Thrombectomy beyond large vessel occlusion - Ep.1/3 - SLICE Worldwide 2022

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Thrombectomy beyond large vessel occlusion – definitions, outcome and functional eloquence

Endovascular thrombectomy (EVT) is well established as a highly effective treatment for acute ischemic stroke (AIS) due to proximal, large vessel occlusions (PLVOs). With iterative further advances in catheter technology, distal, medium vessel occlusions (DMVOs) are now emerging as a promising next potential EVT frontier. DMVOs cause 25% to 40% of AISs, arising as primary thromboemboli and as unintended consequences of EVT performed for PLVOs, including emboli to new territories (ENTs) and emboli to distal territories (EDTs) within the initially compromised arterial field. DMVOs produce clinical syndromes that are highly heterogenous but frequently disabling. While intravenous fibrinolytics are more effective for distal than proximal occlusions, they fail to recanalize one-half to two-thirds of DMVOs.


Two key distinctive anatomic features of distal, medium cerebral arteries profoundly affect endovascular procedure conduct and endovascular device design: vessel distance/tortuosity and vessel size.

Vessel distance/tortuosity

Vessel size

Distal arteries: have ≥1 additional branch steps than proximal arteries and also loop around neuroanatomic structures such as the corpus callosum (ACA), insula and temporal lobe (MCA), and temporal and occipital lobe (PCA).

Large vessels: > 2.0 mm 

the intracranial internal carotid artery (ICA; typical diameter, 3.8 mm), the M1 segment of the MCA (2.7 mm), the basilar artery (3.2 mm), and the vertebral artery (2.8 mm)

Medium vessels: 0.75 – 2.0 mm

the M3 MCA arteries (typical diameters at origin, 1.1–1.5 mm), M4 MCA, A2 to A5 ACA, and P2 to P5 PCA

Small vessels: <0.75 mm  

deep penetrator arteries (typical lenticulostriate artery diameter, 0.5 mm), long pial penetrator arteries, and surface pial arteries (typical pial artery diameter, 0.2 – 0.7 mm). 

Since distance tortuosity and size are each important aspects, the Summit group recommended adopting a general label that captures both—DMVOs. Where appropriate, the use of labels using either of the distinctive anatomic features alone was also endorsed: distal vessel occlusions (DVOs) or medium vessel occlusions (MVOs/MeVOs) (contrasted with proximal vessel occlusions [PVOs] or large vessel occlusions [LVOs], respectively).

In recent publications medium vessel occlusions   have been defined as occlusions of the M2, M3, A2, A3, P2, or P3 segment.

There is wide agreement in the literature that the proximal, large artery category includes the intracranial ICA, M1 MCA segment, intracranial vertebral arteries, and basilar artery. 

Similarly, a general consensus recognizes the distal, middle artery category as including the M3 and M4 MCA segments, A2 to A5 ACA segments, P2 to P5 PCA segments, PICA, AICA, and SCAs. 

However, categorization of M2 MCA, A1 ACA, and P1 PCA has varied. Positioning of the M2 MCA within any classification system is particularly challenging, as M2 MCA angioarchitecture is highly heterogenous across patients.



M2 MCA branch patterns include bifurcation, trifurcation, tetrafurcation, and candelabra.


M2 MCA vessel segments range in size from 1.1 to 2.1 mm in diameter.

M2 MCA occlusions may occur in dominant segments that are similar to M1 in size (though still more distal and branched/tortuous to reach) or in nondominant segments that are similar to M3 and other much small arteries in size. 

The 12 cortical branch arteries range from 1.1 to 1.5 mm in diameter at their origin.



The outcome of DMVO-AIS under medical therapy is characterized by frequent disability and, among M2 high mortality. 

In the multicenter STOP Stroke study (Screening Technology and Outcome Project), at 6 months, M2 occlusions were associated with functional dependency/death (modified Rankin Scale [mRS] score, 3–6) in 60% of patients and mortality in 24%. 

Proximal ACA and PCA occlusions were associated with functional dependency/death (mRS score, 3–6) in 77% and mortality in 8%.

Functional eloquence:

The clinical syndromes of hemispheric DMVOs reflect highly focal compromise of cognitive, motor, sensory, and visual function subserved by the cortex and superficial white matter. Common are partial rather than global aphasia, fractionated rather than complete hemiparesis or hemianesthesia, as well as partial or complete hemivisual field defects. 

While presenting deficits of more distal DMVOs can be subtle, more proximal DMVOs generally produce severe initial symptoms. In the multicenter STOP Stroke study, initial mean National Institutes of Health Stroke Scale (NIHSS) scores were: 

M2 – 11.5 NIHSS

A1 – 8.5 NIHSS 

A2 – 12.4 NIHSS 

P1 – 16.3 NIHSS 

P2 – 10.4.21 NIHSS

With improving imaging modalities and catheter technology advancement, distal, medium vessel occlusions (DMVOs) are being increasingly recognized as a target for endovascular therapy.  

DMVOs may arise as primary (initially found occlusions) or secondary (unintended sequalae) from LVO treatment. They may both be disabling and it is crucial to predict their clinical importance on patient's outcome. This is only possible when we deeply understand the functional anatomy and technical thrombectomy boundaries.

For more information, refer to the publication: 

Saver, Jeffrey L., et al. "Thrombectomy for distal, medium vessel occlusions: a consensus statement on present knowledge and promising directions." Stroke 51.9 (2020): 2872-2884. 

There you will find, among other things, valuable images relating to functional anatomy based on selected types of occlusions.

Ongoing distal thrombectomy trials: DISCOUT, DISTALS and ESCAPE MeVO



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