Vertebrobasilar Infarcts and Ischemia

Clinical Vignette

A 68-year-old male with a history of myocardial infarction 5 years ago, hypertension, hypercholesterolemia, and diabetes presents with the acute onset of rotational vertigo with nausea and vomiting and hearing loss in the right ear. Left-beating nystagmus is present in center gaze and increases velocity in left gaze. The head impulse test, originally known as the Halmagyi-Curthoys test, demonstrates catch-up saccades with head impulses to the right. There is no skew deviation and no other abnormal eye movements. No other focal neurologic symptoms or signs are noted except a mild peripheral neuropathy. When asked, he recalls having some episodes of leg numbness lasting 5 or 10 minutes recently, and 2 months ago he had a drop attack in which his legs gave out and buckled without loss of consciousness.

Introduction

The history and presentation of the patient described in the vignette, whom the otolaryngologist (or neurologist) may be called upon to examine for new onset of Meniere’s disease or other inner ear disorder causing the hearing loss, should alert the clinician to the possibility of a posterior circulation stroke. The next sequence of events for this patient may be gradual onset of increasing lethargy, bilateral leg spasms and hyper-reflexia, and ultimately the recognition of an impending basilar artery stroke. It is important to note that vascular supply for the inner ear labyrinth is usually the same as for the anterior cerebellum: the anterior inferior cerebellar artery (AICA), which is a small branch of the larger basilar artery. Thus an AICA infarct presenting with acute vertigo and hearing loss can be the first sign of an impending basilar artery stroke resulting from a small embolic piece of the growing basilar artery atherosclerotic clot dislodging and traveling into the AICA, in an artery-to-artery embolic stroke. Occlusion of the basilar artery is often fatal and portends a poor prognosis if not treated quickly. Reasons for heightened suspicion of infarct include (1) sudden onset of symptoms; (2) combined vertigo and unilateral hearing loss; (3) vascular risk factors of age, diabetes, myocardial atherosclerotic disease, and hypercholesterolemia; and (4) past history of transient ischemic attacks (TIAs) in the posterior circulation.

In this review, we first present “state of the science” information regarding interventions and outcome studies demonstrating the superiority of treating large cerebral vessel occlusions with endovascular thrombectomy and intravenous tissue plasminogen activator (tPA). In addition, we review the most recent studies analyzing typical presentation of patients with posterior circulation strokes: clinical presentation, magnetic resonance imaging (MRI) and computed tomography (CT) findings, stroke etiologies, as well as important considerations for managing patients with these conditions.

The second section addresses the anatomy and vascular supply of the posterior cerebral circulation and inner ear, with particular attention to major branches and the labyrinthine circulation. The third section describes common presentations of stroke syndromes in the posterior circulation related to symptoms of vertigo and dizziness, with particular stroke syndromes and their presentation described in detail. We elaborate on critical components and key questions of the clinical examination and on the relationship of stroke with migraine.

Exciting New Horizons in Stroke Treatment

Vertebrobasilar Ischemia and Infarcts

Historically, ischemic strokes were treated supportively, and a delay in diagnosis did not have significant consequences. With the development of proven treatments, it is now widely acknowledged that “time is brain” and that each minute of untreated stroke in the frontal lobes allows an estimated 1.9 million neurons to be destroyed. More recently, there has been rapid development of effective interventions for the treatment of stroke, with outcome measures demonstrating that early intervention results in better patient recovery and also extends the post-stroke time interval for additional potentially brain-saving interventions. The presentation of the patient described earlier should result in hospitalization and expedited referral to specialists in cerebrovascular disease, preferably within a leading stroke center. TIAs within the last 72 hours and recent increase in the frequency of TIAs also merit rapid assessment and management.

Acute Ischemic Stroke

Several landmark trials have rapidly transformed the standard of care for acute ischemic stroke treatment, with the most dramatic results demonstrating unequivocal benefit of acute endovascular intervention for a select group of patients with large vessel occlusions in the anterior circulation. These dramatic changes in the treatment of acute stroke make it even more imperative that the clinician who first sees a patient with a potential stroke recognizes the key signs and initiates immediate evaluation. Technological advances and workflow efficiencies have facilitated an even more rapid delivery of acute stroke interventions, and the world of stroke medicine has become exquisitely dynamic, ever evolving, and rapidly advancing.

During a ground-breaking investigational multicenter randomized trial involving 39 centers in the United States and Europe, entitled Solitaire With the Intention For Thrombectomy as Primary Endovascular Treatment (SWIFT-PRIME), investigators stopped the study early because of clear demonstration of superior patient outcomes with thrombectomy with a stent retriever plus intravenous tPA. This protocol was reported to reduce disability at 90 days following anterior circulation stroke, with 60% of patients demonstrating functional independence compared with only 35% of patients who received conventional intravenous treatment demonstrating functional independence at 90 days. Data from multiple studies (e.g., SWIFT PRIME, Evaluation Study of Congestive Heart Failure and Pulmonary Artery Catheterization Effectiveness [ESCAPE trial], Extending the Time in Emergency Neurological Deficits - Intra-Arterial [EXTEND-IA trial], Endovascular Revascularization with Solitaire Device Versus Best Medical Therapy in Anterior Circulation Stroke within 8 Hours [REVASCAT trial]) were pooled to investigate the efficacy and safety of stent thrombectomy in anterior circulation ischemic stroke with the primary analysis including 787 patients, demonstrating that thrombectomy was highly effective and significantly reduced disability. This has led to the highest level of recommended guidelines in the United States, Europe, and Canada supporting mechanical stent thrombectomy within 6 hours of ischemic stroke for patients with large vessel stroke caused by internal carotid and middle cerebral artery occlusions. The meta-analysis of studies conducted in participating countries demonstrated that modern thrombectomy devices achieve faster and more complete reperfusion, with a common odds ratio for improvement in the modified Rankin score at 2.4 and a P value of 10 ⁻¹⁰ . Quality-of-life scores at 24 months follow-up remain high for patients with stroke using this protocol, with no significant difference for patients who received intravenous thrombolysis plus thrombectomy versus thrombectomy alone. A review of pooled data from five separate trials also demonstrates the efficacy of this treatment in important subsets of patients: patients 80 years and older, patients in whom thrombectomy was performed more than 5 hours after stroke onset, and patients not eligible for intravenous alteplase. Taken together, these data illustrate that endovascular thrombectomy is a highly effective treatment for large vessel occlusive stroke in the anterior circulation, and rapid multimodal imaging to identify these patients is crucial.

As mentioned, imaging has an important role in the selection of patients who are likely to benefit from endovascular reperfusion therapy following anterior circulation stroke. In a retrospective study of patients who had undergone endovascular reperfusion, perfusion imaging-selected patients were more likely to have positive outcomes compared with noncontrast CT-selected patients, whereas CT-selected patients and MR-selected patients had roughly similar outcomes. One study using CT perfusion demonstrated superiority of endovascular therapy initiated within 18 hours of symptom onset. Imaging is absolutely essential in the analysis of the acute presentation of a potential cerebrovascular event: evaluating for cerebrovascular stenosis, atherosclerotic burden, silent cerebrovascular cortical infarcts, and perfusion deficits, as well as evaluating for vascular anomalies, tumors, metastatic disease, or other diseases such as multiple sclerosis, cerebellar atrophy syndromes, and Chiari malformation.

Endovascular Therapy in the Posterior Circulation

Vertigo as a presenting symptom in a patient with an evolving or impending stroke localizes to the posterior circulation, and as with the patient with anterior circulation stroke, it may be wise to consider the patient presenting with an acute vestibular syndrome as having a posterior circulation stroke until proven otherwise. This is particularly true for patients who have risk factors for stroke.

Large vessel atherosclerotic disease of the vertebral or basilar arteries accounts for approximately one-third of ischemic events in the posterior circulation. The question has been raised regarding whether the patient with vertebrobasilar TIAs secondary to atherosclerosis or large vessel disease is likely to suffer a recurrence and if so, should vertebrobasilar stenosis be investigated in patients with potential TIAs localized to the posterior circulation. A pooled analysis of prospective studies demonstrated that the 90-day risk of stroke from the first event (TIA or stroke) in patients with vertebrobasilar stenosis was 24.6% compared with 7.2% in patients without stenosis. Identification of stenosis in the posterior circulation in the acute setting of a TIA or stroke should prompt the managing healthcare provider to initiate preventative measures and to educate the patient regarding warning signs of stroke, which some studies suggest occur in one of four patients within 90 days. Therefore, imaging studies, including MRI/magnetic resonance angiography (MRA; see later discussion), in this group of patients with potential posterior circulation events would likely be high yield and may warrant active medical therapeutic preventatives or preemptive interventions.

Basilar artery occlusion, a subset of posterior circulation strokes, is potentially amenable to thrombectomy similar to large vessel occlusions in the anterior circulation. Basilar artery occlusion often mimics other nonstroke entities and can be misdiagnosed as meningitis, vestibular neuritis, or seizures because of the nonspecific clinical manifestations and the common stuttering symptom course. It is important for the clinician to know that dizziness and vertigo are common early prodromal symptoms of basilar artery occlusion.

Not surprisingly, posterior circulation strokes are associated with a delayed time to intervention compared with anterior circulation strokes. In a large systematic review of 71,010 patients with ischemic stroke in the Austrian Stroke Unit Registry, the onset-to-door times (ODTs) and door-to-needle times (DNTs) were compared for anterior and posterior circulation strokes. Overall, the ODT for posterior circulation stroke was 170 minutes compared with 110 minutes for anterior circulation stroke. Furthermore, posterior circulation strokes were associated with a delay in DNT. The FAST algorithm (Face, Arm, Speech, Time) is designed to help identify strokes for patients and their families, ambulance personnel, and persons in the field. However, in an analysis of 736 strokes, 14.1% of patients did not have any FAST symptoms at presentation, and of these, 42% had gait imbalance or leg weakness and 40% had visual symptoms, symptoms not included in the FAST mnemonic. With the addition of balance disturbance and eye movement abnormalities, a significantly higher proportion of patients with stroke would be correctly identified. The authors propose BE-FAST as an acronym to reduce the proportion of missed strokes: B alance, E yes, F ace, A rm, S peech, T ime. Missed strokes often are posterior circulation strokes, which commonly present with dizziness or nystagmus and diplopia. In summary, posterior circulation strokes are associated with delays in OTD and DNT and are also more likely to be missed by current emergency examination protocols. Therefore, it is essential to educate healthcare providers and the public that “time is brain” and “time is brainstem.”

Imaging in identification and management of posterior circulation stroke

In an editorial, Liebeskind recognized the importance of addressing patients with the most complex conditions and the role that advanced multimodal imaging has played in the rapid advancement of stroke interventions, as well as the need for precision medicine. As such, precision medicine in acute stroke depends largely on the phenotypic characterization provided by diagnostic neuroimaging. A study using imaging-based scores for the prediction of early stroke risk after TIA demonstrated an increased sensitivity in the identification of these high-risk patients. Traditional parameters used to predict stroke include the following: age, blood pressure, clinical symptoms, duration, and diabetes (ABCD2), useful for nonspecialists in the community who may not have access to diagnostic neuroimaging. A proposed ABCD3-I model (age, blood pressure, clinical symptoms, duration, diabetes, dual events, and imaging) uses additional prognostic utility of detailed MRI variables, which include vascular imaging information. An example of MRI data for potential risk stratification is the high incidental cerebellar cortical infarcts (35%) seen on the initial MRI of patients presenting with vertebrobasilar stroke or TIA and vertebral artery stenosis. If validated, the presence of cerebellar cortical infarcts and vertebral artery stenosis on an MRI may be indicative of an impending vertebrobasilar stroke. The gathering of high-resolution MRI findings in patients presenting with cerebrovascular ischemia needs to be recognized as an important subset of precision medicine.

Brief Summary of the Anatomy of the Posterior Circulation

The vertebrobasilar vascular system provides blood supply to the posterior region of the brain, which includes the brainstem, cerebellum, and inner ear. The vertebrobasilar system represents about 20% of cerebral blood flow. When vertigo is the presenting symptom of a TIA or stroke, the cerebrovascular region involved is the vertebrobasilar system. Ischemic attacks in the anterior circulation, that is, carotid system, may present with lightheadedness but will not present with rotational vertigo. Vertigo can result from ischemia of the inner ear, brainstem, or cerebellar structures perfused by the posterior circulation. Posterior circulation TIAs and strokes represent about 20% of all TIAs and strokes.

The vertebral arteries originate from the subclavian arteries, which in turn originate from the thoracic aorta. The two vertebral arteries are only rarely the same size, and in 72% of participants in an anatomic study, one vertebral artery was at least double the size of the other. The vertebral artery is designated into four sections: V1, which includes the origin and the extracranial prevertebral portion; V2, which is the extracranial foraminal or cervical artery; V3, which is the extracranial postforaminal artery at the atlantic region; and V4, which is the intracranial artery and includes the junction with the basilar artery. The most common sites of stenosis are at V1 and V4, particularly at the origin of the vertebral artery as it comes off of the subclavian artery and at the vertebrobasilar junction. There is a predilection for development of atherosclerotic plaques at sites with turbulent flow such as bifurcation points and at the origin of arteries.

The two vertebral arteries converge to form the midline basilar artery, which courses along the ventral surface of the pons. The basilar artery gives off penetrating median, paramedian, and short circumferential branches. The posteroinferior cerebellar artery (PICA) is the largest branch of the vertebral artery. It generally originates 1–2 cm below the basilar artery but in angiograms has been shown to sometimes originate below the level of the foramen magnum. The PICAs supply the lateral medulla, including the vestibular nuclei and the posteroinferior cerebellum. The largest circumferential branches are the AICAs, which arise from the proximal third of the basilar artery. In addition to supplying the labyrinth, the AICAs supply the lateral pontine tegmentum, the brachium pontis, the flocculus, and part of the anterior cerebellum. An AICA infarct typically involves the middle cerebellar peduncle. There is often a close relationship between the PICA and ipsilateral AICA; if one PICA is hypoplastic, the ipsilateral AICA may be large and the contralateral PICA small.

The vascular anatomy of the inner ear has been studied in many species. The inner ear consists of the auditory and vestibular end organs, for hearing and balance, respectively. The vestibular labyrinth is composed of three semicircular canals (SCCs) and two otolith organs, the saccule and utricle. The vestibular end organs are involved in sensing head movement and position, more precisely, angular acceleration and linear acceleration. Vascular supply for the inner ear is provided by the internal auditory artery (also known as the labyrinthine artery), which arises most commonly as a branch of the ipsilateral AICA and may also be a branch of PICA or the basilar artery. Within the internal auditory canal, the internal auditory artery branches into two arteries: (1) the common cochlear artery, which feeds most of the cochlea, and the posterior vestibular artery, which feeds the inferior saccule and posterior SCC, and (2) the anterior vestibular branch, which feeds the horizontal and anterior SCC, the utricle, and a small portion of the saccule. The labyrinthine arteries are end arteries—they do not anastomose with other major arterial branches; therefore, if blood flow is interrupted for 15 seconds or more, the auditory nerve fibers become inexcitable. In contrast, there are abundant anastomoses in the brainstem and overlying territories in the cerebellum, and thus the inner ear appears to be more vulnerable to interruption of blood flow than the brainstem or cerebellum.

Before dividing into the two posterior cerebral arteries (PCAs), the basilar artery gives off the superior cerebellar arteries (SCAs). The cerebellum is fed by the AICAs, PICAs, and SCAs with prominent anastomoses provided by these arteries on the cerebellar surface. These arteries feed areas of the brainstem related to wakefulness and consciousness, and there is a high degree of collateralization in this critical area. In contrast, there is little collateralization within the inner ear. This fact may explain why vertebrobasilar insufficiency can present with isolated labyrinthine infarctions and cochlear hearing loss.

The cause of vertebrobasilar insufficiency or ischemia (VBI) is usually atherosclerotic disease of the subclavian, vertebral, or basilar arteries. However, clinicians should realize that it has been estimated that one in five posterior circulation infarcts is cardioembolic and another one in five infarcts occurs from intra-arterial emboli originating in the extracranial and intracranial vertebral arteries. Emboli travel to the distal arterial branches, often causing an isolated cerebellar infarction in the distribution of the SCA, AICA, or PICA. Therefore patients with isolated cerebellar infarctions should be evaluated carefully for an embolic source. The evaluation should include a search for a cardiac source of embolism and visualization of the intracranial and extracranial vertebrobasilar arteries, including the origin of the vertebral artery. There may be a tendency for cardiac catheterization embolic events to involve the posterior vertebrobasilar system.

Another potential cause of VBI results from the fact that the extracranial vertebral arteries are susceptible to dissection, which refers to a tear in an artery, usually involving the middle layer, referred to as the medial coat. Sudden neck movements, such as neck manipulation, or even trivial motions such as extending the neck to take medications, can be an inciting event. Dissections usually occur in portions of the extracranial arteries that are mobile rather than at the origins of arteries (e.g., at the distal extracranial vertebral artery V3). The extracranial and intracranial vertebrobasilar system is well visualized using present-day noninvasive imaging modalities. There are heritable disorders of connective tissue disease such as Ehlers-Danlos type IV, which are associated with arterial dissection, and can occur at multiple sites. However, transient emboli are often not visualized and very small infarcts in communicating areas may not be seen on MRI. Therefore, the diagnosis of posterior circulation TIA or stroke is sometimes made on a clinical basis. Dissection in the vertebral artery, in addition to neurologic signs and symptoms, also causes severe neck pain, a clinical hallmark of vertebral artery dissection.

Microvascular ischemia may also be the cause of age-related atrophy of inner ear structures, a well-studied phenomenon in human temporal bones. There is significant loss of spiral ligament volume of approximately 12% and a 32% loss of stria vascularis volume in humans older than 60 years compared with those younger than 40 years. In the Fisher rat, there is an age-related decrease in blood flow of 75% in capillaries of the posterior crista ampullares. Cells in the inner ear, especially those such as the strial marginal cells and the auditory and vestibular hair cells may be exceptionally vulnerable to ischemia because of the high volume density of mitochondria, which is associated with high energy demand.

The Posterior Circulation: Mechanisms of Stroke and Transient Ischemic Attacks

A large study of 407 consecutive patients with posterior circulation events, of whom 59% had strokes without TIA, 24% had TIAs followed by strokes, and 16% had only TIAs, demonstrated that embolism was the most common stroke mechanism, of which 24% were cardiac, 14% were intra-arterial, and 2% were combined cardiac and intra-arterial. In 32%, large artery atherosclerotic occlusive disease caused hemodynamic vertebrobasilar ischemia. Distal infarcts, meaning those within the PCA, SCA, and top of the basilar artery territory, had a high likelihood of cardiac or artery-to-artery embolism. Areas that commonly demonstrated stenosis (>50%) included the origin of the vertebral artery in 131 patients (32.2%), intracranial vertebral artery in 132 patients (32.4%), basilar artery in 109 patients (26.8%), and PCA in 38 patients (9.3%). Fourteen percent of patients had penetrating and branch artery disease, meaning disease of the branch artery without disease of the parent artery. Most PCA infarcts were embolic. Patients with cardiac emboli had a poorer prognosis than patients with other stroke mechanisms. Consistent with prior studies, lateral medullary strokes were most often caused by atherosclerotic vertebral artery disease (see section on PICA territory infarcts).

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here