Collateral Score on CT Angiography: Prognostic Significance in Stroke Thrombectomy
Collateral score on CT angiography is a critical prognostic marker that predicts functional outcomes, infarct progression, and mortality in stroke patients undergoing thrombectomy—better collaterals correlate with smaller infarct volumes, better 90-day functional independence, and benefit from reperfusion therapy even in patients with low ASPECTS scores. 1
Prognostic Value for Clinical Outcomes
Good collateral circulation on CTA independently predicts favorable functional outcome (mRS 0-2) at 90 days after thrombectomy. 1
- In the PROACT-II trial, patients with good collaterals who received intra-arterial thrombolysis had significantly better outcomes compared to controls, while patients without collaterals showed no benefit from treatment 1
- The presence of collaterals directly affects CT infarct appearance and clinical presentation—more collaterals correlate with smaller infarcts on CT and lower stroke scale scores 1
- In posterior circulation strokes specifically, a 6-point CTA collateral score correlates with good outcomes (mRS ≤3 at 3 months) in patients undergoing thrombectomy 1
Impact on Infarct Progression and Tissue Viability
Collateral status governs the pace and severity of cerebral ischemia, distinguishing fast versus slow progressors 2
- Good collateral circulation on CTA is associated with significantly lower 24-hour infarct volumes (17.2 cm³ vs 97.8 cm³ for poor collaterals, p<0.01) 3
- Each 1-point increase in collateral score reduces lesion water uptake (edema formation) by 1.9% in follow-up CT 4
- Collateral scores significantly correlate with relative cerebral blood volume and relative cerebral blood flow on CT perfusion 5
Treatment Selection and Decision-Making
Collateral assessment should guide thrombectomy decisions, particularly in challenging scenarios:
- Patients with low ASPECTS (≤5) but good collaterals (CS 2-4) still benefit significantly from endovascular recanalization (OR 3.0 for good outcome, p=0.003) 4
- In the DAWN trial (6-24 hour window), collateral flow was a significant predictor of 90-day mRS 0-2 in the endovascular arm: 43.7% with good collaterals achieved favorable outcome versus only 17.7% with poor collaterals (p=0.026) 2
- Collateral status may serve as a selection criterion for thrombectomy in low ASPECTS patients who would otherwise be excluded 4
Optimal Assessment Methods
Multiphase CTA provides superior prognostic accuracy compared to single-phase CTA: 3
- Good collateral status on multiphase CTA (mCTA) was independently associated with functional independence at 3 months (OR 5.0,95% CI 1.99-12.6, p<0.01), while single-phase CTA collaterals were not 3
- mCTA evaluation showed lower 24-hour NIHSS scores (5 vs 8.5, p=0.04) and discharge NIHSS scores (2 vs 4.5, p=0.04) compared to single-phase assessment 3
Among single-phase CTA scoring systems, the Maas and Miteff scores demonstrate best performance: 5
- Maas score achieved 96% sensitivity for favorable outcome prediction 5
- Miteff score best differentiated between favorable and poor outcome groups in ROC analysis 5
- All validated CTA collateral scores show very good to substantial inter-rater reliability 5
Critical Caveats and Pitfalls
The benefit of collaterals depends on achieving successful recanalization—collaterals alone without reperfusion do not guarantee good outcomes 1
- In the BASICS trial for posterior circulation strokes, there was no association between patent collateral circulation and good outcome, likely because recanalization rates and timing varied 1
- Collateral effects are location-dependent: excellent transcortical collaterals may preserve cortex but permanent hemiplegia can still result from infarction of non-collateralized deep structures like the internal capsule and basal ganglia 1
Timing of collateral assessment matters—collaterals can fail over time, and late window patients (6-24 hours) still show diverse collateral grades on both CTA and DSA 2