What is considered a large vessel occlusion in the brain?

Definition of Large Vessel Occlusion in the Brain

A large vessel occlusion (LVO) in the brain is defined as an occlusion of the internal carotid artery (ICA), proximal middle cerebral artery (M1 and M2 segments), vertebral artery, basilar artery, or proximal posterior cerebral artery (P1 segment). 1, 2

Anatomical Classification of LVOs

Anterior Circulation LVOs:

• Internal carotid artery (ICA)
• Middle cerebral artery (MCA)
  • M1 segment (proximal/horizontal segment)
  • M2 segment (insular segment)
• Anterior cerebral artery (ACA)
• Tandem lesions (extracranial ICA stenosis/occlusion with concurrent ipsilateral intracranial occlusion)

Posterior Circulation LVOs:

• Vertebral artery
• Basilar artery
• Posterior cerebral artery (P1 segment)

Clinical Significance

LVOs account for approximately 20-30% of all ischemic strokes 3 and are associated with:

• Higher morbidity and mortality compared to non-LVO strokes
• Poor response to intravenous thrombolysis alone (recanalization rates may not exceed 10%) 1
• Higher rates of permanent disability (40-69%) and death (16-55%) 1
• Primary indication for mechanical thrombectomy

Diagnostic Identification

Imaging Features:

• Hyperdense vessel sign on non-contrast CT (particularly hyperdense basilar artery has 71% sensitivity and 98% specificity) 1, 2
• Definitive diagnosis requires vascular imaging:
  • CT angiography (CTA) - gold standard for rapid identification
  • MR angiography (MRA)
  • Digital subtraction angiography (DSA)

Clinical Features Suggesting LVO:

• Cortical symptoms (aphasia, neglect) are more sensitive indicators for LVO than motor deficits alone 4
• Posterior circulation LVOs may present with:
  • Loss of consciousness
  • Headache, nausea, vomiting
  • Dizziness, vertigo, ataxia
  • Double vision, visual field defects
  • "Locked-in syndrome" in proximal basilar occlusions 1, 2

Treatment Implications

The identification of LVO is critical for treatment decisions:

• Mechanical thrombectomy is indicated for:
  • Anterior circulation LVOs within 24 hours of last known well
  • Basilar artery occlusion within 12 hours (Class I recommendation) 1
  • Basilar artery occlusion between 12-24 hours (Class IIa recommendation) 1
  • Basilar artery occlusion beyond 24 hours may be considered on case-by-case basis (Class IIb) 1

Important Considerations

• LVO strokes have worse outcomes than non-LVO strokes due to the larger volume of affected brain tissue
• Recanalization rates with intravenous thrombolysis alone are significantly lower for LVOs (34%) compared to mechanical thrombectomy (70%) 1
• The benefit of recanalization is dependent on baseline stroke severity - patients with NIHSS >15 benefit most from early recanalization 5
• Posterior circulation LVOs (particularly basilar artery occlusion) are often devastating with mortality rates of 45-86% without treatment 1

Pitfalls in LVO Identification

• Posterior circulation LVOs are often misdiagnosed due to non-specific symptoms
• Relying solely on motor deficits may miss LVOs presenting primarily with cortical symptoms
• Standard NIHSS has limitations in detecting posterior circulation strokes 1
• Approximately 10-30% of LVOs are due to underlying intracranial atherosclerotic disease, which may require different treatment approaches 6

Understanding the definition and implications of LVOs is crucial for rapid triage, appropriate treatment selection, and improved patient outcomes in acute stroke care.