IntroductionStroke is the second leading cause of death world wide, with a 26% increase in global stroke deaths during the past two decades alone.1

In India, the pooled data reveal that ischemic stroke occurs in 68 to 80%, of which extracranial carotid disease accounts for 25 to 26% and intracranial carotid disease for 30% of ischemic stroke cases.2 The 2­year risk of subsequent ipsilateral ischemic stroke in the context of best medical therapy has been estimated at 10 to 15% for extracranial carotid disease and 15 to 20% for intracranial atherosclerotic disease.3 These data of high recurrence rate with medical treatment are reshaping our understanding of management of ischemic stroke.

Historical Background and Existing EvidenceShortly after the development of the extracranial–intracranial (EC–IC) arterial bypass by Yaşargil in Zurich, the procedure became widely used for the treatment of ischemic cerebrovascular disease including patients with symptomatic internal carotid artery (ICA) occlusion.

However, when in 1985, the International EC–IC Bypass Trial, a prospective randomized trial, failed to demonstrate the superiority of EC–IC bypass to the best medical treatments available for reducing the risk of ischemic

A Relook at Cerebral Bypass Procedures in Adult Ischemic Stroke: Current Trends and PracticesRoopesh Kumar V. R. , Vishwaraj Ratha, and Sunil Kapilavayi

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stroke,4 the frequency declined sharply. Unfortunately, this study had several design flaws along with exclusion of utilization of hemodynamic criteria for selecting high risk in surgical arm.5

Since then, multiple groups have reported that superficial temporal artery–middle cerebral artery (STA–MCA) cortical branch anastomosis can significantly reduce symp­toms related to hemodynamic ischemia.6,7 Subsequently, in the 1990s, the analysis of hemodynamic impairment to predict the risk of stroke became more important, and it was recommended to identify candidates for bypass surgery according to hemodynamic criteria.

Recent, Ongoing, and Future StudiesThe St Louis Carotid Occlusion Study (STLCOS) showed that symptomatic pati ents with carotid occlusion and stage II hemo­dynamic impairment (increased oxygen extraction fraction with positron emission tomography [OEF–PET]) were at significantly increased risk of ipsilateral stroke at 2 years compared to patients without stage II hemodynamic impairment (26.5 vs. 5.3%).

The “Carotid Occlusion Surgery Study (COSS),” a prospective, multicenter random­ized controlled trial (RCT) designed to assess whether STA–MCA bypass (plus best medical therapy) was superior to best medical therapy

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alone in stroke prevention in patients with ipsilateral­to­contralateral hemispheric OEF ratios greater than 1.13, showed perioperative ipsilateral stroke rates to be significantly less in the medical group (2 vs. 14.4%), and bypass surgery was shown to provide no clinical benefit over medical therapy.8

However, subsequently published COSS data showed high rates of bypass patency and revealed that it improved but did not normalize cerebral hemodynamics. The OEF improve ment greatly reduced the risk of subse quent stroke in these patients, and the sur gical group had much lower rates of recur­rent ipsilateral ischemic stroke after post­operative day 2 as compared with the medical group (9 vs. 22.7%).

COSS faced criticism as it excluded those patients from surgical arm with symptom recurrence despite medical therapy that could ultimately benefit from surgery.

The “Japanese EC–IC Bypass Trial (JET),” a multicenter RCT, was designed to assess whether STA–MCA bypass (plus best medical therapy) is superior to best medical therapy alone in reducing subsequent ischemic events in patients with recently symptomatic hemodynamic (at least stage I on SPECT) cerebral ischemia from chronic ICA or MCA occlusion. An interim analysis reported a statistically significant reduction of major stroke and death (primary outcome) in the surgical arm (5.1 vs. 14.3%).9

PathophysiologyCerebral parenchyma heavily relies on a continuous delivery of oxygen and glucose to function. This cerebral blood flow (CBF) is deter mined by the interplay between cere­bral perfusion pressure (CPP) and the resist­ance provided by arteriolar constriction. Vasodilation in response to lowered CPP or increased metabolic demand increases cere­bral blood volume (CBV), which may be global or regional. This helps the parenchyma

in maintaining its cerebral metabolic rate of oxygen (CMRO2) until CBF drops below a critical level. Additionally, the tissue extracts a greater fraction of oxygen from the blood when perfusion is poor (OEF). This is increased with marginally perfused tissue.

When the compensatory mechanisms of vasodilation and OEF are exceeded by reduced CPP, CMRO2 declines, and subsequent infarc­tion occurs. With large­vessel occlusion, this is most common in the deep white matter and gray matter of the border zones (watershed areas) of vascular territories.10

Broadly, an ischemic cerebrovascular event goes through several differing processes1: hypo perfusion,2 thrombosis at the site of sten­osis,3 thromboembolism, and direct occlusion of small perforating vessels.4 Severe hemo­dynamic impairment is a powerful predictor of subsequent stroke in patients with carotid artery occlusion.10

Hence, the therapeutic strategies in stroke have been developed with two main aims:

1. Restoration of cerebral flow.2. Minimization of the deleterious effects

of ischemia on neurons.

To summarize, the critical effects of cerebro­vascular ischemia pivot over the existence or absence of collateral circulation in the brain, which perhaps influences the most on the hemodynamic stages (Table 4.1). Cerebral flow augmentation through bypass procedures thus appears to be a natural option in increasing the blood flow through collaterals resulting in good outcomes.11,12

When Does Stroke Occur?The spectrum can vary from being asymp­tomatic to fluctuating clinical symptoms, including recurrent transient ischemic attack or minor/major stroke, depending on the patient’s collateral circulation and cerebral vasoreactivity associated with hemodynamic factors (Fig. 4.1).

A Relook at Cerebral Bypass Procedures in Adult Ischemic Stroke 29

Asymptomatic cases: This happens because of a robust and effective compensat ory collateral circulation, which potentially pro­vides enough arterial blood for parenchymal consumption.13

Inadequate brain perfusion: Inadequacy in compensatory collateral circulation may result in ischemic symptoms. This deficiency in collateral flow may be blood pressure dependent and has the potential to become hemodynamic infarct. Reversible low perfu­sion presents as recurrent transient ischemic attack (TIA), while severe ischemia can present as minor/major stroke.14

Embolus detachment: Some patients with complete carotid occlusion having full hemo­dynamic compensation may still have cerebral infarcts. The emboli primarily originate from a narrow common or external carotid artery, from a proximal or distal ICA stump.14

Cognitive function impairment: One of the most devastating effect of repeated ischemic event is impairment in cognitive function15 and it affects not only the ipsilateral but also the contralateral hemisphere.16 A suc cessful bypass procedure may improve this attri bute, which essentially none of the other non surgical options have addressed, and enhance the global cognitive function as well as attention and psychomotor processing speed.

ImagingThe past 20 years has seen considerable pro gress in imaging techniques enabling physi cians to identify conditions including hemo dynamic ischemia and poor collateral circulation as well as patients at high risk of recurrent stroke.

Table 4.1 Stages of cerebral hemodynamic impairment in stroke

The three stages of hemodynamic failure

Stage Pathophysiology Flow Metabolism Hemodynamics

CBF CMR CVR OEF

I Drop of CPP(lack of collaterals)

↓Cerebral vasodilation

Normal Normal Reduced Normal

II Further drop of CPP

↓Cerebral vasodilation exhausted

↓OEF increase

Reduced Normal Exhausted Increased

III Further drop of CPP

↓CVR and OEF exhausted

↓Ischemia

Reduced Reduced Exhausted Exhausted

Abbreviations: CBF, cerebral blood flow; CMR, cerebral metabolic rate; CPP, cerebral perfusion pressure; CVR, cerebrovascular reserve; OEF, oxygen extraction fraction.Note: CVR: How much cerebral perfusion can increase from a baseline value after stimulation. It is Measured via transcranial Doppler, Xenon-CT, SPECT, PET, and MRI by acquisition of an initial CBF measurement at rest and a subsequent CBF measurement after a vasodilatory stimulus (i.e., acetazolamide [ACZ] or hypercapnia challenge).

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• Immediate diversion of blood flow in the event of large-vessel occlusion• Chronic compensation by secondary collaterals such as the ipsilateral ophthalmic

artery, ipsilateral pcom a, aca and leptomeningeal collaterals• Effective leptomeningeal collateral circulation and the presence or more than 2

collaterals have been associated with good clinical condition without severe disability

Disease remains stable in asymptomatic patients

Recurrence rate for symptoms in the region is relatively low at 2–8% annually

If asymptomaticMedical management metabolic imaging on follow-up

If symptomatic• Cerebrovascular flow

augmentation procedure recommended (bypass)

• Reduces the risk of recurrence • Reduces vascular cognitive

decline

Embolus from CCA, ECA, ICA

Sufficient collateral circulation Insufficient collateral circulation

Stage I (compensated)• Autoregulatory compensatory

vasodilation maintains normal CBF• Oxygen extraction fraction (OEF)

remains normal.

Stage II (misery perfusion state)• Compensation is exhausted and CBF is

reduced• Cerebral metabolic compensation

occurs by increasing OEF

Stage III (decompensated state)• CBF and OEF are reduced to the point

of ischemia and ultimately infarction • Risk or ischemic symptoms may be as

high as 30% per year

Stages of hemodynamic failure

Natural history of cerebrovascular ischemic event

Fig. 4.1 Natural history of cerebrovascular ischemic events.

Anatomical ImagingComputed tomography angiography (CTA), mag netic resonance angiography (MRA), digital subtraction angiography (DSA), and ultra sound can directly measure the length of the occluded artery, the degree of reverse

filling by the distal blood flow of the occluded artery, the shape of the ICA residue at the occlu sion, and the degree of compensation by the collateral circulation, among other features.

CT perfusion provides quantitative evalu­ation of cerebral perfusion by generating maps of CBF, CBV, and mean transit time (MTT). This

A Relook at Cerebral Bypass Procedures in Adult Ischemic Stroke 31

can provide immediate and detailed infor­mation on hemodynamic parameters with good predictive value regarding the hypo­ or hyperperfusion in patients after cerebral bypass surgery and correlates with single­photon emission computed tomography (SPECT).17

Metabolic ImagingPositron emission tomography (PET). It utilizes a biological compound tagged with radio active isotope, typically introduced through intravenous injection or inhalation to image the distribution of that compound in the body. To measure CVR, CBF images are acquired with 15O­water at baseline and following administration of the vasoactive stimulus. The advantage of 15O­water PET is that CBF can be measured directly and quantitatively, and PET is generally considered the gold standard of CBF and OEF measurement.18 OEF is considered as the strongest indicator for revascularization procedure, especially when the ipsilateral­to­contralateral OEF ratio is greater than 1.13.

Single-photon emission computed tomo-graphy. It utilizes a biological compound intro duced through intravenous (IV) inha­lation or ingestion, but unlike PET, the radio­active isotopes in SPECT undergo decay via isomeric transition in which the isotope decays from a metastable state with higher energy to a more stable state through the emission of a gamma photon. The most com­mon SPECT compounds utilized for CVR imaging are 99mTc­exametazime, 99mTc­bicisate, and 123I­iodoamphetamine. These radioactive isotopes have much longer half­lives than those utilized in PET. ACZ SPECT is also the most common method for assessing CVR in clinical setup.

Approaches for Eliciting Cerebrovascular ResponsesTo evaluate CVR, external agents are most commonly utilized to induce changes in

micro vascular resistance using two major classes of stimuli: intravenous pharmacological agents and respiratory challenges.

Acetazolamide. ACZ is the most common pharmacological agent for eliciting vasodila­tion, which acts by decreasing the pH leading to reduced vascular resistance and vasodila­tion in compliant microvasculature. The usual intravenous dose is 1,000 mg and imag ing is performed 15 to 20 minutes after adminis­tration. If intravenous preparation is not avail­able, oral ACZ can be given as 500 mg every 15 minutes for three doses, and following the last dose, tracer is injected and scan is done 1 hour later.

Breath-hold. The most basic method for modulating Pco2 with a respiratory challenge is with a breath­hold. A recent review has summarized breath­hold protocols in detail. It is easy to administer, and does not require exogenous contrast agents and mostly used when ACZ is not feasible.

Indications for Surgical TreatmentIn the shadows of the mentioned trials, the selection criteria for cerebrovascular flow aug­mentation bypass procedures have become very limited. Nonetheless, best medical ther­apy is still not curative, and in severe hemo­dynamic insufficiency, it fares poorly. Recent reappraisal of these trials has demonstrated that bypass surgeries have excellent outcomes in select cases and cannot be rejected as an option anymore.

• Are there methods to identify select subgroups of patients who could benefit from bypass surgery?

• Are there methods to reduce peri opera­tive complications in the very early postoperative period?

There are distinct subgroups of patients with exhausted brain vascular reserve capacity with symptomatic oligemia exacerbated by any form of hemodynamic challenge, for whom COSS was not specifically designed.

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This sub group would benefit immensely with bypass, if performed with low enough morbidity.

It is pertinent to note that a significant reduction of perioperative ischemic compli­cations could change the current statement of “no benefit from bypass” in COSS trial.19 Technical innovations, like systematic use of mini­craniotomy (2–2.5 cm) along with use of indocyanine green (ICG), can lower the perioperative complication rates and be beneficial even for trial­eligible patients.

The following subgroups of patients suffer­ing from ischemic cerebrovascular disease will benefit immensely as well:

• Patients with chronic retinal ischemia resulting in progressive visual loss.

• Patients presenting with ongoing hemo­dynamic symptoms despite optimal medical therapy.

• Patients who develop ischemic symp­toms with postural changes or blood pres sure variation (for instance, patients with debilitating orthostatic hypoper­fusion syndrome or limb­shaking TIAs).

• Patients with symptomatic carotid occlu sion and particularly marked hemo dynamic impairment (more severe than the OEF ratio > 1.13 used in COSS), who may have a significant risk for subsequent stroke.

• Patients harboring multiple extracranial arterial occlusions, not amenable to carotid endarterectomy or stenting, who are symptomatic despite best medical therapy.

Among individuals suffering from acute or evolving stroke, patients who could benefit from bypass surgery might be the ones presenting with the following criteria:

• Acute stroke or stroke in progress (fluctuating or worsening symptoms) despite maximal applicable medical and interventional treatment.

• Major cerebral artery occlusion, with documented region of penumbra and with a small area of infarction (to avoid

hemorrhagic conversion of an acute infarction).19

The eventual benefit of bypass surgery over medical therapy in these individuals will most likely not be testable in RCTs.

To summarize, cerebrovascular flow aug­mentation bypass procedure is highly effec­tive in stage III hemodynamic failure and symptomatic patients with stage I or II hemo­dynamic failure. However, in asymptomatic patients with complete occlusion of ICA and stage I hemodynamic failure, the role of revas­cularization is questionable9,20 (Table 4.2).

The following are some illustrative cases where cerebral revascularization has proven benefit over and above the best medical therapies alone.

Case 1A 45­year­old male patient developed acute­onset left hemiparesis, which improved partially over the next few days. CT cerebral angiogram showed near­total occlusion of supraclinoid ICA on right side. CT perfusion showed significantly reduced cerebral blood flow with large areas of penumbra suggesting stage III hemodynamic failure. He under­went right STA–MCA anastomosis following which he had no further symptoms. At 2­year follow­up, he remains asymptomatic and imaging showed improved cerebral perfusion in right hemisphere (Fig. 4.2a–g).

Case 2A 26­year­old man had recurrent episodes of slurring of speech and right hemiparesis of 5­day duration. Magnetic resonance imaging (MRI) showed multiple watershed infarcts in the left hemisphere and CT cerebral angio­graphy showed left supraclinoid ICA total occlusion. DSA revealed left supraclinoid ICA occlusion just beyond the ophthalmic artery with partial reformation of left M1 through anterior cerebral artery (ACA) collaterals. Despite medical management and intensive

A Relook at Cerebral Bypass Procedures in Adult Ischemic Stroke 33

Fig. 4.2 (a) CT angiography of the brain showing significant stenosis of right clinoidal ICA. (b) CT angiography of supratentorial circulation showing normal vessel candelabra. (c) CT perfusion of the brain showing significant ischemic penumbra in right hemisphere. (d–f) Postoperative CT angiography of the brain showing good patency of STA-MCA bypass. (g) Postoperative CT perfusion showing improved cerebral perfusion in the right hemisphere.

care monitoring, he continued to have recur­rent episodes of TIAs in the intensive care unit. He underwent emergency left STA–MCA anastomosis. Follow­up CT cerebral angio­graphy showed improved collateral circulation on left MCA territory. Three­month follow­up CT perfusion showed good improvement in cerebral blood flow. His symptoms were also resolved (Fig. 4.3a–g).

Case 3A 65­year­old woman had right hand weak­ness and speech slurring for 10 days with acute progression to hemiplegia and aphasia in the same day. MRI brain showed left hemi spheric watershed infarct and MRA showed bilateral ICA occlusion. DSA revealed bilateral ICA occlusion at origin, 80% stenosis

(a)

(d)

(e) (f) (g)

(b) (c)

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Fig. 4.3 (a) Diffusion-weighted MRI of the brain showing multiple infarcts in left hemisphere in deep basal ganglia and centrum semi ovale. (b) CT angiography showing occlusion of left ICA and proximal MCA. (c) DSA showing no filling beyond left supraclinoidal ICA. (d, e) Inadequate cross-circulation and collaterals from contralateral and posterior circulation. (f, g) Postoperative CTA showing good patency of left STA-MCA bypass and improved collateral circulation.

(a)

(d) (e) (f)

(g)

(b)

(c)

A Relook at Cerebral Bypass Procedures in Adult Ischemic Stroke 35

(g)

(a)

(d)(b) (c)

of left vertebral artery, and right vertebral artery sup plying the whole of supratentorial compartment. Emergency left STA–MCA anast omosis was performed. She gradually improved, and at 3­month follow­up, CT per­fusion study showed improved cerebral blood flow in the supratentorial compartment. Her hemiparesis and aphasia also improved,

making her independent and communicate verbally (Fig. 4.4 a–g).

ConclusionDespite many trials showing the superiority of medical therapy, it is still not curative, and many patients with hemodynamic failure

Fig. 4.4 (a) Diffusion-weighted MRI of the brain showing large infarct in left deep centrum semi ovale. (b) DSA showing occlusion of right ICA immediately distal to bifurcation. (c) DSA showing occlusion of left ICA immediately distal to bifurcation. (d) DSA showing filling of entire supratentorial circulation through right vertebral artery. (e–g) Postoperative CTA showing good patency of left STA-MCA bypass and improved collateralization in left hemisphere.

(e) (f)

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continue to develop stroke. It is imperative to understand the stages of hemodynamic impairment in adult ischemic events as cerebrovascular flow augmentation through EC–IC bypass procedure results in significant improvement in hemodynamic parameters and lowers the stroke recurrence in selected symptomatic stages II and III hemodynamic failure.

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