DOPPLER5 (1040793), страница 2
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A correct diagnosis was made in 82%. of cases, with a 90%. reliability in those cases of subclavian steal syndrome, missing vertebral artery, and normal findings. In five patients Doppler recordings showed one with subclavian steal, two of aplasia or occlusion, and two of stenoses, which were not confirmed by arteriography, Keller et al. conclude that the peroral route to the vertebral artery is more reliable than those in the region of the mastoid.
Post-operative follow-up examinations in patients operated on for subclavian steal, showed that in the 11 patients the original reverse Flow was reverted to flow into the skull. Recurrence of flow reversal occurred in two patients on the second and third post-operative day. The diagnosis was of rethrombosis of the vertebral artery which was confirmed by contrast angiography.
5.2 PERI-ORBITAL DOPPLER INVESTIGATIONS
Analysis of the blood-velocity/time waveforms from the carotid bifurcation has been discussed in the previous section. It is clear that there are difficulties in the assessment of carotid artery disease by assessment of the waveform shape alone. However, further information can be gained, particularly concerning the internal carotid artery, from peri-orbital Doppler signals. There has been much discussion as to the suitability of the ophthalmic, supraorbital and frontal arteries for the diagnosis of carotid disease. The evidence will be presented for each so that a decision can be reached as to which is the best vessel to study, in order to increase the accuracy of peri-orbital Doppler investigations.
The investigation of blood-velocity waveforms in the vicinity of the orbit seeks to establish the flow direction in particular vessels, and to study changes in flow direction due to manual compression of supply arteries. The orbit is particularly interesting in that it is the area of anastomosis between the terminal branches of the internal and external carotid arteries. The distal internal carotid artery becomes the ophthalmic artery which then branches to form the supraorbital artery and the frontal artery. Both of these anastomose with terminal branches of the external carotid in over 90% of the population. In the normal situation, flow is out of the orbit, because of the pressure gradient between the ophthalmic artery and the collateral bed. Brockenborough (1969) and Maroon et al. (1969, 1970) showed that if flow is monitored over one of these terminal branches of the internal carotid, manual compression of the ipsilateral superficial temporal artery produces an enhanced signal, in a direction out of the orbit, in the normal circulation. If the pressure in the internal carotid artery is reduced because of occlusive disease, there will be a flow from the higher pressure temporal artery through the collaterals to the supraorbital/frontal arteries. In this situation flow will be into the orbit. If manual compression of the ipsilateral superficial temporal artery is now applied, flow into the orbit will be decreased. This type of examination is known as the temporal artery occlusion test (TAOT). The positive response to the TAOT coupled with the reversal of flow direction is highly suggestive of the presence of a severe lesion of the internal carotid artery.
Muller (l971) investigated 40 patients with angiographically proven unilateral or bilateral internal carotid occlusions. In 85% of subjects it was found that there was reverse flow in the ipsilateral frontal artery. The remaining subjects, with one exception, showed a blood-velocity/time waveform oscillating about zero, which was assumed to be due to balanced internal/external carotid artery pressures. In one case where flow direction was out of the orbit it was found on arteriography that the ophthalmic artery originated from the middle meningeal artery. Muller stated that he had never seen flow into the orbit in a normal subject.
Kaneda et al. (I978) investigated both the ophthalmic artery and the frontal artery in 69 patent internal carotids and also showed no cases of retrograde flow in normals. Hodek-Demarin and Muller (1979) investigated flow direction in the frontal artery in acute and chronic cases of internal carotid artery occlusion. In 80%. of the acute cases, where the Doppler examination was made less than three weeks after the first symptoms, Flow was reversed, and in 20%. there was no detectable flow. In 62%. of the chronic cases flow was reversed 20% showed no Flow and 18% showed a normal flow direction, indicating the development of intracranial collaterals. These authors then showed that if the mean of the zero-crossing outputs from each common carotid are subtracted. and compared with the mean of the frontal artery waveform, there is a linear correlation, in both the acute and chronic groups. This indicates that patients with a small difference in mean velocity between the common carotid arteries, showed a higher, reversed frontal artery flow (p < 0.01).
Gosling and King ( 1978) applied the TAOT examination in 200 patients where a whole spectrum of atherosclerotic lesions from irregularities of the intima to total occlusion of the internal carotid was found. The test was positive in only 15%. of cases. Barnes et al. (1977) using the simple TAOT investigation found that in patients with angiographically proven lesions of greater than 50%. stenosis of the internal carotid, the result was positive in 64%. of cases.
From the results of the work quoted above it appears that the temporal artery occlusion test is quite good in detecting disease of the internal carotid artery when the stenosis is greater than 50%, but poor when the stenosis is less than 50%, The consensus of opinion is that it is better to monitor the blood flow signal from the frontal artery, just above the inner canthus with the probe pointing medially and cephalad. The frontal artery. in general, gives a stronger signal than the supraorbital artery. and if the latter vessel is used confusion can arise between it and the lateral palpetral artery. which is a branch of the superficial temporal artery.
Barnes et al. (l977) have developed an improved cerebro-vascular examination which dramatically improves the level of correct diagnosis. The prove is placed over the frontal artery as described above, and the examination consists of three phases, firstly, the determination of Flow direction in the frontal artery, secondly, manual compression of a11 branches of the external carotid artery, and thirdly, common carotid compression.
5.2a Frontal Artery Flow Direction
The direction of Flow is detected, which in the normal subject is out of the orbit. However, normal flow direction does not exclude significant internal carotid disease. It is possible that intracranial collateral circulations from the opposite carotid or vertebrobasilar system can supply the diseased vessel and produce Flow out of the orbit. According to Barnes et al. (1977) it is also possible for the normal Flow direction to be produced by an extra-cranial circulation. Stages 2 and 3 of Barnes method overcome these difficulties. 5.2b Compression of All External Carotid Branches This series of manoeuvres determines the source of any extra-cranial collaterals. Each of the superficial temporal, infraorbital and facial arteries on both sides of the head are compressed manually and sequentially. Consider the effect of these manual compressions of various vessels on the normal internal carotid. Compression of the ipsilateral superficial temporal artery causes augmentation of antegrade flow (out of the orbit). If the contralateral superficial temporal artery or both infraorbital arteries are compressed, there should be no change in the flow in the frontal artery. Bilateral compression of the facial arteries may augment the flow signal.
If a significant internal carotid lesion exists ( > 50%. stenosis) a similar compression procedure will determine the extra-cranial collaterals. Frontal artery flow will be retrograde if the major collateral supply is the superficial temporal artery. Compression of the superficial temporal artery will reduce, stop or reverse the flow. Compression of the facial and/or infraorbital vessels has no effect. It is possible. however, that either the ipsilateral or contralateral facial artery is the major collateral supply. In the case of contralateral facial artery collateral supply, the flow direction is antegrade because of blood being transported across the bridge of the nose by the dorsal nasal artery, into the ophthalmic artery and out via the frontal artery. Occlusion of the superficial temporal arteries will have no effect on retrograde flow but will enhance antegrade flow. Compression of the facial arteries on both sides will then determine whether the ipsilateral or contralateral facial arteries is the major source of supply. Collateral supply can also be via the ipsilateral infraorbital arteries. In this situation both superficial and temporal artery compression produce no effect but infraorbital compression obliterates the flow.
5.2c Common Carotid Compression
The previous section has dealt with the study of the possible extra-cranial collateral development in cases of occlusive disease of the internal carotid artery. It is possible, however, for the diseased internal carotid to be supplied via intra-cranial collaterals. If flow is retrograde in the frontal artery then the collaterals are extra-cranial; if flow is antegrade then common carotid compression distinguishes between the possible collateral pathways.
Normally, ipsilateral common carotid compression diminishes, stops or causes reversal in frontal artery flow. If frontal artery flow is unchanged or enhanced by ipsilateral common carotid occlusion then significant Internal carotid disease is present. If frontal artery flow is diminished or stopped by contralateral common carotid compression, then this vessel is supplying the blood via the circle of Willis to the opposite hemisphere. If frontal artery flow is unaffected or augmented by sequential compression of each common carotid then internal carotid artery obstruction is present with intracranial collateral supplied via the vertebrobasilar system.
In order to simplify this system, Barnes et al. have produced an algorithm, shown in Table 5.2. Their results using this system are .impressive, in that for diameter stenoses greater than 50% of the internal carotid artery, stenosis or occlusion was correctly identified in 98.7%
of cases, involving 150 vessels. The simple TAOT test, as mentioned previously, detected 64% of the known cases of arterial disease.
Lye (l978) produces some interesting figures concerning the frequency of occurrence of the various collateral pathways in internal carotid disease. The superficial temporal artery is the major collateral supply in 64%. of cases. The ipsilateral and contralateral facial arteries are 20%. and 2%., the ipsilateral infraorbital artery 7'/., and contralateral internal carotid 7%.
Barnes (1977) assessed the overall accuracy, sensitivity, and specificity of the modified compression techniques, when compared with contrast arteriography. He further assessed the predictive value in selecting or excluding patients for carotid endarterectomy. The overall accuracy is defined as the ratio of true positive and true negative tests divided by the total number of confirmed results by contrast arteriography. Sensitivity is defined as the percentage ratio of the true positive results divided by the sum of the true positive and false negative results. The specificity is defined as the percentage ratio of the true negative result divided by the sum of the true negative and false positive results. The predictive value of a positive non-invasive test result in predicting the need for carotid endarterectomy is defined as a percentage ratio of the number of true positive results associated with stenosis divided by the number of true positive results associated with stenosis or occlusion, plus the number of false positive diagnoses. The predictive value of a negative non-invasive study, in excluding patients for endarterectomy is defined as the percentage ratio of the number of true negative studies divided by the number of true negative plus the number of true negative plus the number of false negative results. Barnes shows that the overall accuracy of Doppler peri-orbital studies in detecting lesions > 50%. stenosis in the extra-cranial portion of the internal carotid artery is 98%.. The sensitivity in detecting significant carotid disease is 95%.. and the specificity in normal carotid bifurcations is 98%.. However, for lesions of less than 50%, the Doppler examination has been shown to be abnormal in only 9%. of cases. Including these minor stenoses together with the major stenoses, the Doppler examination is only 56%. sensitive in detecting carotid disease of potential clinical significance.
The predictive value of these peri-orbital Doppler techniques has been shown by Barnes, in the selection or exclusion of patients for carotid endarterectomy, to be 39%. In other words only 39%. of those presenting with abnormal Doppler signals are candidates for carotid endarterectomy. Conversely, more than one third of the vessels associated with a normal Doppler examination were suitable for an endarterectomy.
Otis et al. ( 1979) have studied the effectiveness of the TAOT examination for the evaluation of carotid endarterectomy and have found that in patients with a pre-operative positive test, there are persistently abnormal findings post-operatively in 46%. of patients. It is suggested that a reason for this is the persistence of the collateral circulation. Operative angiography and Doppler imaging showed that in all cases the post-operative images were all within normal limits.
Kaneda et al. (1978) have reported spontaneous recanalization of the internal carotid artery detected using a Doppler probe over the ophthalmic artery. Results were confirmed by arteriography.
5.3 DOPPLER IMAGING OF THE EXTRA-CRANIAL
CEREBRAL ARTERIES
It was shown in a previous section that Doppler scanners produce images of moving blood inside a blood vessel. It is important to realize at this stage that no surrounding anatomy is displayed. Doppler scanners can either operate in a continuous wave mode (Reid and Spencer, 1972) or in a pulsed mode (Mozerskey et al., 1971, Fish, 1971). An obvious advantage of a pulsed ultrasound, range-gated system is that Doppler images can be produced in three orthogonal directions, thus producing three-dimensional information about the internal lumen of the blood vessel, as illustrated in Fig. 5.5. The major clinical questions are (1) how good are Doppler imaging systems at imaging the extra-cranial carotid circulation, i.e., how do the results compare with arteriography in the assessment of all grades of severity of stenosis, and (2) are the pulsed and continuous wave systems of equal reliability in detecting the presence of occlusive disease?
5.3a Carotid Arteries
(i) Pulsed Doppler Imaging Systems
Barnes et al. (1976) used a 5 MHz system in comparing the results from 82 carotid arteries with contrast arteriograms. Ultrasonic and contrast carotid arteriograms were interpreted by independent observers. Each vessel was assessed for the presence of stenosis or complete occlusion. Stenoses were classified from 0-24%. diameter stenosis, 25-49%, 50-74%, and > 75%. Barnes et al. noted that the distal, extra-cranial internal carotid was some 33%. smaller in diameter than the junction of the internal carotid. The 0-24%. range was applied to any artery with a diameter equal to or greater than the distal Internal carotid. The variability of the normal dilatation of the carotid bifurcation and the limit of resolution and sensitivity of the ultrasonic imaging justified the classification of normal vessels with those with less than 25%. stenosis. The results are shown in Table 5.3a. The ultrasonic arteriogram correctly identified all 14 occluded internal carotid arteries. The estimation of the degree of occlusion by the two techniques agreed within the same group, in 51.5%. of cases, and was within one grade to the correct diagnosis in 70.6%. The overall agreement, stenoses and occlusions, was 60%, and to within one grade of the true assessment. 76%. In all but four of the diagnostic errors the ultrasonic arteriogram overestimated the degree of stenosis. Non-visualization of the vessel in these four cases was thought to be due to calcification of the vessel wall. In this study, five major ulcerating lesions were detected arteriographically but the ultrasonic technique did not visualize any.
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