CAROTID ENDARTERECTOMY

Description

Carotid endarterectomy is performed through a neck incision along the medial aspect of the sternocleidomastoid muscle. The carotid bifurcation is the most common location of carotid occlusive disease, and control of the common, internal, and external carotid arteries must be obtained. An incision is made along the common carotid extending through the plaque to an area of nondiseased internal carotid artery. The plaque is then elevated from the adventitia along with the intima. Finally, the arteriotomy is closed with a synthetic or autologous patch, which decreases the risk of restenosis compared with primary closure.

Indications

Vascular surgeons are particularly stringent about the indications for carotid endarterectomy because this is one of the few prophylactic operations. Indications for carotid endarterectomy are based on two large, multicenter, randomized trials comparing best medical therapy (antiplatelet) and surgical therapy for carotid stenosis. The North American Symptomatic Carotid Endarterectomy Trial (NASCET) examined patients with a previous history of stroke, transient ischemic attack, or amaurosis fugax within 3 months of enrollment.¹ During 2 years, the risk of stroke in patients with a 70% to 99% stenosis was reduced from 26% to 9% with carotid endarterectomy. The risk of stroke in patients with a 50% to 69% stenosis was reduced from 22% to 16% during 5 years. Given the less substantial benefit seen with the moderate-grade lesions, most vascular surgeons would recommend carotid endarterectomy to symptomatic patients with 50% to 69% stenosis only if they have a substantial life expectancy and low risk of complications related to the surgical intervention. Finally, subgroup analysis revealed an increased risk of stroke in medically treated patients with contralateral carotid occlusions and ulcerated plaques.

The Asymptomatic Carotid Atherosclerosis Study (ACAS) randomized asymptomatic patients to medical and surgical therapy.² Carotid endarterectomy reduced the 5-year risk of stroke from 11% to 5% in patients with 60% to 99% stenosis compared with medical therapy. Given an absolute stroke risk reduction of 6% during 5 years, further studies have shown that surgical therapy is most beneficial in patients with 80% to 99% stenosis and in men compared with women. This equates to changing the outcome of approximately 1 in 20 to 30 patients, making the safety and risk reduction in the conduct of intervention for asymptomatic patients of paramount importance. This fact significantly affects the decision-making process regarding carotid imaging for either carotid endarterectomy or carotid stenting. In the ACAS trial, there was a 1.5% risk of stroke from diagnostic angiography alone.

Contraindications

Cardiopulmonary comorbidities preventing surgical intervention and total occlusion of the internal carotid artery are the only absolute contraindications to carotid endarterectomy. The risks related to cardiopulmonary comorbidity may be reduced by the use of local or regional anesthesia, thus limiting the systemic effects of general anesthesia. Carotid endarterectomy is not performed on patients with occlusion of the internal carotid artery as the thrombus typically extends to the ophthalmic artery. Relative contraindications to surgery are based on the exclusion criteria of the NASCET and ACAS trials, including, among others, patients with recent myocardial infarction (symptomatic coronary artery disease), uncontrolled diabetes mellitus, and advanced age. Anatomic risk factors include previous neck irradiation or dissection, tracheal stoma, carotid lesions at or above the second cervical vertebra, contralateral vocal cord paralysis, and previous carotid surgery.

Further studies examined carotid endarterectomy and found that it can be safely performed in patients deemed at high risk, including those 80 years or older and others with significant comorbid conditions, with combined stroke and mortality rates comparable to those found in the NASCET and ACAS studies. Contralateral occlusion was the only predictor for moderately increased perioperative risk of stroke and reduced long-term survival.³

Overall, outcomes are based on the surgeon’s experience, and decisions about intervention are largely based on the track record of the team performing the procedure.

Outcomes and Complications

The major complications related to carotid endarterectomy include perioperative stroke, cranial nerve injuries, cardiopulmonary complications, and restenosis. Recurrent stenosis can be in the form of three types occurring at different time stages: (1) residual disease from an incomplete endarterectomy usually can be seen immediately on postoperative imaging; (2) intimal hyperplasia frequently occurs between 2 months and 2 years; and (3) recurrent atherosclerotic disease most often occurs beyond 2 years. Open surgical repair of recurrence in any of these forms carries a higher risk of intraoperative complications, including cranial nerve damage secondary to postoperative scar formation. For this reason, carotid stent angioplasty is frequently used to treat recurrent disease.

One rare complication is hyperperfusion syndrome, which is characterized by a severe, unilateral headache occurring 3 to 7 days after endarterectomy. This has been attributed to the sudden restoration of normal arterial pressures in a vascular bed that has not seen pulsatility and normal pressures for a long period, thus leading to atrophy of the arteriolar restrictive mechanisms and impaired ability of the cerebrovascular circulation to autoregulate after the reestablishment of cranial blood flow. MRI may show reversible vasogenic edema similar to that observed in the posterior leukoencephalopathy syndrome.⁴ Treatment includes control of the hypertension that is frequently associated with this phenomenon to prevent cerebral hemorrhage until cerebral autoregulation is reestablished, typically for a few days to 1 week.

Imaging Findings

Preoperative Assessment

Duplex ultrasonography is the most common technique used to assess patients suspected of having extracranial cerebrovascular disease. B-mode ultrasonography is used to define location of the stenotic lesion, and Doppler examination is used to measure velocities across the stenosis (Fig. 87-1). B-mode ultrasonography not only delineates the location of a stenosis but also describes the characteristics of the plaque itself. Plaque ulceration as well as plaque calcification or hemorrhage may be seen. B-mode ultrasonography can also evaluate plaque echogenicity and the presence of thrombus. Soft, friable plaques on ultrasound examination are typically less stable than echoic plaques are. Mobile thrombus on ultrasound examination has been associated with an increased risk of stroke.

FIGURE 87-1 Duplex ultrasound examination of a patient with left internal carotid artery stenosis. Note the gray-scale lesion on the B-mode image and elevated Doppler velocity measurements across the stenosis.

Velocity measurements across a stenosis are the most important aspect of the preoperative assessment for the vascular surgeon because these measurements are directly correlated with degree of stenosis. Strandness ⁵ first reported a sensitivity of 99% and a specificity of 84% with duplex ultrasound criteria for the evaluation of carotid disease compared with conventional angiography (Table 87-1). More recently, it has been observed that diagnostic criteria must be altered in the setting of a contralateral severe stenosis or complete occlusion of the internal carotid artery because of a compensatory increase in carotid blood flow. By use of the criteria of AbuRahma,⁶ the accuracy of duplex ultrasound measurements can be increased to 96%. No matter which criteria are used, they must be validated by individual vascular laboratories as this technique may be operator and instrument dependent.

TABLE 87-1 Duplex Ultrasound Criteria for Diagnosis of Internal Carotid Artery Stenosis

Stenosis	Criteria
Stenosis	Strandness	AbuRahma
Normal	PSV < 125 cm/sec	PSV < 125 cm/sec
	No SB	No SB
	Flow reversal in bulb
1%-15%	PSV < 125 cm/sec	PSV < 125 cm/sec
	No or minimal SB	Minimal SB
	Flow reversal in bulb absent
16%-49%	PSV > 125 cm/sec	PSV < 140 cm/sec
16%-49%	Marked SB	EDV < 140 cm/sec
50%-79%	PSV > 125 cm/sec	PSV ≥ 140 cm/sec
50%-79%	EDV < 140 cm/sec	EDV < 140 cm/sec
80%-99%	PSV < 125 cm/sec	PSV > 140 cm/sec
80%-99%	EDV > 140 cm/sec	EDV < 140 cm/sec
Occlusion	No flow	No flow

EDV, end-diastolic velocity; PSV, peak systolic velocity; SB, spectral broadening.

Modified from Strandness DE Jr. Extracranial arterial disease. In Strandness DE Jr (ed). Duplex Scanning in Vascular Disorders, 2nd ed. New York, Raven Press, 1993, pp 113-158; and AbuRahma AF, Richmond BK, Robinson PA, et al. Effect of contralateral severe stenosis or carotid occlusion on duplex criteria of ipsilateral stenoses: comparative study of various duplex parameters. J Vasc Surg 1995; 22:751-761.

Many surgeons will rely on duplex ultrasonography alone in making the decision to perform a carotid endarterectomy. Additional imaging is required in any case in which anatomic factors preclude a complete assessment by ultrasound examination. These include extreme tortuosity, inability to see normal distal internal carotid, or any factor that does not allow complete characterization of the carotid bifurcation and confirmation of a normal distal extracranial internal carotid artery. Furthermore, additional testing should be performed in the presence of the so-called string sign. String sign may occur in two scenarios that are treated differently. In the first, there is slow flow through a high-grade stenosis with an underfilled but normal internal carotid artery. In this case, carotid endarterectomy is beneficial. In the second case, there is a long-segment high-grade stenosis extending into the distal internal carotid artery. These types of lesions are more difficult to treat surgically and are associated with a higher perioperative stroke rate. Combining duplex ultrasonography with either computed tomographic angiography (CTA) or magnetic resonance angiography (MRA) increases the overall accuracy of determining degree of stenosis.

CTA has been used to delineate extracranial cerebrovascular and carotid arch anatomy and has the advantages of minimal discomfort for the patient, relatively low radiation doses, and demarcation of calcific plaques in both the carotid arteries and the aortic arch. Recent three-dimensional reconstructions provide the surgeon with a greater ability in preoperative planning (Fig. 87-2).

FIGURE 87-2 Postprocessed view of three-dimensional CTA shows a left carotid stenosis with calcification (white) and plaque (yellow).

MRA is another tool of the vascular surgeon and can be particularly useful in patients with an allergy to intravenous contrast material. Time-of-flight MRA has a tendency to overestimate the degree of stenosis because of local blood flow turbulence. Overestimation of time-of-flight MRA can be reconciled with the performance of gadolinium-enhanced three-dimensional MRA.⁷ Both techniques also demonstrate intracranial vessel anatomy along with patency of the communicating arteries.

Catheter angiography (Fig. 87-3), previously considered the gold standard for the assessment of extracranial cerebrovascular disease, has been used less frequently in uncomplicated patients because of the approximately 1.5% risk of stroke. Catheter angiography is now typically reserved for evaluation of patients in whom results from duplex ultrasonography and either CTA or MRA are discordant.

FIGURE 87-3 Digital subtraction angiography showing high-grade stenosis of the internal carotid artery (arrow).

Preoperative Planning

Preoperative planning uses the same techniques discussed before, including duplex ultrasonography, CTA, and MRA. These studies should alert the surgeon of abnormal anatomy, such as an unusually high or low bifurcation or excessive tortuosity, in an effort to avoid injury to important surrounding structures.

Postoperative Surveillance

Postoperative surveillance is accomplished with duplex ultrasonography. This provides an inexpensive, efficient method of monitoring not only the durability of the surgical result but also the progression of disease on the contralateral side. As discussed earlier, recurrent stenosis occurs in three forms and is typically time related. Residual disease can be manifested in the form of postoperative symptoms of amaurosis fugax, transient ischemic attack, or stroke. These patients are brought immediately back to the operating room for re-exploration. If residual disease is identified at the first postoperative duplex scan, patients can be observed if they are asymptomatic or re-explored if the plaque looks unstable. Patients with intimal hyperplasia are most often asymptomatic because these lesions have a very low embolic potential. Therefore, patients with intimal hyperplasia are typically observed until there is progression to a high-grade stenosis or the rare development of symptoms. Recurrent atherosclerotic disease is treated in a manner similar to the primary development of occlusive disease as these lesions are inherently more unstable than their intimal hyperplastic counterparts.

CAROTID-SUBCLAVIAN BYPASS

Description

Carotid-subclavian bypass may be performed through a remote cervical incision just lateral to the sternocleidomastoid muscle. The jugular vein is reflected medially to expose the common carotid artery. The anterior scalene muscle is then divided to expose the subclavian artery. Once proximal and distal control of each artery is obtained, bypass is typically performed with a prosthetic graft as patency is superior to autogenous conduit in this position.⁸ On occasion, concomitant disease of the ipsilateral common carotid artery precludes its use as an inflow vessel. In these cases, the contralateral common carotid may be used. The prosthetic graft would then be tunneled across the midline through the retropharyngeal space; this is a more direct path and avoids erosion of the overlying skin or interference with possible subsequent sternotomy or tracheostomy.

Indications

Indications for repair of subclavian stenosis include upper extremity disabling claudication, rest pain, and tissue loss. Patients with subclavian steal syndrome are also candidates for repair. Subclavian steal occurs when there is a stenosis of the first portion of the subclavian artery causing distal pressure to drop below that at the vertebrobasilar junction, with subsequent reversal of flow through the basilar and ipsilateral vertebral arteries. This can lead to vertebrobasilar symptoms of presyncope, syncope, or drop attacks. Finally, carotid-subclavian bypass should be performed in those patients with asymptomatic proximal subclavian stenotic lesions who are undergoing coronary revascularization through the internal mammary artery, as well as in patients with symptomatic coronary steal syndrome from a patent internal mammary artery coronary bypass graft.

Contraindications

Severe cardiopulmonary comorbidity is the only absolute contraindication to carotid-subclavian bypass. As described before, this procedure may be performed through a small incision, and local regional anesthesia may have fewer cardiovascular effects than standard general anesthetic techniques.

Duplex ultrasonography may be used to screen for subclavian stenosis but is limited by the bony structures of the mediastinum. Its usefulness lies in its ability to assess for reversal of blood flow in the vertebral artery with upper extremity exercise (Fig. 87-4) and the adequacy of the common carotid artery for inflow.

FIGURE 87-4 Duplex ultrasonography of common carotid and vertebral arteries showing reversal of flow within vertebral artery in a patient with subclavian steal syndrome. Note the opposite color of the vertebral artery (blue) compared with the common carotid artery (red).

CTA and MRA are useful tools to assess for lesions of the proximal subclavian artery. As a screening tool, CTA may be more practical because the examination is more widely available and generally quicker to perform.

In cases in which the diagnosis is questionable, digital subtraction angiography may be helpful to demonstrate reversal of flow through the vertebral artery.

Preoperative Planning

Once a subclavian stenosis has been identified, preoperative planning may be accomplished with CTA or MRA (Fig. 87-5). Both evaluate the exact location of the lesion as well as assess possible inflow and outflow vessels. Full appraisal of the aortic arch, its branches, and neck extracranial cerebrovascular vessels is necessary to rule out other pathologic processes. Resolution of the aortic arch and these large-caliber vessels with CTA and MRA is excellent, and they may replace standard angiography.