In the fast-paced world of neurointervention, the phrase “time is brain” is both a truth and a limitation. While rapid revascularization is essential in acute ischemic stroke, not all brains die at the same pace.

The critical factor behind this variability?
Collateral circulation.

As we refine our approach to mechanical thrombectomy and expand the window of intervention, understanding how and when collaterals compensate for arterial occlusion becomes central to patient selection, procedural planning, and long-term neurological outcomes.

What Are Collateral Pathways?

Collateral circulation refers to alternate routes of blood flow that bypass the primary site of arterial occlusion. In the cerebral circulation, these include:

• Leptomeningeal anastomoses between ACA, MCA, and PCA branches
• Circle of Willis components (ACom, PCom)
• External-to-internal carotid connections in chronic ICA stenosis
• Retrograde ophthalmic artery flow, in some cases

These pathways act as biological buffers—delaying infarct progression and extending the ischemic penumbra’s viability.

Collaterals: The Real Clock Behind the Clock

Not all patients present equally. Two patients with the same MCA occlusion can have dramatically different outcomes:

• Patient A: Dense hemiplegia, large infarct core within 90 minutes
• Patient B: Mild aphasia, viable penumbra even after 6 hours

The difference?
Patient B has robust collaterals.

This variability is why collateral quality is a key determinant in:

• Functional outcome
• Infarct core expansion rate
• Response to thrombectomy
• Risk of hemorrhagic transformation

Assessing Collaterals in Real Time

Modern imaging allows rapid collateral assessment. Tools include:

CT Angiography (CTA)

• Single-phase or multiphase (preferable)
• Collateral scores assess extent and timing of retrograde flow
• Multiphasic CTA reveals slow-filling but effective collateral perfusion

Perfusion Imaging (CTP or MR-PWI)

• Mismatch between cerebral blood flow (CBF) and mean transit time (MTT) suggests collateral-mediated salvageable tissue

Digital Subtraction Angiography (DSA)

• Real-time assessment during intervention
• Venous-phase delay and collateral fill patterns guide procedural urgency and expectations

Clinical Decision-Making: When Collaterals Inform Strategy

Understanding collateral status can guide:

1. Patient Selection in Extended Windows

• DEFUSE-3 and DAWN trials leveraged perfusion mismatch (reflecting collateral status) to safely treat patients up to 16–24 hours from symptom onset

2. Pre-Procedure Risk Stratification

• Poor collaterals = faster infarct progression = higher hemorrhagic risk
• May prompt earlier decision to defer thrombectomy in borderline cases

3. Procedure Planning

• Longer procedures in patients with poor collaterals increase infarct risk
• Consider shorter, more aggressive retrieval attempts or earlier rescue therapy

4. Prognostication

• Collateral scores correlate with NIHSS improvement, mRS outcome, and mortality

Practical Considerations for Interventionists

• In the angio suite, observe venous phase delay and collateral fill before proceeding
• In borderline cases, use perfusion mismatch and collateral status to determine futility vs benefit
• Avoid over-intervening in patients with poor collaterals and large infarct cores
• Communicate with stroke neurologists—collateral quality helps set expectations with families and guide rehabilitation plans

 Final Thought: We’re Not Just Reopening Arteries—We’re Preserving Viable Brain

Mechanical thrombectomy is not simply a race against time—it is a race against tissue loss.

Collateral circulation is the hidden variable that determines who has time—and who doesn’t.

By integrating collateral assessment into every step of stroke care—from imaging to the angio table—we can tailor interventions more wisely, select patients more ethically, and improve neurological recovery more meaningfully.

Because beyond the clot, there’s a network—silent, dynamic, and absolutely vital—that makes all the difference.

Coming Soon from TriVasc Academy:
Downloadable collateral scoring cheat sheets, imaging interpretation guides, and real case reviews highlighting the impact of collaterals on stroke intervention outcomes.