A Classification for the Anterior Inferior Cerebellar Artery-Subarcuate Artery Complex Based on the Embryological Development

Abstract

Objective To hierarchize the anterior inferior cerebellar artery (AICA)-subarcuate artery (SAA) complex's variations in the surgical field. Background The AICA's "subarcuate loop" (SL) presents multiple variations, closely related to the SAA. AICA-SAA complex's variations may represent major issues in cerebellopontine angle (CPA) surgery. As the spectrum of configurations is originated during the development, a systematized classification was proposed based on the interaction between the petrosal bone and the AICA in the embryonic period. Methods The variations were defined as follow: Grade 0: free, purely cisternal AICA, unidentifiable or absent SAA; Grade 1: purely cisternal AICA, loose SL, SAA > 3 mm; Grade 2: AICA near the subarcuate fossa, pronounced SL, SAA <3 mm; Grade 3: "duralized" AICA, unidentifiable SAA, or included in the petromastoid canal (PMC); and Grade 4: intraosseous AICA, unidentifiable SAA, or included in the PMC. The classification was applied to a series of patients assessed by magnetic resonance constructive interference in steady state sequence. Surgical examples were also provided. Results Eighty-four patients were evaluated, including 161 CPA. The proportions found in the gradation remained within the range of previous publications (Grade 0: 42.2%; Grade 1: 11.2%; Grade 2: 35.4%; Grade 3: 10.6%; and Grade 4: 0.6%). Moreover, the degrees of the classification were related to the complexity of the anatomical relationships and, therefore, to the difficulty of the maneuvers required to overcome them. Conclusion The proposed AICA-SAA complex classification allowed to distinguish and objectify pre- and intraoperatively the spectrum of variations, to thoroughly plan the required actions and instrumentation.

Keywords: anterior inferior cerebellar artery; cerebellopontine angle; petromastoid canal; subarcuate artery; subarcuate fossa; subarcuate loop.

Fig. 1

Schematic digital art interpretation of the classification, right CPA superior view. (A) Grade 0 (cisternal AICA, unidentifiable SAA); (B) Grade 1 (SAA >3 mm); (C) Grade 2 (SAA <3 mm); (D) Grade 3 (AICA adhered to the dura mater); (E) Grade 4 (AICA with intraosseous path). AICA, anterior inferior cerebellar artery; CPA, cerebellopontine angle; SAA, subarcuate artery.

Fig. 2

Identification of each degree of the proposed classification, MRI CISS sequence. (A) Grade 0; (B) (mirrored): Grade 1; (C) Grade 2; (D) Grade 3; and (E) (mirrored): Grade 4. The dotted area in (A) symbolizes the region focused in each example. AICA, anterior inferior cerebellar artery; CISS, constructive interference in steady state; MRI, magnetic resonance imaging; SAA, subarcuate artery.

Fig. 3

Identification of each degree of the proposed classification, intraoperative recordings. (A) (Right park bench position, 90-degree right rotation): Grade 1; (B) (left park bench position, 90-degree left rotation, mirrored): Grade 2; (C) (left park bench position, 90-degree left rotation, mirrored): Grade 3; and (D) (right semisitting position): Grade 4. Dashed lines indicate where the AICA is attached to the dura mater (C) or encased by bone (D). AICA, anterior inferior cerebellar artery; SAA, subarcuate artery.

Fig. 4

Length of the SAA according to laterality (average ± 95% CI). There were no significant differences ( p  = 0.9634, n  = 75, one-way ANOVA). ANOVA, analysis of variance; CI, confidence interval; SAA, subarcuate artery.

Fig. 5

Absolute frequencies of the SAA according to length and laterality. A Gaussian distribution with a peak incidence of 1 mm was evident. SAA, subarcuate artery.

Fig. 6

Histogram of relative frequencies of the SAA according to length and laterality. A concordant unimodal distribution was found on both sides, which also was reflected in the global distribution. SAA, subarcuate artery.