The prevalence and anatomy of recurrent artery of Heubner: a meta analysis with neurosurgical considerations.

BACKGROUND: The recurrent artery of Heubner (RAH) is typically the largest medial lenticulostriate branch of the anterior cerebral artery (ACA). Neurosurgical procedures such as aneurysm treatment on the anterior part of the circle of Willis can result in damage of the RAH leading to neurological deficits. The aim of this study was to identify the gaps and provide comprehensive data on the prevalence and anatomical characteristics of the RAH with neurosurgical considerations. METHODS: The major electronic databases were thoroughly searched to identify the eligible studies. The information concerning study type, geographical origin, prevalence of the RAH, course and origin of the RAH, symmetry of origin and number of RAHs in each hemisphere, and morphometric data were extracted. The PRISMA guidelines were rigorously followed throughout the study. The AQUA tool was used to evaluate the reliability of included studies. RESULTS: A total of 34 studies (n = 3645 hemispheres) were included in the meta-analysis. The analysis revealed that the RAH was present in 97.5% (95%CI: 95.5-98.6) of the hemispheres, originating most frequently from the A2 segment (42.2%, 95%CI: 35.0-49.7) or the ACoA-ACA junction (41.6%, 95%CI: 34.0-49.6), and coursing anteriorly (47.6%, 95%CI: 38.7-56.6) or superiorly (43.9%, 95%CI: 34.4-53.8) in relation to ACA. Almost a quarter of patients had more than one RAH, which was on average 22.82 mm (SD: 1.35, 95%CI: 20.16-25.47; I2 = 99.1%, p < 0.01) long and reached 0.76 mm (SD: 0.05, 95%CI: 0.66-0.85; I2 = 99.4%, p < 0.01) in diameter. CONCLUSIONS: As the RAH is present in the majority of the population, it is important to be aware of the wide variations in its anatomy. This will help to prevent postoperative neurological deficits by avoiding undesirable complications during surgeries that are performed in close proximity to the anterior segment of the circle of Willis.

The anterior communicating artery variants: a meta-analysis with a proposed classification system.

Morphological and morphometric variants of the anterior communicating artery (AComA) have been described by multiple studies; however, a complete classification system of all possible morphological variants with their prevalence is lacking. The current systematic review with meta-analysis combines data from different databases, concerning the AComA morphological and morphometric variants (length and diameter). Emphasis was given to the related clinical implications to highlight the clinical value of their knowledge. The typical AComA morphology occurs with a pooled prevalence (PP) of 67.3%, while the PP of atypical AComA is 32.7%. The identified AComA morphological variants (artery's hypoplasia, absence, duplication, triplication, differed shape, fenestration, and the persistence of a median artery of the corpus callosum- MACC) were classified in order of frequency. The commonest presented variants were the AComA hypoplasia (8%) and the anterior cerebral artery (ACA) fusion (5.9%), and the rarest ones were the MACC persistence (2.3%), and the AComA triplication (0.7%). The knowledge of those variants is essential, especially for neurosurgeons operating in the area. Given the high prevalence of AComA aneurysms, an adequate and complete classification of those variants is of utmost importance.

Morphology and morphometry of the anterior cerebral-anterior communicating artery complex.

PURPOSE: The variations of Cerebral arterial circle (Circle of Willis) are not rare. The study is done to assess the pattern of the component vessels of its anterior part with regard to Right- Left variations and compare with other studies. MATERIAL AND METHOD: 56 formalin fixed cadavers were analyzed. The diameter, length and variations of the following vessels were observed: Anterior Cerebral Artery (proximal A1 segment to distal A2 segment) and anterior communicating artery. Statistical analysis was done by Statistica 14.0.1 software. RESULT: 53.57% had normal anterior cerebral artery- anterior communicating artery complex. Length and diameter were slightly more on left side. Unilateral hypoplasia, Azygos and Tripple Anterior Cerebral Artery was present in 21.43%, 3.57 and 1.78% respectively. Aplastic, double and fenestration type anterior communicating artery were present in 5.3%, 8.92% and 1.78% respectively. CONCLUSION: Anatomical knowledge of variations should be updated regularly for neurosurgeons, radiologists and clinicians to avoid unexpected consequences.

[Normal Anatomy and Variants of the Anterior Cerebral and Anterior Communicating Arteries].

The anterior cerebral artery(ACA)and anterior communicating artery(AcomA), two important arteries of the cerebral hemisphere, are involved in various disorders, including aneurysms, steno-occlusive diseases, and arteriovenous shunts, among other such conditions. Additionally, these vessels contain normal variants associated with their embryology and development. The ACA particularly shows characteristic variants such as a persistent primitive olfactory artery and an infraoptic course, among other such variations, which is attributable to the fact that it is embryologically older than the middle and posterior cerebral arteries. Aneurysm formation is associated with these variants. Therefore, knowledge of the normal anatomy and variants is important for diagnosis and treatment of conditions involving these arteries. In this article, we describe the normal anatomy and variants of the ACA and AcomA, focused on their embryological development.

Microsurgical anatomy of the anterior cerebral artery and the arterial supply of the cingulate gyrus.

PURPOSE: The cingulate gyrus is a potential surgical area to treat tumours, psychiatric diseases, intractable pain and vascular malformations. The aim of the study was to define the topographic anatomy and arterial supply of the cingulate gyrus located on the medial surface of the cerebral hemisphere. METHODS: We studied thirty-six hemispheres, each hemisected in the midsagittal plane. The vertical thickness of the cingulate gyrus was measured at the anterior commissure (AC), posterior commissure (PC), and genu levels of the corpus callosum. The branches of the anterior and posterior cerebral arteries supplying each zone were noted separately. The arterial pathways were transformed to digital data in AutoCAD to identify the condensation and reduction areas. RESULTS: The mean AC-PC distance was 27.17 ± 1.63 mm. The thinnest region was the genu level of the corpus callosum (10.29 mm). The superior internal parietal artery (SIPA), inferior internal parietal artery (IIPA) and pericallosal artery (PrCA) supplied all zones of the cingulate gyrus. The anterior zone received the greatest supply. The arterial condensation and reduction areas on both sides of cingulate gyrus and its x, y, and z coordinates specified. CONCLUSIONS: The target cingulotomy (TC) area was determined for anterior cingulotomy. The properties of the TC area are that the thinnest region of the cingulate gyrus is supplied relatively less than other areas and is close to the anterior cingulotomy areas in the literature. The arterial reduction area (ARA) was found to be suitable for corpus callosotomy in terms of avoiding haemorrhage.

Endoscopic endonasal surgical anatomy through the prechiasmatic sulcus: the key window to suprachiasmatic and infrachiasmatic corridors.

BACKGROUND: Classically, the transtuberculum and transplanum approaches have been utilized to reach the suprachiasmatic and infrachiasmatic corridors. The aim of this study was to provide a better understanding of the key endoscopic endonasal anatomy of the suprachiasmatic and infrachiasmatic corridors provided through selective removal of the prechiasmatic sulcus (SRPS). METHOD: A SRPS was performed in 16 sides of 8 alcohol-fixed head specimens. Twenty anatomical measurements were collected on the suprachiasmatic and infrachiasmatic corridors. The transplanum and transtuberculum approaches were also performed. RESULTS: In the suprachiasmatic corridor, the SRPS exposed the anterior communicating artery (AComm) and the post-communicating segment of the anterior cerebral arteries in all the cases, while the pre-communicating segment of the anterior cerebral arteries, recurrent arteries of Heubner, and fronto-orbital arteries were visualized in 75% (12/16), 31% (5/16), and 69% (11/16) of cases, respectively. In the infrachiasmatic corridor, the ophthalmic segment of the internal carotid artery and superior hypophyseal arteries were always visible through the SRPS. The mean width and height of the prechiasmatic sulcus were 13.2 mm and 9.6 mm, respectively. The mean distances from the midpoint of the AComm to the anterior margin of the optic chiasm (OCh) was 5.3 mm. The mean width of the infrachiasmatic corridor was 12.3 mm at the level of the proximal margin of the ophthalmic segment of the internal carotid artery. The mean distances from the posterior superior limit of the pituitary stalk to the basilar tip and oculomotor nerve were 9.7 mm and 12.3 mm, respectively. CONCLUSIONS: The SRPS provides access to the main neurovascular and cisternal surgical landmarks of the suprachiasmatic and infrachiasmatic corridors. This anatomical area constitutes the key part of the approach to the suprasellar area. To afford adequate surgical maneuverability, the transplanum or transtuberculum approaches are usually a necessary extension.

Embryology and variations of the recurrent artery of Heubner.

PURPOSE: The recurrent artery of Heubner (RAH) is a very fascinating artery for various reasons. The first one is its early development in the embryologic and phylogenic life. The second one is the discrepancy between its diameter and its functional importance and the last but not least reason is the many variations of origin and course of this artery. METHODS: For more than a century, numerous studies furnished important information about variations in origin, course, and parenchymal territory of the RAH. The most important anomaly concerning the RAH is the accessory middle cerebral artery and well-illustrates the complexity of its embryologic development. CONCLUSION: For these reasons, authors provide a review of anatomical variations of this artery that could be encountered with a particular attention given to the history and embryological knowledge.

Clinical and morphological risk factors for the recurrence of anterior communicating artery aneurysms after clipping or coiling.

BACKGROUND: The aim of this study was to evaluate clinical and morphological factors associated with recurrence in anterior communicating artery (AcomA) aneurysms after clipping or coiling. METHODS: We retrospectively reviewed the clinical and radiologic features of consecutive 214 patients with AcomA aneurysms treated between January 2012 and December 2016 in a single tertiary institute. Univariate and multivariate analyses were performed to identify the relationship between clinical and morphological variables and recurrence. RESULTS: Of 214 patients, 166 were unruptured aneurysms and 109 were treated with coiling. Overall recurrence rate was 13% (28 out of 214 aneurysms) during mean 36.9 ± 18.4-month follow-up. Multivariate logistic regression analysis showed that size greater than 10 mm (OR = 5.651; 95% CI, 1.317-24.242; p = 0.020), smoking (OR = 3.474; 95% CI, 1.342-8.996; p = 0.010), coiling (OR = 2.98; 95% CI, 1.005-8.832; p = 0.049), and anterior direction of aneurysm (OR = 3.77; 95% CI, 1.12-12.66; p = 0.032) were significantly associated with recurrence of AcomA aneurysms after treatment. CONCLUSIONS: The results of this study demonstrated that coiling, large aneurysm, anterior direction, and smoking history may be independent risk factors for the recurrence of AcomA aneurysms. Therefore, careful follow-up should be needed especially in large AcomA aneurysms with anterior direction after coiling.

Morphometric evaluation of Anterior Cerebral Artery (on Digital Subtraction Angiography)-Potential implications.

PURPOSE: The aim of the present study was to perform a morphometric analysis of ACA and to establish significant differences, if any, with sex and age via Digital Subtraction Angiography (DSA). METHODS: This cross-sectional study was conducted on 70 patients (>20 years) for the evaluation of ACA by Digital Subtraction Angiography done on a Philips biplane system clarity (Allura FD20/20). Length and diameter of ACA were taken manually on the DSA console using auto-calibration. Statistical analysis was done. RESULT: Mean diameter and length of ACA was found to be greater in female. The f-ratio value for diameter and length of the same was 1.43 and 0.92 respectively. The length of ACA was found to be statistically significant for different age groups. The mean length of ACA followed a decreasing trend with age, whereas mean diameter of ACA was nearly the same in all the age groups. CONCLUSION: The results of our study show that the length of ACA was found to be statistically significant for different age groups.

Increased tortuosity of ACA might be associated with increased risk of ACoA aneurysm development and less aneurysm dome size: a computer-aided analysis.

OBJECTIVES: We decided to perform computer-aided analysis of the anterior cerebral artery (ACA) to check for a potential correlation with anterior communicating artery (ACoA) aneurysm presence and growth. METHODS: We retrospectively analyzed the ACA anatomy of 121 patients with ACoA aneurysms along with 121 age, risk factors, and vessel side-matched control patients without an ACoA aneurysm. We obtained their medical history and digital subtraction angiography (DSA) data from their medical records. For each patient's DSA, we extracted curve representing the course of their ACA and calculated its relative length (RL), sum of angle metrics (SOAM), triangular index (TI), product of angle distance (PAD), and inflection count metrics (ICM). RESULTS: Patients with ACoA aneurysm had significantly higher RL (0.64 ± 0.23 vs. 0.56 ± 0.22; p < 0.01), SOAM (0.27 ± 0.19 vs. 0.18 ± 0.15; p < 0.01), PAD (0.12 ± 0.13 vs. 0.09 ± 0.11; p = 0.02), and TI (0.57 ± 0.14 vs. 0.44 ± 0.15; p < 0.01). In multivariate logistic regression analysis, after adjustment for possible confounders, SOAM (OR, 1.34; 95% CI, 1.12-1.63; p < 0.01) and TI (OR, 1.84; 95% CI, 1.47-2.35; p < 0.01) remained independently associated with higher risk of ACoA aneurysm. Additionally, we found significant negative correlations between TI and aneurysm dome size (R = - 0.194; p = 0.047). CONCLUSIONS: Increased tortuosity of ACA might increase the risk of ACoA aneurysm development and decrease the risk of aneurysm growth. KEY POINTS: • Anterior cerebral artery's sum of angle metrics is associated with hypertension as well as with history of ischemic stroke and myocardial infarction. • Increased tortuosity of anterior cerebral artery might be associated with anterior communicating artery aneurysm development. • Tortuosity of anterior cerebral artery is negatively correlated with anterior communicating artery aneurysm dome size.

Microsurgical anatomy of the subcallosal artery.

BACKGROUND AND PURPOSE: The subcallosal artery [SCA, a branch of the anterior communicating artery (ACoA)] is not well described in the literature. However, the memory disorders that can occur after surgical repair of ruptured ACoA aneurysms might be related to infarction of the SCA. The objective of the present study was to perform a thorough anatomical assessment of the SCA. METHODS: The study was carried out over a 6-month period in a University Hospital's anatomy laboratory, using brains extracted from human cadavers. The brains were injected with colored neoprene latex and dissected to study the SCA's origin, path, termination, diameter, length, and vascularized territories. RESULTS: 21 cadaveric specimens were studied. The mean ± standard deviation diameter and length of the SCA were 0.83 ± 0.57 mm and 38.14 ± 25.11 mm, respectively. The predominantly vascularized territories were the paraterminal gyrus (100%), the parolfactory gyrus (78.95%), the rostrum (84.21%) and genu (78.95%) of the corpus callosum, the lamina terminalis (78.95%), the anterior commissure (63.16%), the anterior cingulate gyrus (47.37%), and the fornix (26.32%). When the SCA supplied the fornix and the anterior cingulate gyrus, it was significantly longer and broader (p < 0.05). CONCLUSION: Anatomic knowledge of the SCA is crucial-especially for the treatment of ACoA aneurysms.

Bilateral carotid-anterior cerebral artery anastomoses associated with bilateral ophthalmic arteries arising from the middle meningeal arteries diagnosed by magnetic resonance angiography: a case report.

Anastomosis between the ophthalmic segment of the internal carotid artery and the A1-A2 junction of the anterior cerebral artery (ACA), described as carotid-ACA anastomosis or infraoptic course of the ACA, is rare and known to demonstrate right-sided predominance. We report a case of bilateral carotid-ACA anastomoses associated with bilateral ophthalmic arteries arising from the sphenoidal branch of the middle meningeal artery that were diagnosed by magnetic resonance (MR) angiography. Identification of rare arterial variations on MR angiography requires careful scrutiny of source images.

Duplicate origin of the anterior cerebral artery diagnosed by magnetic resonance angiography: a case report.

An anterior cerebral artery (ACA) of duplicate origin results from the fusion of two arteries that arise from the terminal segment of the internal carotid artery (ICA) to form a ring. This variation is extremely rare and differs from proximal ACA fenestration, supraclinoid fenestration of the ICA, and duplicate origin of the middle cerebral artery. We report a case diagnosed incidentally by magnetic resonance angiography.

The extended endoscopic endonasal transplanum transtuberculum approach to the anterior communicating artery complex: anatomic study.

BACKGROUND: When performing a transplanum transtuberculum approach, dealing with the anterior communicating artery (ACoA) complex is inevitable. The aim of this study is to provide quantitative anatomical information regarding the ACoA complex and its bony and neural relationships, when exposed through this approach. METHOD: The endoscopic endonasal transplanum transtuberculum approach was performed on ten human cadaver heads. In each specimen, radiological studies were performed. A three-dimensional model of the approach was reconstructed. Measured parameters were: exposure of the vessels; distance between the proximal anterior cerebral artery (A1) and the optic chiasm; dimension of the bone opening. The feasibility to perform clip placement was graded as "possible" or "not possible". RESULTS: Dimension of bone opening varied from 88 to 53 mm(2). The ACoA was exposed for 3 mm ± 2 mm, A1 for 17 mm ± 9 mm, the distal anterior cerebral artery (A2) for 12 mm ± 3 mm, the recurrent artery of Heubner (RAH) for 16 mm ± 4 mm. Clip placement was possible on the ACoA, A2, and distal segment of A1 in all cases, and on the proximal segment of A1 in one instance. The distance between A1 and the optic chiasm measured 9 mm ± 2 mm. CONCLUSIONS: The ACoA, A2, and the distal segment of A1 can be visualized and controlled through the transplanum transtuberculum approach. The relationship between A1, gyrus rectus, and optic chiasm is the main determinant for the exposure and control of the vessel. The olfactory nerve can represent a surgical landmark for the identification of the A1 origin. The whole course of the RAH can be visualized trough this approach.

Ophthalmic artery arising from the anterior cerebral artery diagnosed by MR angiography.

Extremely rarely, the ophthalmic artery (OphA) arises from the A1 segment of the anterior cerebral artery (ACA). Discovery of this anomalous OphA during angiography or surgery has been reported in several patients. We report a case in which an OphA of ACA origin was diagnosed by magnetic resonance (MR) angiography and confirmed by selective cerebral angiography. To our knowledge, this is the first report of this variation with MR angiography. Careful observation of MR angiographic images is important for detecting rare arterial variations. Partial maximum-intensity-projection images are useful in identifying tiny anomalous arteries on MR angiography.

[Microsurgical anatomy of perforating branches of anterior communicating artery]. / Anatomie microchirurgicale des branches perforantes de l'artère communicante antérieure.

INTRODUCTION: The anterior communicating artery (ACoA) gives perforating branches to the optic chiasma, the hypothalamus and the corpus callosum. Perforating branches are variable (number, direction). Nevertheless, their knowledge is crucial during surgery of this area to spare injuries leading to ischemic post-operative complications. OBJECTIVE: The objective was to update the anatomical knowledge about perforating branches of the ACoA. METHODS: The study was led on a series of seven brains taken from human cadavers. An injection of latex neoprene was performed for every case. The region of interest was observed under operating microscope. Were examined: the length of the ACoA, its diameter, its orientation, its configuration and perforating branches (number and areas). RESULTS: Three cases on five presented with an anatomical variation at the level of the ACoA. The average length of AcoA was 2.1 millimeters (min: 2, max: 2.2). The average diameter of the ACoA was 1.67 mm (min: 1.1, max: 2.1). The average number of perforating branches was 4.2 (min: 2, max: 6). The presence of a median artery of the corpus callosum seemed to correlated with a low number of perforating branches. Branches supplying the optic chiasma seemed to be more numerous.

Larger A1/M1 diameter ratio predicts embolic anterior cerebral artery territorial stroke.

BACKGROUND AND PURPOSE: In contrast to middle cerebral artery territory strokes, anterior cerebral artery strokes (ACAS) occur rarely. The low frequency of ACAS, in relation to middle cerebral artery territory strokes, may be explained by differences in ACA and middle cerebral artery anatomy influencing their respective flow-directed embolism rates. We aimed to determine whether variability in ACA anatomy, and in particular A1 segment diameter, is associated with embolic ACAS. METHODS: Consecutive patients admitted to Boston Medical Center with embolic ACAS were reviewed. Ipsilateral and contralateral A1 diameters, M1 diameters, and terminal internal carotid artery bifurcation angles were measured from computed tomographic angiography and MRI angiography images. We compared these measurements between cases of ACAS and consecutive cases of embolic middle cerebral artery territory strokes. RESULTS: The study comprised 55 individuals (27 ACAS, 28 middle cerebral artery territory strokes) with mean age of 69 years. In multivariate regression analysis, larger ipsilateral A1 diameters (odds ratio per 1 mm increment: 8.5; 95% confidence interval, 1.4-53.3) and ipsilateral A1/M1 diameter ratio (odds ratio per 10% increment: 1.8; 95% confidence interval, 1.2-2.9) were associated with ACAS, whereas larger ipsilateral M1 diameters was protective for ACAS (odds ratio per 1 mm increment: 0.8; 95% confidence interval, 0.0-0.9). CONCLUSIONS: Larger ipsilateral A1 diameters and A1/M1 diameter ratio are associated with embolic ACAS. These findings suggest that A1 diameters and M1 diameters are important in determining the path of emboli that reach the terminal internal carotid artery.

Microsurgical anatomy of the anterior cerebral artery in Indian cadavers.

BACKGROUND: The microanatomy features of cerebral arteries may be variable and may be different in different ethnic groups. AIM: To study the anterior cerebral artery (ACA) anatomy in North-West Indian cadavers. MATERIALS AND METHODS: Microanatomy features of the ACA were studied in 15 formalin fixed human cadaveric brains under microscope. The outer diameter, length, and number of perforating branches with respective anomalies were measured for each of the following vessels: ACA (proximal A1 segment to distal A2 segment), anterior communicating artery (ACoA), Recurrent artery of Heubner (RAH), and callosomarginal artery and photographed for documentation. RESULTS: The mean length and external diameter of right and left A1 segment was 12.09 mm and 12.0 mm and 2.32 mm and 2.36 mm respectively. Narrowing, duplication, and median ACA were seen in 6.6%, 3.3% and 6.6% of the vessels respectively. Complex ACoA type was seen in 40% cadavers. RAH originated at an average point of 0.2 mm distal to ACoA, but in one cadaver it arose 5 mm proximal to ACoA. Double RAH was found in 26.6%. The course of RAH in relation to A1 was superiorly in 60%, in anteriorly 30% and posteriorly in 10% of cadavers. The orbitofrontal artery (OFA) and frontopolar artery (FPA) arose from A2 in 83.3% to 40% respectively. The mean distance of OFA and FPA from ACoA was 4.17 mm and 8.5 mm respectively. After giving rise to central, callosal and cortical branches, pericallosal artery terminated near the splenium of the corpus callosum or on the precuneus as the inferomedial parietal artery. CONCLUSION: Knowledge of the microvascular anatomy is indispensable and it is mandatory to be aware of the possible variations in the anomalies to minimize morbidity.

Extra- and intracerebral course of the recurrent artery of Heubner.

BACKGROUND: The aim of the current study was to analyse the extra- and intracerebral course of the recurrent artery of Heubner (RAH) to provide detailed information for neurosurgeons operating in this area. MATERIALS AND METHODS: The material for this study was obtained from cadavers (ages 31-75 years) during routine autopsies. A total of 70 human brains (39 male and 31 female) were examined. The material was collected not later than 48 h post-mortem. People who died due to neurological disorders were not included into the study. Right after dissection the arteries were perfused with either acrylicpaint emulsion, polyvinyl chloride or Mercox CL-2R resin, through the Circle of Willis or electively through the RAH. The obtained material was analysed using a stereoscopic light microscope, magnification 2-40´. RESULTS: The RAH was present in 138 hemispheres with a mean of 1.99 RAH per hemisphere (275 RAH in total). The mean RAH length was 25.2 mm and the mean RAH diameter, in its place of origin, was 1 mm. In 168 (61%) cases the RAH ran superiorly, in 88 (32%) cases anteriorly, in 11 (4%) cases inferiorly and in 8 (3%) cases posteriorly to the A1 segment. In 70.2% of the cases the course of the RAH was parallel to the anterior communicating artery A1 segment, and in 29.8% of the cases the RAH arched towards the olfactory tract. As the extracerebral course of the RAH was always tortuous,its length was 1 to 5 times the distance between its place of origin and the most lateral point of anterior perforated substance (APS) penetration. The intracerebral course of the RAH was almost always univectorial - towards the head of the caudate nucleus.The course of RAH branches depended on their number. When the number of RAH sand their branches was low, they separated immediately after penetrating the APS and formed multiple small branches. When the number of RAHs and branches was high,post-APS branching was less frequent and occurred in distal segments. CONCLUSIONS: The origin and course of the RAH is highly variable. The RAH, in its extra- and intracerebral course, may join with the middle group of the lenticulostriate arteries or directly with the middle cerebral artery. This artery should be routinely identified during anterior communicating artery aneurysm clipping to prevent postoperative neurological deficits.

Microsurgical anatomy of the abducens nerve.

OBJECTIVE: To clarify the oriented classification, relationships, and variations of the abducens nerve and provide a detailed description of its microsurgical anatomic features. METHODS: A microsurgical anatomic dissection of the abducens nerve was performed in 100 specimens obtained from 50 adult cadaveric heads fixed in formalin and two adult cadaveric heads stained with hematoxylin and eosin for histological examination. Important neurovascular and structural relationships of the abducens nerve were observed. RESULTS: The abducens nerve was divided into five segments (cisternal, petroclival, internal carotid artery, fissural, and intraconal). It coursed in the petroclival venous confluence and there was a complex anatomic relationship. Two new types of abducens nerve variations were found. In one type, the duplicated nerve is split into two branches for a limited length in the cavernous sinus (CS). The other is a complex type, which has a complex course and pattern. This type of duplicated abducens nerve has a communicating branch in the cistern and numerous fasciculi in the CS. In addition, the two branches do not accompany each other for the entire course in the CS. CONCLUSION: The vulnerability of the abducens nerve results from diverse factors. The inferolateral trunk, which arises from the intracavernous segment of carotid artery (also called the artery of the inferior CS), is an important landmark for finding the abducens nerve and sympathetic nerve. Variations of the abducens nerve are not rare. Keeping variations of the nerve in mind is important during skull base operations and transvenous endovascular interventions. Understanding the relationship of the abducens nerve with adjacent structures will help us in preparing for safe surgery.