Chapter 3
The Carotid and Vertebral Arteries; Transcranial Colour Doppler
The Carotid and Vertebral Arteries
CAROTID DISEASE AND STROKE
Stroke is a major cause of morbidity and mortality. In the United Kingdom, there are approximately 150 000 first strokes each year and some 53 000 deaths a year as a result of stroke; in the United States the equivalent figures are 795 000 strokes and 137 000 deaths each year. There are significant direct and indirect costs for society, in addition to the impact on the individuals and their families.1
ECST & NASCET
Two major trials have shown that endarterectomy for symptomatic patients with significant stenoses confers a significant advantage over medical management in terms of reducing morbidity and mortality.1,2 The European Carotid Surgery Trial (ECST)2 data showed that, during the follow-up period, there was an 18.6% reduction in the risk of ipsilateral major stroke for the surgical patients with stenoses > 80% diameter reduction. The equivalent figure for the North American Symptomatic Carotid Endarterectomy Trial (NASCET)3 study was a 17% reduction in the risk of ipsilateral stroke if surgery was undertaken in patients with > 70% diameter reduction. The risk of surgically related death or stroke was 7% in ECST and 5.4% in NASCET and because of this small but real risk, endarterectomy for lesser degrees of stenosis, or in asymptomatic patients with stenosis, needs to be considered more carefully.
The two major trials used different methods for assessing the degree of carotid stenosis on arteriograms (Fig. 3-1), which resulted in a stenosis measured at 80% diameter reduction in ECST corresponding to a 50% diameter reduction by NASCET measurements. The ECST data were therefore reviewed using the NASCET criteria and this showed a similar 18.7% risk reduction in patients with a stenosis of 70–99% diameter reduction using the NASCET criteria.4
For symptomatic patients with less severe degrees of stenosis, endarterectomy is of marginal benefit, conferring an absolute risk reduction of 4.6% for patients with a 50–70% stenosis (NASCET) but no benefit for lesser degrees of stenosis.5,6 For patients with asymptomatic stenoses > 60% (NASCET), the absolute risk reduction from endarterectomy was only 3% over 3 years.7
The American Academy of Neurology reviewed the evidence from the various trials and has provided recommendations for the management of patients with carotid stenosis8 (Box 3-1).
INDICATIONS FOR CAROTID ULTRASOUND
Ultrasound of the extracranial cerebral circulation is used predominantly in the assessment of patients with symptoms which might arise from disease in the carotid arteries, such as amaurosis fugax and transient ischaemic attacks (TIA), in order to identify those patients with significant carotid stenosis who will benefit from surgery. There are also other indications for which ultrasound of the neck vessels is of value and the main indications for ultrasound of the carotids are shown in Box 3-2.
Cerebral Ischaemic Symptoms
1. Those without significant disease.
2. Those with mild disease (< 50% diameter reduction), who will benefit from medical therapy if they are symptomatic.
3. Those with more severe disease (50–70% diameter stenosis), who will be treated medically and may be followed to assess progression of disease, particularly if they are symptomatic.
4. Those patients with severe disease (> 70% diameter reduction) who will benefit from surgery if they are symptomatic.
5. Those patients with a complete occlusion, who are therefore not candidates for surgery.
The relationship between the presence of carotid artery disease and the development of cerebral ischaemic symptoms is not straightforward and detailed discussion of this subject is beyond the scope of this book. However, patients who have suffered from temporary ischaemic symptoms, such as TIA, reversible ischaemic neurological deficits, or amaurosis fugax, are significantly more likely to suffer a stroke than asymptomatic subjects: 36% of patients who have a TIA will have an infarct within 5 years of the TIA, compared with an annual stroke rate of 1% for asymptomatic, elderly individuals.9 Therefore it is reasonable to investigate patients with reversible ischaemic cerebral symptoms in order to identify those with a 70% or greater stenosis who will benefit from endarterectomy. Those with lesser degrees of stenosis can be treated medically and followed up; those who progress to more than 70% diameter stenosis can then be considered for surgery.
The situation regarding the examination of patients with asymptomatic carotid bruits is also complex. The authors, along with many people, would wish to know the status of their arteries if they were found to have an asymptomatic carotid bruit. However, a Cochrane Review of surgery for asymptomatic stenosis7 concluded that ‘for most people with a narrowing of the carotid artery which is not causing any symptoms a surgical operation carries a risk and has little benefit’. The review found that the absolute risk reduction from surgery on patients with > 60% stenosis (NASCET) was only 1% a year, for centres with a surgical complication rate of < 3%. More recent studies have concluded that as ‘best medical therapy’ continues to improve, surgery for asymptomatic carotid stenosis is no longer indicated.10,11 Ultrasound will therefore have a role in the identification of those patients who might be considered for endarterectomy, in those centres which offer surgery. However, if there is a policy not to offer surgery to asymptomatic patients, then it might be argued that an ultrasound examination is unnecessary.
Patients at Risk of Perioperative Stroke
Arterial disease is usually a generalised process, although it affects different arterial territories to varying degrees. Therefore, patients undergoing surgery for conditions such as coronary artery disease, peripheral arterial disease and aortic aneurysms may also have significant carotid disease; there is concern that perioperative morbidity from strokes can be increased in these patients as a result of emboli or inadequate perfusion. Diabetics can also have severe arterial disease and are at risk from perioperative strokes when undergoing major surgery. A review of carotid artery disease and stroke during coronary artery bypass surgery12 showed that patients without carotid disease had a < 2% stroke rate and this only rose to 3% in patients with an asymptomatic stenosis > 50%; it also drew attention to the role of aortic arch disease in the aetiology of strokes in coronary artery bypass graft patients. For patients with symptoms, ultrasound assessment of the carotids is valuable to allow decisions on modification to surgical bypass technique, or whether carotid endarterectomy should also be considered in addition to the surgery for the primary condition. However, this decision would depend on the relative urgency of the primary condition and many centres, whilst taking note of the carotid disease, will proceed with the main operation and consider subsequent endarterectomy in symptomatic patients.
Postendarterectomy Patients
1. Early occlusion due to thrombosis, occurring within the first 24–48 hours after the operation.
2. Stenosis developing over 12–18 months due to neointimal hyperplasia.
3. Recurrence of atheroma over a period of several years, resulting in restenosis.
Colour Doppler ultrasound provides a rapid and straightforward method for diagnosis of these complications.
Routine follow-up of asymptomatic patients is not justified by the pick-up rate for developing significant recurrent stenoses,13 but any patient suffering symptoms related to the operated side should be examined by colour Doppler in the first instance.
Pulsatile Masses
Colour Doppler ultrasound provides a rapid technique for the assessment of pulsatile neck lumps. There is a variety of causes for these; the main ones are listed in Box 3-3.
Carotid Dissection
Dissection of the carotid artery may develop from a variety of causes.14
1. It may occur spontaneously, usually consequent upon atheromatous change.
2. It may result from the extension of an aortic arch dissection.
3. It may develop following trauma to the neck, such as occurs in whiplash injuries.
4. As a result of iatrogenic causes, such as carotid catheterisation.
Colour Doppler can be used to identify different flow patterns on either side of the flap, or the presence of a thrombosed channel, and monitor subsequent progress.
Epidemiological Studies and Monitoring of Therapy
The carotids provide a convenient window for the assessment of the whole arterial system. Patterns of development of atheroma vary in different arterial circulations. It was initially thought that changes in the carotids might allow some prediction of severity of disease in other vessels, such as the coronary arteries; in addition, their examination could also provide a method for assessing the rate of progression, or regression of disease, if treatment regimens were being investigated, or epidemiological studies were being performed. However, more recent consideration suggests that the value of using carotid disease as a marker of atherosclerosis lies more in refining the subject’s classification based on other risk factors.15,16
ANATOMY AND SCANNING TECHNIQUE
The main steps in the examination are given in Box 3-4. The patient lies supine, with their neck a little extended by placing a pillow under their shoulders. The patient should be comfortable and excessive extension of the neck should be avoided. In addition, some patients with carotid or veretebral disease may find that neck extension compromises the flow of blood to the cerebral circulation, so if the patient appears to be asleep it is worth checking that they have not lost consciousness. Some patients may not be able to lie supine; if this is the case they can usually be examined adequately in a sitting position.
A high-frequency transducer (7–14 MHz) is used17 and the examination starts with a transverse scan of the carotid artery from as low in the neck as possible, to as high in the neck as possible behind the angle of the mandible. This approach will allow the depth and course of the vessels to be ascertained, together with the level of the bifurcation and the orientation of its branches (Fig. 3-2). In addition, areas of major disease will be identified and can be noted for further assessment.
FIGURE 3-2 The left carotid bifurcation on transverse scanning from a lateral approach using power Doppler. The external carotid artery (ECA) and a small branch vessel lie anteriorly with the internal jugular vein (IJV) lying laterally, the internal carotid artery (ICA) lies behind the ECA.
Identification of the Internal and External Carotid Arteries
The common carotid artery on the right arises from the brachiocephalic artery behind the right sternoclavicular joint (Fig. 3-3), where the origin can usually be seen on ultrasound. On the left it usually arises directly from the aorta, so that its origin on the left cannot be seen on scanning from the neck. The level of the carotid bifurcation is usually at about the level of the upper border of the laryngeal cartilage but it may vary considerably. The two branches of the common carotid artery are the internal carotid artery and the external carotid artery. It is essential that they are identified positively, otherwise there is the possibility that disease in one vessel will be mistakenly attributed to the other, which may lead to further inappropriate investigations. The external carotid artery is usually the easier of the two branches at the bifurcation to recognise positively and the criteria to look for are listed in Box 3-5.
The external carotid artery has branches just above the bifurcation (Fig. 3-4); the superior thyroid, ascending pharyngeal and lingual arteries may all arise from the external carotid artery below, or around, the level of the angle of the mandible.
FIGURE 3-4 The external carotid artery showing a branch arising just above the bifurcation and fluctuations induced in the spectrum by tapping the superficial temporal artery at the level of the zygoma.
The external carotid artery is nearly always the more anterior of the two branches. In one study it lay anteromedial to the internal carotid artery in 48.5% of bifurcations studied, anterior in 34.5% and anterolateral in 13%; other positions accounted for only 4% of vessels.18
The external carotid artery supplies the relatively high-resistance vascular bed of the facial muscles, pharynx, tongue and scalp. Therefore the external carotid artery has relatively less diastolic flow, which makes it appear more pulsatile on colour Doppler and to have a characteristic waveform on spectral Doppler with relatively low diastolic flow (Fig. 3-5A). In addition the dichrotic notch of the pulse wave is usually more prominent in the external carotid artery spectrum than in the internal carotid artery spectrum.
FIGURE 3-5 (A) The external carotid artery and the characteristic spectrum seen in normal vessels. There is relatively low diastolic flow and a prominent dichrotic notch compared with the spectrum from the internal carotid artery. (B) The internal carotid artery and its characteristic spectrum with more flow throughout diastole. The normal area of reversed flow in the carotid bulb is clearly visible.
The superficial temporal artery is one of the terminal branches of the external carotid artery, and if this is tapped by a finger as it passes over the zygoma it will produce rapid, clear fluctuations in the waveform in the external carotid artery, whereas there is generally little or no effect in the ipsilateral common carotid artery or internal carotid artery (Fig. 3-4).
Once the external carotid artery has been identified positively then it can be assumed that the other large vessel arising from the carotid bifurcation is the internal carotid artery. This vessel is nearly always the more posterior of the two branches and tends to run deeply and more posteriorly. It does not have visible branches at this level but the bulge of the carotid bulb is usually apparent in subjects without severe disease. Colour Doppler will show the normal area of reversed flow in the carotid bulb, sometimes referred to as the boundary layer separation zone. The spectrum from the internal carotid artery is less pulsatile and more sustained than that of the external carotid artery, with relatively high diastolic flow (Fig. 3-5B).
Diseased vessels may be more difficult to distinguish as plaques can obscure visual details; local and remote disease can lead to alterations in the normal patterns of flow, so that distinction on the basis of the appearance of the waveform may be impossible; ‘internalisation’ of external carotid artery flow may be seen in patients with a severe stenosis or occlusion affecting the internal carotid artery and in whom external–internal collaterals around the meninges and cerebral circulation result in increased diastolic flow in the external carotid artery (Fig. 3-6). In addition some high bifurcations may be very difficult to see well enough to allow reliable assessment; in this situation scanning transversely with colour Doppler switched on may allow localisation of the internal and external carotid arteries, so that some spectral information can be acquired.
Standard Velocity Measurements
Once the bifurcation and its branches have been identified and assuming that no areas of significant disease are present, it is good practice to take peak systolic velocity measurements from the common carotid artery, the internal carotid artery and the external carotid artery in order to have a record of the examination. These are obtained using spectral Doppler from the upper common carotid artery 2–3 cm below the bifurcation; the internal carotid artery from 1 to 2 cm above the bulb, or as high as possible, in order to allow the normal bulbar turbulence to settle; and from the lower external carotid artery. For routine measurements the sample volume is set at about one-third of the total diameter and placed in the centre of the vessel in order to avoid the natural turbulence at the edge of the lumen and ‘wall thump’ from inclusion of the vessel wall in the sample volume. The Doppler angle is kept as low as possible, ideally in the range 45–60°; it is good practice to try to keep to a specific angle, such as 55° or 60°, in order to improve the reproducibility of results between examinations. For tight stenoses it is better to reduce the size of the sample volume, as this allows the area of the peak systolic flow signal to be better defined. Colour Doppler allows more accurate assessment of the direction of flow in a stenosis as this may not always be parallel to the walls of the vessel (Fig. 3-7). The precise final location of the sample volume is chosen using a combination of the audible Doppler signal and the spectrum so that the clearest, highest-frequency audible signal and the best spectral trace are obtained; in stenoses, the sample volume should be moved through the length of the stenotic segment in order to locate the peak signal. The gain for the spectral display is set to use the full grey scale without over- or under-saturation.
Measuring the Intima-Medial Thickness
This is not always required but should it need to be measured, for instance as part of a population survey, then it can be measured on an image of the distal common carotid wall where the echoes from the intima-media complex are most easily distinguished. The machine settings should be set to give a clear, uncluttered image of the vessel wall and the position of the transducer adjusted to show the characteristic double-line appearance of the vessel wall (Fig. 3-8). The image should be magnified as much as possible to make the measurement easier to perform. The intima-medial thickness (IMT) is best demonstrated in the upper common carotid artery on the posterior wall 1–2 cm below the bifurcation where the vessel is usually at right angles to the ultrasound beam. The internal carotid artery is more difficult to assess as the vessel slopes obliquely away from the transducer face in many cases. A minimum of three measurements over a 1 cm segment of the upper CCA are taken. Automated edge detection systems are available on some ultrasound systems but with careful attention to detail it is possible to measure the IMT manually with satisfactory, reproducible accuracy. The precise upper limit of the normal range is a matter of some discussion. IMT is known to vary with age, gender and race16 but values of less than 0.8 mm correlate well with lack of coronary artery disease, whereas an increasing thickness above this level is associated with increasingly severe coronary artery disease, an increased risk of myocardial infarction and also stroke.19
The Vertebral Arteries
Once both carotids have been examined, the vertebral arteries are assessed. The vertebral artery on each side is the first branch of the subclavian artery (Fig. 3-3). It passes posteriorly and upwards to the vertebral foramen in the transverse process of the sixth cervical vertebra (V1 segment) (Fig. 3-9), and from there it passes upwards in the vertebral canal to the level of the axis (C2) (V2 segment). It emerges from the vertebral canal at C2, passing behind the lateral mass of the atlas (C1) to enter the skull through the foramen magnum (V3 segment) and runs anterior to the brain stem (V4 segment) to join with the vessel from the other side in front of the brain stem to form the basilar artery.20
FIGURE 3-9 (A) The vertebral artery passing from its origin (V1 segment) to enter the vertebral canal and then running cranially (V2 segment) between the vertebral transverse processes (arrows). (B) The vertebral artery looping round the atlas (V3 segment).
There may be marked variation in the size of the vertebral arteries and their relative contribution to basilar artery flow; when there is disparity of size, the left artery is usually the larger of the two and in 7–10% of individuals there are significant segments of hypoplasia, which result in the artery not being visible.21 Clear visualisation of the vein but not the artery suggests that the artery may be either thrombosed or congenitally absent.
