Which intracranial arteries are classified as large‑vessel occlusions (LVOs)?

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Large Vessel Occlusions: Anatomic Definition

Large vessel occlusions (LVOs) are defined as occlusions of the intracranial internal carotid artery (ICA), M1 and M2 segments of the middle cerebral artery (MCA), basilar artery, vertebral artery, and the A1/A2 segments of the anterior cerebral artery. 1

Anterior Circulation LVOs

The anterior circulation LVOs include:

  • Intracranial internal carotid artery (ICA) - including tandem ICA-MCA lesions
  • M1 segment of the middle cerebral artery - the most common occlusion site (33.3% of all LVOs) 2
  • M2 segment of the middle cerebral artery - second most common (27.3% of all LVOs) 2
  • A1 and A2 segments of the anterior cerebral artery 1

The M1 segment can be further characterized as proximal (involving the proximal half or occluding lenticulostriate vessels) or distal (involving the distal half or sparing lenticulostriate vessels). 3

Posterior Circulation LVOs

Posterior circulation LVOs (pc-ELVO) account for approximately 20% of all strokes but only 1% of all ischemic strokes. 4 These include:

  • Basilar artery - the most clinically significant posterior circulation LVO, now with Class I evidence for thrombectomy within 12 hours 5
  • Vertebral artery - particularly when causing basilar territory compromise 5
  • P1 segment of the posterior cerebral artery - though variably classified as medium vessel occlusion in some studies 6

Important Clinical Context

The prevalence of LVO among confirmed acute ischemic stroke patients is approximately 18.6% (95% CI 17.3%-20.0%) when using this broad anatomic definition. 2 This is lower than many earlier estimates that ranged from 13-52%, which varied based on different definitions and detection methods.

Key Diagnostic Considerations:

  • CT angiography (CTA) is the diagnostic cornerstone for identifying LVOs, with high accuracy and interrater reliability 1
  • For posterior circulation, CTA confirmation of basilar or vertebral artery occlusion is required for thrombectomy consideration 5
  • The target downstream territory (TDT) varies by occlusion site - for ICA occlusions, this typically includes both ACA and MCA territories unless anterior communicating artery collaterals supply the ACA 3

Distal/Medium Vessel Occlusions:

P2 and more distal posterior cerebral artery segments are considered distal or medium vessel occlusions, not large vessel occlusions, and thrombectomy for these remains unproven. 5 Similarly, M2 occlusions occupy a gray zone - they are included in the LVO definition but have lower rates of thrombectomy treatment compared to M1 occlusions. 2

The distinction matters clinically because LVOs are less likely to recanalize with alteplase alone and should be considered for endovascular therapy, whereas more distal occlusions have higher spontaneous recanalization rates. 1

References

1
Guideline

management of acute ischemic stroke due to large-vessel occlusion: jacc focus seminar.

Journal of the American College of Cardiology, 2020

2
Research

Large Vessel Occlusion in Acute Ischemic Stroke Patients: A Dual-Center Estimate Based on a Broad Definition of Occlusion Site.

Journal of stroke and cerebrovascular diseases : the official journal of National Stroke Association, 2020

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