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. 2026 Feb 20;168(1):43. doi: 10.1007/s00701-025-06749-4

Microsurgical anatomy of the anterior inferior cerebellar artery- systematic review with meta-analysis

Jonasz Tempski 1,2,5,✉, Grzegorz Fibiger 1,2,3,4, Radosław Chudy 4, Jakub Pękala 1,2, Katarzyna Majka 4, Oliwia Andrasz 4, Ignacy Jastrzębski 4, Dominik Łazarz 1,2, Kamil Możdżeń 4, Stanisław Adamski 5, Jerzy A Walocha 1,2, Przemysław Pękala 1,2, R Shane Tubbs 6
PMCID: PMC12923414  PMID: 41718792

Abstract

Purpose

The anterior inferior cerebellar artery (AICA) is a major vessel of cerebellopontine angle (CPA) and one of main cerebellar arteries. Many studies described a variety of AICA variants, however it has never been systematically reviewed and pooled in order to provide the neurosurgeons with the standardized knowledge in regard to most common types, including their clinical associations. This study aimed to determine AICA’s microsurgical anatomy, including its morphometry, branching, origin, relationships with cranial nerves, as well as their respective neurosurgical considerations, such as tumor resection, decompression and bypass procedures.

Methods

Major databases (PubMed, Embase, Science Direct, Scopus, and Web of Science) were systematically searched up to November 2024. A total of 64 studies (n = 15,529 arteries) were included.

Results

The AICA originated from the basilar artery (BA) in 90.1% (95% CI:77.2–98.4; p < 0.001) of the general population, and had a unilateral branching pattern in 85.5% (95% CI: 78.4–89.6;p < 0.001). It was in contact with the abducens nerve (CN VI) up to 82.2% (95% CI: 54.7–100.0; p < 0.001) in general population. Regarding the facial-vestibulocochlear complex (CN VII-VIII), we found AICA to be located between CN VII and CN VIII in 60.6% (95% CI: 11.5–56.1;p < 0.001). AICA loops, were as common as 58.3% (95% CI:34.7–80.2;p < 0.001) in the general population, ranging up to 77.1% (95% CI:18.7–100.0;p < 0.001) in the South Korean population and just 18.0% (95% CI:1.0–45.1;p < 0.001) in the Turkish population.

Conclusions

Neurosurgeons should remember that the AICA can present with many variants, which can produce various neurological symptoms, and increase the risk of iatrogenic lesions during numerous neurosurgical procedures.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00701-025-06749-4.

Keywords: Anterior inferior cerebellar artery, Cerebellopontine angle, Compression syndrome, Posterior circulation, AICA loop, Anatomical variations, Meta-analysis

Introduction

The anterior inferior cerebellar artery (AICA) is one of the most significant vessels of the cerebellopontine angle (CPA). In most cases, it originates from the middle portion of the basilar artery (BA) [28, 31], specifically the lateral section. It runs posterolaterally, ventrally to the pons in the prepontine cistern, entering the CPA and continuing its course through the anterior portion of the cerebellum [24]. The main trunk subsequently bifurcates into superior (caudal) and inferior (rostral) trunks at the level where the facial-vestibulocochlear (VII-VIII) nerves exit [8, 16, 37].

The area supplied by the AICA includes the anterior inferior surface of the cerebellum, the flocculus, the choroid plexus of lateral ventricles, the lower third of lateral pontine area, sometimes the middle third or superior part of the lateral region of medulla [24].

The CPA region is clinically and anatomically significant due to the presence of numerous cranial nerves and their exit points, with which the AICA has many reciprocal relationships.

Therefore, the possible contacts of these nerves with AICA in a space where they are so concentrated, and the formation of loops can cause a variety of cranial nerve compression syndromes (CS), characteristically connected with the facial (VII) and vestibulocochlear (VIII) nerves, presenting as tinnitus, hearing loss and hemifacial spasm. In such cases microvascular decompression (MVD) can be needed to relieve such symptoms [55].

In addition to vascular formations, including loops or aneurysms, CPA tumors can cause cranial nerve syndromes, potentially distorting the normal anatomy of the neurovascular structures, leading to an increased risk of iatrogenic AICA lesions, during surgical resection.

Additionally, the AICA can be used during bypass surgeries, especially the occipital artery-AICA bypass. In such a complex procedure correct identification of vessels is crucial, but can be complicated by the many different subtypes of AICA, as well as AICA loops.

All the aforementioned situations can lead to iatrogenic lesions involving either ischemic or hemorrhagic events, both of which result in similar clinical symptoms including vertigo, dizziness, facial paresis, ataxia, nystagmus or even death [8, 46].

This study aimed to review the microsurgical anatomy of the AICA, focusing on its origin, variations, branching patterns, and neurovascular relationships. The review should support clinicians and neurosurgeons, with detailed knowledge regarding this vessel. It could provide important insights during all the aforementioned neurosurgical procedures.

Materials and methods

This systematic review and meta-analysis was prospectively registered in PROSPERO (CRD42024533886).

Search strategy

In November 2024 the main online databases (PubMed, Embase, Web of Science Core Collection, Scopus, SciElo) were searched extensively to extract all studies that included relevant information regarding the AICA and its anatomy. We also reviewed all the major anatomical, neurosurgical journals (Annals of Anatomy, Journal of Anatomy, Anatomical Record, Clinical Anatomy, Surgical and Radiologic Anatomy, Anatomical Science International, Folia Morphologica, Journal of Neurosurgery, World Neurosurgery, Acta Neurochirurgica, etc.), and clinical journals related to the anatomical structure of the study. Finally, the authors searched the references of all incorporated studies for additional articles eligible for inclusion in the meta-analysis. No data or language restrictions were imposed on.

the search for articles(64 studies were in English, 1 in Chinese, 2 in Spanish and 1 in Portuguese). Search terms used in this systematic review and meta-analysis included: "anterior inferior cerebellar artery" AND ("anatomy" OR "origin" OR "course" OR "branch" OR "morphometry" OR "variant" OR "variation" OR “distance”).

The detailed process by which the studies were included in our meta-analysis is summarized in the PRISMA flow chart (Fig. 1) [57]. Guidelines for writing evidence-based anatomical meta-analyses were followed [30].

Fig. 1.

Fig. 1

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PRISMA flow chart for study identification, evaluation, and inclusion in the meta-analysis

Eligibility assessment

Eligible articles included in our study were assessed by four independent reviewers: OA,RC,KM,IJ. All studies describing extractable data about the anatomical characteristics and prevalence of the AICA were included. The following exclusion criteria were used: (1) animal studies, conference abstracts, case studies, letters to editors, and reviews; (2) studies with incomplete or irrelevant data (e.g. when the distance measurements were given without signifying specific direction along which the AICA was measured or when they lacked mean values). There were no language or date restrictions. Publications in languages other than English were included and assessed by medical professionals fluent in the original language. Any disagreement about the eligibility of a study was resolved by consensus among reviewers, in some cases after consulting with the authors of the original study, if possible and necessary.

Data extraction

Three independent reviewers: JT, GF, KM extracted data regarding prevalence and origin, including laterality, ethnicity, number of branches, relationship with specific cranial nerves, Chavda classification, loop formation and morphometric data concerning the mean diameter of the AICA.

Quality assessment

To assess the possible risk of bias, the Anatomical Quality Assurance (AQUA) Checklist and the Anatomical Quality Assessment Tool were used. The AQUA guidelines have been validated by the Federative International Committee for Scientific Publications (FICSP) of the International Federations of Associations of Anatomists (IFFA) [29]. A methodological assessment of the included studies, revealed a predominantly "high" risk of bias in two key areas: study objectives, characteristics and methodological rigor. The first concern stemmed from the absence of clearly stated demographic data, such as the sex and ethnicity of the specimens. The second was attributed to incomplete reporting by the authors, particularly regarding the number of researchers involved in dissections or radiological assessments and their respective levels of expertise.

Statistical analysis

Statistical analysis was performed by JP using MetaXL 5.3 by EpiGear International Pty Ltd (Wilston, Queensland, Australia). The random effects model was implemented to approximate the pooled prevalence. The I2 statistic and Chi2 tests were used to determine the heterogeneity of the included studies. The I2 statistics were interpreted in terms of four intervals: 0–40% (“might not be important”), 30–60% (“might indicate moderate heterogeneity”), 50–90% (“might indicate substantial heterogeneity”), and 75–100% (“might represent considerable heterogeneity”) [Cochrane Handbook]. Cochran’s Q p-value < 0.10 was used to define significant heterogeneity among studies in the Chi2 test [Cochrane Handbook].

Results

Study selection

The process by which the studies used in this article were identified and selected is presented in the PRISMA flow chart (Fig. 1). An extensive search through the major online databases revealed a total of 2013 studies. Searching through the references of the included articles, revealed an additional 9 articles. After duplicates were removed 862 records remained. We evaluated their full texts to determine their eligibility; 130 articles were excluded due to either lack of the data or irrelevance, and 668 were excluded based on article type, such as case reports or reviews. Finally, 64 articles were included in our meta-analysis following the predefined inclusion criteria.

Study characteristics

A total of 64 studies (n = 15529 arteries) were included and consisted of 37 Anatomical body donor studies studies, 23 radiological studies, two surgical studies and one mixed-methods study (including both radiological and anatomical body donor data). The characteristics of the incorporated data are shown in Table 1. The geographic origins of the studies included six continents (Australia, Africa, Asia, North America, Europe, South America), most being from Asia (26 studies = 5084 AICAs.).

Table 1.

List of studies included into meta-analysis

Study ID Country, Continent Type of study Number of AICA
Akar 1995 [3] USA, North America Anatomical body donor study 22
Akgun 2013 [4] Turkey, Asia Radiological 270
Alonso F 2017 [5] Grenada, North America Anatomical body donor study 50
Ates 2008 [7] USA, North America Anatomical body donor study 12
Pai BS 2007 [58] India, Asia Anatomical body donor study 50
Ballesteros 2020 [8] Colombia, South America Anatomical body donor study 184
Bayav 2021 [9] Turkey, Asia Radiological (MRI) 498
Best 2013 [10] Germany, Europe Radiological (MRI) 80
Brunsteins 1990 [11] Argentina, South America Anatomical body donor study 26
Caille 1974 [12] France, Europe Radiological (angiography) 100
de Abreu Junior 2016 [1] Brazil, South America Radiological (MRI) 64
Delion 2017 [17] France,Europe Anatomical body donor study 50
De Vilalta 2019 [16] USA,North America Anatomical body donor study 12
Di Russo 2019 [18] USA,North America Anatomical body donor study 22
Di Stadio 2020 [19] Italy,Europe Radiological (MRI) 5244
Dos Santos 2020 [20] Brazil,South America Anatomical body donor study 88
Ensari 2017 [21] Turkey,Asia Radiological (MRI) 88
Erdogan 2013 [22] Turkey, Europe Radiological (MRI) 374
Esmer 2010 [23] Turkey, Asia Anatomical body donor study 44
Fomkina 2016 [25] Russia, Asia Anatomical body donor study 48
Gerald 1973 [27] USA,North America Radiological (angiography) 102
Habibi 2011 [28] Iran,Asia Anatomical body donor study 62
Hou 2020 [31] China, Asia Radiological (DSA) 1000
Jiménez-Castellcmos 1992 [34] Spain, Europe Anatomical body donor study 30
Kawashima 2005 [37] USA, North America Anatomical body donor study 44
Kim 1990 [39] South Korea, Asia Anatomical body donor study 52
Kim 2019 [38] South Korea,Asia Radiological (MRI) 98
Kwiatkowska 2023 [40] Poland, Europe Anatomical body donor study 200
Macchi V 2004 [42] Italy, Europe Anatomical body donor study 80
Mahmood 1991 [43] USA, North America Anatomical body donor study 40
Ma 2020 [41] China,Asia Radiological (MRI) 204
Mandiola 2002 [44] Brazil, Mandiola Anatomical body donor study 140
Marinkovic 1995 [46] Serbia, Europe Anatomical body donor study 50
Marinković 1994 [45] Serbia, Europe Anatomical body donor study 28
Martin 1980 [47] USA,North America Anatomical body donor study 50
McDermott, A.-L. 2003 [48] United Kingdom, Europe Radiological (MRI) 664
Mejía-Quinones 2022 [49] Colombia,South America Radiological (MRI) 204
Mercier 2008 [50] France, Europe Anatomical body donor study 50
Meybodi 2023 [51] USA,North America Cadavery (Endoscopy) 34
Morita 1989 [52] Japan, Asia Surgery 8
Murali 1991 [53] India,Asia Anatomical body donor study 46
Naidich 1976 [54] USA,North America Anatomical body donor study 63
Nithish 2024 [81] India,Asia Radiological 228
Pekcevik 2014 [59] Turkey, Asia Radiological (CTA) 682
Ramesh 2009 [60] India,Asia Anatomical body donor study 100
Reka 2019 [56] India, Asia Radiological (MRI) 80
Reisser 1991 [61] USA,North America Anatomical body donor study 1327
Reyes-Soto 2024 [62] Mexico, South America Anatomical body donor study 100
Ryu 1999 [63] Japan,Asia Anatomical body donor study 34
Salgado-Lopez 2020 [64] USA,North America Anatomical body donor study 25
Samanvitha 2024 [65] India,Asia Radiological (MRI) 224
Saraf 2022 [66] India,Asia Radiological (MRI) 262
Shrontz 1986 [67] USA,North America Anatomical body donor study 54
Sirikci 2005 [68] Turkey, Asia Radiological (MRI) 280
Su 2024 [69] China,Asia Radiological (CTA) 164
Sunderland S 1948 [70] Australia,Australia Anatomical body donor study 420
Ucerler 2008 [73] Turkey, Asia Anatomical body donor study 40
Vejarano 2019 [74] Colombia, South America Anatomical body donor study 58
Watt 1935 [75] Canada, North America Anatomical body donor study 64
Wende 1975 [76] Germany,Europe Radiological (angiography) 238
Woischneck 1991[77] Germany, Europe Anatomical body donor study 52
Yegin 2023 [78] Turkey, Asia Surgery 218
Yurtseven 2004 [79] Turkey, Asia Anatomical body donor study and Radiological (MRI) 188
Zafar 2024 [80] Pakistan, Asia Radiological (MRI) 116
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AICA Anterior Inferior Cerebellar Artery, MRI Magnetic Resonance Imaging, CTA Computed Tomography Angiography, DSA Digital Subtraction Angiography

Some studies lacked data on the ethnicity of the specimens, so we decided to distribute studies from the same countries and continents into separate sub-groups and subsequently pool the data to achieve the specified results. We present the results in the general population below; the data for the sub-groups are shown in their respective tables.

Branching of AICA

The AICA did not branch in 2.9% (95% CI:0.7–6.1; p < 0.001) of the general population.

There was a single AICA branch in 85.5% (95% CI:78.4–89.6; p < 0.001) of the general population.

Double and triple branching occurred in 10.4% (95% CI:6.0–15.5; p < 0.001) and 0.7% (95% CI:0.0–2.3; p < 0.001) of the general population, respectively.

Four branches were present in 0.5% (95% CI:0.0–1.9; p < 0.001) of the general population.

The detailed subgroup analysis is presented in Electronic Supplementary Material Table 7.

Hypoplastic AICA

The AICA was hypoplastic in 30.6% (95% CI:17.3–45.6; p = 0.016) of the general population.

Electronic Supplementary Material Table 8 presents the number of hypoplastic AICAs in the general population.

Origin of the AICA

The basilar artery (BA) was the most common origin of the AICA, with prevalence of 90.1% (95% CI:77.2–98.4; p < 0.001). The internal carotid artery (ICA) and vertebral artery (VA) were less common: 32.6% (95% CI:22.1–43.9; p = 0.697) and 10.1% (95% CI:0.0–28.8; p < 0.001), respectively. The least common origin points included, the posterior inferior cerebellar artery (PICA) and the AICA-PICA common trunk, which were noted in 6.9% (95% CI: 2.2–13.7; p = 0.075) and 9.5% (95% CI:4.1–16.8; p < 0.001), respectively (Fig. 2).

Fig. 2.

Fig. 2

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Variable origin of the anterior inferior cerebellar artery (AICA) and arterial circulation at the base of the brain

The detailed subgroup analysis is presented in Table 2.

Table 2.

AICA origin in different populations

AICA origin point Population Stydy type no. of studies (no. of AICA) prevalence [%](95% CI) I2 [%](95% CI) P-value
Basilar Artery(BA) General Mixed studies 14 (1794) 90.1 (77.2–98.4) 97.2 (96.3–97.8) p < 0.001
Anatomical body donor study 11 (592) 96.3 (90.4–99.7) 86.4 (77.4–91.8) p < 0.001
Imaging (Angiographic studies) 3 (1202) 50.9 (0.0–100.0) 99.3 (98.9–99.5) p < 0.001
Asian Mixed studies 5(1194) 91.6 (78.1–100.0) 93.6 (88.8–96.4) p < 0.001
Anatomical body donor study 4 (194) 94.7 (79.9–100.0) 92.4 (83,8–96.5) p < 0.001
European Mixed studies 4 (230) 78.0 (0.0–100.0) 99.0 (98.6–99.4) p < 0.001
Anatomical body donor study 3 (130) 96.2 (89.8–100.0) 63.6 (0.0–89.6) p = 0.064
European (French) Mixed studies 2 (150) 57.0 (0.0–100.0) 99.6 (99.2–99.7) p < 0.001
USA Mixed studies 4 (186) 95.6 (81.3–100.0) 90.4 (78.5–95.7) p < 0.001
Anatomical body donor study 3 (184) 99.2 (96.7–100.0) 0.0 (0.0–8.2) p = 0.893
Internal Carotid Artery(ICA) General Mixed studies 2 (71) 32.6 (22.1–43.9) 0.0 (0.0) p = 0.697
Posterior Inferior Cerebellar Artery (PICA) European Anatomical body donor study 2 (78) 6.9 (2.2–13.7) 0.0 (0.0) p = 0.075
Vertebral Artery (VA) General Mixed studies 3 (1115) 10.1 (0.0–28.8) 95.4 (89.8–97.9) p < 0.001
Anatomical body donor study 2 (115) 14.7 (0.0–56.0) 95.7 (87.7–98.5) p < 0.001
Asian Mixed studies 2 (1052) 3.6 (2.6–4.9) 0.0 (0.0) p = 0.691
Common AICA-PICA trunk General Mixed studies 7 (1444) 9.5 (4.1–16.8) 87.4 (77.3–93.0) p < 0.001
Anatomical body donor study 6 (444) 9.5 (4.1–16.8) 87.4 (77.3–93.0) p <.001
European Anatomical body donor study 3 (158) 8.7 (4.7–13.6) 0.0 (0.0–61.2) p = 0.765
Asian Mixed studie 3 (1102) 11.1 (2.2–24.6) 88.1 (66.7–95.7) p < 0.001
Anatomical body donor study 2 (102) 7.2 (2.9–13.1) 0.0 (0.0) p = 0.611
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Basilar artery (BA) as the origin of the AICA

The most prevalent origin of the AICA, was the lower half of the BA, in 95.6% (95% CI:82.9–100.0; p < 0.001) of the general population. The two-thirds part of the AICA was most prevalent: 40.3% (95% CI:0.0–82.3; p < 0.001); of the general population.

The detailed subgroup analysis is presented in Electronic Supplementary Material Table 9.

Bifurcation pattern of AICA

The most common pattern of AICA bifurcation, two trunks (rostral and caudal), was found in 73.4% (95% CI:48.6–85.9; p = 0.008) of the general population. There was no bifurcation in 26.6% (95% CI:14.1–41.4; p = 0.008).

The detailed subgroup analysis is presented in Electronic Supplementary Material Table 11.

Location of AICA bifurcation

When the bifurcation is meatal, the AICA mostly branched in the premeatal area: 55.2% (95% CI:0.0–100.0; p < 0.001) of the general population. It branched in the meatal and postmeatal zones in 40.9% (95% CI:0.0–100.0; p < 0.001) and 3.9% (95% CI:0.0–47.1; p < 0.001), respectively.

The detailed subgroup analysis is presented in Electronic Supplementary Material Table 12.

AICA perforators

Electronic Supplementary Material Table 13 presents the data regarding the prevalence of AICA perforating arteries in different geographical groups.

The arteries were found in 48.5% (95% CI:18.6–79.0; p < 0.001) of the general population.

AICA-PICA common trunk

Electronic Supplementary Material Table 14 summarizes the prevalence of the AICA-PICA trunk in different geographical groups.

The AICA-PICA common trunk originated from the BA in 11.3% (95% CI:4.7–20.1; p = 0.093) of the general population.

Morphometric analysis of AICA

A total of 13 studies (n = 1256 AICAs) were included in the analysis of AICA diameter.

The characteristics of these studies are displayed in Table 3.

Table 3.

Morphometry of AICA- diameter of AICA in different populations

Morphometric parameter Population Type of study No. of studies (No. of AICA) Diameter [mm] (95%–CI) I2 [%]
Total AICA diameter General Mixed studies 9 (742) 1.1 (1.0–1.2) 94.8
Anatomical body donor study 7 (268) 1.1 (1.0–1.2) 92.5
Imaging study 2 (474) 1.2 (1.1–1.3) 89.3
European Anatomical body donor study 2 (80) 1.2 (0.9–1.4) 85.4
Asian Mixed studies 4 (584) 1.2 (1.1–1.3) 94.4
Anatomical body donor study 2 (110) 1.2 (1.0–1.4) 91.0
Imaging study 2 (474) 1.2 (1.1–1.3) 89.3
USA Anatomical body donor study 2 (24) 1.0 (0.9–1.1) 78.6
Proximal AICA diameter General Anatomical body donor study 2 (228) 1.2 (0.9–1.5) 97.6
AICA diameter depending on laterality Right General Mixed studies 4 (348) 1.1 (0.8–1.4) 98.3
Anatomical body donor study 3 (184) 1.2 (0.8–1.5) 97.3
Left General Mixed studies 4 (174) 1.0 (0.8–1.2) 91.8
Anatomical body donor study 3 (92) 1.1 (0.8–1.4) 84.2
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The estimated diameter of the AICA was 1.1 mm (95% CI:1.0–1.2) and its distance from the vertebrobasilar junction (VBJ) was 10.4 mm (95% CI:9.2–11.6) in the general population.

The diameters of right and left AICAs were 1.1 mm (95% CI:90.8–1.4) and 1.0 mm (95% CI:90.8–1.2), respectively.

The right and left sides were 9.1 mm (95% CI:96.9–11.3) and 9.3 mm (95% CI:98.3–10.4) from the VBJ, respectively, in the general population.

AICA and cranial nerves (CN) relationship

Table 4 summarizes the relationships between the AICA and specific cranial nerves (CN) in different geographical populations.

Table 4.

AICA and Cranial nerves relationships

Cranial nerve Relationship Population Type of studies No. of studies (No. of AICA) prevalence [%](95% CI) I2 [%](95% CI) P-value

Trigeminal nerve

(CN V)

 AICA in contact with CN V General Mixed studies 4 (360) 23.9 (4.0–51.5) 95.1 (90.3–97.5) p < 0.001
Anatomical body donor studies 3 (352) 11.6 (0.0–32.6) 94.5 (87.2–97.6) p < 0.001
European Anatomical body donor studies 2 (252) 17.8 (0.0–62.4) 97.2 (92.9–98.9) p < 0.001
Asian Mixed study 2 (108) 41.5 (0.0–100.0) 95.9 (88.2–98.6) p < 0.001

Abducens nerve

(CN VI)

 AICA in contact with CN VI General Anatomical body donor studies 4 (324) 82.2 (54.7–100.0) 95.8 (92.1–97.8) p < 0.001
European Anatomical body donor studies 3 (280) 72.1 (32.2–100.0) 96.6 (92.9–98.4) p < 0.001
Facial nerve (CN VII) AICA in contact with CN VII Asian (Indian) Mixed studies 2 (270) 31.9 (19.7–45.4) 67.9 (0.0–92.8) p = 0.077
CN VII between AICA General Anatomical body donor studies 2 (234) 3.1 (1.2–5.8) 0.0 (0.0) p = 0.537

Vestibulocochlear nerve

(CN VIII)

 AICA in contact with CN VIII General Mixed studies 2 (126) 13.0 (0.0–44.8) 94.1 (81.2–98.1) p < 0.001
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The most prevalent association was with the abducens nerve (CN VI), which was found in contact with AICA in 82.2% (95% CI:54.7–100.0; p < 0.001) in the general population.

The trigeminal nerve (CN V) was in contact with the AICA in 23.9% (95% CI:4.0–51.5; p < 0.001) of the general population.

The vestibulocochlear nerve (CN VIII) was in contact with AICA in 13.0% (95% CI:0.0–44.8; p < 0.001) of the general population.

The facial nerve (CN VII) was found between the AICA branches in 3.1% (95% CI:1.2–5.8; p = 0.537).

AICA and facial-vestibulocochlear complex(CN VII-VIII) relationship

Table 5 summarizes the relationship of the AICA to the facial-vestibulocochlear complex (CN VII-VIII) in different geographical populations.

Table 5.

AICA and VII-VIII nerve complex relationship

Population Type of studies No. of studies (No. of AICA) Relationship of arteries to facial-vestibulocochlear complex
between superior Posterior (dorsal) inferior anterior(ventral) I2%(95% CI) P-value
General Mixed studies 8 (1054) 60.6 (11.5–56.1) 3.7 (0.0–11.3) 7.5 (0.0–15.9) 12.7 (0.0–21.1) 15.5 (0.0–23.6) 98.0 (97.3–98.6) p < 0.001
Anatomical body donor studies 6 (648) 64.5 (10.5–70.2) 2.1 (0.0–13.0) 4.3 (0.0–17.0) 16.0 (0.0–31.8) 12.7 (0.0–28.0) 97.5 (96.2–98.4) p < 0.001
USA Anatomical body donor studies 2 (84) 83.4 (25.8–81.6) 0.9 (0.0–9.1) 7.9 (0.0–21.3) 6.9 (0.0–19.9) 0.9 (0.0–9.1) 84.4 (36.2–96.2) p = 0.011
Asian Mixed studies 4 (520) 58.9 (2.4–68.8) 3.9 (0.0–19.1) 12.4 (0.0–30.3) 3.2 (0.0–17.7) 21.6 (0.0–39.3) 98.3 (97.2–98.9) p < 0.001
Anatomical body donor studies 2 (114) 73.0 (0.0–100.0) 0.6 (0.0–31.7) 5.4 (0.0–48.6) 3.3 (0.0–42.8) 17.7 (0.0–69.8) 97.5 (93.7–99.0) p < 0.001
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The AICA was found between the facial-vestibulocochlear complex (CN VII-VIII), in 60.6% (95% CI:11.5–56.1; p < 0.001).

The second and third most common associations, AICA anterior (ventral) and inferior to CN VII-VIII complex, were found in 15.5% (95% CI:0.0–23.6; p < 0.001) and 12.7% (95% CI:0.0–21.1; p < 0.001), respectively.

Least common relationships, posterior (dorsal) and superior relationships of the AICA to the CN VII-VIII complex, were found in 7.5% (95% CI:0.0–15.9; p < 0.001) and 3.7% (95% CI:0.0–11.3; p < 0.001), respectively.

AICA loops

Table 6 shows the total number, origin relationship, and Chavda classification of AICA loops in different geographical populations.

Table 6.

AICA loops and CHAVDA classification

AICA Loops Population Type of studies No. of studies (No. of AICAs) prevalence [%](95% CI) I2 [%](95% CI) P-value
Total AICA Loops General Mixed studies 19 (8397) 58.3 (34.7–80.2) 99.6 (99.6–99.7) p < 0.001
Anatomical body donor studies 8 (541) 78.5 (51.8–97.1) 97.2 (95.9–98.1) p < 0.001
Imaging Studies 10 (7638) 47.2 (14.5–81.1) 99.8 (99.7–99.8) p < 0.001
European Mixed studies 5 (6364) 50.8 (0.0–100.0) 99.9 (99.8–99.9) p < 0.001
Anatomical body donor studies 2 (82) 49.8 (0.0–100.0) 99.0 (98.0–99.5) p < 0.001
Imaging studies 3 (6282) 51.4 (0.0–100.0) 99.9 (99.9–100.0) p < 0.001
Asian Mixed studies 8 (1524) 45.6 (27.8–64.0) 97.9 (97.0–98.5) p < 0.001
Imaging studies 7 (1254) 43.3 (30.1–57.1) 95.3 (92.1–97.2) p < 0.001
Turkey Mixed studies 3 (804) 18.0 (1.0–45.1) 97.9 (96.3–98.9) p < 0.001
Imaging studies 2 (586) 26.2 (5.5–53.5) 95.4 (86.5–98.4) p < 0.001
India Imaging studies 3 (570) 52.4 (25.7–78.5) 97.2 (94.5–98.6) p < 0.001
South Korea Mixed studies 2 (150) 77.1 (18.7–100.0) 97.7 (94.3–99.0) p < 0.001
AICA loop origin in relation to IAC Far from IAC General Mixed studies 4 (526) 39.4 (21.0–59.4) 91.8 (82.3–96.2) p < 0.001
Anatomical body donor studies 3 (152) 47.9 (39.1–56.8) 14.7 (0.0–91.1) p = 0.310
European Mixed studies 2 (404) 26.5 (13.3–42.1) 69.3 (0.0–93.1) p = 0.071
In IAC General Mixed studies 10 (7656) 18.9 (11.1–28.0) 97.8 (97.8–98.4) p < 0.001
Anatomical body donor studies 7 (1986) 22.1 (13.7–31.7) 91.9 (85.9–95.4) p < 0.001
Imaging studies 3 (5670) 11.8 (2.6–25.4) 97.0 (94.1–98.5) p < 0.001
European Mixed studies 2 (5274) 9.8 (0.0–30.1) 89.8 (62.5–97.3) p = 0.002
Asian Mixed studies 4 (522) 24.4 (10.7–41.3) 90.0 (77.2–95.6) p < 0.001
Anatomical body donor studies 2 (96) 29.5 (7.8–56.8) 84.2 (34.9–96.2) p = 0.012
Imaging studies 2 (426) 18.5 (1.6–44.4) 92.5 (74.5–97.8) p < 0.001
USA Anatomical body donor studies 2 (1352) 10.8 (5.9–16.8) 29.8 (0.0–100.0) p = 0.233
Chavda classification of AICA loops CHAVDA I General Mixed studies 14 (2650) 37.6 (26.4–49.6) 97.1 (96.2–97.8) p < 0.001
Anatomical body donor studies 4 (178) 59.6 (47.1–71.4) 60.0 (0.0–86.6) p = 0.058
Imaging studies 10 (2472) 29.6 (17.6–43.2) 97.8 (97.0–98.4) p < 0.001
European Mixed studies 3 (1068) 52.7 (17.6–86.6) 98.9 (98.2–99.3) p < 0.001
Imaging studies 2 (1038) 39.7 (2.3–84.9) 99.4 (99.0–99.7) p < 0.001
Asian Mixed studies 8 (1404) 27.3 (18.4–37.1) 92.9 (88.3–95.7) p < 0.001
Imaging studies 7 (1370) 24.3 (15.9–33.8) 92.7 (87.6–95.8) p < 0.001
India Imaging studies 3 (570) 24.8 (9.1–44.5) 95.4 (89.7–97.9) p < 0.001
Turkey Imaging studies 2 (586) 13.4 (0.4–35.9) 94.8 (84.0–98.3) p < 0.001
South America Mixed studies 3 (178) 50.6 (43.2–57.9) 0.0 (0.0–85.3) p = 0.492
Anatomical body donor studies 2 (114) 52.4 (41.4–63.3) 19.5 (0.0–100.0) p = 0.265
Brasil Mixed studies 2 (152) 48.7 (40.8–56.6) 0.0 (0.0) p = 0.960
CHAVDA II General Mixed studies 11 (2498) 15.4 (10.1–21.5) 92.8 (89.1–95.2) p < 0.001
Imaging studies 10 (2472) 15.1 (9.6–21.4) 93.5 (90.1–95.8) p < 0.001
European Imaging studies 2 (1038) 16.2 (0.4–43.0) 98.7 (97.2–99.4) p < 0.001
Asian Imaging studies 7 (1370) 15.1 (10.4–20.4) 83.4 (67.4–91.6) p < 0.001
India Imaging studies 3 (570) 19.6 (16.3–23.1) 7.4 (0.0–90.4) p = 0.340
Turkey Imaging studies 2 (586) 9.6 (7.4–12.2) 0.0 (0.0) p = 0.959
South America Mixed studies 2 (90) 11.7 (2.7–25.0) 55.5 (0.0–89.2) p = 0.134
CHAVDA III General Mixed studies 9 (2346) 4.0 (2.0–6.6) 85.8 (75.0–92.0) p < 0.001
Imaging studies 8 (2320) 3.5 (1.6–6.0) 86.4 (75.3–92.5) p < 0.001
European Imaging studies 2 (1038) 4.2 (0.5–10.4) 92.7 (75.4–97.8) p < 0.001
Asian Imaging studies 6 (1282) 3.0 (0.9–6.0) 83.3 (64.9–92.0) p < 0.001
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The prevalence of AICA loops in the general population was 58.3% (95% CI:34.7–80.2; p < 0.001).

The origin of the AICA loop was far from the internal acoustic canal (IAC) in 39.4% (95% CI:21.0–59.4; p < 0.001). It originated in the IAC in 18.9% (95% CI:11.1–28.0; p < 0.001).

A Chavda type I was the most prevalent type of AICA loop: 37.6% (95% CI:26.4–49.6; p < 0.001). Chavda types II and III were found in 15.4% (95% CI:10.1–21.5; p < 0.001) and 4.0% (95% CI:2.0–6.6; p < 0.001), respectively, of the general population.

Laterality of AICA loops

Electronic Supplementary Material Table 17 presents the laterality of AICA loops in different geographical groups.

Unilateral AICA loops were the most prevalent: 70.7% (95% CI:49.4–88.4; p < 0.001).

AICA loops were bilateral in 29.3% (95% CI:11.6–50.6; p < 0.001) of the general population.

AICA loops in relation to the porus acusticus

Electronic Supplementary Material Table 18 presents the relationships of AICA loops to porus acusticus (internus) in different geographical populations.

The AICA loops were medial and lateral to the porus acusticus in 34.5% (95% CI:0.0–91.7; p < 0.001) and 2.2% (95% CI:0.0–37.2; p < 0.001). However, the most common configuration included an AICA loop at the porus acusticus: 63.3% (95% CI:5.6–100.0; p < 0.001).

Branches of AICA

Electronic Supplementary Material Tables 19, 20, 21 presents the respective branches of AICA, including the internal auditory artery (IAA) (labyrinthine artery), subarcuate artery (SA) and recurrent perforating branch (RPB) and their prevalences in different geographical populations.

The SA was unilateral in 94.6% (95% CI:79.1–100.0; p < 0.001) and bilateral in the remaining 5.4% (95% CI:0.0–20.9; p < 0.001) of the general population.

The RPB was unilateral and bilateral in 49.7% (95% CI:20.2–89.5; p < 0.001) and 37.0% (95% CI:9.3–78.3; p < 0.001), respectively. Three or four branches of RPB were noted in 11.4% (95% CI:0.0–41.3; p < 0.001) and 1.9% (95% CI:0.0–18.8; p < 0.001).

Lastly, the IAA presented with unilateral and bilateral patterns in 17.5% (95% CI:4.7–33.8; p < 0.001) and 52.3% (95% CI:31.7–69.6; p < 0.001), respectively. There were three and four branches in 24.4% (95% CI:9.2–41.8; p < 0.001) and 5.8% (95% CI:0.0–15.9; p < 0.001) of the general population.

Discussion

Our meta-analysis involved the assessment of 64 studies (n = 15,529 AICAs) and is, to the authors knowledge, the first and most up-to-date study analyzing AICA, including the clinical background, involving microsurgical and neurosurgical procedures.

Most notably, such a connection can be found in case of AICA involvement in compression syndromes(CS), as well as treatment of choice, which is microvascular decompression (MVD).

The whole concept of MVD is surrounded by controversies, as many radiological and Anatomical body donor studies have yielded different and incompatible results [48]. Hence, the authors of this study found it crucial to include this aspect and data regarding the possible prevalence of AICA CS involving specific cranial nerves.

Secondly, the concept of vascular compression was first described in 1875, when a patient presenting with hemifacial spasm was diagnosed with a vertebral artery aneurysm compressing the facial nerve[48]. However, it attracted little attention, until the subject was revisited by Dandy in 1934, in connection with the fifth nerve and trigeminal neuralgia [15]. The concept of CS was reintroduced yet again by Janetta in 1975, who based his thesis on his observation and on data presented by the aforementioned authors. He was the first to perform MVD for a dysfunctional eighth nerve in a case of vertigo [32, 33, 48].

The pathological basis for neurovascular compression syndromes (NVCs), especially those associated with cranial nerves, involves compression or irritation of nervous structures [36, 81]. NVCs most commonly result from the compression of cranial nerves at either the root exit or entry zone.

When AICA is the culprit, the cranial nerves involved in CS can include the trigeminal nerve (CN V), producing trigeminal neuralgia, the abducens nerve (CN VI), producing diplopia and failure of eye abduction, the facial nerve (CN VII), producing hemifacial spasm (HFS) and vestibulocochlear nerve (CN VIII), producing tinnitus, vertigo and even hearing loss [36, 55, 81].

We found the AICA to be most commonly in contact with the abducens nerve (CN VI) up to 82.2% (95% CI:54.7–100.0; p < 0.001). However, we believe that the symptoms presented by the compression are often neglected by either the patient or the doctor and are managed conservatively e.g. with prism glasses, similarly to the case presented by Arulratnam et al. (2020) [6] (Fig. 3).

Fig. 3.

Fig. 3

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Posterior view via a retrosigmoid craniotomy in microvascular decompression procedure (MVD)

In the case of trigeminal nerves we found 23.9% (95% CI:4.0–51.5; p < 0.001) of patients with AICA contact, which can potentially produce compression syndrome and possible trigeminal neuralgia. The authors strongly believe that if suspected in the differential diagnosis, we should include imaging studies such as MRI [5] and even high resolution 3-T MRI, if available, for best visualization of possible neurovascular conflict.

A similar paradigm of CS is especially relevant to the AICA, due to the potential for loop formation. The loops are vascular formations associated with coil-like structures, most commonly distorting the facial and vestibulocochlear nerves, however other nerves can also be implicated [26, 81]. Some authors, including McDermott et al. [48], significantly associated auditory symptoms with a special subtype of loop, those embedded in the intrameatal area, with auditory symptoms, classified as Chavda II [48, 81]. However, in most studies, type I Chavda loops predominated (Fig. 3).

We found that 37.6% of loops (95% CI:26.4–49.6; p < 0.001) were Chavda type I, which as the aforementioned authors observed, was the most prevalent type of loop. When the origin was in the IAC, 18.9% were a Chavda type I (95% CI:11.1–28.0; p < 0.001).

We found that 58.3% (95% CI:34.7–80.2; p < 0.001) of the general population presented with AICA loops though this was not universal, as the Turkish population presented with only 18.0% (95% CI:1.0–45.1; p < 0.001) loops. The greatest predisposition to AICA loops was in the South Korean population up to 77.1% (95% CI:18.7–100.0; p < 0.001). However, those findings are based on limited sample size, and can result from different study methodologies, rather than only geographical variations. Therefore more studies should be constructed to either confirm or contradict aforementioned results.

Nevertheless, we should give special consideration to those geographic variations as they can potentially increase the chance of iatrogenic lesions during neurosurgical procedures or produce CS themselves.

Another clinical consideration concerns the resection cerebellopontine angle (CPA) tumors and skull base surgery in general. In such cases, proper identification of neurovascular structures, including the AICA and its differentiation can be crucial, when a safe depth of resection and prevention of possible iatrogenic vascular lesions are planned [2].

Most sequelae from surgical interventions in the CPA region are dictated by the high density of vascular and cranial nerves in this area. We should also remember that not only can the anatomy of vascular structures change, but also that tumor mass itself can shift those structures, thus making the anatomy more unpredictable. Nervous components of the CPA can be easily monitored using neurophysiological techniques, which are available in neurosurgical centers [2, 71, 72]. However there is no equivalent for vascular components. Of course, we should not forget about neuronavigation and other pre- and intraoperative imaging modalities, including intraoperative fluorescence, which helps to differentiate vascular structures [14, 35].

However, for smaller intracranial vessels, including the AICA it may not be always effective [77, 81]. In our study, we found the AICA to measure 1.1 mm (95%:CI:1.0–1.2) in the general population. Therefore, thorough knowledge of the vessel anatomy can provide additional help, during many neurosurgical procedures.

Our study found that the AICA originated from the BA in most cases, up to 90.1% (77.2–98.4) of the general population. However, in the French population, this ratio was only 57.0% (0.0–100.0). Other common origin points included the ICA up to 32.6% (22.1–43.9) and the VA up to 10.1% (0.0–28.8). These different origin points can make it more difficult to identify critical anatomical landmarks and lead to devastating clinical sequelae.

Particularly, such cases could take place during occipital artery (OA) bypass surgery, when AICA could be mistaken for another vessel or could be originating from another point inhibiting the possible aneurysm trapping.

Lastly, the authors believe that with development of new imaging modalities, including previously mentioned 3-T MRI and 3D-FIESTA sequence, the prevalence of AICA loops reported throughout the literature will increase, as they will be much easier to distinguish especially pre-operatively, rather than during CT, angiography or classical MRI studies [13]. Additionally, those new imaging techniques should provide additional help in solving the dispute regarding CS and MVD, providing more coherent and compatible results.

At the time of writing this publication the authors believe it to be the first to pool all the data regarding the origin, relationship to cranial nerves, diameter, branching and loop patterns of the AICA. The main limitation of this meta-analysis is the overall dominance of Anatomical body donor studies and possible variations/disparity in dissection methods, which are not always specified. For those reasons, the heterogeneity in most studies was high.

Conclusions

Although the AICA has been investigated in numerous anatomical body donor and radiological studies, to the authors' knowledge this is the first study, including the data regarding origin, relationship to cranial nerves, diameter, branching and loop configurations. It was conducted not only to provide data on the anatomical idiosyncrasies of the AICA, but also to provide insight into the connection of AICA anatomy to clinical aspects of many neurosurgical procedures. Anyone performing such interventions should be aware of possible cranial nerve characteristics and compression syndromes, including the ethnic predispositions of AICA loops.

Supplementary Information

Below is the link to the electronic supplementary material.

Supplementary file1 (DOCX 21 KB) (20.7KB, docx)

Author contributions

JT led the project, wrote the original article, and extracted data. OA,RC,KM,IJ,GF,KM were responsible for data selection,extraction and revisions. Additionally GF,KM and SA were responsible for writing of the article. JP was in charge of revisions and performed statistical analysis of the study. DŁ created figures for the study and provided useful insight into the review process, with JAW,PP and RST contributing to the conception of the study.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Data Availability

Data extracted from included studies; template data collection forms; data used for analyses; analytic code; any other materials used in the review are available upon request from the corresponding author.

Declarations

Ethical approval

Not applicable.

Consent to participate

Not applicable.

Clinical trial number

Not applicable.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Associated Data

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Supplementary Materials

Supplementary file1 (DOCX 21 KB) (20.7KB, docx)

Data Availability Statement

Data extracted from included studies; template data collection forms; data used for analyses; analytic code; any other materials used in the review are available upon request from the corresponding author.

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