Head CT with hyperdense left MCA sign
He was not a candidate for IV tPA due to his recent use of the blood thinner apixaban. After determining that he would be a good candidate for mechanical thrombectomy, the risks and benefits of the procedure were explained to his wife, who agreed to proceed. He was taken urgently to the interventional radiology suite. The cerebral angiogram confirmed a proximal occlusion of the left MCA (Fig. 7.2). A stent retriever was deployed and the clot was retrieved. A repeat angiogram showed near complete revascularization of the left MCA (Figs. 7.3 and 7.4). This was graded as TICI 3 revascularization. Postprocedure, the patient’s neurological exam drastically improved, with resolution of his aphasia and near-complete resolution in his right arm’s weakness (NIHSS 4). He was admitted to the neurology service for further surveillance. MRI the following day demonstrated an ischemic stroke in the left basal ganglia, which was much smaller than the territory originally at risk at the time of presentation (Fig. 7.5).
Angiogram demonstrating left MCA thrombus
Angiogram with stent retriever crossing thrombus
Angiogram demonstrating restoration of flow in the left MCA
MRI with resultant ischemic stroke
7.2.1 Stroke Systems of Care
Mechanical thrombectomy has demonstrated benefits in carefully selected patients; however most patients with acute ischemic stroke do not receive this therapy. The primary reason for this is delayed arrival to the hospital. As with IV-tPA, mechanical thrombectomy is a time-sensitive procedure with the greatest benefit if pursued early. Therefore, patient recognition of the signs and symptoms of a stroke and calling 911 is the first step to ensure acute stroke care is delivered. The second challenge is the prompt triage of patients with acute ischemic stroke to a center that can offer mechanical thrombectomy . In the US, certain hospitals are designated as Primary Stroke Centers, or Comprehensive Stroke Centers (CSC) by the Joint Commission. Primary stroke centers should have the capability to diagnose and treat stroke patients, including thrombolytics, but they may not have an ability to perform mechanical thrombectomy. Therefore, it is essential that prompt recognition of large-vessel occlusion occurs in hospitals that do not have the capability to administer such therapy, so that urgent transfer can occur to a CSC. At present, because of geographic distances and the difficulty of identifying a large-vessel occlusion in the field, patients with acute stroke may not be preferentially transferred to a CSC by emergency medical services. Patients may instead be brought to the closest hospital which is often a primary stroke center. Often times, the delay in transport to a CSC can cause the patient to be ineligible for thrombectomy, either because they arrive >6 h after symptoms onset, or sufficient time has passed to allow the infarct to fully develop. Future efforts in improving stroke systems of care need also focus on protocols for urgent transfer of patients to appropriate stroke centers where they can receive the full spectrum of care.
7.3 Initial Evaluation
The initial evaluation of a patient with acute ischemic stroke is summarized elsewhere (see Chap. 6 on Acute Ischemic Stroke). Here, we emphasize the aspects of clinical management specifically related to mechanical thrombectomy. During the teams’ initial evaluation, a judgment is made as to whether the patient is likely to have a large-vessel occlusion. A large-vessel occlusion is defined as acute thrombosis in the proximal segment of the anterior, middle, or posterior cerebral artery, the basilar artery, or the internal carotid artery. The presence of a large-vessel occlusion will determine if the patient can proceed to mechanical thrombectomy.
The determination of a large vessel occlusion can be made in several ways. In some centers, a neurologic examination suspicious for a “large-territory” infarction that would include a proximal vascular territory is enough evidence to warrant consideration for mechanical thrombectomy. A conventional cerebral angiogram to confirm a large-vessel occlusion is the first step in mechanical thrombectomy. Because of this, some centers have adopted a protocol to rely on the neurologic examination prior to transfer of the patient to the angiogram suite. However, all of the large-scale clinical trials that demonstrated a benefit of mechanical thrombectomy on functional outcome included at minimum a head CT and vascular imaging.
Other centers have created protocols which incorporate vessel imaging, or other advanced imaging techniques to examine the ischemic penumbra to confirm a large-vessel occlusion prior to recommending mechanical thrombectomy. These protocols are often the result of an ability to quickly obtain advanced imaging, and reflect the selection used in the published trials that confirmed the benefit of mechanical thrombectomy [13, 14, 16]. Options for advanced imaging include a CT or MR angiogram, a CT or MR perfusion study.
7.3.1 Patient Selection for Mechanical Thrombectomy Using Imaging
There are several methods supported by the literature to select patients that may benefit from mechanical thrombectomy. The most commonly used methods include the CT-based scoring system of the ASPECTs score and CT and MR perfusion. The purpose of patient selection is to offer mechanical thrombectomy to patients that are most likely to benefit from revascularization. Examples of patients that are unlikely to benefit include those with a minimal ischemic penumbra, suggesting that there is no “tissue to save” or patients with a large stroke who are more likely to experience hemorrhagic transformation of the stroke if revascularization is achieved. The key principles, no matter what advanced imaging technique is used, is to select patients with a small ischemic core infarct, and large ischemic penumbra. The ratio of the penumbra to core would be large in patients who are most likely to benefit from reperfusion.
One method of image selection incorporates the CT-based ASPECTs score. The ASPECTS score was developed in 2000 as an attempt to quantify the size of the ischemic infarct on a CT scan in order to predict outcome after mechanical thrombectomy . The benefit of this scoring system is that all centers that administer tPA have the ability to quickly obtain a CT scan. The ASPECTS scoring system is qualitative and ranges from 0 to 10, with higher numbers indicating less ischemia. In this scoring system, a point is subtracted for each of ten territories where there is evidence of ischemia. For information and training on this scoring system, see www.aspectsinstroke.com. Two large randomized controlled trials demonstrated benefit to mechanical thrombectomy in patients with an ASPECTS score > 6 or >7 [14, 17]. These trials help establish a cutoff from which treatment may be beneficial to select patients for mechanical thrombectomy.
The benefit of CT perfusion as compared with an ASPECTS-based scoring system is that the ischemic penumbra can be visualized along and the core infarct area can be estimated. Three of the recently completed trials that demonstrated a benefit for mechanical thrombectomy utilized CT perfusion for some of the participants [14–16]. CT perfusion displays the physiologic function of the brain in the form of perfusion maps. A perfusion map is a view of the brain where each pixel measures blood flow to that area, with different colors assigned to represent a measurement over time. For example, one type of perfusion map is called the mean transit time (MTT). This map measures the mean amount of time it takes for the contrast to get to each pixel and can provide an estimate of the size of the ischemic penumbra. Other types of perfusion maps include the cerebral blood flow (CBF) and cerebral blood volume (CBV) maps . Regions of the brain with reduced CBV and CBF can represent the area of core ischemic damage. There are several vendor software packages and institutional methods that are used to determine thresholds, or cutoffs to distinguish the ischemic core from the penumbra. These thresholds are based on a relatively few number of studies, and each institution that interprets CT perfusion should establish its own standards for decision-making.
In institutions where MRI scanning is readily and quickly available, MR perfusion techniques are often used to select patients for mechanical thrombectomy. The benefit of MRI over CT is that a more accurate determination can be made of the ischemic core. This is because of the higher sensitivity and specificity of MRI in detecting hyperacute ischemia . Ischemic core can be accurately estimated using the diffusion-weighted imaging (DWI) sequence in the MRI protocol. MR perfusion begins with administration of gadolinium which allows for the generation of maps similar to those created during CT perfusion. The ischemic penumbra can be defined by MTT or time-to-maximum (Tmax) maps which highlight areas of hemodynamic compromise. The ratio of the ischemic penumbra to the core infarction measured on DWI can be used to augment decision-making. As with CT perfusion, several different thresholds exist to measure the ischemic penumbra and several vendor software packages are available to assist in the measurement of the volumes. In MRI, the amount of poorly perfused tissue that is still at risk of infarction is often called the diffusion–perfusion mismatch. It is this number which can accurately predict whether there is a substantial amount of “brain to save” through revascularization.