms and should, therefore, be detectable with the "ideal" test. In considering the features of ulceration or irregularity, large craters (>2mm) are thought to have a greater propensity for initiating embolic events than small craters ( < 2mm). 13 It was against this background of diagnostic requirements that the various aspects of standardization were considered. A major difficulty encountered in the reporting and interpretation of studies of carotid disease is the absence of a uniform reporting protocol that can be applied to all the major diagnostic modalities. For example, different methods are used to estimate the degree of stenosis as determined by arteriography and the various noninvasive diagnostic tests. It is proposed that percent diameter reduction should be used as the standard and should be determined by the mean value of measured luminal diameter expressed as a ratio of the diameter of the normal internal carotid artery immediately distal to the diseased area. It is recognized that this approach is a compromise because of inherent accuracies in determining the true degree of stenosis. Biplanar views, for example, may not accurately represent the true three-dimensional extent of disease. For lesions that are confined to the bulb region, considerable intramural disease may be present without reducing the cross-sectional area of the lumen of the distal internal carotid artery. Finally, although the method described above uses parameters that can be measured, variability is also likely to occur because of poor definition of the wall-flow interface, particularly when digital subtraction studies are used. In considering these issues it was recognized that B-mode ultrasonography may be capable of defining cross-sectional areas of a vessel at a specific location, but this appears to be of little practical value when no other diagnostic technique exists for useful comparison. Although the above methods of measurement ca. be used for measuring arteriogram and B-mode images, stenotic lesions of the carotid system are classified with the flow velocity information into a number of different relatively broad categories. It is clear that it would also be important to standardize these categories, if possible, into similar categories that would also be clinically useful. The suggested categories of stenosis are listed in Table I. The rationale for proposing this classification is that each of the categories can be fairly well identified with the major diagnostic tests, and it also provides clinically relevant grades of disease. The next area of attention is-related to determination of a useful classification for defining the surface characteristics of lesions that can be identified by arteriography and ultrasound imaging. It is well recognized now that neither arteriography nor ultrasonography is capable of accurately predicting the presence or location of ulceration. 14 It is, therefore, proposed that lesions be classified as smooth or irregular rather than ulcerated, with the latter being subdivided into minor ( < 2 mm) or major ( > 2 ram) (Table I). Finally, it was decided to address the issue of plaque morphology, which although not discemable with arteriography, could be interpreted from ultrasound images and may have clinical relevance, s'12 It is recommended that in view of studies performed to date, plaque characterization be confined to whether the plaque is homogeneous or heterogeneous, recognizing that further research may produce a more comprehensive and clinically appropriate classification (Table I). When collating an approach for the description of disease characteristics that appear to have the most clinical value as defined above, it is possible to classify carotid bifurcation disease in a manner similar to that

3 Volume 15 Number 3 March 1992 Standards in noninvasive cerebrovascular testing 497 used for the TNM classification of tumors. The hemodynamic characteristics of the lesion could be defined by H (stenosis), the surface character by S (smooth-irregular), and the plaque characterized by P (homogeneous-heterogeneous). This allows lesions to be classified in a manner that takes into consideration all of the clinically important features and establishes a comprehensive classification. For example, a lesion listed as P2, $2, H4 represents a lesion producing greater than 80% diameter reduction (H4), that is heterogeneous (P2), with surface irregularity < 2 mm ($2). If diagnostic modalities are used that are capable of defining only two of these features, the classification can still be used. With a clear definition of the disease characteristics to be identified, it becomes possible to address the issue of nomenclature and test features outlined earlier. NOMENCLATURE One of the basic considerations for the formulations of standards in any field is the necessity to recognize the need for the adoption of a uniform language or nomenclature particularly as it applies to the test methods. This, therefore, was a primary consideration of the committee with the assumption that this would facilitate comparing results. If this endeavor were to have the desired effect, it would also be necessary for the professional community to adopt these recommendations. The individual characteristics of each test alluded to in the introduction will be discussed within the framework of the suggested nomenclature. The testing methods used for the detection of carotid disease have traditionally been divided into direct tests that use instrumentation to examine the vessels of interest (i.e., the carotid bifurcation) for either anatomic or physiologic anomalies (Table II) and indirect tests that detect elevations of ocular or periorbital physiology. These two major categories can also be subdivided into primary and secondary tests. The primary test constitutes those that are considered to be appropriate for widespread use, whereas the secondary tests may bc used on occasions but are not considered sufficiently accurate for widespread use. Inherent in this classification, in addition to the nomenclature issue, is the desire to refine cerebrovascular testing so that a minimum number of tests are used to establish the diagnosis. PRIMARY DIRECT TESTS B-mode ultrasonography (Table III) Principle of tests. The principle of this technology is that when ultrasound is transmitted into Table II. Classification of testing procedures Direct tests Primary B-mode ultrasonography Duplex ultrasonography Color duplex ultrasonography Transcranial doppler ultrasonography Secondary Ultrasonic arteriography Carotid Doppler scan Quantitative phonoangiography Indirect tests Primary Oculopneumoplethysmography (OPPG) Secondary Periorbital Doppler Peri0rbital photoplethysmography Periorbital pulse-wave plethysmography tissues, it is absorbed, attenuated, and reflected depending on the density of tissue transversed in the transmitting frequency of the sound waves. Processing of the return signal enables a two-dimensional image of tissue to be produced, with the differences in tissue density being depicted with a gray scale display, is When imaging vessels the lumen is echofree and black, whereas the wall and surrounding tissue display varying degrees of echo density. When insonating the carotid vessels, an image is produced from which an interpretation of luminal diameter, wall thickness and composition, and surface characteristics of the plaque may be made. Test methodology. The test is performed by coupling the ultrasonic scanhead to the skin of the neck with acoustic gel. The entire length of the cervical portion of the carotid arteries should be scanned to obtain multiple plane views in both longitudinal and transverse axis. The views from at least two planes should be obtained from the common carotid and internal carotid arteries at the site of the disease in addition to the proximal external carotid arteries. Diagnostic biplanar views should be obtained in the region in which disease is considered to be most severe. The origins of the vertebral arteries should also be examined as should accessible portions of the subclavian arteries. Diagnostic criteria. Disease severity is determined primarily by evaluating the percent diameter stenosis as defined earlier. In addition, the surface characteristics of the lesion and the acoustic features of the visualized plaque can be determined. Currently, this latter process is performed subjectively with heterogeneous plaques being those in which major differences in acoustic density are identified. Occlusion of vessels may be inferred by the lack of

4 498 Thiele et al. Journal of VASCULAR SURGERY Table III. Performance characteristics of test procedures Test name Advantages Disadvantages Results B-mode ultrasonography Duplex ultrasonography Color duplex ultrasonography Ultrasonic arteriography Oculopneumoplethysmography OPPG Easy image visualization Valuable for normal or minimally diseased vessels Image and Doppler capability The standard test Image and Doppler capability Color flow image Rapid visualization of vessels Areas of interest rapidly identified Cheaper equipment Acceptable results in good hands Inexpensive equipment Good results for> 60% stenosis Difficulty with severe, critical lesions Spec 95% Difficulty with occlusions Sens 60%-80% Not individual test Long learning curve Knowledge of hemodynamics necessary Knowledge of Doppler necessary Detailed knowledge of Doppler required I(mowledge of hemodynamics necessary Expensive equipment Least easy to use Difficulty with anatomic variations Contraindicated with ocular pathology Inaccurate with bilateral disease Cannot differentiate occlusion Spec, Specificity; Sens, sensitivity; PPV, positive predictive value; NPV, negative predictive value. PPV 80%-90% NPV 50%-60% Spec 90% Sens 90% PPV 90% NPV 90% Spec 90% Sens 90% PPV 90% NPV 90% Spec 60% Sens 90% PPV 80% NPV 60% Spec 60% Sens 80% PPV 90% NPV 60% expanse or pulsation of the walls and/or longitudinal pulsation of the internal carotid artery. Advantages. The test can be performed quickly, and the instrumentation is simple to use, in large part because of the fact that it provides a visual image of the anatomy of the vessel being examined. Disease is generally readily identifiable, and the sonographic features of atherosclerotic plaque are readily appreciated. This technique is particularly useful when homogeneous plaque or minimal disease is present. Disadvantages. Although the vessel anatomy is readily defined, it may be difficult to distinguish between occluded and patent arteries in spite of attempts to identify the secondary characteristic of wall motility. Because the acoustic characteristics of plaque are variable, definition of the surface of the lesion may also be difficult and require considerable subjectivity. 13 This is likely to lead to both overestimation and underestimation of the severity of stenosis. This has been recognized as a problem in the identification of severe or critical lesions in a number of studies. 13 In addition to these particular features, technical difficulties may be encountered in performing the examination because of the size of the scanhead and the difficulty in identifying the more distal portions of the internal carotid arteries. Appropriate use. Although this technology was developed as a single technique, recent studies have raised the question of its usefulness for the detection of critical and severe stenosis) 6 It is, therefore, proposed that when used as a routine diagnostic investigation, it should be accompanied by OPPG examination. When used in this mode, it is useful for the evaluation of both symptomatic and asymptomatic patients. The ability of this technology (B-mode ultrasonography) to identify relatively minimal degrees of disease and its associated high specificity and negative predictive value make it a useful test for excluding the need for arteriography in certain groups of patients. Duplex ultrasonography (Table III) Principle of test. This technology uses combined imaging and Doppler velocity detection capabilities, thus providing information regarding the morphologic features of the vessel wall and rate of flow of blood in the lumen. Image information is displayed in gray scale format, whereas the Doppler flow information is usually processed with Fast- Fourier transform analysis and displayed in time frequency format with gray scale representation of amplitude. Color coding of the frequency content of the Doppler signal is used in some instruments. The diagnosis of severity of disease is made on the basis of changes in the Doppler shifted frequency information augmented with interpretation of the morphologic B-mode image. Test methodology. The examination with this technology is performed in a manner similar to that outlined for B-mode ultrasonography. All of the cervical portion of the carotid vessels should be examined to the bifurcation, and both image and Doppler information should be obtained. Hard copy Doppler spectral information should be obtained by sampling at the specific locations in the proximal common carotid artery, the internal carotid artery in

5 Volume 15 Number 3 March 1992 Standards in noninvasive cerebrovascular testing 499 the region immediately distal to the most severe disease where the most severely disturbed signal can be obtained, and from the external carotid artery immediately distal to disease. It is important when obtaining Doppler spectral information that angle be controlled, with the ideal ranges being from 55 degrees to 65 degrees. As with B-mode ultrasonography, the origin of the vertebral and accessible portions of the subclavian artery should also be examined and spectra obtained from these focal locations. In certain instances it may also be necessary to obtain signals in the intravertebral portions of the vertebral arteries. Diagnostic criteria. Duplex ultrasonography identifies disease primarily by interpretation of the Doppler wave spectra? 7 The criteria can be divided into primary diagnostic criteria and secondary diagnostic criteria. The primary diagnostic features include the peak systolic frequency or conversion to velocity, the end-diastolic frequency or velocity, a qualitative assessment of the magnitude and timing of spectral broadening, and the contour of the frequency envelopes. This latter feature is important with regard to whether or not frequency in the common carotid artery reaches the zero baseline. For lesions through the ranges of normal to severe, peak systolic frequency and spectral broadening are the most appropriate criteria, whereas for critical lesions, end-diastolic frequency is the most valuable) 8 With instrumentation providing continuous-wave Doppler, peak systolic frequency may be used for the diagnosis of critical lesions as well. It is important to be aware of the fact that interpretation of Doppler diagnostic criteria is dependent on many factors including the transmitting frequency, the angle at which sampling is performed, the sample volume size, the pulse repetition frequency, and whether continuous or pulsed ultrasonography is used. It is for this reason that diagnostic criteria should be established for each instrument model and not extrapolated from different instruments. Furthermore in reporting of results, these characteristics should be listed. Secondary diagnostic criteria include the use of frequency or velocity ratios ~9 such as that ofpourcelot pulsatility index and internal carotid artery peak systolic frequency or velocity expressed as a ratio of ipsilateral common carotid frequency or velocity.19'2 The exact role of the secondary criteria in the diagnosis of the carotid disease remains illdefined, and for that reason they are included as secondary diagnostic criteria. Advantages. This technology has enjoyed the most widespread acceptance of all the instruments currently available for the diagnosis of carotid disease. Rapid production of an image facilitates sub- sequent sampling with the Doppler instrumentation from which a definitive diagnosis of the degree of stenosis can be made. The categories of degree of stenosis should be those as listed earlier. This test in numerous centers has been documented as having both a high sensitivity and specificity with associated acceptable positive and negative predictive values. It is the current standard for ultrasonographic diagnosis of clinically significant carotid disease. Disadvantages. Interpretation of the Doppler spectral criteria requires a long learning experience, whereas high quality performance of the examination itself also requires considerable learned expertise. Difficulties are encountered in relating the physiologic changes associated with disease to the actual morphologic characteristics of disease. The instrumentation is expensive, although fairly reliable. Difficulties are still encountered on occasion in the diagnosis of internal carotid occlusion, 21 although difficulties may also be experienced in identifying a nondiseased carotid bifurcation. In this area it is important to evaluate the flow patterns in the region of the carotid bulb to determine whether or not flow separation is present. 22 Appropriate use. Duplex ultrasonography is ideally suited for evaluating both asymptomatic and symptomatic patients. Diagnostic information obtained from this instrumentation serves as a decision point for further therapeutic considerations such as arteriography or carotid endarterectomy. Color duplex ultrasonography (Table III) Principle of test. In addition to the technology described above for duplex ultrasonography, this instrument uses a large number of sampling sites to determine the backscattered frequency and to visually depict this information as a real-time flow image. This development has occurred because of advances in computer technology, which enables the rapid processing of large amounts of information. The instrument simultaneously analyzes Doppler information obtained from more than 300 small sampling sites in the zone of insonation. This frequency information is subprocessed and displayed in a color coded format rather than a gray scale format. The color depiction of the frequencies facilitates identification of focal areas of abnormal flow patterns. Test methodology. The test is performed in a manner similar to that described for duplex ultrasonography with the addition of a hard copy of the real-time color-flow image. No data are available to date regarding the accuracy of this instrument when the color coded-flow mapping system is used without the single sample volume generated spectral information. It is for this reason that, at this stage, no

6 500 Thiele et al. Journal of VASCULAR SURGERY specific diagnostic criteria for the color coded images are included. Advantages. This technology generally is simple to use by virtue of the fact that it provides a real-time anatomic and flow image of the vessels being examined. Although this information can be obtained fairly rapidly initially, additional time is required for determining the optimum location of the sample volume for discrete spectra. Disadvantages. The major drawback of this instrumentation currently is its high cost and the fact that prospective validation studies clearly defining the advantages of the color-flow component have not been reported. It is also apparent that a detailed knowledge of Doppler technology is essential for meaningful interpretation of the color images. Appropriate use. In the duplex ultrasound mode, the use of this instrumentation is similar to that described herein. Transcranial Doppler ultrasonography Principle of test. As with all Doppler scans, this test uses the ability of Doppler to detect direction of fl0w, and uses frequency:shifted data to identify focal areas of stenosis. This test in its early development was performed with a handheld probe, 23 but more recently a jig providing automatic identification of an XY axis enabling mapping has been developed and tested. Test methodology. Pulsed Doppler instrumentation of low transmitting frequency is used to obtain flow signals from the vessels in the region of the Circle of Willis by insonating through a temporal acoustic window, an orbital acoustic window, and/or the foramen magnum. The anterior cerebral artery, the middle cerebral artery, and the posterior cerebral artery can usually be insonated through the temporal window, as can the anterior communicating channels: The terminal portions of the vertebral artery and the proximal portion of the basilar artery can usually be identified by the foramen magnum window. Although pulsed Doppler is used, the small size of the vessels insonated compared to the sample volume size of the pulsed Doppler results in signals similar to those obtained from a continuous-wave Doppler when processed with a Spectrum analyzer. Diagnostic criteria. Diagnostic features include blood flow directionality, frequency amplitude, and frequency envelope contour, z4 Lack of knowledge of the exact angle of insonation of the vessels renders absolute values of either frequency or velocity of dubious significance, although very high values may be diagnostic of severe disease. Advantages. This technique allows direct eval- uation of the character and directionality of blood flow in the region of the Circle of Willis and provides some information about the resistance in this vascular bed from interpretation of the contour of the frequency envelope. This may be correlated with intrinsic disease or, more specifically, the vascular tone of the cerebrovascular bed. Disadvantages. Because the vessels being examined cannot be visualized, a considerable degree of operator skill is required for accurate determination of which vessel is being examined. In part, this has been overcome with the development and application of a jig apparatus that defines the XY axis of the Doppler probe enabling a flow map to be developed. Appropriate use. The major application of these studies relates to the determination of flow patterns in the region of the Circle of Willis, the presence oi siphon disease, and the presence or absence of vasospasm in the cerebral system. 25 Monitoring of cerebral blood flow during carotid surgery is also being performed with this method but its usefulness has yet to be determined? 6 SECONDARY DIRECT TESTS Ultrasonic arteriography (Table III) Principle of tests. This instrument uses a position-sensing arm and Doppler ultrasound scanhead to insonate tissue and obtain backscattered frequency information from multiple sampling sites in serial fashion. A threshold frequency for the backscattered signal is used, above which the information is accepted and displayed on an oscilloscope screen. By traversing the position-sensing probe over the entire area of the vessel, a flow image is gradually developed that defines the lumen of the vessel. 27 In some instruments this information was color coded. These insmunents also have the capability of analyzing discrete Doppler data obtained from a specific location by use of standard and Fast-Fourier transform analysis techniques. Diagnostic criteria. The initial primary diagnostic criteria were obtained by visual interpretation of the flow image, which depicted areas of disease as indentations in the flow image. Coupling of this instrumentation with a spectrum analyzer has provided diagnostic Doppler display data. The spectral criteria for diagnosis are similar to those used with duplex ultrasonography. Advantages. The instrumentation built for performing this test is relatively inexpensive. Disadvantages. The lack of a simultaneous anatomic image results in considerable difficulty being encountered in performance of the test. A very long learning period is necessary if acceptable results

7 Volume 15 Number 3 March 1992 Standards in noninvasive cerebr~ascular testing 501 are to be obtained. Patient movement interrupts the registration of the flow image and necessitates restarting the test. Absence of Doppler information obtained from the lumen deep to areas of calcification results also in false-positive studies. Identification of occluded arteries in particular is difficult, and knowledge of the location of the sample volume for obtaining Doppler information for diagnosis is lacking. Appropriate use. In the hands of highly experienced individuals, this test may be used for screening asymptomatic patients but is gradually being replaced by duplex instruments. PRIMARY INDIRECT TESTING METHODS.)culopneumoplethysmography (Gee) (Table III) Principle of test. A lesion in the common or internal carotid artery that produces a pressure gradient will be associated with concomitant reduction in the perfusion pressure of the ipsilateral ophthalmic artery in the absence of collaterals. This may be detected with pressure sensing apparatus applied to the globe. Increasing the negative pressure deforms the globe until blood flow ceases. The absence of arterial pulsations is detected by the apparatus, whereas rapid release of the vacuum is accompanied by restoration of pulsations. A conversion table is used to determine the absolute ocular systolic pressure. 28 Positive tests are usually produced by lesions equal or greater than 60% diameter stenosis. Test methodology. The test is performed with the patient supine or sitting with the eye cups attached to the lateral sclerae after installation of topical anesthetic. A vacuum pressure of 300 mm Hg can be applied for normotensive patients and 500 mm Hg for hypertensive patients. Rapid deflation is associated with return of pulsations that are recorded on hard copy, and pressure conversion is subsequently carried out. Diagnostic criteria. The interpretation of the test is based on (1) a comparison of the ocular systolic pressures (OSP) obtained from each eye, (2) a comparison of OSP to brachial systolic pressure (OSP-BSP), and (3) a comparison of the amplitude of pulsations from each side. Unilateral carotid stenosis greater than 60% diameter reduction uncompensated by collaterals is recognized by (1) a difference of 5 mm Hg or greater in the observed OSP and (2) both OSP-BSP ratios lying above a discriminant formular line. The side on which the lowest pressure is detected is the diseased side. If the ocular pressures differ by less than 5 mm Hg Table IV. Appropriate use Asymptomatic patients Evaluation of neck bruits 6-month evaluation for disease > 60% diameter reduction Symptomatic patients Evaluation of all symptomatic patients Selected instances in place of arteriography General guidelines Direct primary tests preferred Dual modalities appropriate if complementary (anatomic and physiologic) Single fee Quality control but one OSP-BSP ratio lies below the discriminant line, this side is abnormal. Bilateral severe carotid stenosis is detected by (1) no difference in the OSP but both ratios of OSP-BSP falling below the discriminant line, or (2) the OSP being unequal with both falling below the discriminant formular line. 29 Advantages. This test has been used widely for many years and is easy to learn, simple to perform, and provides reproducible objective data. It can also be performed rapidly with a minimum of discomfort. With regard to its performance characteristics, it has a high positive predictive value for unilateral stenosis in particular. Disadvantages. The disadvantages are related to the low positive predictive value in patients with bilateral severe disease and the inability to differentiate between severe stenosis, critical stenosis, and occlusion. Furthermore, the test is not capable of detecting disease progression and provides no informarion referable to the surface characteristics or intramural pathology of the atherosclerotic plaque. The test is contraindicated in patients with significant ocular disease or those who have undergone recent ophthalmic surgery. Difficulties are also experienced in severely hypertensive patients. Appropriate use. The test is most appropriately performed for the examination of patients with cervical bruits and also as an adjunctive test for B-mode imaging of the carotid bifurcation. In view of the information regarding the importance of disease progression and the differentiation between severe and critical lesions, this test is becoming less frequently used in asymptomatic patients. SECONDARY INDIRECT TESTS The secondary indirect tests include the periorbital Doppler, s ocular periorbital photoplethysmography, sl and pulse-wave delay oculoplethysmography. s2 In view of the rapid acceptance of the direct methods of testing for diagnosis of carotid disease,

8 502 Thiele et al. Journal of VASCULAR SURGERY these tests are gradually becoming relegated to relatively infrequent use and are, therefore, not discussed in detail. The major disadvantages of these tests are their insensitivity to bilateral disease, 33 the inability to differentiate between varying degrees of severe and critical stenosis, the inability to differentiate between stenosis and occlusion, and the lack of localization of disease when a positive test is obtained. They have served as important evolutionary tests in the field of noninvasive testing for carotid disease. GENERAL GUIDELINES FOR USE The diagnosis of carotid disease should be obtained with the tests that are most accurate and most capable of identifying the characteristics that are of clinical importance. For this reason the primary tests as outlined herein are considered to be the initial diagnostic approaches of choice for all patients with asymptomatic cervical bruits. When used in this manner the information so obtained may serve as a decision point for subsequent therapy. In patients in whom a nonoperative approach is chosen either because of contraindications to surgery or the disease itself, it is appropriate for these studies to be repeated at six monthly intervals in an attempt to identify disease progression. Direct tests only should be used for the evaluation of disease progression and should be instituted in patients who at the initial examination have degrees of stenosis at category III or greater (Table IV). If more than one test is to be used for the primary evaluation of a patient with a cervical bruit, then the tests should be complementary. An anatomic test, for example, may be combined with a physiologic test. An example of this would be the use of high resolution B-mode ultrasonography combined with OPPG. Dual anatomic or physiologic tests are not considered appropriate. In addition, even if dual tests are to be used, a single common fee for carotid testing should be required. In symptomatic patients it is considered appropriate for the initial evaluation to be a primary noninvasive diagnostic test to determine the need and/or type of arteriography. In certain patients under unusual circumstances, the primary diagnostic test may serve as the only preoperative investigation but clear documentation by virtue of hard copy diagnostic information should be retained. The availability of many different instruments with similar basic operating characteristics mandates that diagnostic criteria obtained with one instrument should not be extrapolated to another. It is important for each laboratory to establish the appropriate diagnostic criteria to be used unless it is using instrumentation exactly similar to that reported by other investigators. This will clearly necessitate periodic reviews of accuracy of information compared independently to arteriography. This latter feature is only a small component of the ongoing quality assurance activities that should be part of every noninvasive diagnostic laboratory. Correlation of noninvasive diagnostic findings with other diagnostic tests providing similar information should be an ongoing and routine part of the activities of these diagnostic facilities. REFERENCES 1. Whisnant JP, Martin MG, Sayre GP. Atherosclerotic stenosi~--, of cervical arteries-clinical significance. Arch Neurol 1961; 5: Fields WS, Lemak NA. Joint study of extracranial arterial occlusion VII. Subclavian steel- a review of 168 cases. JAMA 1972;222: Veda K, Took JF, McHenry LC. Carotid and vertebrobasilar transient ischemic attacks: clinical and angiographic correlation. Neurology 1979;29: Ehrenfeld WK, Hoyt WF, Wylie EJ. Embolization and transient blindness from carotid atheroma. Arch Surg 1966; 93: Lusby RJ, Ferrell LD, Ehrenfeld WK, Stoney RJ, Wylie EJ. 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