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.

Definitions:

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.

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.

Outcome:

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

https://clinicaltrials.gov/ct2/show/NCT05030142

https://clinicaltrials.gov/ct2/show/NCT05152524

https://clinicaltrials.gov/ct2/show/NCT05151172