FOTOPLETISMOGRAFÍA SUPRAORBITAL

La fotopletismografía supraorbital no es estrictamente una técnicaphotoplethysmography is not strictly a plethysmographic technique.(15) Rather, its operation is based on the principles of photodensitometry, using a near-infrared light-emitting diode (LED) light source and a phototransistor with a linear response over a wide range of intensities. The absorption of light by hemoglobin reduces the amount of backscatter to the receiver and is dependent on the volume of blood in the superficial dermal venous plexus.( 54,55) A flat probe that contains both an infrared light source and phototransistor receiver is attached to the patient's forehead. Qualitative information about the patency of the ICA can be elicited by various compression maneuvers of the ECA. Supraorbital photoplethysmography has been used in conjunction or in sequence with other noninvasive diagnostic methods such as OPG, CPA, and Doppler ultrasound, but is not used as a stand alone diagnostic test. Barnes et al(56) studied 156 ICAs in 78 consecutive patients "evaluated for cerebrovascular disease." They chose a very liberal definition of stenosis (>= 50) which accounted for the high sensitivity (100) and poor PPV (67). The authors claimed that in 10 of these 18 vessels, angiography underestimated the stenosis by 33 to 49, and, therefore, the SPPG results should not be considered false positives. Barnes later demonstrated that the predictive value of SPPG, validated by surgical specimens of carotid artery stenosis, was about 40.(56) Duke et al(57) studied 272 vessels with SPPG and carotid angiogram and found a similarly high false-positive rate (15). Magnetic Resonance Angiography Investigators are presently attempting to establish MRA as a reference method for the study of extracranial CAD (Table 8). In an independent technology assessment on the clinical utility of MRA for the determination of blood flow and vessel morphology, Handelsman et al(75) concluded that MRA has demonstrated accuracies comparable to CA, but CA remains the definitive preoperative study. This collection of studies, most of which were performed retrospectively in study populations "at risk or symptomatic for carotid artery disease," reveals that the reliability of OPG-K, OPG-G, and OPG-Z in broadly defined populations is extremely variable. The effectiveness of OPG in the detection of bilateral disease is dismal (Table 9). Even with the sequential use of CPA, as is the case with OPG-K, sensitivity does not significantly increase. It is also interesting to note that OPG attains its highest sensitivity when both the differential time delays of the eyes (or eye and ear) and the SOB index are used to define a positive test. Some earlier studies used only the pulse differential criteria and missed bilateral disease in particular. Is it possible to differentiate the clinical utility of one OPG device over another, even in well-described subgroups, i.e., asymptomatic patients with or without carotid bruits? Based on the published information to date, this question cannot be answered. Each OPG method is significantly less accurate than the "gold standard" of carotid angiogram, although OPG-G has been demonstrated to attain 100 specificity in small study populations (66-72 patients) in which the prevalence of disease is high (55 to 88). AbuRahma and Diethrich,(38) in a comparison of the three OPG methods, concluded that all OPG methods are "valuable," and the OPG-Gee was particularly useful in bilateral CAD. They reported overall accuracies of 88 to 95, and accuracy in bilateral disease ranged from 66 (OPG-Z) to 90 (OPG-G). They neglected to report the sensitivity and specificity of OPG in patients with bilateral disease and the number of those patients in each group. They failed to report any patient selection criteria, the comparability of patients among the three testing groups, and whether the investigators were blinded. The comparability of the OPG devices would have been better demonstrated had the test subjects been randomly selected patients who had undergone testing with all three OPG devices as well as with the same reference method. The prevalence of carotid disease in the sample populations was very high (range, 65 to 70), and the false-negative rates ranged from 9 to 23. When prevalence is very high in a study population, it is possible for a comparatively poor diagnostic test to have a very high PPV. For example, at a disease prevalence of 70, even a test with 60 sensitivity will have a PPV of 93. This study and many others that were published supporting the clinical utility of OPG demonstrate this mathematical relationship quite well (Tables 4, 5, and 7). The clinical utility of OPG has been particularly difficult to address for two reasons. First, recent innovations in imaging systems, such as DU, have not only replaced the reference method of carotid arteriography in many cases but have also become the initial diagnostic screening test for those patients believed to be at risk of stroke. Second, very few studies reported posttest outcomes in symptomatic patients. Even though the correlation of imaging and nonimaging systems may seem fraught with complexities, some authors did report comparisons of presurgical OPG and an imaging test (duplex, arteriography) to the ultimate "gold standard," the surgical specimen.(25) The accuracy of OPG was approximately 60 (severe stenosis on arteriogram = significant delay in eye-to-eye pulse wave forms and SOB pressure indices below .68 mm Hg). If stroke or neurologic impairment of vascular origin are the critical clinical endpoints to be avoided in symptomatic patients, a highly sensitive noninvasive test that indicates reduced flow or mechanical obstruction in carotid vessels is of great value. As previously noted from Table 8, sensitivity and yield (the number of true positives found in the sample population) for OPG-K, OPG-G, and OPG-Z were 72 and 31, 81 and 45, and 74 and 35, respectively. In reviewing additional clinical evidence to evaluate the utility of diagnostic tests in CAD, two controversies were identified with respect to imaging tests: the use of duplex imaging as a reference method and the most accurate method of estimating intraarterial stenosis.(77) Alexandrov et al,(25) in reaction to the belief that ultrasound methods have been inaccurate (when compared with cerebral angiography) in screening for carotid stenosis, prospectively analyzed 45 patients undergoing carotid endarterectomy from June 1992 to March 1993, using the North American Symptomatic Carotid Endarterectomy Trial (NASCET) and the European Carotid Surgery Trial (ECST) methods of measuring carotid stenosis and compared those results with direct visualization of the arteriogram ("eyeballing") and color duplex ultrasound. The reference method was the carotid artery plaque that was surgically removed during endarterectomy from 15 of the 45 patients. All patients had severe carotid stenosis and were symptomatic, having suffered either transient ischemic attacks or minor stroke (undefined in this population). Carotid angiography was performed by intraarterial digital subtraction technique (DSA) through the femoral artery, and biplanar images were obtained for all ICAs. Both the European and American carotid trials had previously developed linear methods to determine the degree of stenosis (1 - [the diameter of the residual lumen divided by an estimation of the "normal" vessel lumen] x 100). However, these authors recalculated carotid stenosis by angiography using the area luminal reduction, essentially the same formula but squaring the diameter measurements (1 - [d2/n2] x 100), where d equals the diameter of the residual lumen and n is the diameter of the normal vessel, and correlated the linear and area measurements. Peak systolic velocity was measured, and a conversion curve (linear to area stenosis) was used to calculate the degree of stenosis by duplex. The surgical plaque samples were preserved in formalin, photographed, decalified, and rephotographed, and planimetric tracings were taken of the narrowest lumen and the carotid bulb. Both area cross-sections were calculated by computer and percents of stenosis were derived (1 - [area of the residual volume divided by the area of the carotid bulb]). The mean percents of stenoses (± SD) by angiography were calculated: NASCET (linear) 63 ± 18, ECST (linear) 73 ± 16, eyeballing 76 ± 19, NASCET (area) 82 ± 17, ECST (area) 90 ± 12. These angiographic measurements were significantly different from duplex (P < .01) except for the NASCET area and eyeballing measurements. There was also a significant (P = .006) difference between NASCET and ECST measurements. For example, a patient estimated to have 55 stenosis calculated by the NASCET linear method would be classified as a 70 stenosis using the ECST linear, 78 by NASCET area, and 88 by ECST area measurements. Stenosis of 80 by DU correlated to an 80 stenosis by NASCET area. Having established these differences, they compared the planimetric measurements of the 15 surgical specimens to angiographic and ultrasonographic results and found that NASCET consistently underestimated ECST, and both underestimated the planar measurements of the surgical specimens, whereas DU and the two areal measurements were not significantly different from each other.