Interventional Radiology · Neurointervention · Original ASITN/SIR 2003 scale

ASITN/SIR Angiographic Collateral Flow Grading

A five-grade digital subtraction angiography scale describing the speed and completeness of collateral filling of an ischaemic vascular territory.

Purpose

To standardise angiographic assessment and reporting of collateral blood flow to an ischaemic vascular territory in acute arterial occlusion.

Grade	Angiographic collateral filling	Practical interpretation
0	No collateral vessels are visible at the ischaemic site.	Absent collateral flow
1	Slow collateral filling reaches the periphery of the ischaemic site, but a substantial residual filling defect persists.	Slow and incomplete
2	Rapid collateral filling reaches the periphery of the ischaemic site, but only part of the ischaemic territory is supplied and a residual defect persists.	Rapid but incomplete
3	Collateral flow is slow but completely fills the ischaemic vascular bed by the late venous phase.	Slow but complete
4	Collateral flow rapidly and completely supplies the vascular bed of the entire ischaemic territory by retrograde perfusion.	Rapid and complete

How to use it

The scale is fundamentally a DSA grading system and evaluates both the speed and territorial completeness of collateral filling.
Grades 1 and 2 are incomplete: grade 1 fills slowly, whereas grade 2 fills rapidly but still supplies only part of the threatened territory.
Grades 3 and 4 are complete: grade 3 becomes complete only by the late venous phase, whereas grade 4 is both rapid and complete.
Study-specific dichotomisation varies; commonly used groupings include grades 0–1 as poor, grade 2 as intermediate and grades 3–4 as good, or grades 0–2 versus 3–4.
The full grade and angiographic context should be stated because collapsing the scale may conceal clinically relevant differences.

Common mistake

Do not confuse ASITN/SIR collateral grade with TICI or eTICI reperfusion grade: ASITN/SIR describes collateral supply to the threatened territory, whereas TICI-based systems describe antegrade reperfusion after treatment.

Exam pearl

Remember the sequence: 0 absent, 1 slow-incomplete, 2 rapid-incomplete, 3 slow-complete and 4 rapid-complete.

Viva questions

What is the ASITN/SIR collateral grading system?: It is a five-grade DSA scale, from 0 to 4, that assesses the speed and completeness of collateral filling of an ischaemic vascular territory.
Enumerate the ASITN/SIR collateral grades in one line.: Grade 0 is absent flow, grade 1 slow-incomplete, grade 2 rapid-incomplete, grade 3 slow-complete and grade 4 rapid-complete collateral filling.
How do grades 1 and 2 differ?: Both are incomplete. Grade 1 shows slow filling to the periphery, whereas grade 2 shows rapid peripheral filling but supplies only part of the ischaemic territory.
How do grades 3 and 4 differ?: Both completely fill the ischaemic bed. Grade 3 is delayed and becomes complete by the late venous phase, whereas grade 4 is rapid and complete.
Which ASITN/SIR grades are generally considered good collaterals?: Grades 3 and 4 are commonly considered good because collateral filling is complete. However, dichotomisation varies between studies, so the full grade should be reported.
What is the main pitfall when interpreting an ASITN/SIR collateral grade?: The scale requires adequate angiographic coverage and sufficiently late acquisitions. Missing contributing arterial injections or the late venous phase can underestimate collateral completeness.
How does ASITN/SIR collateral grading differ from eTICI grading?: ASITN/SIR grades collateral supply to the threatened vascular bed, while eTICI grades the extent of antegrade reperfusion achieved in the target territory after thrombectomy.
Should the ASITN/SIR grade alone determine eligibility for thrombectomy?: No. Collateral status provides prognostic and physiological information but must be integrated with vessel occlusion, infarct core, clinical deficit, time window and current treatment guidelines.