Editorial–a New Method to Classify the Collateral Patterns in the Posterior Circulation

Abstract

Several studies have reported on collaterals in anterior
circulation arterial occlusion seen in patients with acute
ischemic stroke (1–4). The presence of collateral flow in
basilar artery occlusion has been recognized in small studies (5,6). Moscow and Newton (5) reported nine cases of
basilar artery occlusion located proximal to the origins of
the superior cerebellar artery. The posterior cerebral and
superior cerebellar arteries filled in all nine patients via
collateral flow either through the posterior communicating artery (n=7) or pial anastomoses from the middle cerebral arteries to the posterior cerebral arteries (n=2). Subsequently, Archer and Horenstein (6) provided more indepth data regarding frequency and type of collaterals on
the surface of the cerebellum in 20 patients with basilar
artery occlusion and three patients with bilateral vertebral
artery occlusions. Drake (7) reported that three of the
seven patients in whom basilar artery was surgically
occluded to treat intracranial aneurysm survived with minimal deficits presumably due to collateral formation. Such
observation highlighted that collateral flow can develop
and adequately supply the posterior circulation distribution in the event of basilar artery occlusion.
Numerous studies have found that a large proportion of
patients with basilar artery occlusion have a poor outcome
despite recanalization following intravenous or endovascular thrombolytic treatment (8–11). This observation has
prompted the investigators to look for factors such as collateral circulation that are associated with patient outcomes. Brandt et al. (11) determined the variables affecting recanalization and clinical outcome in 51 patients with
basilar artery occlusion who were treated with either intraarterial or intravenous thrombolysis. The initial collateral
state was graded as follows: 0, no collaterals; 1 (minimal),
anterograde or retrograde collaterals with partial or slight
filling; 2 (moderate), anterograde or retrograde collaterals
with filling of the superior cerebellar arteries; and 3 (maximal), collateralization with anterograde and retrograde
channels or maximal bilateral filling of the superior cerebellar arteries. Collateral circulation was moderate or
maximal in 32 patients and was associated with lower
mortality independent of recanalization. However, collaterals that form over the surface of the cerebellum are not
accounted for by this method.
A new classification scheme was proposed to categorize
collateral circulation in patients with basilar artery occlusion or stenosis (see Table). The classification distinguishes the collateral flow in four grades. Grade 1: Retrograde filling of basilar artery through posterior cerebral
artery with filling of the superior cerebellar artery; Grade
2: Retrograde filling of basilar artery through posterior
cerebral artery without filling of the superior cerebellar
artery; Grade III: Bilateral anastomoses of cerebellar or
posterior cerebral arteries; and Grade IV: Unilateral anastomoses of cerebellar or posterior cerebral arteries. The
reason for differentiation between Grade I and Grade II is
based on the prognostic significance of superior cerebellar
artery filling through collateral circulation observed by
Brandt et al (11). Grades III and IV do not have filling of
the basilar artery past the site of occlusion and therefore
constitute a group in which the territories of penetrating
and paramedian arteries are at high risk of ischemia.
If we apply the classification scheme to the individual
patient angiographic data presented as a schematic by
Archer and Horenstein (6), we find the following categories in the 23 patients with vertebral artery or basilar artery
occlusion: Grade I (n=9); Grade II (n=3); Grade III (n=7),
Grade IV (n=1), and no collaterals (n=3). The presence of
Grade I collaterals appeared to be associated with lower
rate of comatose presentation. Six of the nine patients who
were not comatose had Grade I collaterals and two of the
eleven patients who were comatose had Grade I collaterals. Further data are required to determine the prognostic
significance of various patterns of collateral formation.
The four grades in the current grading scheme may allow
greater level of risk stratification in patients with vertebral
or basilar arterial occlusions.